Rick (W-A-one-R-K-T) wrote:
I've pretty much decided to go ahead and build a T2FD "radiating dummy
load", hopefully for use from 2 to 24 MHz.

This will be almost (but not quite) exclusively for receiving, wherein my
limiting factor is usually noise anyway, rather than antenna efficiency.
If somebody appears on one of the frequencies I scan that I want to talk
to, if he's not S9 or better I'll likely switch over to a "real" antenna.
But, the occasional use for transmitting means I'll need the appropriate
power resistor on the load end (100 watts or so).

You guys have suggested googling for "T2FD" and "TTFD" and I have done
that, but I haven't yet found anything that really lays out how to design
and build one of these things, what the formulas are, etc.

Some of the Google hits suggest using baluns and resistor values all the
way from 11:1 balun, 550 ohm resistor, and 50-ohm coax to 4:1 balun,
300-ohm resistor, and 75-ohm coax. Some seem to imply that one should use
a resistor value equal to the coax impedance times the balun ratio, and
others say that the resistor value should be some percent higher than the
coax impedance times the balun ratio (one site suggested 75 ohm coax, 4:1
balun, and 390-ohm resistor).

Obviously I'd like to do this a cheaply as possible and I do have a W2AU
4:1 balun sitting on the shelf here, so would 50 or 75 ohm coax, 4:1
balun, and anywhere from 300 to 390 ohms for the terminating resistor work
as well as a higher resistance value and a higher ratio, scarce, and more
expensive balun?

I have seen formulas, which I can no longer find, that say what the length
should be for a given minimum frequency, and what the spacing between the
conductors should be. I assume that the spacing of the conductors has a
lot to do with the balun ratio and terminating resistor.

I guess if I can find the length, conductor spacing, and terminating
resistor value needed for 2 MHz minimum and 4:1 W2AU balun I should be all
set.

Thanks...


The terminating resistor of a B&W antenna is 600 ohms resistive i.e.
there is a 600 ohm resistor connected to the ends of the folded dipole
antenna. It's purpose is to swamp the current leftover when the standing
wave is formed. The balun is of necessity a 12 to 1 ratio balun. Other
values may be used to other effects.
The biggest problem is the balun. I haven't seen any specs for making
one. The resistors need to be carbon non inductive high wattage.

Let us know how you do.

Dave WD9BDZ