[TowerTalk] Coaxial Antennas

[Tom Rauch]

> This might be OK, but according to TLW (v1.0), the impedance of a 33"
> shorted stub of RG-58A at 50.125 MHz is 13.22 + j 206.73 ohms.  This is a
> Q
> is only 15.6 (a typical copper wire coil Q might be 200 or
> more).  Considering just the driven element, there is a total of 26.44
> ohms
> of loss resistance appearing at the feedpoint.  If the total feedpoint
> impedance at resonance is 50 ohms, the loss due to the driven element is
> 3.2 dB.  There is also loss in the director and reflector.  This could
> easily explain the marginal performance of your mini-yagi.

That's true. Many people assume coaxial stubs are low loss loading systems,
but they really are not. They are worse than linear loading.

> N0KHQ uses the velocity factor of the coax to determine the length of the
> elements, but it seems fortuitous that this gets reasonably close to the
> resonant length for an element.

I certainly agree with that also!!! It's likely just sheer luck it worked
out. The same thing when people wind X wavelength of wire in a helically
loaded antenna and sometimes happen to hit some winding distribution  that
just by pure luck or perhaps Edisonian engineering happens to work.

It doesn't surprise me when any coaxially loaded antenna fails to show
reasonable signal strength performance. I expect it.

By the way.....when I measure cables on good lab quality equipment (like a
HP4191A or HP8753E) the cables fall almost in exact agreement with TLA. It
is a VERY good program compared to most others. Stubs indeed have Q's well
below 30. On the other hand aircore coils with #10 wire can provide Q's in
the 300-400 range! Linear loading isn't so hot either, but it sells for some
reason.

73 Tom