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...


Rick, pretty much the only good treatment of the terminated tilted folded
dipole, (T2FD) I've found in Practical Wire Antennas by John Heys, an RSGB
book published in 1989.

Apparently there are also articles in June 1949 QST, and CQ Antenna Roundup
1963.

While some describe the T2FD as a broadband resistor, it's most salient
characteristics are it's broadband coverage over about a 4:1 ratio, and
with the 20 - 40 degree tilt, an aperiodic vertically polarized radiator.
It can be fed with 300 ohm line.

The optimum tilt of the antenna is about 30 degrees.

My skills with ascii art aren't that good, but I'll try to describe the
antenna.

Non Inductive resistor
B | C
____________________________________________ _____________________
| | | |
| | | E |
______________________________________________ _____________________
A | | D
| |
300 - 600 ohm feedline

Of course it must be tilted at 30 deg from vertical.


The length of each leg when measured from the centre of the wires across the
spreaders to the feedpoint or terminating resistorshould be
50,000/f(kHz) X 3.28 feet. The total top length and the lengths AB and CD
will be twice this calculated length. Frequency f is the lowest operating
frequency of the antenna, although it will work at half this frequency with
reduced efficiency. The spacing between the two radiator wires in feet is
found by dividing 3000 by the frequency in kHz and multiplying this by
3.28.

The terminating resistor must be non-inductive if you want the antenna to
work over a large frequency range. It will still work in an inductive
resistor is used but it will become resonant on one or more frequencies and
the feeder must be used as a tuned line.

The terminating resistor is to some extent determined by the impedance of
the feedline. With 300 ohm twin lead, the optimum resistor value is about
400 ohms. With 450 ohm line, a 500 ohm resistor is fine and 600 ohm line
needs 650 ohms. Keep in mind that 50% duty cycle of CW or even lower of SSB
wattage needed in the resistor. Of course RTTY or FSK will dissipate even
more power.

The article suggests that 24 270 ohm 2W carbon film resistors, wired into
four lines of six resistors in series, and then paralleled would produce a
final resistance value of 405 ohms, and would allow 300 watts on SSB.

Hope this adds to the discussion.

Bob, VE7HS