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

[Robert Lay]

Dear Jim,

I am confused by your expression, "center loaded", so I will assume that you
mean "center fed".

There are many antenna designs using off-center feed points that supposedly
offer more tractable impedance levels when operating multi-band.

There are many versions of the "Windom" antenna available from Radio Works,
for example.

In my personal opinion, those antennas are an attempt to provide "no
brainer" solutions to a problem which is not that difficult to deal with in
the more straightforward and conventional manner.

It has been my experience that there is too much emphasis on the
"characteristic impedance" of the lines in question. It really doesn't
matter very much whether the transmission line is 200 ohms or 450 ohms or a
600 ohms or what.

More important is the question of what impedance will be seen looking into
that line at a given frequency and with a given antenna impedance
terminating that line. More often than not, we have no idea what that
impedance will be, and yet there seems to be this overly confident view that
everything will be OK if the balun is designed to transform the 450 ohm
impedance of the ladder line to the 50 ohms of the coax. Nothing could be
further from the truth. The typical "truth" of the matter is that we will
generally have to provide a complex transformation of the impedance seen
looking into the line from whatever it is to approximately 50 ohms, and we
usually have to do that with no knowledge of what that impedance is.

That's where a good, balanced, link coupled tuner becomes invaluable.
Although most of them still cannot transform every impedance encountered,
they will do the job more often than not. Occasionally, you will run into a
situation where a small, additional element of capacitance or inductance in
the feed line is needed for a gross adjustment before attaching the tuner.
For example, in a case that I had with my 80 m antenna, I found that it was
very helpful to add a small, 1" dia inductor of about 10 turns in each leg
of the balanced line, before I connected the tuner. Then I could tune
through the entire 80 m band with the Johnson Matchbox. It definitely helped
that I had measuring equipment available to tell me whether the impedance
looking into the line was high, low, capacitive, inductive, or what.

I hope this helps to explain how the traditional approach works, but I'm
afraid that in the case of the antenna that you describe, there may not be a
good explanation of "what the ladder line is for. A good example is the
number of variations of the G5RV - most of which were completely unknown to
G5RV himself. His original design used ladder line entirely, with NO COAX,
so one might well ask, "what is the coax for".

Bob Lay (W9DMK) in Dahlgren, VA
http://www.qsl.net/w9dmk
----- Original Message -----
From: "Jim Isbell" <millenniumfalcon@cableone.net>
Cc: <antennas@mailman.qth.net>
Sent: Friday, October 31, 2003 4:51 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?