Rick (W-A-one-R-K-T) wrote:

I'm aware of the B&W "radiating dummy load" (TTFD) antennas but I'm not
quite ready to give up that much efficiency and I'm DEFINITELY not ready
to blow $300 on one.

I think you need to rethink this. A T2FD antenna is a radiating dummy load
on frequencies that it is not really cut for. A squashed rombic is more
accurate. On frequencies near resonance it is mostly antenna, as the
resonance drops it becomes an oddly shaped feedline with a dummy load
on the end.

The T2FD was developed by the U.S. Navy and is a popular millitary and
government antenna. The B&W one is designed for that market, you can make
one very easily from copper wire, strips of insulating material and some
resistors. The requirement for the resistor is that it be around 600 ohms
and non resonant, so depending upon exactly what you transmit with,
and how you transmit, the power requirment may be less than you think.

The resisitor must be able to withstand the peak power and the average
power. Carbon resistors tend to withstand much higher peaks for very
short times than they are rated. Average power is often far less than
peak, for example while AM and RTTY will heat the resistors far more
than other modes, CW and SSB will not. One figure I read was 12%.

One book, I have is Practical Wire Antennas, by John Heys, which discusses
it in simple, easy to make terms. The book seems to out of print,
replaced by Practical Wire Antennas 2 ISBN 1-905086-04-0.

I think it came from his book, but I remember a high power resistor
made from a combination of low power resistors in a series parallel
form.

I'm sure you could build one for far less than $300, most of the
money going to the resistor and if you decided to buy one, a
9:1 balun.

Geoff.


--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 U.S. Voice: 1-215-821-1838
Visit my 'blog at http://geoffstechno.livejournal.com/

Geoffrey S. Mendelson wrote:
Rick (W-A-one-R-K-T) wrote:


I'm aware of the B&W "radiating dummy load" (TTFD) antennas but I'm not
quite ready to give up that much efficiency and I'm DEFINITELY not ready
to blow $300 on one.


I think you need to rethink this. A T2FD antenna is a radiating dummy load
on frequencies that it is not really cut for.

But so what... in ham terms, an antenna that radiates 25% of its
feedline power would be considered egregiously inefficient, but hey,
that's only a 6dB hit. Easy to make it up with an amplifier on Tx, and
on Rx, you're usually external noise limited anyway, so, from a system
standpoint, the T2FD isn't a bad approach.

Rick (W-A-one-R-K-T) wrote:
I'm quite active in CAP and MARS and so I need to be able to operate on a
fairly long list of frequencies between about 2.2 MHz and about 27 MHz
(actually up to 24 MHz would probably be OK, not much goes on above there).


Actually, almost any antenna with a 5 dB attenuator would work. a 5 dB
pad would give you a return loss of -10dB, which is around 1.9:1 VSWR..

According to Cebik's models, the T2FD is down about 6 dB relative to a
dipole, worst case, so it would be almost the same.

All you need to do is build yourself a BIG 6dB attenuator that can
dissipate the power.