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.

Cebik has a good web page at:

http://www.cebik.com/wire/t2fd.html

Looks like a very useful page. And, the section down at the bottom
labelled "Terminator Resistor Losses" tells the sad tale. With both
of the designs Cebik modelled, the power loss into the termination
resistor is only rarely less than 50%. For the longer of the two
models (165', wide spacing) the losses between 4 MHz and 9 MHz range
from 4 dB to over 6 dB, with an average loss of around 5 dB.

It looks as if it might be possible to model/design a T2FD with an
intermediate length and spacing, which might result in average losses
in the 4 - 9 MHz range which are lower than either the 100' or 165'
models. However, from the shape of these curves I doubt that you'll
get the average loss down much below 5 dB. Even getting the average
loss down below 3 dB looks as if it won't happen, and I don't think
you're going to ge the worst-case loss down below 3 dB at all.

You might want to go back and reconsider the possibility of using a
feedpoint-mounted autotuner - in particular, one which has a
frequency-based tuning memory capability and fast-retune feature, and
which does not require a large amount of RF power to initiate a tune.
Some such tuners require only a couple of watts of RF in order to
tune, and can re-tune to a previously-memorized frequency and
match-set within a couple of hundred milliseconds. I believe that an
autotuner of this sort would be compatible with low-power ALE, as long
as you had "pre-programmed" it by doing a full tuning cycle at each
frequency in the range you're interested in, and given it a chance to
search for and "memorize" a suitable match.

The lady who has demo'ed ALE at a number of hamclubs in my area
(regrettably I don't recall her callsign) uses an ALE-equipped
military radio, and a Motorola (or Harris?) longwire autocoupler
hooked to a random length of wire and a counterpoise.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Wed, 22 Nov 2006 19:29:39 +0000, Dave Platt wrote:

the section down at the bottom
labelled "Terminator Resistor Losses" tells the sad tale.

Yes, it does.

My heartfelt thanks to all of you for all of your assistance and advice.
I think I have enough info now to guide my experimentation and come to a
conclusion.

Doesn't look promising...