[Antennas] Loading of dipoles and off center loading of dipoles

Sun, 2 Nov 2003 20:58:33 -0600

Jim,
I have a 40 meter length off-center-fed antenna. It's 22' on short end, 47'
on long side, with a 1:4 Van Gordon balun at the feed point. Antenna is fed
with 50 ohm coax to balun. The antenna was designed with Eznec 3.0 for a low
angle of radiation on 20 meters with an antenna height of 25' in center, 15'
on ends, shallow inverted vee. The antenna can be matched pretty with well
on all bands 40 to 10 with my internal tuner in my Yaesu FT-900CAT. Main
problem is feed line radiation somewhat cured with a large coax choke, about
22' of RG-58U on a 7" plastic form. The choke is in the attic about 10' from
the feedline attachment to the antenna. The antenna modeling and actual SWR
were very close. I can send an Eznec 3.0 file if anyone is interested. The
lengths were determined both by modeling and experimentation. Hope this is
of interest.
Darryl, KK5IB
----- Original Message ----- 
From: "Jim Isbell" <[email protected]>
Cc: <[email protected]>
Sent: Friday, October 31, 2003 3:51 PM
Subject: [Antennas] Loading of dipoles and off center loading of dipoles


> I am using a 66' dipole, center loaded with 57.5 feet of 450 Ohm ladder
> line fed with a 1:1 ferrite bead balun of RG58. and about 20 more feet
> of RG58 to the transmitter.  I built this from instructions without
> questioning anything about the construction.  It works fine.
>
> But now I am thinking of changing to an off center fed dipole so I can
> run 40, 20, 15, and 10 meters instead of just 40 meters.  In my reading
> I find that feeding it at the 2/3 by 1/3 point the feed impedance will
> be 200 to 300 ohms and I can use the same ladder line and a 4:1 balun to
> feed it from the RG58.
>
> In the off center fed dipole I can see the ladder line as matching the
> high impedance of the feed point and the 4:1 balun to match the ladder
> line to the coax.
>
> But here comes the question...............  What the hell is the ladder
> line for on the center loaded dipole?  I assumed it was to match the
> impedance of the center fed dipole to the 50 ohm coax, but I now find
> that the center of the dipole is ALREADY 50 ohms or there abouts, so
> what is the ladder line for?
>
>
> - - -
>
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> Larry Wilson KE1HZ [email protected]
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