[TowerTalk] Testing a Balun

[Jim Brown]

On Sat, 6 Dec 2008 10:51:57 -0500, KC2PCR wrote:

>My apologies in advance for a poor understanding in the area of antenna
>current & impedance, it isn't from a lack of trying ;)

http://audiosystemsgroup.com/RFI-Ham.pdf   includes an extensive 
tutorial on common mode chokes, widely known as "current baluns." I 
believe that the W2AU balun is a transformer (voltage) balun. The 
tutorial will explain a lot of that. 

Testing these things is complex, depending on what you want to learn 
about them. If you want to know if they're "broke," I'd start with a 
transmitter, a calibrated wattmeter, and a dummy load. Load it to 100 
watts with your rig and see what the wattmeter reads. 

Also see if the balun gets warm with a long "keydown" in this test. If 
it does (in this test) and you have significant power in the wattmeter, 
it's almost certainly a voltage balun. 

An Ohmmeter with a good low Ohms scale can also tell you things. If it's 
broke, you'll likely should see a dead short between one conductor and 
the other on each side where the coax inside has melted. A good one 
should show the resistance of the coax (tens or hundreds of miliOhms) 
from end to end. If it's a transformer, you should also see the 
resistance of the coax. 

As others have noted, you can use a dummy load and an antenna bridge to 
figure out whether the impedance match is "right." If you want to know 
how well it works as a choke, you need far more sophisticated test gear. 
See my tutorial. 

More important -- simple, cheap "baluns" like the W2AU, W2DU, and copies 
thereof are very old ideas and vastly inferior to what you can do 
yourself for the same or less money by following the advice in my 
tutorial. 

73,

Jim Brown K9YC