[Antennas] QRP/P Direct Feed of 1/4 and 3/4 Wires?

Fri Nov 5 20:48:30 EST 2004

Dear Jay,
Just because this antenna is to be for QRP does not alleviate the need for
a proper match (for transmitting. In fact, it is probably MORE important in
QRP than it is for those running hundreds of watts. The reason being, that
when you are close to the S/N threshold, a dB or even a fraction of a dB can
make the difference between copying and not.
You said not one word about the band you will be on or the bandwidth over
which you expect to operate, so all we have to go on is that you will want a
1/4 wave or a 3/4 wave end fed. I assume you intend this antenna to be
horizontal - more or less (at least when 3/4 wavelength is involved).
OK - so portable it is! That in itself is no impediment. If you can fold up
a hank of wire that is long enough to make a decent antenna, it won't hurt
you one bit to make an L-Section matching section that would fit into a
match-box (1.5" x 2" x 1/2"), and here's how you do that ....
Design an L-section impedance match for the antenna based on its measured
impedance in the center of the bandwidth in which you will operate. This
will consist of a single small mica capacitor and a single air wound coil on
a ceramic insulator.
For information on how to design and build such a matching section (you will
want one for each band you intend to operate, see the ARRL Antenna Book. You
might also find my programs, LSECT or SmartSmith of value in doing the
design (See my QSL Web Site below). you can find someone with the necessary
RF bridge to measure your antenna, or a replica thereof, in order to know
what impedance it will have.
Bob Lay (W9DMK), Dahlgren, VA
w9dmk at crosslink.net
Home Pages: http://www.qsl.net/w9dmk
http://zaffora.f2o.org/W9DMK/W9dmk.html
http://meltingpot.fortunecity.com/albania/784