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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?
Coro says to use an antenna tuner. What do you all think? (Michael
Pelikan)
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.
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