[ARC5] Backwave

Mon Oct 26 12:14:11 EDT 2015

My thanks to Bill KU8H for summarizing the comments thus far.  I certainly
did not intend to start such a far-ranging discussion.  I am VERY aware of
the original purpose of the selector relay but had read of success in using
it for keying by simply bending its contacts slightly so the oscillator is
powered first so that chirp is minimized.   That is not an option since the
relays were excised long ago according to the usual ham modifications in
both CQ and QST.

Bill, the distance was over 100 miles and the power supply was only 155
volts for all three feeds; plates, screens and oscillator.  QRP worked
well.   Obviously band conditions were excellent so that even the micro or
milliwatt backwave signal was heard at that distance.  I am looking for a
solution not just for the backwave but for a non-invasive break-in keying
that includes the oscillator B+ as could have been done with the
now-missing selector relay.  I was thinking possibly of using an
opto-isolator and a high voltage N-channel MOSFET or high volt NPN and also
a mechanical relay for the PA cathodes given the slight millisecond or so
difference between the electronic "relay" for the oscillator and the
mechanical one for the cathodes.  Was wondering if anyone had experience
with such a solution.

Also working on a QRO capability with a Heath HP-20.  Am using a hi-volt
N-channel MOSFET and zeners on the 300+ volt line inside that supply to
freeze that to about 270 for screens and then to a resistor for the 200 or
so for the oscillator but there is also significant backwave with that.

Tnx de Rich KB8TAD

On Mon, Oct 26, 2015 at 9:51 AM, Bill Cromwell <wrcromwell at gmail.com> wrote:

> Hi,
>
> I went back to the original post from Rich, KB8TAD,  and reread it. The
> first time I read it I took it that the friend with whom he is working on
> the transmitter project was hearing the backwave at a distant location. The
> message is ambiguouis about that detail. QRM from the backwave has been
> discussed in the 'classic' literature and that is where my my mind went.
> Maybe Rich can clarify that. Other replies have been about the backwave in
> the adjacent receiver.
>
> Dennis says the backwave is "exceeding weak" as transmitted but does not
> say weaker than... what. Later he says that the frequency shift cannot be
> heard at a local friend's home. Maybe he means the backwave cannot be heard
> there and *that* is encouraging. There is a significant population of hams
> who work the world with a few (single digit) milliwatts demonstrating that
> signals escaping from our transmitters *can* be heard at great distances
> and *can* cause QRM. If the backwave cannot be heard in the local
> neighborhood it probably cannot be heard, nor cause QRM to more distant
> hams. Dennis has apparently checked for that.
>
> Thanks to Neil, ZL1ANM, for reviewing how those circuits worked
> originally. A couple of others pointed out those transmitters work fine "as
> designed". I have used keying relays in the past to key high voltages but
> that was in my home in the middle of the night and NOT in a B-17 at
> altitude! The mechanical clattering was unacceptable and I moved to
> blocked-grid keying. Any keying system has it's own caveats but mechanical
> noise is not an issue with blocked-grid keying. The relays are one of the
> reasons I have not had interest in using the transmitters with the
> receivers that I do use. Thanks to Dennis's comment about the keying
> transistor I will review that and maybe look for transmitters to "go with".
> Meanwhile I am happy with old ham transmitters with blocked-grid keying,
> including keyed oscillator and with one or two stages of isolation between
> the oscillator and the output stage.
>
> I have a half dozen of the command receivers that I have been gradually
> 'moving' back in the direction of "original" operation. If I live long
> enough they will be used with remote controls (probably home brewed)
> through the connector on the back. Maybe not museum pieces but as others
> have said..working "as designed". Meanwhile they are 'on the air' where
> radios belong. I do use mechanical relays with one switch for the T-R
> function but NOT for CW keying. Simple. Effective. Tolerable at 1 AM.
>
> 73,
>
> Bill  KU8H
>
>
> On 10/25/2015 11:16 PM, Dennis wrote:
>
>> I run the osc in my T-22 continuously and key the PA cathodes via a
>> transistor.  The back wave is fairly strong in the receiver but exceeding
>> week as transmitted.  Key down the osc pulls down about 2 KHz.  Because I
>> spot the rig key down into a dummy load, the osc runs 2 KHz above the
>> transmitting/listening freq. so a decent IF filter or notch eliminates the
>> potential self QRM.  On 80M the pull would be less (1 KHz?).  Osc and PA
>> screens are regulated so minimal classical chirp.  One cannot hear the very
>> fast 2 KHz pull in the home receiver or at a local friend's house.
>>
>> Dennis AE6C
>>
>>
>>
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