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Questions on broadband antenna design (e.g. T2FD)
Michael Coslo wrote:
C. J. Clegg wrote: On Tue, 21 Nov 2006 23:24:59 +0000, Dave Platt wrote: 2. How do I determine the minimum power rating for the terminating resistor for an antenna that will be driven by 100 watts maximum? Ummm... I'd say that you'd need a resistor capable of dissipating 100 watts, continuous, when used under ambient-free-air conditions in the highest operating temperature you'll encounter. I'd probably de-rate it by at least 50% (200 watt resistor) just to be sure, especially if you're going to be operating RTTY or any other high-duty-cycle mode. One of the unfortunate things about a T2FD is that there are going to be frequencies where most of your power warms up the feet of the birds perching on the termination resistor :-( Yeah, this is exactly what I'm trying to avoid, by careful choice of design parameters like length, resistor value, balun type. Feeding the antenna with 100 watts and having all 100 of those watts dissipated in the resistor, at any frequency within my range of 4 to 9, isn't going to work. If I can't keep the efficiency above 50 percent across the range, then it probably isn't going to be worth doing. Narrowing the frequency range isn't going to help much with the design. I'm no expert, but if I read my signs correctly, the T2FD has the same ups and downs in SWR as does most other multiband antennas. IOW, it relies on frequency harmonic relationships. Pah! I should have said wavelengths, not harmonics!! - 73 de Mike KB3EIA - |
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