I've been gathering information on the T2FD for a long time, intending to build
one and to model it with EZNEC. It is one of a half a million other things on
my list. So far, I've made the 390 ohm terminating resistor, computed the
x,y,z coordinates for EZNEC, and gathered all the articles I could find on the
topic.
These are the articles:
1. "An Experimental All-Band Nondirectional Transmitting Antenna" by G. L.
Countryman, W1RBK, (W3HH), QST, June 1949, page 54.
2. "Performance of the Terminated Folded Dipole", G. L. Countryman, W3HH, CQ,
November 1951, page 28.
3. "A Little Gem for QRP" W. Brandon Randolph, W8VFT, 73 Magazine, May 1984,
page 26.
4. "T2FD - The Forgotten Antenna", Guy Atkins, Fine Tuning's Proceedings 1990,
page A10.1
The T2FD is also described in several antenna books: Practical Wire Antennas (a
RSGB book, I think?) and some of the Joe Carr books.
The reference on the internet given by Peter, KA1AXY, <http:swl.sds.se/antenn>
contains the information in reference 4.
According to the references, the 30 degree angle with the ground and the 6 foot
high lower support are important to the performance of the antenna. IMHO, If a
6 foot high lower support is important, grounding must be important.
On the terminating resistor: In reference 3, the author uses 10 3900 ohm 2 watt
resistors between two small disks of copperclad circuit board to make a 390 ohm
20 watt resistor and he used 300 ohm twinlead down to a balanced tuner (a
zmatch might be appropriate). Reference 1 states that the terminating resistor
should be non-inductive and have a minimum rating equal to 35% of the input
power to the final stage. A balanced to balanced transformer (bal-bal ?)to a
50 ohm resistor (from an inexpensive dummy load) might give a workable high
power terminator with input impedance determined by the number of turns on the
primary. Another alternative: Caddock makes non-inductive power resistors (30
watts or so) in a TO220 package and in a wide range of values. (I've used
50ohm, 1% Caddock resistors to make QRP dummy loads.)
Anyone interested in the references or my XYZ calculations for antenna
modelling is welcome to email me privately.
73, Rich, N3SLR, Baltimore, MD
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