Re: KGD antenna QrpProject ?

Hmmm. It looks to me as if this may be a sort of L-match.

The basic antenna is a dipole, with each arm being inductively loaded
at its base with a coil wound from magnet wire. The inductive loading
would allow the relatively short dipole (which would have a very low
radiation resistance, and a high capacitive reactance) to resonate
properly on the 40-meter frequency.

Even when resonated, and even with the relatively high losses in the
coils, the antenna's feedpoint impedance would probably be on the low
side. If I'm understanding the German manual correctly, it's on the
order of 20 ohms.

It sounds to me as if the antenna uses an L-match arrangement to raise
the feedpoint impedance. This would be done by:

- Lengthening the tuning rods at the end slightly (or adding a turn or
two more to the coils than would be required for resonance). This
would increase the resistive portion of the feedpoint impedance
very slightly (trivially), and would add some inductive reactance
in series with the resistance. With the correct adjustment, the
feedpoint impedance would be equivalent to 50 ohms resistive, in
parallel with some amount of reactance.

- Place a shunt capacitor directly across the feedpoint, with the
capacitor's value chosen to have a capacitive reactance equal in
magnitude (but opposite in sign) to the antenna's parallel
reactance. The two reactances would add, resulting in a zero
reactance, and thus a feedpoint impedance of 50 ohms resistive.

A similar matching arrangement is sometimes used with mobile HF
antennas such as screwdrivers and bug-catchers.



Yes it is.
I found the description on my old ARRL Handbook (1994 p33.22)
Thanks a lot...that was simply but I had some difficulties to find the start
of the trail...


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Relevant Pages

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