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

Mon Apr 1 09:36:06 EDT 2013

I have run 88 and 102/105 foot doublets fed with 450 Ohm ladder line of whatever length was needed to connect antenna to the eave-mounted balun.  A good tuner (I used an LDG AT-200 Pro) should handle just about anything.

73 - Mac, K2GKK/5

Sent from my iPhone

On Apr 1, 2013, at 7:26, bcarling at cfl.rr.com wrote:

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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