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Author Topic: OCF dipole question  (Read 3950 times)
AA2HA
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Posts: 16




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« on: January 17, 2013, 09:59:14 AM »

Hi to everyone.

No subject is as confusing and contradictory (form one "expert" to the next) in the hobby as antenna theory. I own books that directly contradict each other, etc.
 Anyway, I have a question that perhaps someone may be able to shed some light upon. I decided to try my first OCF dipole as an experiment. One leg is 180' and the other is 90' (using AWG 10 copper wire). It's fed with about 100 feet of coax (LMR-400), I'm using a DX Engineering 4:1 balun. The center is about 70' and the ends are about 20'. The SWR was extremely impressive on 160, 80, 40, 20, and 6 and not too bad on 10. The only band I can not tune with the internal tuner is 30 meters. (This is all after a little trimming).
 I tested the antenna, on-air, with good receive and signal reports and permanently mounted it. I performed some A/B tests with my 80 meter dipole and in most cases, the OCF antenna worked better, much to my surprise.
 Anyway to my question: I added a Poly-Phaser protector and grounded the antenna. The resonant frequency went up to the high portion of the band on some bands, and the lower portion on others. Thinking the Poly-Phaser may be the culprit, I removed it and just connected the ground (about 6-7 feet of 10 ga. THHN to a 10-foot 3/4" rod) and had the same results.
 With the ground disconnected, the SWR is lower on the bottom portion of the 160 M band, when it's connected to the ground rod, it changes to the upper portion of the band.
 I'm not exactly what forces are in play here. I would use my antenna modeling software, if only I could figure out how the heck to use it. Any ideas what may be going on here? I should consider myself lucky; install a switching relay at the ground connection and cover most of each band by switching the ground in and out. Smiley
 This doesn't happen with my 1/2 wave 80 Meter dipole, but does with the OCF dipole. (Scratching my head) Huh
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W5DXP
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« Reply #1 on: January 17, 2013, 10:16:25 AM »

I'm not exactly what forces are in play here.

Seems obvious that you have common-mode signals flowing on the outside braid of your coax. That is a known problem with OCF dipoles especially if your 4:1 balun is a voltage balun instead of a current balun. The band-aid that the Carolina Windom people put on their very successful system is a common-mode choke about 20 feet down from the 4:1 balun. If you can dissipate/reflect the common-mode signals at that common-mode choke point, your lightning arrestors should have little effect. If you accomplish that feat, the coax will only radiate for the first 20 feet, just like the Carolina Windom.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
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« Reply #2 on: January 17, 2013, 10:46:49 AM »

As W5DXP alludes to, the OCFD is known for common mode current: that means that the
coax is not well decoupled from the antenna, and, in fact, the outside of the coax is
actually part of the antenna.  Anything you do that changes the routing of the coax or
what is attached to it (such as, in this case, grounding the shield) is actually changing
your antenna:  of course it changes the resonant frequencies, etc.

Using a current balun at the feedpoint, and adding an additional ferrite choke to the
coax, might provide enough decoupling that the antenna behavior will be more consistent.
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KU3X
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« Reply #3 on: January 17, 2013, 03:41:23 PM »

Just like mentioned above, "You need a good 4:1 Guanella current balun ."
Also, the OCF you are using is not to be used on 15, 30 or 60 meters, hence the high SWR on those bands.
Barry

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K0ZN
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« Reply #4 on: January 17, 2013, 07:28:13 PM »


The cold, hard fact is that an OCF is NEVER a fully balanced antenna nor is it a properly fed UN-balanced antenna. It CAN'T be. Such being the case, it is nearly
impossible to keep *some* RF off the outside of the coax simply because the currents in the un-equal antenna sections are not equal and don't off set.
Keep in mind that a balun is designed/intended to work into a BALANCED load....and an OCF is NOT balanced. This cannot be a happy marriage! This is the classic "square peg in to a round hole" situation. No matter how people or "experts" try to twist or justify it, an OCF is an antenna that is NEVER optimized electrically. This does not mean they don't or won't radiate: they do, but not in a mode that is balanced or optimized. To Wit: there will be current on the feed line and the amount will be impacted by the frequency.

You NEVER get something for nothing in the antenna game!

73,  K0ZN
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KE6EE
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« Reply #5 on: January 17, 2013, 07:57:07 PM »

You NEVER get something for nothing in the antenna game!
73,  K0ZN

On the other hand it's very easy to get nothing for something in the game!  Roll Eyes
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KU3X
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« Reply #6 on: January 18, 2013, 11:13:04 AM »


...................... Such being the case, it is nearly impossible to keep *some* RF off the outside of the coax simply because the currents in the un-equal antenna sections are not equal and don't off set.......
73,  K0ZN

I disagree with this statement. I use an OCF80 and have a 4:1 Guanella Current balun at the feed point of this antenna. I attached a current probe to the coax (RG 213) and transmitted on all of the bands the antenna is designed to operate on (6, 10, 12, 17, 20, 40 and 80 meters). The current meter was placed on its lowest scale. Not one time did any RF show up on the outside of the shield of the transmission line.

The key here is to use the proper balun.

Barry
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W5DXP
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« Reply #7 on: January 18, 2013, 03:26:04 PM »

Not one time did any RF show up on the outside of the shield of the transmission line.

You were detecting the magnetic field with your current meter. When the magnetic field of a standing wave is at a minimum, the electric field of a standing wave is at a maximum. How do you know that there wasn't a high electric field at the point you measured a low magnetic field? The higher the SWR, the lower will be the current measurement at a standing wave current null.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
N7WR
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« Reply #8 on: January 20, 2013, 06:26:26 AM »

In the research I did before homebrewing my OCFD (180ft/90 ft on the legs) I read somewhere that the value of the CURRENT balun will vary with the height of the feedpoint.  What I read indicated that if the feedpoint is over 65 ft in height a 6:1 current balun should be used, so that is what I did as mine is fed @ 90 ft with the ends being up about 30 feet.  From the balun my coax run to the remote antenna switch is about 100 ft and the output of the remote coax switch goes to a Polyphazer before the 200 ft run of hardline back to the entry panel at the shack.  My SWR's on the bands the antenna is designed to work on are just fine.
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AA2HA
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« Reply #9 on: January 21, 2013, 07:55:08 PM »

Thanks for all the input. I am using a 4:1 current balun for the OCF dipole : http://www.dxengineering.com/parts/com-bal-41150e

 I suppose the common mode current makes sense. It's very interesting to note that 9 out of 10 contacts on 80 meters have the OCF dipole about one S-unit or more on both receive as well as received signal over the 1/2 wave 80 meter dipole.
 perhaps when the weather warms up a bit (it's going to be in the single digits at night and the teens/days for the next week) I'll experiment with an isolator and see what results I find. That's the great thing about our hobby- we're always learning and even after 23 years, there are still new avenues to explore.
 In the spring, I'll likely put a "fan diople" back up, but for now, the OCF was an interesting experiment and it appears, much to my surprise, to work quite well, especially on 160 and 80 (my two most active bands.)
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N4CR
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« Reply #10 on: January 21, 2013, 10:49:31 PM »

..but for now, the OCF was an interesting experiment and it appears, much to my surprise, to work quite well, especially on 160 and 80 (my two most active bands.)

Other than being known for common mode currents which can lead to RF in the shack, OCF dipoles convert energy sent to them about the same as any wire of the same length. When you send energy to an antenna, it either radiates or gets converted to heat. So assuming you have an efficient feeding system, it radiates.

If you put an Un-Un in the feedline about 30-40 feet away from the feed point, you can choke off more of that common mode current and keep even more RF out of the shack. Where does it go? It radiates in a vertical orientation.

And that is one reason the OCFD can appear to work differently than a purely horizontal dipole is due to the vertically oriented radiation coming from the feed line portion. This can be either good or bad depending on the propagation conditions at the time. I chose to call it different.

I had one up for years but I got tired of the RF in the shack so I put up a ladder line fed doublet which is a balanced antenna with about the same multiband performance the OCFD had. I have no RF in the shack with that one even with an amplifier.

I keep a small relative field strength meter sitting between my radio and my amplifier. RF in the shack is easy to detect with that.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
W5DXP
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« Reply #11 on: January 22, 2013, 04:52:50 AM »

It's very interesting to note that 9 out of 10 contacts on 80 meters have the OCF dipole about one S-unit or more on both receive as well as received signal over the 1/2 wave 80 meter dipole.

That's exactly as it should be because a 180'/90' OCF is an end-fire antenna on 80m while the 80m dipole fires broadside. I'll bet most of your contacts are in the directions that the OCF wires are running.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W1JKA
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Posts: 1763




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« Reply #12 on: January 22, 2013, 10:23:32 AM »

Re: W5DXP reply#11

          Your post clears up something that I suspected but was not sure of .My homebrew 40M new Carolina windom definently gets out more than my 40m dipole at same height but in opposite direction.Until now I had thought it had more to do with the vertical 10 ft. Q section rather than the end fired aspect.Thanks

       
 
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WB6BYU
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« Reply #13 on: January 22, 2013, 11:36:33 AM »

Actually the vertical section is too short to radiate much on 40m anyway.

This is a particular issue for NVIS operation:  you want maximum radiation nearly
straight up for good local coverage (when the ionosphere is cooperative.)  An
80m dipole center-fed with open wire line has both half wavelengths of wire in
phase, so maximum radiation is perpendicular to the wire.  With an OCFD of the
same length the two half wave sections are out of phase (as would be the case
if the wire were fed at one end) so there is a null perpendicular to the wire
(including straight up as such antennas are normally installed horizontally) and
maximum radiation is at roughly 45 degrees to the direction of the wire.

So for any OCFD, the radiation patterns and signal strengths should be the same
as a dipole on the fundamental (where the OCFD is a half wavelength), but the
patterns will be significantly different on the second harmonic.
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AE5JU
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« Reply #14 on: January 30, 2013, 10:12:45 PM »

Your analyzer needs to be grounded to earth ground when testing the OCFD, otherwise you can get false readings as the system "floats".

When you connect the coax directly to the analyzer, if not grounded, then you will get one set of readings.

When you measure through the lightning protection... that is grounded, the shield tied to earth ground, you get another set of readings.

When the coax is connected to your radio, the radio is connected to the ground, then the radio is really seeing the SWR as you would measure with your analyzer grounded.
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