[ARC5] Getting an "ARC-5 " Transmitter Running- Preparing The Transmitter

[David Stinson]

Mike Morrow wrote:

> Hi David,
> 
> It makes one wonder why, when these rigs were in their ham heyday, 
 > so much ham "re-design" was thought to be necessary....

I guess I'll risk showing how dumb I am again
and try to provide some information....

I believe it was a result of the mis-tuning of the output
circuit by the average user, which lead to harmonic-related
"pink tickets" for a few people.  Word of that spread panic
instead of thoughtful introspection, and out came the chain saws
and dynamite.  Very much like the way people react to problems
even today ;-).

 >... it would be interesting to see measurements of
> the spurious output spectrum when the set is operated
 > all original and under design conditions...

I made these measurements in 1996 using several transmitters.
I used then-state-of-the-art spectrum analysis equipment
borrowed from Uncle Sam in Nevada for the purpose.
I tested the rigs into resistive and reactive loads,
running them straight-through, with a series capacitor,
with an UnUn and with combinations thereof.
While the exact figures are buried somewhere in storage,
I can provide a summary:
When operated as they were designed into either resistive
or design-tunable reactive loads, the transmitters
are exceptionally clean, with excellent harmonic attenuation
and no significant HF or VHF spurs- all the anti-TVI
flailing around and hacksawing was complete folly.

When tuned as the average ham of 1950 did-
loading coil at zero, link at 100%-
harmonic attenuation was severely reduced, especially
of the third harmonic IIRC, though
never less than 20 db down from the fundamental.
I don't doubt there were some pink tickets for that.
However- even in the mistuned state, there were NO
significant HF or VHF spurs radiated from any transmitter.
I think all those "ARC-5 TVI" reports had more to do with
poor TV front ends and 21 MC TV IF strips than the transmitters.

One exception-  I also did measurements on the ARC-type-12
transmitters, although only into a 50-ohm resistive load.
These rigs double in the final and there is a very significant
.5Fo peak in the output when feed to a dummy load.
At 144 MC Fo, the 72 MC peak was only about 3 db down,
which seems a shame to waste that power, but perhaps
the costs of better selectivity was a factor.
I assume the designers meant the antenna to be the final
selective element.

I hope this is helpful.

73 OM DE Dave AB5S