[TowerTalk] OCFD: Should I be able to do better? (TowerTalk Digest, Vol 221, Issue 30)

Fri May 28 13:17:15 EDT 2021

Joe, thanks so much for your clear and concise answers to the questions 
I asked.

It's funny that so many folks have immediately proposed a fan dipole 
instead of an OCFD. Now that I'm looking at it, I think I should have 
worded the subject line differently. I didn't mean, "Can I do better 
than an OCFD?" I meant, "With this data on my OCFD, did I do something 
wrong building it, or is this the way it's supposed to look?"

The choice of a fan dipole is especially funny, in that that is the type 
of antenna that this OCFD replaced. I had to take my old fan dipole down 
when my neighbor asked nicely that I unbolt the 12 foot 2x4 from his 
fence, that was supporting the southern end. Without that support, the 
conducting part of my antenna was about two feet from the 240vAC 
powerline into my house. With the shorter end of the OCFD to the south, 
this is not a problem.

Also, I just wanted to experiment with antennas. Antenna parts are 
cheap, and I can build myself all the types of antennas I'm interested 
in. My next attempt will be a DIY version of the AS-2259 NVIS antenna 
for 80/40/20M, to connect to the Maryland Hospital Net once a month. 
Then, I'd like to build something like the vertical DX Commander SOTA / 
POTA EXPEDITION Antenna Kit for portable operations.

Regarding the "...6.68MHz, with an VSWR of 1.012 and an impedance of 
50.1-j576m ohms" that's exactly what the nanoVNA-saver software 
reported. I ran another, shorter sweep and got resonance at 6.954MHz, 
with a VSWR of 1.023 and an impedance of 51.1-j14.7m ohms, as can be 
seen in the screen shot at 
https://www.dropbox.com/s/tvtne58p2lm6r7b/Screenshot%20from%202021-05-28%2013-09-38.png?dl=0.

Thanks, again, for your advice. I've really enjoyed this discussion.

-Kevin
KC3KZ

On 5/27/21 3:01 PM, Joe Subich, W4TV wrote:
> 
>> 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
> The SWR/Impedance values you report are typical of a 40 meter OCFD.
> The values are certainly within the range of the auto-tuner in most
> quality transceivers *and* represent minimal additional feed line
> losses.
> 
>> The closest resonant frequency is 6.68MHz, with an VSWR of 1.012 and
>> an impedance of 50.1-j576m ohms.
> 
> That is *NOT* a resonant frequency - not with -j576 Ohms.  Your measured
> value at 7.15 MHz (+j7.11 Ohms) is much closer to resonance.  I would
> estimate the actual resonant frequency ( j0.0 Ohms) to be about 7.13
> MHz.  Note: *Resonance* is defined as j0 ohms regardless of the real
> part of the impedance.
> 
>  > Are these reasonable results for an OCFD?
> 
> As above, the values are reasonable for an OCFD.
> 
>> I'm used to tuning monoband dipoles, where I calculate the percentage
>> off, then use this percentage to shorten or lengthen the dipole,
> 
> As others have suggested, you may be better served to use a parallel
> dipole - particularly since you do not appear to be interested in 17
> and 12 meter coverage.
> 
> 73,
> 
>     ... Joe, W4TV
> 
> 
> On 2021-05-27 12:30 PM, 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_project.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%2007-39-52.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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> 
> 