[Boatanchors] The 100-foot Doublet - was Antenna Question

[Al Klase]

Gang,

I've used a 100-foot doublet fed with balanced line and an antenna tuner 
for more that 20 years at four different QTH's.  It's a practical 
solution to all-band operation, and give reasonable results, with 
minimal fuss.  Being a balance antenna, a doublet is not dependent on 
having a good RF ground, and it rejects local noise as long as the tuner 
is balanced or uses a balun.

A doublet is a center-fed antenna that is not necessarily a resonate 
length.  The feed-point impedance of a center-fed wire varies from about 
50-75 ohms, when the two elements are each an odd multiple of 1/4 
wavelength, to several thousand ohms, when the elements are even 
multiples of 1/4 wavelength.  As a compromise, we feed it with low-loss 
balanced line with an impedance that is roughly the geometric mean 
(SQRT(50 * 3000)) of the possible extreme feed-point impedances, say 
300-450 ohms.  We then depend upon the antenna tuner to arrive at a 
conjugate match.

100 feet is "'tweener" not an 80-meter dipole, nor a 40-meter dipole.  
The intention is to avoid "inconvenient" matching points, like an 
80-meter dipole, that looks like two end-fed half-waves on 40-meters.  
BTW, 102 feet is the specified length for the G5RV antenna, which 
undertakes the fools errand of implementing this sort of antenna system 
without a tuner.

You can use 300-ohm TV twin lead up to a couple hundred watts.  The 
450-ohm "window" line should tolerate most of a full gallon.  Start with 
some extra length.  The excess can be hung up in a loose coil away from 
metal objects.  Be prepared to cut off a few feet if you have trouble 
matching all bands.  It might be reasonable to keep an extra piece of 
feedline handy for extreme cases.  I terminate my balanced line with 
dual-banana  plugs.  This makes it easy to change connections or plug 
the line into a grounded jack when not in use.

The antenna can be used as a T om 160 meters.  Short the two sides of 
the feedline together, and work it against the best RF ground you can 
muster.

Enough!
Al

-- 
Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/