Re: T2FD, etc.

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From: L. B. Cebik (cebik@utkux.utcc.utk.edu)
Date: Tue Aug 27 1996 - 22:35:49 EDT


Joe,

You are correct. The antennas with resistive components to swamp out SWR
are very much less efficient than vitually any "normal" antenna. It does
not matter whether the arrangement is a folded dipole with a resistor near
300 ohms or a standard dipole with a resistor near 50 ohms. The amount of
power that is radiated will be proportional to the ratio of the swamping
resistor and the impedance that the antenna sees at a given frequency
without the resistor in accord with standard parallel impedance
calculations. This applies equally to incoming and outgoing signals.

The losses will vary with the impedance of the antenna, which will differ
on each band (apart from the swamping resistor). Where power is of little
concern, as in military applications, the losses are not too significant.
However, for QRP work, the losses can be very significant (depending upon
one's circumstances). If you have a natural impedance of 50 ohms and a
swamping resistor in this vicinity, then you will get a net 25 ohm
impedance, a 2:1 SWR, and one have of your power converted to heat. THat
is a 3 dB loss, like cutting your power from 2 watts to 1 watt, and
received signals will be half an S-unit down. That is livable. However,
consider antenna impedances of a feww hundred ohms in parallel with a 50
ohm resistor and calculate how much power is heat and how much signals may
be down. The degree of difference, of course, can only be determined if
you can also have an unswamped antenna of similar proportions side-by-side
for the comparison. Like so many other cases, the antenna may seem fine
if no comparison is possible.

For most (but probably not all) QRP situations, a dipole with parallel
feeders and an ATU is likely to be more efficient as an all band
antenna--and likely cheaper in the long run.

-73-

LB, W4RNL


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