[TowerTalk] OCFD: Should I be able to do better?

[Wes Attaway]

Here is my 2-cents opinion based on currently using both an OCF and two fan
dipoles.

1. Kevin's available mounting situation may favor an OCF over a true dipole.
It doesn't appear that he can effectively deploy fan dipoles.  It seems that
for this reason he has chosen the OCF design.
2. I have a homebrew OCF that covers 80M to 10M.  It is rarely as effective
as my two 40/20 fan dipoles but it is not a "bad" antenna and actually works
extremely well on 80M.  
3. The OCF required a lot of "fiddling" with the lengths of the two sides in
order to get reasonable SWR on several bands.  There is nothing magic about
the old 1/3-2/3 split.  You can vary the split ratio slightly one way or the
other to get the low SWR points near (or close) to where you want them.
4. The OCF does pick up more noise than the dipoles.  However, I have an
extremely low noise environment so the noise is not a big factor for me.
5. Jim's cookbook has some real good info on noise, chokes and antennas.
His choke designs are better than anything you can buy.  It is easy to buy
some cores and wind your own chokes and baluns.
6. I also built and deployed a 12M vertical dipole antenna based on info on
Jim's website.  It works just fine (when the band is open).  It also works
on 10M with slightly higher SWR.
7. I run a TenTec Orion II at 100W and work plenty of DX on all bands
(except 160).

     -----------------------------------
      Wes Attaway (N5WA)
(318) 393-3289 - Shreveport, LA
Computer/Cellphone Forensics
      AttawayForensics.com
     -----------------------------------

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Lux,
Jim
Sent: Thursday, May 27, 2021 12:49 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] OCFD: Should I be able to do better?

On 5/27/21 9:30 AM, Kevin Zembower via TowerTalk wrote:
> Hello, all,
>
> I'm trying to build my first Off-Center Fed Dipole for 40-10M. I'm
> following the directions at

If you're looking for multiband, then a "fan dipole" is probably a 
better bet - it's symmetrical, just needs a choke at the feed, etc.

The basic design is a 40m dipole, a 20m dipole, and a 10m dipole in 
parallel, with 6-12" between the wires. (Commercial product is something 
like an Alpha-Delta DX-CC).  the 40m will also resonate on 15m.  
Adjustment is a bit fiddly, because there is interaction among the 
dipoles.  What I do is "get it close" and then use an autotuner - the 
mismatch if it's slightly off resonance isn't huge, so the loss in your 
transmission line won't be huge.

Rig up a choke with a couple 31 mix 2.4" cores per Jim's choke cookbook. 
A single choke will have trouble covering the whole 2 octave band, 
although give it a shot, it could work ok - I have a box of cores, so I 
just do more chokes because it's easy.

Easy to build. Works great as an inverted V on a single support.


If you don't have a tuner, make sure your support is on a pulley you can 
hoist and lower.  Get your measuring tool of choice out, cut the wires a 
bit long, and do the "fold back or crumple" the wires to shorten it 
iteratively until it matches where you need it to match.   With 
something like a NanoVNA it's easy to see where the resonances are 
(although you can't get the whole 5-30 MHz band at sufficient resolution 
with the NanoVNA 100 points) and a few tries will tell you how much 
crumpling you need to do.  There is a *lot* of interaction between the 
wires.



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