On Fri, 06 April 2001, "Maurizio Panicara" wrote:
>
> I didn't actually calculate it, but in open or shorted lines (stubs) used as
> inductances or capacitors there are extra losses that finally decrease the
> circulating current in the parasitic element, expecially if the line is a
> low impedance coax.
> This loss is not so evident on 160m but exists and is surely bigger in
> coaxial cables than with open wire lines, increasing quite rapidly with
> frequency.
> For maximizing gain and directivity it's convenient to reradiate all the
> energy from a parasitic element and so it's better to design an added
> reactance of this kind taking care to use the shortest lenght of cable that
> produces the desired inductance or capacitance.
>
> 73,
> Mauri I4JMY
>
This is a very good point that is often forgotten when using open or
short-circuited coaxial stubs as reactive elements. These stubs operate with
infinite SWR and as a result, the line losses are MUCH higher than they would
be for a line with 1:1 SWR. The net result is that the impedance presented by
the stub back to the antenna can contain a large series resistive component.
The ARRL Antenna Handbook contains a section which discusses how to use the
Smith Chart to calculate the amount of resistance with a lossy line. When you
plug this resistance into an antenna modeling program, you might be surprised
how much it can degrade the gain of a parasitic array.
Incidentally, to set the record straight, "Petrezewski" is really Dave
Pietraszewski, K1WA. Back in the 1960's I lived in the same town as him when
he was K1THQ. He designed this sloper system as a research project in his
student days at WPI and had one running for 40 meters at his QTH. He (or maybe
it was K1ZZ, then K1ZND, who operated from THQ's place) won DX Contests back
then with what now would be considered a rather modest antenna farm. However,
I haven't heard K1WA on the air for years now.
73, John W1FV
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