On Sun, 14 Mar 1999 08:44:36 -0600 "Roy Koeppe" <royanjoy@ncn.net>
writes:
>
>A question I have is : Is there a problem with too much of a good
>thing?
>I wind my chokes so that no resonances fall within a desired
>operating
>frequency. The belief is that it contributes to stability by not
>having
>another potential tuned circuit to deal with.
>Guess I'm of the old school that remembers putting a resistor across
>a
>choke to de- Q it without really examining the reason.
>I have noted that in a 6M amp, when I place the series resonance
>around
>70MHz the amp is extremely easy to tame. That is not the case when
>the
>resonance is around 50MHz. Something to do with poor input to output
>isolation in a GG amp ??
>
>Hi Carl; you wrote the above (and I'm sure you meant to say PARALLEL
>resonances in the above),
Thats what happens when you get interrupted while typing. I had meant to
say placing the series resonance above 70MHz in one instance and then
another design had the parallel resonance at 50MHz which was not entirely
stable until I moved that above 60MHz.
The final design took some juggling around since I also had to worry
about second harmonic RF and series resonance.
>
>Well, when a choke is resonant ABOVE the operating frequency, I too
>notice
>things go easier. I'm used to that condition on 160M, where a
>self-parallel-resonant choke, an inch in diameter requires about 350
>turns!
>So in this case I went for a parallel resonance closer to 80M or
>even
>above. (For the reasons you listed in an earlier post--difficulties on
>the
>higher bands with many turns). With the resonance above F-Op, the
>choke
>reflects what I have heard called, "negative inductive reactance" into
>the
>tank circuit. It has that familiar effect of canceling some of the
>capacity
>of C1, forcing us to increase the value of C1 higher than predicted.
>More
>circulating current also flows in the choke winding.
>
>Now, when the choke's fundamental resonance is right on F-Op, no
>reactance
>is reflected in, and the choke's Z is very high indeed. I'm doing this
>on
>80M. But, the same choke on 40M reflects "negative capacitive
>reactance"
>into the tank, causing C1 to be smaller than predicted, and again
>some
>additional circulating current flows in the choke. All this tested by
>dipping, and on the air. (This amp now covers only 80M at present,
>purist
>style). Above 40M a series resonant mode happens, so this
>single-segment
>choke is not for multi-band use at all.
>
>About your instability situation, the type of reactance your tank
>"sees"
>with the 50 mHz or the 70 mHz resonance inserted, must be influenced
>in
>much the same manner as if you placed a small shunt cap across the
>choke to
>ground, or did not do so. The reactance of the higher-resonace choke
>must
>counteract some other undesireable VHF resonance, maybe? That's all I
>can
>come up with, but it's fun imagining :-)
I havent heard anyone comment on the Drake choke designs which have a
fixed C across the upper half of the choke. I should see if I still have
one of those chokes in the junk box and run some tests. The L-4B for
instance covered 6.5 to 30MHz continuous and was/is very popular in
foreign embassy service. I believe the L-7 used the same choke.
73 Carl KM1H
>73, Roy K6XK
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