Terminated Folded Dipole
How to feed a T2FD correctly
Date: March, 2000
Original source: rec.radio.shortwave
Question: I've got a 166 ohm
situation and not, as recommended, a 200 ohm; will this effect
the design quality of this antenna?
John H. Carver Jr.: I really don't think it's going to
hurt you at all. In a receive only situation there's a
lot of slop to things. Besides, of all the articles I've read on the net, no
one really agrees about the value of the reisistor.
Several sources say 390 ohms for a receive situation and if you're using the
antenna to transmit you have to use another whole set of values.
The best way to tell would be just to put the thing up and let your ear tell
you. I've done a lot of crazy things with TTFDs and
they always seem to work okay.
A receiving antenna is a very forgiving thing.
Question: It regards the target impedance for
the T2FD. The sites mentioned in this thread all have Guy Atkins' specs: using
a 4 to 1 balun into a 75 ohm coax (which then presumably goes into a 50 ohm
input). Arnie Coros' specs
call for using a 6 to 1 balun to feed over a 50 ohm coax into a 50 ohm input
(and he adds that the 6 to 1 may be hard to find).
So to follow onto this thread: could someone
discuss how "bad" a mismatch it is to run a 75 ohm coax into a 50 ohm
input? I mean, is it a matter of being a "mere 25 ohms" or does the 75 ohm being 50% greater than 50 ohm tip the
scales?
Dale Parfitt: I really doubt the antenna looks
like 75 Ohms resistive except across a narrow spectrum.
With that said, a 75 Ohm (or a 37 Ohm) to 50 Ohm
mismatch results in a 1.5: VSWR- nothing to worry about.
John H. Carver Jr.: And if you look at the January 1988
Poplular Communications for Joe Carr's antenna column
you'll see slightly different numbers again.
This antenna is also mentioned in Joe Carr's Receiving Antenna Handbook,
Antenna Toolkit and Practical Antenna Handbook. Arnie
Coro always talks about using tuners because he uses most of his antennas to
transmit with also, although, in my experience, using a tuner can't hurt
anything.
While normally I do try to be careful about matching and balancing and all that
because I do believe that the more efficent the
antenna is the more signal you'll pull in, in all actuality receiving antennas
are very forgiving and there's room for all sorts of mismatch without really
affecting reception.
If I had just built a new antenna and only had 50 ohm coax where I needed 75
ohm or vice versa I would use what I had rather than buy new. Believe me, your ear will never hear the difference. Now
transmitting is a whole different ball game.
As I said earlier, I try to be careful about things but I'm not a fanatic about
it. Sometimes people get all hung up on numbers from test equipment that is
testing things that will make no difference to your ear. I've been building
antennas for thirty or forty years now and while I do use test equipment and I
surely wouldn't transmit on an antenna that had a bad SWR, I still use my ear
for the final test meter. Especially on receive only antennas.
Receiving antennas are fun to build and it's virtually impossible to screw
anything up, so build away and have fun.
T2FD -- a match for any antenna
Tests were conducted at the home of
Fred Carlisle, a DXer from rural
The 500 ft. and 50 ft. antennas were
switched in and out of the receiver's high impedance input,
the T2FD fed the 50 ohm coax input. The receiver used was a
As the results show, the T2FD
antenna did not begin to drop off in performance until down in the 120 meterband. This is well below the 60 meterband
design frequency of this particular T2FD.
|
FREQ/ |
T2FD
(NW-SE) |
50'
(N-S) |
500'
(E-W) |
|
17680 RNZ |
S9 |
S8-S9 w/buzz |
S9 w/buzz |
|
17700 RM |
S7S8 |
S7 w/buzz |
S7 w/buzz |
|
17795 RA |
S8 |
S6-S7 w/buzz |
S7 w/buzz |
|
15440 RBI |
S6S7 |
S6 w/loud buzz |
S6 w/loud buzz |
|
15435 BSKSA |
S7S8 |
S7 w/loud buzz |
S7 w/loud buzz |
|
15330 RAI |
S5-S6 |
S5 w/loud buzz |
S6 w/loud buzz |
|
12085 RD |
S8-S9 |
S8S9 slight noise |
S8-S9 slight noise |
|
11955 RJ |
S9 10dB |
S9-C9S8 |
S9 10dB |
|
11940 RJ |
S8 |
S9 |
S9 |
|
11505 RB |
S4-S5 |
S4 moderate noise |
S4-S5 slight noise |
|
9895 RM |
S9 10dB |
S7 |
S9 |
|
9425 VoG
|
S6-S7 |
S5 |
S6 |
|
9375 RT |
S8 |
S6 slight noise |
S7 slight noise |
|
7355 WYFR |
S8-S9 |
S6-S7 |
S7 |
|
7335 |
S8-S9 |
S6 |
S7S8 |
|
7255 VoN
|
S7-S8 slight noise |
S5-S6 mod. noise |
S6S7 slight noise |
|
6165 RN |
S9 15dB |
S9 10dB |
S9 |
|
6150 Rl |
S7-S8 |
S7 |
S6 |
|
6135.4 RFO |
S7S8 |
S6 some noise |
S6 |
|
5030 |
S5 slight noise |
barely audible |
S5 slight noise |
|
5000 WWV |
S9 10dB |
S9 10dB |
S9 |
|
4985 R.Bras.Cent.
|
S5 |
S5 |
S6 |
|
4915 Anhanguera
|
S4S5 |
S5 noisy |
S5 some noise |
|
4885 O. |
S6 |
S6 slight noise |
S6 slight noise |
|
4865 L.V. de C. |
S6 |
S6-S7 |
S5-S6 |
|
4765 RM |
S7 |
S7 slight noise |
S6S7 |
|
3990 VOA |
S4-S5 |
S5 |
S5 |
|
3975 |
S5 |
S5 |
S5 |
|
3330 |
S5 strong noise |
S5 strong noise |
S5 strong noise |
|
3385 Rebelde
|
S5 |
S5 some noise |
S6 some noise |
|
2500 WWV |
S6 |
S6-S7 some noise |
S8 |
|
1610 TIS stn. |
S5 |
barely audible |
S8S9 |
T2FD excellent receiving antenna
From: Frank Gilmore, K0JPJ
Date: May 29, 1993
Original source: Fidonet's shortwave
conference
I used a T2FD in 1961. At that time I had lots of land to
experiment with antennas and each weekend the local gang would come over and we
would put up another one.
W8JK antennas were my favorite wire antennas although
they were rather awesome when built for 40... also had
lots of wind resist. and the smallest ice storm would
bring them down.
The T2FD gave a good accounting of itself. If I
remember correctly I built mine from an article in either CQ or the CQ Antenna
Manual. I don't recall it ever appearing in the ARRL Antenna Handbooks. It may
have been in Bill Orr's Radio Handbook at one time.
It was an excellent receiving antenna for SWL use. After I
dropped traffic nets and did little but chase DX on the higher freqs with a beam I used the T2FD on a Hammarlund
HQ-180
