Used Balun- How to tell if it's good???
I purchased a used W2AU 1 to 1 balun at a hamfest.
How do you test to tell if there good??? I used an ohm meter and checked it at the pl259, conductor to shield and it shows continuity, same from pl259 to both sides of the balun, in all there is continuity everyway I test it.
Common sence would tell me it's junk? But never had a balun before so I have not a clue! Bought to set up an 80 meter dipole, have made several over the years but never used a balun.
Did I just buy a useless trinket at the hamfest???
Because of the way the windings interconnect with each other, you will measure continuity from everywhere to everywhere.
If you have an antenna analyzer, connect it to the input of the balun and connect a 50 ohm resistor across the antenna terminals. You should read close to 1:1 from 2 - 30 mhz. Lacking an analyzer, connect your rig through an swr meter to the balun and connect a power resistor (20 - 50 watts, non inductive) across the antenna terminals. Using LOW power, check the vswr at several spots. Again, should be close to 1:1 everywhere across hf. Can't do that? Connect your antenna and see if it will tune ok, not best, but last resort.
W2AU baluns never go wrong unless somebody has put 2 Kw through it
How to test a balun to see if it works:
You need two 25 ohm non inductive resistors and an MFJ 259 antenna analyzer. You will also need a short jumper wire, an 8" length of #16 wire will do.
Put the two resistors in series to make 50 ohms and put that across the output of the balun. Now put the MFJ 259 on the coax side of the balun. Pick the desired operating frequency and you should see a 1:1 SWR on the analyzer. Now attach the one end of the jumper to the center of the two resistors. Attach the other end to the ground lug of the MFJ. If the SWR stays the same, you are ok. If it does not, well.....look for the nearest trash can.
That tests the balun CM impedance when operating into a balanced load. A more severe test is to attach the grounded jumper first to one side of the series pair, and then to the other side, and check how much the SWR changes.
Originally Posted by KU3X
Also, using low value resistors "desensitises" the test. A pair of 100 Ohm resistors in series would be better.
Last edited by G3TXQ; 02-20-2011 at 06:37 PM.
As for testing the balun to determine if it has been damaged, the simple insertion VSWR test that has been proposed will reveal if the magnetic core has been permanently damaged, of if there a permanent internal short (unlikely).
Originally Posted by KB0FSD
But the more important question is why are you going to use a balun on your 80m dipole? If it is to reduce the risk of significant common mode current, this type of balun ('voltage' balun) does precious little in this application. The fact that huge numbers were sold for this purpose does not enhance their suitability, it just shows we (collectively) don't understand baluns very well.
If the objective is the reduces feed line radiation (and related effects), then a balun that tends to force equal but opposite phase feed line currents is needed. A voltage balun tends to force equal but opposite phase feed line currents ONLY when the load is perfectly symmetric (or balanced), a current balun tends to force equal but opposite phase feed line currents for ALL loads.
A 1:1 current balun with high common mode (or choking) impedance is better suited to the application.