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

[bcarling at cfl.rr.com]

I am just picturing how my xyl would react to seeing 80 feet of 450 ohm line piled up on the house someplace even though she's a ham.  Back to good ole 52 ohm coax for me!


David Propper <k2dp at charter.net> wrote:

>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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