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323 posts

Ultimate Geek
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Topic # 115048 12-Mar-2013 10:46
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Hi Im from the old VHF school where your aerial balun has an output which measures basically a short on ohmmeter.
I recently got a Matchmaster 4 element phased array model MDU36 or L3087. At one stage had a perfect picture with all channels and thought it was fine so mounted it in desired position only to find no signal at all!.
Checked all leads eventually back to aerial where the input measured o/c.
Rang up Matchmaster to confirm what output impedance should be approx and they did not know whether it should be low or o/c but said they'd send a replacement balun in any case.
Anyway I have pulled the aerial down, and opened the balun which is just a PCB with connections to the F connector all correct. Very odd layout where both feeds of the phased array are  shorted out on PCB, but the F connector goes to open circuit connections. The only possible way for the 'balun' to work would be through capacitive effect.
Does anybody know about these beasts and if so can you confirm the balun should be o/c output with both phased array feeds joined by pcb track?
Thanks vm.
Alistair.

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2 posts

Wannabe Geek


  Reply # 862703 20-Jul-2013 19:11
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Hello AGeorge,

I didn't see any answers to your questionsand where you were at then is exactly where I am now, almost identical situation, same old school RF background, we are ~30km from sugar loaf in canterbury out near sheffield and the antenna is one of the Hills high gain UHF aerials for fringe area reception, sold by disk smith a couple years ago, our line of site is perfect and all was well in the good old days when analog was available, after a storm or any signs of trouble and I would take a phone up on the roof and fiddle with things till my wife said good on a prime or other analog channel and then we would be good to go on full HD freeview, so now that analog has gone... first really big southerly storm and we end up with 'no channels' found on the freeview box and their is no analog channel to check... bummer about that. So I go check things and get to the same point you were when you posted, I have now added a signal meter in circuit temporarily - one of those led devices from Jaycar, what else are we supposed to use with no analog channels ? anyway this all results in no led's on the meter no matter where I line the aerial (and no channels on freeview scan) so I start metering things out and come to the same point as you, cable is good but when disconnected the antenna (unbalanced) input to the PCB balun shows 0.4 ohms and the balanced aerial connection is open circuit when disconnected, I thought this was a smoking gun initially thinking yeah I know about baluns but I did not know about these types, what puzzles me is this, 

given the last page of this pdf on balun design,

http://www.odyseus.nildram.co.uk/RFMicrowave_Circuits_Files/Balun%20Design.pdf

The indications are that the inductors L1 and L2 are 'realized' by thin and thicker PCB track and the capacitor C1 the same, understandable at 530 odd MHZ, if this is even related to the Hills PCB balun then maybe this is all answered by the DC block for a basic resistance test in the balance side.

Did you figure this out and was the balun good or bad with open circuit on the balanced side ?

I was about to drill the rivets on the PCB balun looking for other components to check and then wondered if there were any - now doubtful about that.. other than the PCB single side that I can see, would hate to ruin a perfectly good UHF balun...

The other good read I found while looking was this,

http://www.minicircuits.com/pages/BalunApplicationNote.htm

Looked like an open circuit on the balanced side - if - sec is the balanced side.

Anyway interested to know what you do next - replace or .. ?

Cheers.

2 posts

Wannabe Geek


  Reply # 862713 20-Jul-2013 19:20
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Not obvious but I was referring to the item, 'III. Marchand Balun3 Transformers' in the second link on that posting... for anyone interested in baluns.



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