[Milsurplus] BC-230 questions and observation

[Bob Camp]

Hi,

I may be able to help a little here.

An electrically short antenna whip antenna is a fairly good capacitor. If
the antenna was a capacitor you would say that it's Q  was in the 10's or
even  the 100's.

A capacitor of 100 pf at 3 MHz looks like about 530 ohms reactive. If it has
a Q of 10 you can model it two ways. One way is to put a 5,300 ohm resistor
in parallel with it. The other way is to put a 53 ohm resistor in series
with it. Both are perfectly valid ways to do it at a single frequency. If
you want to get really exact there is a little more to it, but that's
another story.

Your phantom antenna with 10 ohms in series with the 50 pf cap is the
equivalent of the same capacitor (more or less) with a much larger resistor
in parallel with it.

When you put the antenna (or resistor capacitor combination) in *parallel*
with the tuned circuit you need to use the model that puts the resistor in
*parallel* with the capacitor. That way you are putting a large resistance
across the tank. The net result is a fairly high Q tank.

What's this all mean. Well when you put a short whip across a tank it
doesn't de-q it a whole lot.

Probably of more significance is that a short antenna will generally radiate
better the higher you go in frequency. That combined with the fair to poor
harmonic suppression of a single tuned tank implies that radiation on the
second or third harmonic is possible.

All of this assumes that the radio was set up and  tuned up right in the
first place. That was not as reasonable assumption back then as it is today.
A lot of early radios would tune almost as well on the second harmonic as
they would on the fundamental. The first transmitter I ever ran fell into
that category. In the days before standard coax connectors and for that
matter aircraft grade coax running an antenna to a transmitter was far from
simple. There are a lot of possibilities for error as well as failure in
flight. Another very real possibility is that the operator cranks up the
power when they don't get through. As soon as they do that most of the early
radios would start generating harmonics in a hurry.

    Take Care!

        Bob Camp
        KB8TQ




> However, here's where i have a problem. The short aircraft
> antenna ( or car whip, or any short antenna ) is modeled as
> a low resistance, 10 ohms or less, in series with a capacity of
> something like 50-100 pF. The "phantom antenna" boxes all
> have some circuit like this in them. So on a BC-230 type
> circuit, where approximately does the tap go to load such
> an antenna? In the ARC-5 transmitter, i can understand the