[Elecraft] The use of a doublet

[Ron D'Eau Claire]

The beauty of a doublet is that the length of the radiator and the length of
the feed line is completely unimportant if you have a wide-range tuner and
the radiator is at least 1/2 wave end-to-end. 

With modern tuners -especially most "automatic" types - the feeder can
present an impedance outside their matching range. That requires adjusting
the feed line length to find something that will provide a match on all
bands. 

Such an antenna should work as well or better than a dipole at equivalent
height all frequencies at which it is at least 1/2 wavelength long. If good
open wire line with a moderate impedance in the 450-600 ohm range is used,
the line losses are very low thanks to the moderate SWR. Typically the SWR
on any frequency will be less than 10:1, which won't result in significant
losses if the conductors are of a decent size. Extremely high currents can
flow at some points along the line, and large conductors help minimize the
ohmic losses in the line. For that reason, 300 ohm "twinlead" or 450 ohm
"ladder line" works, but has larger-than-necessary losses because of the
small diameter conductors those lines use. Whenever possible, I use a #12 or
larger wire for my open wire lines. 

At frequencies where the antenna is considerably longer than 1/2 wavelength,
it shows some gain over a dipole with narrower but stronger lobes.

It will work well down to where the antenna is only 1/4 wavelength
end-to-end, showing only a small loss compared to a full size dipole. The
problem there is that the SWR on the feeder jumps up as the impedance drops
quickly at that size. 

Ron AC7AC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Phil LaMarche
Sent: Tuesday, July 14, 2009 9:32 AM
To: 'dw'; 'Elecraft_List'
Subject: Re: [Elecraft] The use of a doublet


DX Engineering shared an antenna with me that I put up and it works
wonderful on all  bands with a tuner.  120 ft and fed with 90 ft of 300 ohm
line into a 1:1 balun of theirs and into the tuner with RE 213.  Tuner is a
Palstar AT2K.  The feed line length must be 90 feet.  YES, it really works.

Phil