The folded dipole gives a wider frequency response as compared to a simple
dipole. The folded dipole has a impedance of about 300 Ohms that probably
matches the impedance of you receiver antenna input.

The only reason I can see them shorting it at those points is to tune it to
a certain band of frequencies. Perhaps it was designed for VHF low band use
and was modified for the FM broadcast band.

John

"Colin B." wrote in message
...
Hey all;

I'm currently using a standard twin-lead folded dipole for my FM
reception, and it works great. (all three worthwhile radio stations are
on the same tower, less than 20km away)

The thing is, this (inherited) antenna has been modified. Specifically,
there's a shorting wire across the two conductors of the twin-lead,
about 3/4 of the way between the centre and each end.

Anyone here know why this would have been done, and what (dis)advantage
it provides?

Thanks,
Colin

Colin-

Check out the following from eHam.net
http://www.eham.net/forums/Elmers/16049
----------------------
Twinlead folded dipole formula? Reply
by WB6BYU on February 21, 2002 Mail this to a friend!
The velocity of typical cheap TV twinlead is usually in the range of
0.8 to 0.85 (and can vary along a single role.) This is important if
you are creating a quarter wave matching stub, but not for the
overall length of a folded dipole antenna. Use the standard single-
wire dipole formulas and it should work fine.

One variation I have seen is to connect a short jumper wire across
the twinlead a short distance in from each end of the folded dipole.
The idea is that, while the overall antenna length is not corrected
for the velocity factor, each half of the antenna is effectively a
quarter-wave stub which does need the correction. The shorting
bar is put on each side about 85% of the way out from the center.
This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)


------------------------------
HTH
Mike Metzger

Colin-

Check out the following from eHam.net
http://www.eham.net/forums/Elmers/16049
----------------------
Twinlead folded dipole formula? Reply
by WB6BYU on February 21, 2002 Mail this to a friend!
The velocity of typical cheap TV twinlead is usually in the range of
0.8 to 0.85 (and can vary along a single role.) This is important if
you are creating a quarter wave matching stub, but not for the
overall length of a folded dipole antenna. Use the standard single-
wire dipole formulas and it should work fine.

One variation I have seen is to connect a short jumper wire across
the twinlead a short distance in from each end of the folded dipole.
The idea is that, while the overall antenna length is not corrected
for the velocity factor, each half of the antenna is effectively a
quarter-wave stub which does need the correction. The shorting
bar is put on each side about 85% of the way out from the center.
This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)


------------------------------
HTH
Mike Metzger

Colin-

Check out the following from eHam.net
http://www.eham.net/forums/Elmers/16049
----------------------
Twinlead folded dipole formula? Reply
by WB6BYU on February 21, 2002 Mail this to a friend!
The velocity of typical cheap TV twinlead is usually in the range of
0.8 to 0.85 (and can vary along a single role.) This is important if
you are creating a quarter wave matching stub, but not for the
overall length of a folded dipole antenna. Use the standard single-
wire dipole formulas and it should work fine.

One variation I have seen is to connect a short jumper wire across
the twinlead a short distance in from each end of the folded dipole.
The idea is that, while the overall antenna length is not corrected
for the velocity factor, each half of the antenna is effectively a
quarter-wave stub which does need the correction. The shorting
bar is put on each side about 85% of the way out from the center.
This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)


------------------------------
HTH
Mike Metzger

Colin-

Check out the following from eHam.net
http://www.eham.net/forums/Elmers/16049
----------------------
Twinlead folded dipole formula? Reply
by WB6BYU on February 21, 2002 Mail this to a friend!
The velocity of typical cheap TV twinlead is usually in the range of
0.8 to 0.85 (and can vary along a single role.) This is important if
you are creating a quarter wave matching stub, but not for the
overall length of a folded dipole antenna. Use the standard single-
wire dipole formulas and it should work fine.

One variation I have seen is to connect a short jumper wire across
the twinlead a short distance in from each end of the folded dipole.
The idea is that, while the overall antenna length is not corrected
for the velocity factor, each half of the antenna is effectively a
quarter-wave stub which does need the correction. The shorting
bar is put on each side about 85% of the way out from the center.
This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)


------------------------------
HTH
Mike Metzger

"Mike Metzger" wrote in
message .com...
Colin-

Check out the following from eHam.net
http://www.eham.net/forums/Elmers/16049
----------------------
Twinlead folded dipole formula? Reply
by WB6BYU on February 21, 2002 Mail this to a friend!
The velocity of typical cheap TV twinlead is usually in the range of
0.8 to 0.85 (and can vary along a single role.) This is important if
you are creating a quarter wave matching stub, but not for the
overall length of a folded dipole antenna. Use the standard single-
wire dipole formulas and it should work fine.

One variation I have seen is to connect a short jumper wire across
the twinlead a short distance in from each end of the folded dipole.
The idea is that, while the overall antenna length is not corrected
for the velocity factor, each half of the antenna is effectively a
quarter-wave stub which does need the correction. The shorting
bar is put on each side about 85% of the way out from the center.
This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

"Mike Metzger" wrote in
message .com...
Colin-

Check out the following from eHam.net
http://www.eham.net/forums/Elmers/16049
----------------------
Twinlead folded dipole formula? Reply
by WB6BYU on February 21, 2002 Mail this to a friend!
The velocity of typical cheap TV twinlead is usually in the range of
0.8 to 0.85 (and can vary along a single role.) This is important if
you are creating a quarter wave matching stub, but not for the
overall length of a folded dipole antenna. Use the standard single-
wire dipole formulas and it should work fine.

One variation I have seen is to connect a short jumper wire across
the twinlead a short distance in from each end of the folded dipole.
The idea is that, while the overall antenna length is not corrected
for the velocity factor, each half of the antenna is effectively a
quarter-wave stub which does need the correction. The shorting
bar is put on each side about 85% of the way out from the center.
This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

"Mike Metzger" wrote in
message .com...
Colin-

Check out the following from eHam.net
http://www.eham.net/forums/Elmers/16049
----------------------
Twinlead folded dipole formula? Reply
by WB6BYU on February 21, 2002 Mail this to a friend!
The velocity of typical cheap TV twinlead is usually in the range of
0.8 to 0.85 (and can vary along a single role.) This is important if
you are creating a quarter wave matching stub, but not for the
overall length of a folded dipole antenna. Use the standard single-
wire dipole formulas and it should work fine.

One variation I have seen is to connect a short jumper wire across
the twinlead a short distance in from each end of the folded dipole.
The idea is that, while the overall antenna length is not corrected
for the velocity factor, each half of the antenna is effectively a
quarter-wave stub which does need the correction. The shorting
bar is put on each side about 85% of the way out from the center.
This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

"Mike Metzger" wrote in
message .com...
Colin-

Check out the following from eHam.net
http://www.eham.net/forums/Elmers/16049
----------------------
Twinlead folded dipole formula? Reply
by WB6BYU on February 21, 2002 Mail this to a friend!
The velocity of typical cheap TV twinlead is usually in the range of
0.8 to 0.85 (and can vary along a single role.) This is important if
you are creating a quarter wave matching stub, but not for the
overall length of a folded dipole antenna. Use the standard single-
wire dipole formulas and it should work fine.

One variation I have seen is to connect a short jumper wire across
the twinlead a short distance in from each end of the folded dipole.
The idea is that, while the overall antenna length is not corrected
for the velocity factor, each half of the antenna is effectively a
quarter-wave stub which does need the correction. The shorting
bar is put on each side about 85% of the way out from the center.
This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

Karl Uppiano wrote:

"Mike Metzger" wrote in
message .com...
Colin-

Check out the following from eHam.net
http://www.eham.net/forums/Elmers/16049
----------------------
Twinlead folded dipole formula? Reply
by WB6BYU on February 21, 2002 Mail this to a friend!
The velocity of typical cheap TV twinlead is usually in the range of
0.8 to 0.85 (and can vary along a single role.) This is important if
you are creating a quarter wave matching stub, but not for the
overall length of a folded dipole antenna. Use the standard single-
wire dipole formulas and it should work fine.

One variation I have seen is to connect a short jumper wire across
the twinlead a short distance in from each end of the folded dipole.
The idea is that, while the overall antenna length is not corrected
for the velocity factor, each half of the antenna is effectively a
quarter-wave stub which does need the correction. The shorting
bar is put on each side about 85% of the way out from the center.
This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

My GUESS here, is that since the guy who wrote the above bit is a ham,
he's using a ham radio tuner which has 50 ohm antenna inputs, and
presumably a balun. I don't know much about it, but I do believe they
use 50 ohm coax for most of their antenna wiring.

I could be utterly off base, though. At any rate, it sounds like for an
FM tuner with 300 ohm antenna inputs, this actually should improve the
SWR a bit, which I think would effectively correspond to improved gain.

Colin

Karl Uppiano wrote:

"Mike Metzger" wrote in
message .com...
Colin-

Check out the following from eHam.net
http://www.eham.net/forums/Elmers/16049
----------------------
Twinlead folded dipole formula? Reply
by WB6BYU on February 21, 2002 Mail this to a friend!
The velocity of typical cheap TV twinlead is usually in the range of
0.8 to 0.85 (and can vary along a single role.) This is important if
you are creating a quarter wave matching stub, but not for the
overall length of a folded dipole antenna. Use the standard single-
wire dipole formulas and it should work fine.

One variation I have seen is to connect a short jumper wire across
the twinlead a short distance in from each end of the folded dipole.
The idea is that, while the overall antenna length is not corrected
for the velocity factor, each half of the antenna is effectively a
quarter-wave stub which does need the correction. The shorting
bar is put on each side about 85% of the way out from the center.
This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

My GUESS here, is that since the guy who wrote the above bit is a ham,
he's using a ham radio tuner which has 50 ohm antenna inputs, and
presumably a balun. I don't know much about it, but I do believe they
use 50 ohm coax for most of their antenna wiring.

I could be utterly off base, though. At any rate, it sounds like for an
FM tuner with 300 ohm antenna inputs, this actually should improve the
SWR a bit, which I think would effectively correspond to improved gain.

Colin

Karl Uppiano wrote:

"Mike Metzger" wrote in
message .com...
Colin-

Check out the following from eHam.net
http://www.eham.net/forums/Elmers/16049
----------------------
Twinlead folded dipole formula? Reply
by WB6BYU on February 21, 2002 Mail this to a friend!
The velocity of typical cheap TV twinlead is usually in the range of
0.8 to 0.85 (and can vary along a single role.) This is important if
you are creating a quarter wave matching stub, but not for the
overall length of a folded dipole antenna. Use the standard single-
wire dipole formulas and it should work fine.

One variation I have seen is to connect a short jumper wire across
the twinlead a short distance in from each end of the folded dipole.
The idea is that, while the overall antenna length is not corrected
for the velocity factor, each half of the antenna is effectively a
quarter-wave stub which does need the correction. The shorting
bar is put on each side about 85% of the way out from the center.
This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

My GUESS here, is that since the guy who wrote the above bit is a ham,
he's using a ham radio tuner which has 50 ohm antenna inputs, and
presumably a balun. I don't know much about it, but I do believe they
use 50 ohm coax for most of their antenna wiring.

I could be utterly off base, though. At any rate, it sounds like for an
FM tuner with 300 ohm antenna inputs, this actually should improve the
SWR a bit, which I think would effectively correspond to improved gain.

Colin

Karl Uppiano wrote:

"Mike Metzger" wrote in
message .com...
Colin-

Check out the following from eHam.net
http://www.eham.net/forums/Elmers/16049
----------------------
Twinlead folded dipole formula? Reply
by WB6BYU on February 21, 2002 Mail this to a friend!
The velocity of typical cheap TV twinlead is usually in the range of
0.8 to 0.85 (and can vary along a single role.) This is important if
you are creating a quarter wave matching stub, but not for the
overall length of a folded dipole antenna. Use the standard single-
wire dipole formulas and it should work fine.

One variation I have seen is to connect a short jumper wire across
the twinlead a short distance in from each end of the folded dipole.
The idea is that, while the overall antenna length is not corrected
for the velocity factor, each half of the antenna is effectively a
quarter-wave stub which does need the correction. The shorting
bar is put on each side about 85% of the way out from the center.
This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

My GUESS here, is that since the guy who wrote the above bit is a ham,
he's using a ham radio tuner which has 50 ohm antenna inputs, and
presumably a balun. I don't know much about it, but I do believe they
use 50 ohm coax for most of their antenna wiring.

I could be utterly off base, though. At any rate, it sounds like for an
FM tuner with 300 ohm antenna inputs, this actually should improve the
SWR a bit, which I think would effectively correspond to improved gain.

Colin

In article ,
Colin B. wrote:

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

My GUESS here, is that since the guy who wrote the above bit is a ham,
he's using a ham radio tuner which has 50 ohm antenna inputs, and
presumably a balun. I don't know much about it, but I do believe they
use 50 ohm coax for most of their antenna wiring.

You're basically correct.

An "antenna tuner" (also known as a "transmatch") is a device which
can transform one impedance into another, in order to reduce or
eliminate an impedance mismatch. In ham radio, it's fairly common to
use an antenna system which doesn't match the 50-ohm impedance
expected by the transceiver... the antenna impedance may be lower, or
higher than 50 ohms, and may have a capacitive or inductive reactance.
For example, a "random wire" antenna may have an impedance which
varies from a few ohms at some frequencies, to thousands of ohms at
other frequencies, and it can have a large amount of reactance as
well. A transmitter intended to operate into a 50-ohm antenna load
will not work effectively into such oddball impedances... in fact, if
the transmitter doesn't protect itself against such impedances, it can
burn up!

A transmatch lets you "tune out" the reactance, and adjust the
resistive part of the impedance, so that the transceiver "sees" a nice
50-ohm load and a 1:1 SWR. This allows the transmitter to achieve its
full rated power.

Adjustable transmatches are usually _not_ included in receive-only
setups, such as television sets and FM radio tuners. A modest
impedance mismatch at the receiver usually has little effect on
signal reception in these applications.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In article ,
Colin B. wrote:

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

My GUESS here, is that since the guy who wrote the above bit is a ham,
he's using a ham radio tuner which has 50 ohm antenna inputs, and
presumably a balun. I don't know much about it, but I do believe they
use 50 ohm coax for most of their antenna wiring.

You're basically correct.

An "antenna tuner" (also known as a "transmatch") is a device which
can transform one impedance into another, in order to reduce or
eliminate an impedance mismatch. In ham radio, it's fairly common to
use an antenna system which doesn't match the 50-ohm impedance
expected by the transceiver... the antenna impedance may be lower, or
higher than 50 ohms, and may have a capacitive or inductive reactance.
For example, a "random wire" antenna may have an impedance which
varies from a few ohms at some frequencies, to thousands of ohms at
other frequencies, and it can have a large amount of reactance as
well. A transmitter intended to operate into a 50-ohm antenna load
will not work effectively into such oddball impedances... in fact, if
the transmitter doesn't protect itself against such impedances, it can
burn up!

A transmatch lets you "tune out" the reactance, and adjust the
resistive part of the impedance, so that the transceiver "sees" a nice
50-ohm load and a 1:1 SWR. This allows the transmitter to achieve its
full rated power.

Adjustable transmatches are usually _not_ included in receive-only
setups, such as television sets and FM radio tuners. A modest
impedance mismatch at the receiver usually has little effect on
signal reception in these applications.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In article ,
Colin B. wrote:

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

My GUESS here, is that since the guy who wrote the above bit is a ham,
he's using a ham radio tuner which has 50 ohm antenna inputs, and
presumably a balun. I don't know much about it, but I do believe they
use 50 ohm coax for most of their antenna wiring.

You're basically correct.

An "antenna tuner" (also known as a "transmatch") is a device which
can transform one impedance into another, in order to reduce or
eliminate an impedance mismatch. In ham radio, it's fairly common to
use an antenna system which doesn't match the 50-ohm impedance
expected by the transceiver... the antenna impedance may be lower, or
higher than 50 ohms, and may have a capacitive or inductive reactance.
For example, a "random wire" antenna may have an impedance which
varies from a few ohms at some frequencies, to thousands of ohms at
other frequencies, and it can have a large amount of reactance as
well. A transmitter intended to operate into a 50-ohm antenna load
will not work effectively into such oddball impedances... in fact, if
the transmitter doesn't protect itself against such impedances, it can
burn up!

A transmatch lets you "tune out" the reactance, and adjust the
resistive part of the impedance, so that the transceiver "sees" a nice
50-ohm load and a 1:1 SWR. This allows the transmitter to achieve its
full rated power.

Adjustable transmatches are usually _not_ included in receive-only
setups, such as television sets and FM radio tuners. A modest
impedance mismatch at the receiver usually has little effect on
signal reception in these applications.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In article ,
Colin B. wrote:

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

My GUESS here, is that since the guy who wrote the above bit is a ham,
he's using a ham radio tuner which has 50 ohm antenna inputs, and
presumably a balun. I don't know much about it, but I do believe they
use 50 ohm coax for most of their antenna wiring.

You're basically correct.

An "antenna tuner" (also known as a "transmatch") is a device which
can transform one impedance into another, in order to reduce or
eliminate an impedance mismatch. In ham radio, it's fairly common to
use an antenna system which doesn't match the 50-ohm impedance
expected by the transceiver... the antenna impedance may be lower, or
higher than 50 ohms, and may have a capacitive or inductive reactance.
For example, a "random wire" antenna may have an impedance which
varies from a few ohms at some frequencies, to thousands of ohms at
other frequencies, and it can have a large amount of reactance as
well. A transmitter intended to operate into a 50-ohm antenna load
will not work effectively into such oddball impedances... in fact, if
the transmitter doesn't protect itself against such impedances, it can
burn up!

A transmatch lets you "tune out" the reactance, and adjust the
resistive part of the impedance, so that the transceiver "sees" a nice
50-ohm load and a 1:1 SWR. This allows the transmitter to achieve its
full rated power.

Adjustable transmatches are usually _not_ included in receive-only
setups, such as television sets and FM radio tuners. A modest
impedance mismatch at the receiver usually has little effect on
signal reception in these applications.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:

In article ,
Colin B. wrote:

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

My GUESS here, is that since the guy who wrote the above bit is a ham,
he's using a ham radio tuner which has 50 ohm antenna inputs, and
presumably a balun. I don't know much about it, but I do believe they
use 50 ohm coax for most of their antenna wiring.

You're basically correct.

An "antenna tuner" (also known as a "transmatch") is a device which
can transform one impedance into another, in order to reduce or
eliminate an impedance mismatch. In ham radio, it's fairly common to
use an antenna system which doesn't match the 50-ohm impedance
expected by the transceiver... the antenna impedance may be lower, or
higher than 50 ohms, and may have a capacitive or inductive reactance.
For example, a "random wire" antenna may have an impedance which
varies from a few ohms at some frequencies, to thousands of ohms at
other frequencies, and it can have a large amount of reactance as
well. A transmitter intended to operate into a 50-ohm antenna load
will not work effectively into such oddball impedances... in fact, if
the transmitter doesn't protect itself against such impedances, it can
burn up!

A transmatch lets you "tune out" the reactance, and adjust the
resistive part of the impedance, so that the transceiver "sees" a nice
50-ohm load and a 1:1 SWR. This allows the transmitter to achieve its
full rated power.

Adjustable transmatches are usually _not_ included in receive-only
setups, such as television sets and FM radio tuners. A modest
impedance mismatch at the receiver usually has little effect on
signal reception in these applications.

So then if I read the original explanation correctly...

The fact that twinlead is used in a folded dipole is coincidental, and
each half of the wire pair is a different part of the whole single-wave
dipole path. In other words, there's a single loop path, and no propagation
velocity to correct for.

But each half of the antenna is in effect a twin-lead quarter-wave dipole,
and has the normal propagation of about 85%. Hence the shorting wire, to
shorten the quarter dipole and correct some SWR mismatch.

The next question then, is how much of a difference would this make for
FM reception? Is it noticable? Measureable? Worthwhile to do?
(And yes, I know that getting a better antenna would be an order of
magnitude better than trying to improve a cheap folded dipole :-)

Thanks!
Colin

Dave Platt wrote:

In article ,
Colin B. wrote:

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

My GUESS here, is that since the guy who wrote the above bit is a ham,
he's using a ham radio tuner which has 50 ohm antenna inputs, and
presumably a balun. I don't know much about it, but I do believe they
use 50 ohm coax for most of their antenna wiring.

You're basically correct.

An "antenna tuner" (also known as a "transmatch") is a device which
can transform one impedance into another, in order to reduce or
eliminate an impedance mismatch. In ham radio, it's fairly common to
use an antenna system which doesn't match the 50-ohm impedance
expected by the transceiver... the antenna impedance may be lower, or
higher than 50 ohms, and may have a capacitive or inductive reactance.
For example, a "random wire" antenna may have an impedance which
varies from a few ohms at some frequencies, to thousands of ohms at
other frequencies, and it can have a large amount of reactance as
well. A transmitter intended to operate into a 50-ohm antenna load
will not work effectively into such oddball impedances... in fact, if
the transmitter doesn't protect itself against such impedances, it can
burn up!

A transmatch lets you "tune out" the reactance, and adjust the
resistive part of the impedance, so that the transceiver "sees" a nice
50-ohm load and a 1:1 SWR. This allows the transmitter to achieve its
full rated power.

Adjustable transmatches are usually _not_ included in receive-only
setups, such as television sets and FM radio tuners. A modest
impedance mismatch at the receiver usually has little effect on
signal reception in these applications.

So then if I read the original explanation correctly...

The fact that twinlead is used in a folded dipole is coincidental, and
each half of the wire pair is a different part of the whole single-wave
dipole path. In other words, there's a single loop path, and no propagation
velocity to correct for.

But each half of the antenna is in effect a twin-lead quarter-wave dipole,
and has the normal propagation of about 85%. Hence the shorting wire, to
shorten the quarter dipole and correct some SWR mismatch.

The next question then, is how much of a difference would this make for
FM reception? Is it noticable? Measureable? Worthwhile to do?
(And yes, I know that getting a better antenna would be an order of
magnitude better than trying to improve a cheap folded dipole :-)

Thanks!
Colin

Dave Platt wrote:

In article ,
Colin B. wrote:

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

My GUESS here, is that since the guy who wrote the above bit is a ham,
he's using a ham radio tuner which has 50 ohm antenna inputs, and
presumably a balun. I don't know much about it, but I do believe they
use 50 ohm coax for most of their antenna wiring.

You're basically correct.

An "antenna tuner" (also known as a "transmatch") is a device which
can transform one impedance into another, in order to reduce or
eliminate an impedance mismatch. In ham radio, it's fairly common to
use an antenna system which doesn't match the 50-ohm impedance
expected by the transceiver... the antenna impedance may be lower, or
higher than 50 ohms, and may have a capacitive or inductive reactance.
For example, a "random wire" antenna may have an impedance which
varies from a few ohms at some frequencies, to thousands of ohms at
other frequencies, and it can have a large amount of reactance as
well. A transmitter intended to operate into a 50-ohm antenna load
will not work effectively into such oddball impedances... in fact, if
the transmitter doesn't protect itself against such impedances, it can
burn up!

A transmatch lets you "tune out" the reactance, and adjust the
resistive part of the impedance, so that the transceiver "sees" a nice
50-ohm load and a 1:1 SWR. This allows the transmitter to achieve its
full rated power.

Adjustable transmatches are usually _not_ included in receive-only
setups, such as television sets and FM radio tuners. A modest
impedance mismatch at the receiver usually has little effect on
signal reception in these applications.

So then if I read the original explanation correctly...

The fact that twinlead is used in a folded dipole is coincidental, and
each half of the wire pair is a different part of the whole single-wave
dipole path. In other words, there's a single loop path, and no propagation
velocity to correct for.

But each half of the antenna is in effect a twin-lead quarter-wave dipole,
and has the normal propagation of about 85%. Hence the shorting wire, to
shorten the quarter dipole and correct some SWR mismatch.

The next question then, is how much of a difference would this make for
FM reception? Is it noticable? Measureable? Worthwhile to do?
(And yes, I know that getting a better antenna would be an order of
magnitude better than trying to improve a cheap folded dipole :-)

Thanks!
Colin

Dave Platt wrote:

In article ,
Colin B. wrote:

PMJI, what does it mean "if you are using a tuner to match 50 ohms to 300
ohms"? I am not familiar with the term "tuner" in this context.

My GUESS here, is that since the guy who wrote the above bit is a ham,
he's using a ham radio tuner which has 50 ohm antenna inputs, and
presumably a balun. I don't know much about it, but I do believe they
use 50 ohm coax for most of their antenna wiring.

You're basically correct.

An "antenna tuner" (also known as a "transmatch") is a device which
can transform one impedance into another, in order to reduce or
eliminate an impedance mismatch. In ham radio, it's fairly common to
use an antenna system which doesn't match the 50-ohm impedance
expected by the transceiver... the antenna impedance may be lower, or
higher than 50 ohms, and may have a capacitive or inductive reactance.
For example, a "random wire" antenna may have an impedance which
varies from a few ohms at some frequencies, to thousands of ohms at
other frequencies, and it can have a large amount of reactance as
well. A transmitter intended to operate into a 50-ohm antenna load
will not work effectively into such oddball impedances... in fact, if
the transmitter doesn't protect itself against such impedances, it can
burn up!

A transmatch lets you "tune out" the reactance, and adjust the
resistive part of the impedance, so that the transceiver "sees" a nice
50-ohm load and a 1:1 SWR. This allows the transmitter to achieve its
full rated power.

Adjustable transmatches are usually _not_ included in receive-only
setups, such as television sets and FM radio tuners. A modest
impedance mismatch at the receiver usually has little effect on
signal reception in these applications.

So then if I read the original explanation correctly...

The fact that twinlead is used in a folded dipole is coincidental, and
each half of the wire pair is a different part of the whole single-wave
dipole path. In other words, there's a single loop path, and no propagation
velocity to correct for.

But each half of the antenna is in effect a twin-lead quarter-wave dipole,
and has the normal propagation of about 85%. Hence the shorting wire, to
shorten the quarter dipole and correct some SWR mismatch.

The next question then, is how much of a difference would this make for
FM reception? Is it noticable? Measureable? Worthwhile to do?
(And yes, I know that getting a better antenna would be an order of
magnitude better than trying to improve a cheap folded dipole :-)

Thanks!
Colin

In article ,
Colin B. wrote:

So then if I read the original explanation correctly...

The fact that twinlead is used in a folded dipole is coincidental, and
each half of the wire pair is a different part of the whole single-wave
dipole path. In other words, there's a single loop path, and no propagation
velocity to correct for.

But each half of the antenna is in effect a twin-lead quarter-wave dipole,
and has the normal propagation of about 85%. Hence the shorting wire, to
shorten the quarter dipole and correct some SWR mismatch.

I'll check my reference books tonight, if I can, and see if I can
confirm or clarify that.

The next question then, is how much of a difference would this make for
FM reception? Is it noticable? Measureable? Worthwhile to do?

My guess is that the benefits are relatively minor, for several reasons:

[1] Most twinlead floppy dipoles are built without the shorting wires.

[2] The FM band is a relatively wide one - the width of the band is
roughly 20% of the center frequency. Any relatively thin
half-wave dipole cut for any frequency in this band is going to
be significantly mis-matched, and have a fairly high SWR
somewhere else in the band.

[3] In my experience, a twinlead dipole's ability to receive a clean
signal is usually limited more by the presence of multipath
reflections and nearby noise, than it is by the antenna's basic
signal sensitivity. Making small adjustments in the dipole's
location and orientation is probably going to make more of
a difference than tweaking with the dipole itself.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In article ,
Colin B. wrote:

So then if I read the original explanation correctly...

The fact that twinlead is used in a folded dipole is coincidental, and
each half of the wire pair is a different part of the whole single-wave
dipole path. In other words, there's a single loop path, and no propagation
velocity to correct for.

But each half of the antenna is in effect a twin-lead quarter-wave dipole,
and has the normal propagation of about 85%. Hence the shorting wire, to
shorten the quarter dipole and correct some SWR mismatch.

I'll check my reference books tonight, if I can, and see if I can
confirm or clarify that.

The next question then, is how much of a difference would this make for
FM reception? Is it noticable? Measureable? Worthwhile to do?

My guess is that the benefits are relatively minor, for several reasons:

[1] Most twinlead floppy dipoles are built without the shorting wires.

[2] The FM band is a relatively wide one - the width of the band is
roughly 20% of the center frequency. Any relatively thin
half-wave dipole cut for any frequency in this band is going to
be significantly mis-matched, and have a fairly high SWR
somewhere else in the band.

[3] In my experience, a twinlead dipole's ability to receive a clean
signal is usually limited more by the presence of multipath
reflections and nearby noise, than it is by the antenna's basic
signal sensitivity. Making small adjustments in the dipole's
location and orientation is probably going to make more of
a difference than tweaking with the dipole itself.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In article ,
Colin B. wrote:

So then if I read the original explanation correctly...

The fact that twinlead is used in a folded dipole is coincidental, and
each half of the wire pair is a different part of the whole single-wave
dipole path. In other words, there's a single loop path, and no propagation
velocity to correct for.

But each half of the antenna is in effect a twin-lead quarter-wave dipole,
and has the normal propagation of about 85%. Hence the shorting wire, to
shorten the quarter dipole and correct some SWR mismatch.

I'll check my reference books tonight, if I can, and see if I can
confirm or clarify that.

The next question then, is how much of a difference would this make for
FM reception? Is it noticable? Measureable? Worthwhile to do?

My guess is that the benefits are relatively minor, for several reasons:

[1] Most twinlead floppy dipoles are built without the shorting wires.

[2] The FM band is a relatively wide one - the width of the band is
roughly 20% of the center frequency. Any relatively thin
half-wave dipole cut for any frequency in this band is going to
be significantly mis-matched, and have a fairly high SWR
somewhere else in the band.

[3] In my experience, a twinlead dipole's ability to receive a clean
signal is usually limited more by the presence of multipath
reflections and nearby noise, than it is by the antenna's basic
signal sensitivity. Making small adjustments in the dipole's
location and orientation is probably going to make more of
a difference than tweaking with the dipole itself.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In article ,
Colin B. wrote:

So then if I read the original explanation correctly...

The fact that twinlead is used in a folded dipole is coincidental, and
each half of the wire pair is a different part of the whole single-wave
dipole path. In other words, there's a single loop path, and no propagation
velocity to correct for.

But each half of the antenna is in effect a twin-lead quarter-wave dipole,
and has the normal propagation of about 85%. Hence the shorting wire, to
shorten the quarter dipole and correct some SWR mismatch.

I'll check my reference books tonight, if I can, and see if I can
confirm or clarify that.

The next question then, is how much of a difference would this make for
FM reception? Is it noticable? Measureable? Worthwhile to do?

My guess is that the benefits are relatively minor, for several reasons:

[1] Most twinlead floppy dipoles are built without the shorting wires.

[2] The FM band is a relatively wide one - the width of the band is
roughly 20% of the center frequency. Any relatively thin
half-wave dipole cut for any frequency in this band is going to
be significantly mis-matched, and have a fairly high SWR
somewhere else in the band.

[3] In my experience, a twinlead dipole's ability to receive a clean
signal is usually limited more by the presence of multipath
reflections and nearby noise, than it is by the antenna's basic
signal sensitivity. Making small adjustments in the dipole's
location and orientation is probably going to make more of
a difference than tweaking with the dipole itself.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!