[ARC5] Re: Harmonic "suppression" in ARC-5s

[Kenneth G. Gordon]

On 14 Dec 2008 at 20:24, Bob Macklin wrote:

> If you us an unmodified ARC-5 Tx with the kind of antenna they were
> designed to be used with they will work just fine.

This is obviously true.

> OTOH:
> 
> If you want to use an ARC-5 Tx with a better antenna then the
> conversion articles are where to start.

I completely disagree: the people who wrote those articles had no real 
idea of, or ignored, how the ARC-5s were DESIGNED to work, and 
either didn't know, or ignored, the fact that they were DESIGNED to 
work into a low impedance, highly reactive, load.

Therefore, although what they advised worked, after a fashion, they 
certainly were not technically correct, and resulted in the ARC-5 being 
incorrectly and inaccurately labeled a harmonic generator and a TVI 
generator.

Although YOU may not have gotten pink tickets or complaints about 
TVI, many other people did when using those conversion manual's 
methods.

> My first Novice transmitter in 1957 was a 40M 6AG7/807 link coupled
> design from an early 50's ARRL handbook. I did not have a LPF and I
> did not get TVI complaints.

So what? I did the same thing. But just because we got away with it, 
doesn't mean we weren't radiating a LOT of harmonics. Mine was in 
1956, BTW.

In addition, I was once told that one reason the FCC chose the 
frequencies they did for the 40 meter Novice frequencies, at least, was 
that any second or third or fourth harmonics that DID occur would fall 
well inside other ham bands, 20 meters, 15 meters, and 10 meters.

> Operating an ARC-5 Tx in it's original configuration is different from
> operating them in typical ham service.

Exactly. 

I repeat: maxing out the link and setting the antenna loading coil to 
zero reduces or completely eliminates the harmonic suppression that 
is built into the ARC-5 transmitters.

It essentially amounts to tapping the antenna directly to the plate tank 
coil: the only harmonic reduction that occurs under such conditions is 
due, almost exclusively, to the plate tank "Q" only, and significant 
harmonics are produced and radiated.

Both the link settings AND the loading coil settings, in conjunction with 
the various capacitances involved, whether antenna or other wiring, 
constitutes, automatically, a kind of harmonic attenuator, since they 
ARE frequency dependent and frequency selective.

If they weren't, then it wouldn't matter how you set either the link or the 
loading coil, as long as the final was tuned to the same frequency as 
its driver was.

You had to adjust the loading coil, and the link, to achieve proper 
power into what ever antenna your airplane carried. And, if you 
changed frequency significantly, you had to RE-adjust those for proper 
power output.

Therefore, those setting ARE frequency dependent, and DO exhibit 
some form of frequency selectivity. Anything that is frequency 
selective AUTOMATICALLY reduces harmonics to a certain extent.

In the ARC-5, you have three "levels" of frequency selectivity, and 
therefore harmonic suppression to the antenna: 1) the plate tank, 
amount of suppression of harmonics determined probably exclusively 
by its "Q", 2) the link and its associated wiring, amount of suppression 
determined by its coupling coefficient (I would imagine) and 3) the 
loading coil and the associated capacitance of the antenna and other 
wiring, amount of suppression determined pretty much by ITS "Q", at 
least.

Eliminate or reduce any of those three, and harmonic suppression 
would ALSO be severely reduced or eliminated.

By cranking the coupling to max, you have pretty much eliminated that 
one.

By setting the loading coil to zero, you have pretty much eliminated 
THAT one.

Whats left? The plate tank "Q" only.

Dave Stinson's data proves what I say. All you have to do is to read 
what he measured when the ARC-5s were operating as originally 
designed, into the loads for which they were originally designed.

They are NOT TVI generators, and DO NOT put out unusual amounts 
of harmonics. In fact, according to Dave's data, when operated as 
originally designed, their harmonic output is fully in compliance with 
MODERN FCC requirements.

Ken Gordon W7EKB