rf everywhere
Cydrome Leader wrote:
In sci.electronics.misc Michael A. Terrell wrote:
Don Pearce wrote:
On Sun, 10 Mar 2013 04:21:03 -0400, "Michael A. Terrell"
wrote:
wrote:
On Sun, 10 Mar 2013 02:43:08 +0000 (UTC), Cydrome Leader
wrote:
Not many folks making hard coax runs anymore.
this stuff was pretty darn old.
are there power levels where they stil use wavegides and the like?
In countries still using analog TV, the UHF final amplifier is often
implemented with klystron in the 100 kW range. The waveguide is quite
large, due to the low frequency.
DVB-T digital TV transmitters typically operate with only 1-10 kW,
consisting of multiple redundant solid state modules, so there is not
much need for waveguides any more.
How do you get that to the antenna without waveguide? Coax losses are
much higher than waveguide, and is less likely to have problems since
there is no dielectric to break down.
Andrew's Heliax is pretty low loss, and good for VHF and UHF runs.
Dielectric amounts to nothing more than a thin spiral spacer - the
rest is all air.
Heliax for 10 KW? Ever had the filter fail on the compressor and get
water in Heliax? I had a stupid SOB for a boss 30 years ago who was too
sheap to replace filtes on schedule and ruined a piece of 3" Heliax used
at 4 GHz. Waveguide is better at high power, and better than Heliax. I
had over 1700 feet of it at one TV transmitter. It carried about 195 KW
of RF to the top of the tower. We had to maintain a set pressure of dry
nitrogen on the waveguide to keep from compressing the sync pulses.
what exciting things happens when you get moisture ingress?
Corrosion, which increases the attenuation. If it gets bad enough,
the conductor will get a hot spot and burn through leading to
spectacular fireworks. The mositure tends to collect in low spots,
which are at regular intervals in helical cable. vertical runs will let
it run down to the first horizontal section and pool.
It also changes the impedance of the air dielectric, which increases
attenuation.
how hot can coax or waveguides run at high powers? I've never been by
large transmitters, so the concept of anything but a power cord running
warm is just strange to me.
IR losses turn current into heat, the frequency doesn't matter. RF
power costs a lot more than power from the utility company, and using a
bigger transmitter to turn that RF into heat costs more for the hardware
& air conditioning.
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