|To:||Bill Fuqua <email@example.com>, firstname.lastname@example.org|
|Subject:||Re: [TowerTalk] Re: TowerTalk Digest, Vol 18, Issue 3|
|From:||Jim Lux <email@example.com>|
|Date:||Wed, 02 Jun 2004 12:50:32 -0700|
At 01:44 PM 6/2/2004 -0400, Bill Fuqua wrote:
At 12:00 PM 6/2/2004 -0400, firstname.lastname@example.org 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.
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?
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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