[Milsurplus] [ARC5] Ant. Series Matching Caps- I Just Don't Get It.

[Ian Wilson]

Dave - a couple of thoughts:

1. when doing this sort of test, the focus tends to be on the hot end of
    the transmitter - but poor RF grounding can easily introduce a stray
    ohm or two of loss, and move the results all over the place

2. you say "nearly identically" - but well, or poorly in terms of watts of
    output compared to last time?

3. measuring gear can go bad over time. I find when measuring power
    that it helps to check with a scope in addition to relying on the power
    meter (this also finds embarrassing things such as transmitting a
    harmonic of what you intended)

I have found a small number of capacitors (all ceramic, IIRC) that seem
to soak up power when used to match ARC-5 TX outputs. Others have
some variation in efficiency but nothing dramatic.

73, ian K3IMW


On Wed, Jan 14, 2015 at 7:04 PM, David Stinson <arc5 at ix.netcom.com> wrote:

> Over a year ago, I was trying different kinds of series caps
> to match the 5-to-12 Ohm outputs of WWII aircraft rigs
> to our 50-Ohm standard.
> I tried several things and discovered that a lot of caps would
> eat-up power.  We talked about it.  Folks said they wouldn't
> handle the RF current and indeed, some of them heated and
> I saw lower power out.  We discussed how "100 pFd wasn't
> always 100 pFd," depending on the cap type.
> I eventually settled on using RF "doorknob" caps as being best.
>
> This evening after work,  I decided to go through several kinds
> of caps and record just what was best/worst etc.
> Remember how some of us have puzzled over how we can
> make a test one day, get a certain result, then days or weeks or
> years later, do it exactly the same way and get radically different
> results?  I know-- sounds crazy, but....
>
> Tonight I tested all these caps on the output of the BC-230
> on 7029 KC and also a good-quality air variable.
> http://home.netcom.com/~arc5/SCR-183/Justcaps.jpg
> They all run between 50 and 68 pFd.
> The ceramics were NPO and N750.
> The little mica is just a mica- nothing special.
>
> With the exception of the red "high voltage" cap, which did
> indeed introduce loss and chirp,  all these caps performed
> nearly indentically.
> The NPOs did seem a little more stable but that might be
> my imagination, too.  Wasn't much difference.
> The air variable did no better than the little 68 puff N750.
>
> I don't understand it.  How can two sets of basicly identical tests
> a year and change apart produce such radically different results??
> I'm starting to believe in gremlins and magic "spooks"
> in our radios.  I don't know what else to think.
>
> The only thing I can suggest when you go to match the output
> of your WWII transmitter is to try several cap types.
> Find the one with least loss and that doesn't introduce any chirp.
>
> No wonder my hair is all falling out....
>
> 73 DE Dave AB5S
>
>
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