[John Byrns]

In article ,
Patrick Turner wrote:

On Jul 30, 2:28*pm, John Byrns wrote:

I have one of these J.W.Miller TRF tuners. *To maintain constant bandwidth
from
the bottom to the top of the MW broadcast band the circuit Q of the coils
must
increase in proportion to the frequency, this can be accomplished by
arranging
the circuit so that series resistance, either part of the coil, or
external,
dominates the in circuit Q of the coil. *Then when coupled into band-pass
pairs
the product of the coupling factor k, and the Q, or kQ, must be kept
constant
across the MW band. *Since the Q is increasing with frequency the coupling
coefficient k must be inversely proportional to frequency. *This is done by
keeping the mutual coupling reactance constant across the band.
*Unfortunately
ordinary capacitors and inductors have reactance which varies with
frequency. *
This problem is solved by using a normal capacitor in series with an
inverse
capacitor who's reactance varies proportionally to frequency, the two
"capacitors" approximating a constant capacitive reactance across the MW
band. *
The inverse capacitor is simulated by using the so called "negative mutual
coupling coil", the center tapped coil that you describe which acts as the
inverse capacitor in the mutual coupling circuit.

I like the Miller tuner more than I like the other WE sets.

But what you said in your concluding paragraph has an abundance of
information which leaves the obscurity around the subject described at
least highly persistant.

I found the Q got higher at the low end of the BC band so in one radio
I used two input coils and two gangs on one set so that each coil
tuned each side of a centre F at the low end, and both together at the
top end where whatever the Q was, it wasn't enough to cut sidebands
much for the wanted 10kHz of AF BW. The two coils had their own copper
can and were bits of ferrite rod and solid wire. The first had a
loosely coupled input primary winding from a bittowire antenna, with
the tuned winding coupling to the second tuned LC via a carbon comp R
= 39k. Seemed to work just fine. But fluorescent lamps ruined
reception and I changed to one hand made coil on a single ferrite rod.
Audio HF remained OK and hum ****ed off, so I is 'appy wiff wotteye
got now.

I think you need to use Litz wire instead of solid wire to wind the coils, like
the crystal set guys do, to get the required Q at the high end of the band.
Then add a small series resistor to each coil to reduce the Q at the low end of
the band; this series resistance has little effect at the high end of the band
when variable capacitor tuning is used. Then you need to do some reading on
filter theory to find out the proper way to couple the two coils. This approach
will provide a flatter response, with less finicky alignment and lower losses
than your ad hoc approach.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/