Re: [TowerTalk] Re: TowerTalk Digest, Vol 18, Issue 3

[Jim Lux <jimlux@earthlink.net>]

At 01:44 PM 6/2/2004 -0400, Bill Fuqua wrote:
> At 12:00 PM 6/2/2004 -0400, towertalk-request@contesting.com wrote:
> > I expect most antennas would fail by arcing limits for low
> > duty cycle modes and by heating on high-duty modes like CW.
> > The failure points I've found were always heat related in
> > the coil. Away from coil self-resonance near the finger
> > stock, otherwise in the area of maximum voltage where the
> > coil is self-resonant.
> >
> > 73 Tom
>
>
>         It turns out that the shorter an antenna is relative to a 
> wavelength is, the higher the Q of the resonant antenna system (antenna, 
> transmission lines, coils, inductors and tuner)  must become to 
> effectively radiate RF energy. And in achieving that higher Q using coils 
> and/or capacitors there is a good chance of increased losses due to the 
> higher AC currents and voltages involved. Also, along with that comes 
> narrower antenna system bandwidth. If a electrically short antenna is 
> broadband then it must have a lot of loss. Screwdriver antennas have a 
> lot of inductance, I suspect that there is appreciable loss in the 
> inductor along with high currents and voltages.
>
> 73
> Bill wa4lav

The loss of the inductor is whatever it is, however, it's spread out over a 
fairly large area (unfortunately with a cover over it... maybe drilling 
some holes to allow cooling air would be a good idea).

As far as what "appreciable" might be... say the coil has a Q of 10 (which 
is actually pretty bad).. Running, say, 500 W through it, you'll dissipate 
50W, which is a bunch, but probably not too bad when spread out over 20-30 
square inches.  (2 inch diameter x 5 inches long = 30 square inches)

Screwdrivers aren't broadband antennas, so there's no "broadbanding via 
loss" - in fact, they're hideously narrow band, particularly on low 
bands.  I'd guess that on 75m, mine has a SWR bandwidth of around 10-15 
kHz.   This admittedly imprecise estimate of bandwidth implies a system Q 
of 100-200. The radiation resistance is only a few ohms, and we can assume 
the inductor  (and the car body, etc.) probably has losses in the same 
ballpark (at resonance, the match is pretty good to 50 ohms, through the 
4:1 transformer, implying that the feedpoint resistance is 12.5 
ohms).  Ballparking, then, the inductor must have a reactance of a few 
kohms... maybe 100 microhenries at 3.8 kHz?


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