In article ,
Rick (W-A-one-R-K-T) wrote:

On page 7-10 of the 20th edition of the ARRL Antenna Book is a description
of an off-center fed dipole. I have the necessary materials around here
so I thought I'd experiment with that a bit and see how it does.

I'm a little confused about the balun.

I don't have a 4:1 or 6:1 current balun, but I do have a 4:1 W2AU-type
balun and an MFJ-915 RF Isolator, what they call a "1:1 current balun"
(actually an unun). I figured on connecting the latter to the former with
a double male PL-259 adapter and then connecting the coax to the other end
of the unun ... rather heavy, and no way to support it at the balun, and
it'll no doubt droop like a 400-pound beer belly, but I guess I'll jump
off of that bridge when I come to it...

The instruction sheet for the MFJ-915 says I should put the unun in the
line at the transmitter end, and yet the discussion of the OCF dipole in
the Antenna Book shows the 4:1 current balun at the antenna and a length
of coax to the transmitter.

So, should I connect the unun to the balun as I have it, or should I
connect it at the transmitter end as MFJ advises?

It does seem that if the objective is to keep RF off the outside of the
coax, the unun really should go up at the antenna end.

What say you?

Based on the experience of a few of my ARES/RACES cohorts with their
OCF dipoles, I'd say that "it depends".

Putting the unun/isolator up at the antenna will tend to block
*conduction* of RF down onto the feedline from the radiator.

However, because the feedline is not located symmetrically with
respect to the radiator, there will be some amount of RF induced onto
the outside of the feedline by the (unequal) RF fields from the two
sides of the radiator. This will allow some amount of RF current flow
on the feedline, and (depending on your shack grounding arrangements)
might possibly allow for some amount of "RF on the case", RF feedback
into the audio chain, and similar annoyances.

If you put the isolator down at the transmitter, you may have more RF
flowing on the outside of the feedline (as it can be induced as above,
and can also flow down via conduction) but the isolator/unun will
present a high impedance to this current flow and keep it away from
the rig.

Due to the differences in the feedline RF current flow, you may find
that the SWR as seen at the rig is better in one configuration than in
the other. Which arrangement gives the better match is likely to
depend on the length of the feedline, height of the antenna, proximity
of the antenna to buildings and trees, and so forth.

In our Field Day use of a Buckmaster 130' OCF, the guy doing the setup
ended up with the isolator located near the rig... he got a better
match to 50 ohms that way, and preferred to keep the induced RF away
from the rig, PCs, sound-card interfaces, and so forth.

I'd suggest that you try it both ways, and use whichever way that
you find you prefer.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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