Subject: Zero-Weight Feedline & SD20 Super-light Antenna
From: Ade Weiss W0RSP (aweiss@usd.edu)
Date: Fri Feb 11 2000 - 01:50:31 EST 


Hi gang:

Light-weight antennas are always of interest, and the first step is the
fabled SD20 20-ft pole.

In a recent post, Lee W0VT made the following suggestion which some
may take as not-too-serious:

"If you really are worried about the weight of a feedline, why use one at
all?  Instead put up an end feed antenna coupled to a tuner! Then the
coax running from the rig to the tuner could be as short as 12 inches
long.  I think RG-174 one foot long would weigh about 2 oz.  Can anyone beat this!"

Actually, it is a serious alternative to using the SD20 as a vertical-mount
or in other schemes.

Joe Everhart N2CX (of NJ-QRPC fame who needs no introduction) has a
piece in the Jan 2000 QRP QUARTERLY with exactly this application.

It is a simple halfwave wire for the lowest desired band of operation, fed 
at the end with either a tuner or an L/C network.

The SD20 raises the middle of the 67-ft wire (40m = lowest band) 20-ft
off the ground, and the two sides are sloped Inv-Vee fashion. The tuner
is connected to one end, with a short counterpoise wire strecthed on the
ground -- 33-ft will do. There is no reason to bring the far end down to
the ground, but keep it as high as possible. The tuner-end also can be
rigged to drop the end vertically to the tuner. 

This is a resonant wire antenna on 40-20-10,  3rd harmonic on 15m,
and a random wire on 30-17-24m. The current maximum is at the center
on 40m, half-way down the wires on 20, and three appear on 15m -- not
ideal but OK.  Any losses will be attributable to ground-proximity on 
40-30-20, but on the higher bands, we're almost at a halfwave height or
more. Incidentally, contrary to common knowledge, a dipole at a halfwave 
height will generally outperform a vertical with less than 16 or
so radials -- even at low angles -- and definitely one with only 4 radials.

The beauty of the system is that there is no feedline, and hence no 
feedline loss -- a very significant plus -- especially considering the
exorbitant amount of power that is wasted in RG174U and ribbon
cable feedline. Add to that resonance as per the above. Add to that some 
gain at 20m and above. 

Joe notes that light guage wire #22 or smaller the top of the pole won't
bend that much. Actually from an efficiency viewpoint, using #18 can
reduce the r.f. resistance of the radiator by about 50%. A perfectly
straight pole-top is an aesthetic criteria -- neither the r.f. nor the 
QRP'r at the other end is going to know the pole is bent! A perfectly straight
pole as a criteria simply has no place in antenna theory!

Check out Joe's detailed description in Jan 2000 QRP QUARTERLY p. 5.

Would I recommend this antenna? Well, if you have a W6MMA vertical
for 40/30m and are going to stick 16 radials under it, I suspect that the
W6MMA is going to outperform the low horizontal on long hauls on 40/30m, 
but the N2CX end-fed Inv-Vee will be better closer in. However, I also suspect 
that on 20m and above, the N2CX will outperform the W6MMA on US paths and very 
likely on DX paths.

The major advantages of the N2CX are efficiency due the absence of loss 
components, plus some gain on the high bands.

Another advantage is utter simplicity. You cut 67-feet of wire, then
more or less 30-ft for the counterpoise, and that's it!

72, Ade