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Author Topic: NEC for modeling OCF?  (Read 6627 times)
WX7G
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« Reply #15 on: August 05, 2010, 07:40:51 AM »

I will have time tonight to prepare a note and a modeling file to send you. I'll be at work today and won't be able to access personal email.  

A balun having a CM (common-mode) impedance of 2500 ohms will reduce the feedline shield current to 1/5 the antenna current. This should be more than low enough to avoid RFI issues. The 2500 ohm balun CM impedance is figured for a worst-case feedline length. At 100 watts the feedline shield current will be 0.1 amps.

My experience with many end-fed antennas using the house AC ground has been that with less than 0.3 amps of antenna current there will be no RFI issues.

I'll measure a DX Engineering 4:1 balun today and report the common-mode impedance.
« Last Edit: August 05, 2010, 08:16:18 AM by DAVE CUTHBERT » Logged
K5LXP
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« Reply #16 on: August 06, 2010, 07:59:30 AM »

The model I'm trying to correlate is actually two OCF's with a common feedpoint. The "double" OCF doesn't exhibit the deep nulls on higher frequencies like a single >1wl wire does, plus the allure of most or all bands "without a tuner" has my interest piqued.  But other than the discrepancy of simulated vs published results I'm not sure how to best place the source.  I've tried split source, single source on a short segment and other variations but I'm not sure which results in a "good" simulation with a 4 wire junction point.  So I'm back to possibly building something up and doing some empirical measurements and seeing how closely it resembles any of the models.

Here's a non-ARRL link to the original QST article:

<http://n9wn.com/data%20files/OCF%20Dipole.pdf>


Mark K5LXP
Albuquerque, NM


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WX7G
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« Reply #17 on: August 06, 2010, 08:35:55 AM »

To model this a short segment (2') is what I use. The four antenna wires attach to the two ends and the one source is at the short segment.

The 20 segment free EZNEC can model this antenna on the lowest band but may not do well on the higher bands. For this antenna I would use 2' segments. 
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K5LXP
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« Reply #18 on: August 06, 2010, 09:49:30 AM »

Thanks, Dave.  That was one of my variations but I questioned it due to a reference in the documentation:

"The source shouldn't be placed on a wire which is shorter than about 0.02 wavelength, particularly if adjacent wires connect at an angle."

and,

"It's usually better to use a split source at wire junctions than to insert a short wire containing the source."

Followed by:

"Rules 1 and 2 can be broken when a short wire is used only for connecting a source to a transmission line."


The only time I've used a short wire to place a source is so I could connect a feedline, never within the antenna model.  So this is why I've been messing around with various sources (I, SI) and connections (short wire, or not) and wondering which is the best/correct way.


Mark K5LXP
Albuquerque, NM
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VK2FXXX
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« Reply #19 on: August 07, 2010, 06:36:07 AM »

Gday here is a link to a modeling program that works well,and its free.                             http://home.ict.nl/~arivoors/
Ive been playing with it to model a resonant feedline ocf dipole that I am now experimenting with.
The  resistive part of the impedances  I measured are reasonably close to the modelled numbers using the above program.
Be warned modeling can be addictive.
73 Brendan
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K0BG
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« Reply #20 on: August 07, 2010, 08:27:07 AM »

Interesting article Mark, and I remember seeing it in QST (Boy! 20 years sure has gone by quickly!).

Once upon a time (circa 1975), there was a guy in Indiana (as I recall), who was marketing a tripole OCF, which he claimed didn't have the CM problems (pixy dust coated wire perhaps). I'd love to know now what the lengths he used were, because it would be easy to model nowadays.
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WX7G
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« Reply #21 on: August 08, 2010, 06:53:36 PM »


I just sent you a personal message with my email address.
Terry
[/quote]
Terry, your email did not make it to my in box. Can you resend it? Thanks.
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NZ5E
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« Reply #22 on: August 09, 2010, 05:54:26 AM »


I just sent you a personal message with my email address.
Terry
Terry, your email did not make it to my in box. Can you resend it? Thanks.
[/quote]

I just resent you a personal message.  I did not get your personal message either.  Let me know if you don't get my personal message and we will do this another way...

I purchased the version of EZNEC with 500 segments and I am already addicted to the software!
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KG6YV
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Posts: 508




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« Reply #23 on: August 12, 2010, 09:22:07 AM »

You should go to Yahoo-Groups, enter OCF antenna and join the OCF group.  There are lots of hams subscribed who have had excellent results desigining and buildin OVF antennas. 

FYI,

Greg
KG6YV
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