Posted by
N2EY on
Oct 28, 2006; 3:28pm
URL: http://elecraft.85.s1.nabble.com/Attic-Antenna-tp394788p394809.html
In a message dated 10/27/06 10:05:38 PM Eastern Daylight Time,
[hidden email] writes:
> > B&W makes a "Folded Dipole", 7-30mhz, 45 feet long -
> > with supposed flat swr? Like $245. Our attic antenna
> > needs to be pretty much left by itself - no trips up
> > there to change frequencies. And 40 thru 10 would
> > be nice.
>
> This *is* resistively loaded. It is not a scam like some, but it is less
> efficient than an ordinary dipole.
It is a version of the T2FD, which has been discussed extensively all over
the 'net.
The best discussion I have seen is on the W4RNL (Cebik) website.
The efficiency of a T2FD varies over its frequency range. At the top end it
may be only a few dB less than a dipole of the same size, while at the bottom
end it is much less efficient. See the graphs on W4RNL's site - they're not
pretty.
While it *is* possible to make contacts with an inefficient antenna, remember
that the T2FD is much less efficient than a dipole of the same size, erected
in the same place.
The T2FD isn't new - it's been around since the late 1940s, and was
originally meant for receiving applications where its low efficiency wasn't really an
issue. It finds use in some military and government applications where its
broadband nature is essential (ALE, spread spectrum). The military and government
folks get around its inefficiency by running more power. Lots more power!
OTOH a T2FD can give you the QRP experience with a 100 watt rig.
For normal ham use it is a very poor choice, IMHO. A trap dipole or tuner-fed
dipole of the same size will do much better.
If you really want to try one in your attic, one can be homebrewed for much
much less than $245. Wire, plastic insulators, homemade balun, noninductive
resistors. Doesn't have to be weatherproof in the attic.
A dipole as long as possible in the
>
> attic, not parallel to wires/pipes if possible, and fed with ladder line
> and a balanced tuner would be close to optimum.
>
>
Agreed. Trap dipole is a second choice. Traps can be made easily from coax
and PVC pipe. You need a dip meter, though.
73 de Jim, N2EY
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