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

Peter Voelpel dj7ww at t-online.de
Thu May 27 14:18:39 EDT 2021


The statement is: " Stick with RESONANT antennas fed with coax and chokes",
not "An OCFD is not resonant"!

Antennas not fed symmetrical and choked CAN pickup more local noise.
If you don´t have any you are just lucky.

73
Peter

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of John
Kemker
Sent: Donnerstag, 27. Mai 2021 19:56
To: towertalk at contesting.com
Subject: Re: [TowerTalk] OCFD: Should I be able to do better?

Since when are properly designed OCFD antennae not resonant?  I've used
OCFD on multiple bands for years and not noticed any more noise than on
similar single-band dipoles.  Admittedly, I haven't measured the noise
figures of those antennae, but your statements don't make sense compared to
my experiences with OCFD.

On Thu, May 27, 2021 at 1:33 PM Jim Brown <jim at audiosystemsgroup.com> wrote:

> Off-center-fed antennas are a really bad idea for today's world. They
> are a sitting duck for all of the noise surrounding us, and, by design,
> they cannot be choked to suppress that noise. Further, from what I've
> seen on their website, and from one of their products that I measured,
> Balun Designs does not have a clue about building effective chokes, nor
> did the unit I measured come close to their published data for it.
>
> Stick with RESONANT antennas fed with coax and chokes from my website (I
> don't sell anything, you build them at a fraction of the cost).
>
> 73, Jim K9YC
>
> On 5/27/2021 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
> > https://www.balundesigns.com/content/OCF%20Antenna.pdf and built the 4:1
> > balun from the directions at
> >
>
http://www.cvarc.org/resources/Tech_Articles/dual_core_balun_construction_pr
oject.pdf
> .
> > The OCFD has arms of 44.0' (13.4m; 64%) and 24' 9" (7.55m; 36%) and the
> > feedpoint is mounted at the peak of my roof, about 30-40' above the
> > ground. The long arm is mostly level, but the short arm slopes down at
> > about a 45 degree angle.
> >
> > I don't know what to expect in performance from this type of antenna. A
> > nanoVNA screen shot of sweeping the antenna from 7-30MHz is available at
> >
> >
>
https://www.dropbox.com/s/6cxy0meqqexy6de/Screenshot%20from%202021-05-26%200
7-39-52.png?dl=0
> .
> > Here are some data at the 40, 20, 15 and 10M bands:
> >
> >       Freq    VSWR    Imp
> > 40M   7.15MHz 1.570   33+j7.11
> > 20M   14.15MHz        2.05    28.4+j17.3
> > 15M   21.2MHz 2.334   36.2-j34.5
> > 10M   28.8MHz 1.329   57.4+j13.3
> >
> > The closest resonant frequency is 6.68MHz, with an VSWR of 1.012 and an
> > impedance of 50.1-j576m ohms. My target for the 40M band was 7.15MHz.
> > 7.15 - 6.68 = 0.47MHz too low. 0.47MHz/7.15MHz = 6.6% too low. 68.78
> > feet overall length x 6.6% = 4.52 ft too long.  4.52ft x 64% = 2.89 feet
> > to remove from long side; 4.51ft x 36% = 1.62 feet to remove from short
> > side. Have I done these calculations correctly?
> >
> > Are these reasonable results for an OCFD? If I need to tune it some
> > more, which direction should I go, longer or shorter? I'm used to tuning
> > monoband dipoles, where I calculate the percentage off, then use this
> > percentage to shorten or lengthen the dipole, but what's the calculation
> > for a multiband OCFD, where all the bands interact?
> >
> > Thanks so much for your advice and guidance. I appreciate whatever
> > suggestions you have.
> >
> > -Kevin
> > KC3KZ
> >
> >
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
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> > TowerTalk at contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> >
>
> _______________________________________________
>
>
>
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-- 
--John E Kemker III
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