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Author Topic: Repairing a "fried" OCFD - balun choice  (Read 2814 times)
AE7UT
Member

Posts: 60




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« on: November 14, 2012, 10:55:10 AM »

My set up:
  Elecraft K3 -  Elecraft KPA500 - Palstar AT2K - 132' OCFD up 35'
  75' run of LMR-400

So I was trying to see if I could get my antenna to work on 160 meters.
I was tuning up when the SWR went through the roof and all the LEDs on
my amplifier lit up and both the K3 and the KPA500 shut down.  I knew right
away I had fried something.  I've narrowed it down to the antenna.  I'm
"assuming" it's the balun.  The balun is at the apex of the antenna up on a
35 ft. push up pole that is very difficult to get up and down.  I'm trying to
anticipate my needs and get it fixed ASAP. 

I was thinking of replacing the balun with these choices.
http://www.balundesigns.com/servlet/the-110/current-balun-4-cln-1-designs/Detail
http://www.balundesigns.com/servlet/the-75/OCF-balun-4-cln-1/Detail
One leg does run about 15 feet from my roof.  Brick house and singled roof.
I've never heard of using a different balun for interactions from local buildings.
Any input on this choice?  Do you think there is a better choice?

I guess my other option is to try and repair the "assumed" bad one or build my own.
Or I could try a different wire antenna.

Thanks for any help you can be.
73
Stan AE7UT
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KG6YV
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Posts: 504




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« Reply #1 on: November 14, 2012, 11:19:57 AM »

The type of balun (imedance ratio) is determined by where the feedpoint is on the dipole relative to the ends.
Are you sure the original balun is a 4:1?  Did you home brew the antenna or buy one?

greg
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WB6BYU
Member

Posts: 12983




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« Reply #2 on: November 14, 2012, 11:26:14 AM »

First step is to put a pulley and halyard your mast to make it easier to lower your
antenna for maintenance, or when you want to try something else.

An 80m OCFD won't be anywhere near 200 ohms on 160m, more likely something
like 10 ohms with LOTS of reactance.  A 4 : 1 balun will try to step that down
to an even lower resistance, and the high reactance means it will have high
voltage across the feedpoint to radiate much power.

But don't assume that the balun is bad until you can get it down and check it
out.  Might be an arc across the coax connector, or melted solder due to high
currents.  Might just be a carbon arc path shorting the output terminals that
can be cleaned off, or a simple internal repair.


However, regarding the choice of a balun, I suspect that the warning about
running a wire close to the house or a metal roof is because that will throw
off the feedpoint impedance in such a way that raises the stress on the balun
core, or puts a voltage across the windings that the insulator can't handle.
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WX7G
Member

Posts: 5908




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« Reply #3 on: November 14, 2012, 11:49:49 AM »

Those two baluns are good choices. The ant cannot be driven on 160 meters as a dipole or as a Marconi without stressing the balun.
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W2WDX
Member

Posts: 188




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« Reply #4 on: November 14, 2012, 01:01:44 PM »

A better solution is run a doublet and get rid of the OCF altogether. From the description is sounds like a Buxcomm OFC. I know of many people (including myself) who have had that balun fail (if that's what happened).

Now remember on 160 the chances you were even close to resonance is slim, both in terms of the 160m band requires more antenna-per-hertz AND the fact that you were way to short to start. It is asking a lot of a piece of wire and its feedline to be close to resonant across such a wide band. Even if the wire is cut for the middle of the band, it may be considerably out of resonance—offering impedance that is a long way removed from 50 ohms—when you try to use it at the extreme ends of the band. Complicating the fact that OCF dipoles have very unpredictable voltages at the feedpoint. Regardless of what the manufacturers claim about low voltage at the feedpoint at every band, the rise in voltage as you go off resonance increase at a higher rate with an OCF design. Everything affects this; height, nearby objects, imbalances due to cutting and tuning, ground type and losses, etc. So the balun can see very high voltages, especially if using an amplifier, way beyond what is practicable to build inexpensively for breakdown voltage in a balun.

Also bear in mind that toroidal baluns or any balun using a core can be rendered useless by repeated near lightning strikes and impulses, which if this occurs can severely increase these problems.

You are better off running as long a regular dipole (doublet) as you can, ditch the LMR-400 (save it for your VHF/UHF) and use ladder line and run the tuner balanced. You probably are losing about 40% of your power with the current set-up, mostly in feedline losses and in the balun if you are not exactly on the frequency of resonance for the antenna. With the balanced system you will have nearly no feedline losses, may not have to use a balun at the feedpoint (assuming you have balanced current and phase), and you will be able to tune (and radiate) almost anywhere, depending on the range of the tuner.

Ditch the OCF. I had one and they are not efficient at all and more problematic for a wire antenna, at best. Even in the best situation. You seem to have the space, use it ... efficiently.

John, W2WDX
« Last Edit: November 14, 2012, 01:59:44 PM by W2WDX » Logged

AE7UT
Member

Posts: 60




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« Reply #5 on: November 14, 2012, 02:02:56 PM »

Thanks for all the replies.

I didn't add the caveat that the antenna still tunes ok and will work
for a while at low power.  Then when I send for over 5 minutes the
SWR starts climbing again.  This only happens with CW and only if
I'm running more than 300 watts.

I love the idea of ladder line but the pathway to my shack has lots of
metal on the route. 

I actually did try a rope and pully at first.  I removed it because of the
racket it made with high winds in my area.  I really need to replace the
mast.  Of course this stuff only happens after we got a foot of snow
last week.

Thanks again.  I'll let you know what I find when I drop the mast this
weekend.

73
Stan AE7UT
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N4CR
Member

Posts: 1650




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« Reply #6 on: November 14, 2012, 03:04:54 PM »

I actually did try a rope and pully at first.  I removed it because of the racket it made with high winds in my area.

The classic solution for this is to wrap the halyard in a spiral coming down the mast and then tie it off. No slapping because there is no slack. Most flag poles and sail boat masts are tied off like this.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
WD8KNI
Member

Posts: 137




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« Reply #7 on: November 14, 2012, 05:42:14 PM »

You can also place a 12" standoff 1-2 feet above the tie off point.  run the down rope over that before tying it off.  Keeps the rope from touching the mast.  You should also run the rope in one continuous loop that way you have twice the weight for the wind to move.  You also will never loose the bitter end.. Fred
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WB6BYU
Member

Posts: 12983




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« Reply #8 on: November 14, 2012, 06:19:05 PM »

Quote from: WX7G
...The ant cannot be driven on 160 meters as a dipole or as a Marconi without stressing the balun.


For example, an EZNEC model of such an antenna used on 160m suggests the feedpoint
impedance is 5 - j1200 ohms.  At 500 watts the feedpoint would see about 12kV @ 10 amps.
If your balun really worked in the theoretical manner that would be only 6kV at 20A on
the coax side.  I doubt you will find any ham balun with enough insulation to handle
that sort of voltage.

But the voltage won't get high in practice due to losses along the coax and in the tuner.
Coax losses are likely to be 15dB or so (depending on the actual behavior of the balun).
With that length of coax the impedance seen by the tuner is predicted to be 0.6-j27 ohms,
and W9CF's handy tuner simulator suggests a tuner loss of about 8dB matching such a
load.

That's a total loss of 23dB, so with 500W out of the rig the actual power at the antenna
is only 2.5W, giving about 900V at a bit less than 1 amp out of the balun.


While the full power doesn't reach the feedpoint, it does reach the tuner, and you will still
have very high voltages and currents in the tuner and at the coax connector in the shack. 
I wouldn't be surprised if you had arcing either in one or the other that left a carbon trace,
and that is causing the arcing at high power now.  So don't just assume the problem is in
the balun - it may have been protected by the high coax losses.
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K9SRV
Member

Posts: 121




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« Reply #9 on: November 14, 2012, 11:17:34 PM »

the balun designs 4114, 4:1 is designed to be able to be used
closed to other objects than a regular 4:1. It's rated 5k and is dual core.
Call bob rumsey @ bd, he will be happy to chat baluns. He has chatted with me like 5  times
about the ONE balun i actually bought from him! Great guy.

On a sidenote, I recently bought the DX Eng. 160 meter dipole kit.
My amp only goes to 40 meters. Iwas wondering if I would be OK cutting some
and making a 40 meter double extended zepp to the 300 ohn HD feedline
that came with the kit, the a Johnson Matchbox. The antenna will be @ about 50 feet.
Also, would anyone be willing to run an eznec radiation pattern for either/both antennas?
I realize at 50 feet and w/100 watts, the thing will be worthless on 160, but I was wondering about 80 meters and up.

Thanks,
John
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AE7UT
Member

Posts: 60




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« Reply #10 on: November 15, 2012, 12:06:31 AM »

I received an email from a very nice ham that made me think.
He conjectured that the balun may be over heating.

I tried the antenna again this evening with the same results.
It tunes fine on 80 meters and works FB for about 20 minutes of
sending.  Then the SWR takes a gradual climb to >10:1.
I switched bands and it had the same but of course instant high SWR.
I then allowed about 10 minutes of "cooling" and had a low 1.0:1
SWR.  After a shorter interval of time, 5 min. sending the SWR climbed.
I then switched immediately to a dummy load and had an 1.0:1 SWR.
So I think it is isolated at least to the antenna or the coax.
This is running 300 watts CW with an outdoor ambient temp of -2 C.


I just need to pull it down and check it out.  Hopefully this weekend
I'll have time.

Thanks for all the help.

Stan AE7UT

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W5WSS
Member

Posts: 1599




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« Reply #11 on: November 15, 2012, 08:20:40 AM »

Well from your most recent post it could be that the episode when you had the problem might have well been a core saturation due to high voltage.

The balun might not be ruined per say.

You could use a multi meter and down at the station end of the feedline check for a short,because even though the balun is up at the antenna a dipole or doublet reads open continuity but regardless IMHO a balun that suffers core saturation needs to be benched checked as well as evaluating the system for proper application of type to apply at the feedpoint etc. So if it has opened up wires burned etc you will know during the examination of the interior.

I would be in the process of making these determinations so that when you remove the balun you can quickly install a suitable replacement that can handle the RF pressure IE over design is better.

The material used in a balun or to be more specific, the toroid material permeability used that the Teflon coated wire is wrapped around might not be adequate for the service you are expecting or the physical wire size in diameter Even though the designer says both is OK!

I would not press that balun or replace it with an exact model In the end one will be faced with a decision as to whether the balun is usable for some other application, if determined to be OK.

There is some debate as to whether a core that has been overstressed becomes more and more easily saturated until it burns up. kind of exponentially susceptible to core breakdown even when the power applied and all other factors are held exact.Just say in.
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W5DQ
Member

Posts: 1209


WWW

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« Reply #12 on: November 16, 2012, 03:17:55 PM »

So I was trying to see if I could get my antenna to work on 160 meters.
I was tuning up when the SWR went through the roof and all the LEDs on
my amplifier lit up and both the K3 and the KPA500 shut down.  I knew right
away I had fried something.  I've narrowed it down to the antenna.  I'm
"assuming" it's the balun.  The balun is at the apex of the antenna up on a
35 ft. push up pole that is very difficult to get up and down.  I'm trying to
anticipate my needs and get it fixed ASAP. 

Most likely you turned the balun in a charcoal briquet Smiley

I did the same thing a few years ago playing with 160M. I had a balun I had homebrewed and tested and the design claimed it could handle full power out so I figured it probably would handle 500W from my small amp even with a high SWR. Everything was coasting along nicely on 160M CW and then everything was toast!!

Once I determined it was the balun, I decided it was just as easy to get a quality high power model so I picked up one of the 5KW models from DX Engineering. While not cheap, it sure beat replacing balun more than once and whether or not it actually will handle 5KW is unknown but I can tell you have have had no more problems with balun failures in the same setup. I keep the power down now to around 350-400W or so when on 160M and the same balun works great on 80-10M feeding an OCFD using ladder line from the ground where balun is up to the antenna / feedline junction of about 40'.

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
KU3X
Member

Posts: 140




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« Reply #13 on: November 16, 2012, 04:33:25 PM »

Balun replacement: Search, "GU4-HF-5KW"
It's a 4:1 Guanella Current Balun

Never operate on a band an OCF is not designed to operate on. An OCF80 will work on 6, 10, 12, 17, 20, 40 and 80 meters. Never use it on 15, 30, 60 or 160 meters.

Never operate any antenna on a lower frequency than it is designed for. In other words, if you make an 80 meter diople, never use it on 160 meters.

Barry, KU3X
www.ku3x.net
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AE7UT
Member

Posts: 60




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« Reply #14 on: November 16, 2012, 06:03:56 PM »

I thought it was:
     Never say never
     Say seldom if ever

Thanks again guys.
Snowing again today.  Prospects don't look good for tomorrow.
The only sunny weekends are when I'm at work.

73
Stan AE7UT
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