[ARC5] KJ4KV RF autotransformer circuit

[Christopher Bowne]

This is a matching circuit that Walt presented in one of his early ER in 
Uniform  columns about the Command sets around early 1990.  It consists of a 
parallel tuned circuit where the feed from the transmitter, which includes a 
series nominal 200 pf cap, is tapped very low down on the coil from the cold 
end.  The antenna feed is tapped onto the coil about twice as many turns above 
the cold end as the TX tap point. The cold end is connected to the TX ground, 
and the coax shield as well.  I am using an old Eico 720 pi net coil in mine, 
the TX tap is at 3 turns above the cold end, and the antenna taps is at 6 turns 
up from the cold end.  It acts as an RF step down transformer so the TX is 
looking into a low impedance load like it was designed to work into on an 
aircraft. Be sure to use a variable cap with decent spacing, the RF voltage 
developed on the parallel tuned circuit from hot to cold end is pretty high at 
resonance.  I was using a command set variable cap but it started flashing over 
where the lead to the fixed plates passed through the frame.  A Cardwell with a 
bit wider spacing and terminals for the fixed plates on an insulated side rail 
works fine.

With the DM-33/BC456 powering the T19, I am getting 55 W output on CW, and 15 W 
on voice,  with best output with the link coupling control on the transmitter at 
about 1/2 to 2/3 of max, above that power comes down a bit.  The voice 
modiulation and waveform are way better than I have ever seen using just a 
variable cap in series wth the coax feed into the TX.  Best output/linearity in 
voice is just like the operating instructions state,tune for max in CW, then 
switch to voice.  Tuning for max soup in voice really distorts the modulation.  


I just swept the force fed audio chain/T19 with my HP-201signal gen fed into an 
RF 50 ohm dummy load.  Started getting some distortion below 100 Hz near 100%, 
but otherwise very clean from there to way above 10K with close to 100% positive 
peaks, with no flat topping.  The negative peaks are the limiting factor on 
getting 100% positive peaks, but they are not far off. I did not actually plot 
the frequency response, but didn't lose much on either end of the spectrum.  
Adjusting the final tuning with a 1K tone at 100% modulaton confirmed the 
correctness of tuning for max in CW as the way to get best results on voice.

I also force fed my TCS with the same audio chain, it is losing alot of high 
end.  IIRC there is a cap across the secondary of the mod transformer in the TCS 
that is likely limiting things.  On the TCS, I was using a reversed 4/1 W2AU 
balun as anvRF step down impedance converter, and a 500 pF variable in series 
with the feed to it from the TX, getting about 20 W out of it on voice, 40W CW, 
tuned for max with the roller inductor with the output switch in DIRECT vs using 
the internal SERIES cap switch setting.




________________________________
 From: David Stinson <arc5 at ix.netcom.com>
To: ARC-5 List <arc5 at mailman.qth.net>
Sent: Sun, January 13, 2013 8:06:37 AM
Subject: [ARC5] KJ4KV RF autotransformer circuit

"KJ4KV RF autotransformer circuit"

Can we get some details on this, please?
Google no help.

73 D.S.

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