Peter Chadwick wrote:
>Phil says:
>
>>How can I test it to assure balanced output into my open-wire
>>feeders?
>
>Best bet is to measure the current in the two feedwires. If you have 2
>identical RF ammeters, put one in each leg and measure the currents. Now
>swap over the meters (the one in the right hand leg goes in the left and so
>on) and measure again. If the results are the same in each leg, you're OK.
>If the answers are unbalanced in both cases, but the unbalance follows the
>meters, you're OK.
>
>If you haven't got a pair of ammeters, get some flashlamp bulbs - the really
>low power 2.5 volt ones. A couple of clip leads so you can clip them across
>about 2 or 3 feet of feeder line, and they'll indicate the current. (i.e. on
>the one feeder wire, coonect the bulb between two points two or three feet
>apart - not across the feeder line). Fix a bit of wood so you can have the
>bulbs with a bit of paper midway between them: put a drop of oil on the
>paper, and when (if!) the bulbs are equal brilliance, it will apparently
>disappear (the grease spot photometer). If the feedpoint is really high
>impedance, and the current too low even at full power to light the bulbs,
>put the bulbs in series with the feeder wires.
>
>It may be worth swapping the bulbs between legs as a check that they show
>equal brightness at equal currents.
>
>You can also try measuring the voltages with a suitable RF voltmeter, but
>that's a more difficult process, especially at high impedance where the
>capacity of the voltmeter input can throw things off.
Somewhere... somewhere... I remember seeing a description of a gadget
with two toroidal current transformers and a switchable rectifier/load.
It allowed you to measure I1, I1 and also (I1-I2). For relative
measurements of (I1-I2) as a fraction of I1, it doesn't even need to be
calibrated.
>
>Finally, I wouldn't worry about any unbalance less than 10%, and in
>practice, I'd accept up to 25%. Bear in mind that the feeders need careful
>dressing for symmetry.
Somewhere else (and this time I think it was an Internet posting by Bill
Sabin, W0IYH) there was a description of a... well, I guess you'd have
to call it a "balbal" - a common-mode choke for balanced feeders. It was
another ferrite device that was transparent to the correct mode of
balanced feeder, but offered a high impedance to common-mode currents.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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