Off-Center Fed Dipole (Buxcom 80M-6M)

[Bill Striegel]

I am trying to get my ham shack back together after Hurricane Katrina
destroyed it. Bought the above anatenna. I went out back today to lay it out
on the ground trying to avoid the trees. The best position still had a tall
water oak, about 12" in diameter about 6 inches from the antenna, about 20'
from the balun, on the long side. It's 136 feet long, about 100 on one side,
the remainder on the other. Would it be worth trying to move the
balun(feedpoint) closer to this tree? Just how much will this affect my
antenna? Yes, there are trees further away from it, I have a wooded
backyard.  So much for an open field. It runs NE to SW.

 

Thanks.

 

Bill Striegel

KE5UD

Ocean Springs, MS

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[K. Rice]

Bill writes:

> I am trying to get my ham shack back together after Hurricane Katrina
> destroyed it. Bought the above anatenna. I went out back today to lay
> it out on the ground trying to avoid the trees. The best position still
> had a tall water oak, about 12" in diameter about 6 inches from the
> antenna, about 20' from the balun, on the long side. It's 136 feet
> long, about 100 on one side, the remainder on the other. Would it be
> worth trying to move the balun(feedpoint) closer to this tree? Just how
> much will this affect my antenna? Yes, there are trees further away
> from it, I have a wooded backyard.  So much for an open field. It runs
> NE to SW.

Hi Bill,

I don't have direct experience of the BuxCom 806136 or 806137, but
I have used OCF dipoles pretty much exclusively since 1997 including
a Fritzel FD4, two Buckmasters, and four of my own design using homebrew
baluns.  All of those except possibly the Fritzel would stand up
just fine in the situation you describe -- the Fritzel lacked
adequate feedline common-mode choking and it was extraordinarily
finicky about installation.

I have put up my 135-ft OCF field dipole (with a 4:1 current
balun and multi-turn feedline choke) in much worse situations than the
one you describe, and it still pulled in stations as well as
any dipole or doublet of equivalent height and length.

This has been borne out in A/B comparisons with a 135-ft doublet
at a similar height located in the same pasture.

Your cypress tree is located near the high-current region of the
antenna and it shouldn't (or wouldn't with my antennas, the Fritzel
excepting) cause any noticeable change in resonant frequency nor would
it likely increase the minimum SWR you can obtain.  I would be
surprised if it caused any noticeable attenuation, either, though
that would be the likely failure mode, if one exists.

If you do have problems with detuning,  here's the troubleshooting drill:

Nearly all of OCF dipoles' reputation for terrain sensitivity is caused
by inadequate feedline common-mode choking, usually owing to an
inadequat 4:1 or 6:1 balun (such as a voltage balun or a lousy current
balun) at the feedpoint.

If you can sniff RF on the feedline with an RF ammeter, or especially
if grasping or moving the feedline changes SWR, then a good 1:1 current
unun located just below the feedpoint will likely restore good behavior.
If the Buxcom balun is as good as reputed, this should not be a problem
with your antenna.

Although the OCFs offer good SWR on several bands, I always hit the
KAT1 or KAT2 before operating.  This is by way of adding a little
harmonic suppression as possible and achieving a conjugate match at the
transceiver for optimum power transfer.  On most bands, it probably
makes no perceptible difference, but I do it anyway as a matter of
good practice when using multiband antennas.

Let us know how it goes.

73,
Ken
K3VV
Coopersburg
 




 
>
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