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Author Topic: Discussion about a 6 Meter balun  (Read 620 times)
N4UE
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Posts: 296




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« on: October 23, 2008, 06:00:07 PM »

Hi. This may get a little long, but I'll try to be as accurate as possible....

I am refurbing my M2 6M7JHV. I had a failure at the end of one of the elements in the 'T' match. M2 uses a press fitting of a piece of copper into a slit in the end of the 'T' elements. One element connection was tight, the other was quite loose.

While checking the rest of the system, I noticed that water appeared to have infiltrated the balun. This despite the waterproof 'F' connectors and multiple layers of good electrical tape and several coats of clear lacquer.

Here's my observation / question:

M2's balun is a 1/2 wave (obviously) made from what appears to be RG6 cable.
Using my trusty MFJ 259B, I measured the resonant frequency to be 48.130 MHz. (X=0 on the MFJ). Since this is a 1/2 wavelength section, the far end was shorted. I used my 756PRO to double check the frequency on the 259B. They were spot on.
Next, I measured the capacity of the line (unshorted, natch) using my LCR bridge which was calibrated to NIST specs. The capacity was 350 pf.

I then made my own balun using RG6 quad. It is EXACTLY the same length as M2's.
However, it measures 50.150 MHz (X=0 on the MFJ).
When I measured it's capacity, it measures 135 pf. Quite a difference, eh?

So, I'm unsure if M2's balun was cut to that frequency on purpose in order to work correctly with the rest of the matching system, or could the original coax have degraded to the point the capacity went up 3X?

I have professional RG6 quad connector attaching tools, so I am confident in my balun being well sealed.

M2 has been pretty helpful, but I think I've bothered them enough! I do NOT want to spend $75 for new parts, when my new mechanical/electrical connections are MUCH more positive.

If you perfer to correspond directly, to save everyone else the agony, here's my e-mails:

n4ue@arrl.net

or

bonddaleena@aol.com

thanks in advance!
ron
N4UE
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WA1RNE
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Posts: 825




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« Reply #1 on: October 23, 2008, 07:09:08 PM »


 Ron, I think you're tests nailed it. I would go ahead and use your homebrew balun.  

 
 Your replacement balun and connectors should work fine and will probably outlast everyone else's.


 ...WA1RNE
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WA7NCL
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Posts: 625




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« Reply #2 on: October 24, 2008, 08:20:03 AM »

Sounds like a velocity factor difference because of the different resonant point measurements with the same length.  It seems like you should just adjust your new 1/2 wave section to what ever length the analyzer says is a half wave at your operating frequency.  Not sure what the low frequency capacitance is telling you.

You could also do a test using a non-inductive resistor as a load for the balun you've made and check the SWR or reflected power at your operating frequency.  If it shows it works, then who cares about the length and capacitance.
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WB2WIK
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Posts: 20603




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« Reply #3 on: October 24, 2008, 03:23:20 PM »

Since if the balun is made of foam dielectric cable it should only be about 7-1/2 feet long end-to-end, that 350pF measurement means the old cable had about 45pF/foot capacitance.

There isn't any cable like that.

If your measurement is correct, there's something wrong with the coax and it should be replaced.

BTW I never, ever "seal" the type F connectors on my M2 beam baluns.  The O-rings and construction make them very waterproof just the way they are, and none of mine have ever taken in a drop of water -- even the ones that have been up >10 years.  Something weird happened in your case, this is not a common problem at all.
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N4UE
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Posts: 296




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« Reply #4 on: October 24, 2008, 05:37:09 PM »

Thanks to all that replied. I was using you 'experts' as a sanity check. I hesitated to ask the nice folks at M2 since I gave this antenna such bad reviews HERE! Yep, buying an antenna in this price range and getting a boom without holes drilled in it, didn't exactly 'make my day'...

The method M2 uses to attach the copper strips from the matching elements, to the 'F' connectors, is poor with a capitol 'P'. Upon dissassembly, one copper strip was still wedged into it's aluminum rod, but the other fell right out.

WB2WIK was right about the coax. There ain't any like that! It appears to be just RG6. However, like I mentioned, water appeared to have seeped in, somehow.
THAT's why I included the capacity readings. Yes, I did it multiple times and get the same exact results.

Steve, since I live here in the "Sunshine State", where we do get hurricanes, rain and humidity, I don't see how it can hurt to 'waterproof' both the 'N' connector and the 'F' connectors.....

By the way, I was at Home Depot this past weekend. In the Plumbing Dept. they have a cool product, relating to antennas. It's a Silicone based non-adhesive tape. It's clear and has a temperature range suitable for the Space Shuttle. It's the self-amalgmating type. It is supposed to be UV proof also. About $5 per roll.
I have some undergoing 'tests' right now.

This antenna is the only one I have ever had fail the 3M electrical tape/wire tied/sealed with clear acrylic, method. However, it was moved 4 times (balun assembled) and did sit a ground level (up OFF of the ground), for 4 years.

Again, thanks fer the input and I hope it plays well at it's new 100' high home! It will be fed with 7/8" CATV Hardline using my own machined adapters. Remember those, Steve?

ron

N4UE
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WB2WIK
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Posts: 20603




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« Reply #5 on: October 24, 2008, 07:46:20 PM »

>RE: Discussion about a 6 Meter balun       Reply
by N4UE on October 24, 2008    Mail this to a friend!


Steve, since I live here in the "Sunshine State", where we do get hurricanes, rain and humidity, I don't see how it can hurt to 'waterproof' both the 'N' connector and the 'F' connectors..... <

::I can, and I have!  Problem is, whatever you seal out, you also seal "in."  If moisture can't get in, it also can't get out.  And there's always moisture, everywhere, unless you're dealing with dry-gas filled cable and pressurized connectors.

I don't seal anything and nothing ever leaks, no matter where I've lived, including Jacksonville.

WB2WIK/6

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