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Author Topic: Repairing a "fried" OCFD - balun choice  (Read 2851 times)

Posts: 1531

« Reply #15 on: November 16, 2012, 10:51:59 PM »

The problem with OCF's is that there is NO condition where everything is optimal with an OCF....they are neither fish (unbalanced) nor fowl (balanced)..... feeding them
is electrically kind of like trying to drive a square peg (a balun is INTENDED/designed to work into a Balanced load...which an OCF is NOT) into a round hole (unbalanced antenna load). To wit: the term Bal-Un...  Unbalanced (coax) input transformed to a balanced output LOAD...again, which an OCF is not.  This is not to say an OCF won't radiate...they do...but it is essentially impossible to avoid some issues with them such as RF on the OUTSIDE of the coax shield. I you dig back through the many, many threads on here about OCF's you will find a repetitive issue of RF on the coax and/or SWR problems.  The joke about OCF's is that they do radiate and *most* of the RF is from the antenna section. (the rest is from the feedline! ...this can be good or bad, depending upon a lot of variables.)

What drives the OCF business is the commercial manufacturers claims of "do all, be all" and fed with coax......usually "too good to be true".  You NEVER get something
for nothing in the antenna game. ALL antennas are a compromise in some aspect....  OCF's are just more so.

Count me in with the "use a doublet/center fed zepp" fed with balanced line (usually ladderline) crowd. The antenna is efficient and BALANCED (avoiding some issues) and will work ANY HF frequency above the frequency the antenna is close to a half wave on. The doublet/CFZ may be a little quieter on receive too because the OCF may pick up vertically polarized man made noise from the coax shield. The "problem" with the doublet/CFZ is that it works best with a good balanced tuner or a good high quality balun specifically designed for High SWR/high voltage service. (voltages can get VERY high in this situation.) Also, in some cases, the line length may have to be
played with in order to present a resonable impedance load on some bands to the tuner. Ladderline or other balanced parallel line is also kind of a pain in some cases to mechanically work with....but it has very low loss compared to coax, especially at high SWR and the SWR on an OCF can be pretty high in many cases. With a doublet/CFZ fed with ladderline, you can tie the two conductors together at the tuner and feed it as a random wire if wanted. i.e. Use an 80 M antenna on 160. This is not the best set up in the world, but it will work pretty decently for average operation. The CFZ is also cheap and non-critical/easy to make.....a lot to like. There is a reason the
doublet aka Center Fed Zepp has been around and widely used (in both Amateur and Commercial stations) for 70 is simple, efficient and it works well over a wide frequency range.

73,  K0ZN

Posts: 141

« Reply #16 on: November 17, 2012, 05:44:15 AM »

"The transmission line will radiate on an OCF?" That is not alway true ! If you use the correct balun the transmission line WILL NOT RADIATE !
I have three Off Center Fed antennas and all three are fed with a 4 to 1 Guanella Current Balun. I also have an RF probe and have checked each
antenna on all of the bands that it is designed to operate on. I put the RF pickup on the transmission line and set the the probe to the lowest
scale. I transmitted on all of the bands and there was ZERO RF detected on the shield of the transmission line.

The OCF80 I use in the primary station uses the GU4-HF-5KW. I run my home brew 3CX1000A7 amp into it on 10, 12, 17, 20, 40 and 80 meters. That's
1500 watts and I have ZERO issues with RF in the shack......and my station is on the second floor.

I have even gone as far as to put a trap and an RF ampmeter on the ground going from the station to the ground rods in the yard to see if any RF was
on the ground system. None there either.

To say, "The transmission line will radiate on an OCF" as a blanket statement is not true.

If you build a simple dipole incorrectly you can have RF issues. Build an OCF correctly and use the proper balun and you should not have any issues.
Barry, KU3X


Posts: 60

« Reply #17 on: November 17, 2012, 08:08:58 AM »

Barry I've been using my antenna for about 2 years and until this
incident never had a problem.  NO RFI that I have noted.

Since I'm going to be taking it down and replacing the balun I might
as well try to maximize it.  Right now it has legs of 44' and 88'.
The area I have is 110' by 55'.  Would you change the length?

Thanks for all the help guys.
Stan AE7UT

Posts: 141

« Reply #18 on: November 17, 2012, 08:31:50 AM »

Perfect ! Don't change any of the measurements. If you put the "GU4-HF-5KW" 4 to 1 Guanella Current Balun at the feed point of what you already have you should do fine. If not that balun, just make sure it is a 4:1 Guanella Current Balun.
I get a very low SWR on 6, 10, 12, 17, 20, 40 and 80 meters. That's not to say the SWR is low across the entire band on every band, I'm just saying that at
one point in every band the SWR is very low. Most of the bands the SWR will be 1:1 at a given frequency.

If you run your coax properly away from the OCF, you'll do fine. Since you didn't have any RF issues before, you won't have any now.

Commom Mode is sometimes mixe up with the use of a poor or no balun.
Make sure you DO NOT use that antenna on 15 meters. The balun is at a voltage point on that antenna and a transmatch DOES NOT change that.

Good luck with your project.

Barry, KU3X

Posts: 493

« Reply #19 on: November 18, 2012, 10:38:40 AM »

Hi Stan:

Do you have room for a center fed dipole?

If so feed this antenna with home made ladder line and a home made link antenna coupler.

With this setup you will have years of multi-band operations without the worries of frying a balun.


Posts: 30

« Reply #20 on: November 28, 2012, 02:46:41 PM »

Your ferrite balun or transformer may not be "fried" if it works again after a time interval. What's happening is the cores are heating and reaching their Currie temperature -- the point where their permiability changes. Once they cool down a bit, they return to their original value. What you then have is a design failure, not a component failure. Live with lower power -- or use larger parts that can dissipate the heat. Also, consider using (quantity) more lower permiability ferrite materials that won't heat up as much and will dissipate less heat because of their lower resistive component. If your antenna is failing, it is likely improperly designed!   Roll Eyes

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