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

[David Propper]

Al,

Well explained description of the antenna. However, one very important note to make for all who use any type of open - wire or "ladder Line/ window line" feed lines: 

DO NOT COIL UP THE EXCESS LENGTH OF LADDER LINE. THIS ESSENTIALLY FORMS A TRANSFORMER WHICH WILL PLACE NUMEROUS CURRENT NODE POINTS WITHIN CLOSE PROXIMITY OF EACH OTHER, PARTICULARLY AT POWER LEVELS OF 1KW !!!!  WHAT WILL AND HAS HAPPENED, IS THAT THE COIL , WHICH NOW HAS INSUFFICIENT INSULATION BASED ON THE CURRENTS FLOWING IN THE WIRE, WILL IGNITE AND CATCH ON FIRE.  I HAVE THE PICTURES TO PROVE IT FROM RUNNING 1KW ON 160 MTRS !!!!!  YOU CAN LOOP EXCESS LADDER LINE IN LARGE "S" OR "U" SHAPED CURVES, BUT DO NOT COIL THE LINE LIKE YOU WOULD WITH A HANK OF COAX CABLE IN 12 OR 18 INCH DIAMETER COILS.


Always think SAFETY first !

73,
Dave, K2DP


On Mar 31, 2013, at 3:45 PM, Al Klase wrote:

> 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/
> 
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