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'SMALL' ANTENNA CRITERIA

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March 6th 08, 06:45 PM posted to rec.radio.amateur.antenna

Roy Lewallen Roy Lewallen is offline

Roy Lewallen is offline

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First recorded activity by RadioBanter: Jun 2006

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'SMALL' ANTENNA CRITERIA

wrote:
On Mar 5, 3:14 pm, (Dave Platt) wrote:
In article ,
Roy Lewallen wrote:

I highly recommend that you decouple the feedline with a current balun
(common mode choke). Better yet would be two, spaced about a quarter
wavelength apart. Small antennas can be made to look a lot better than
they really are if the feedline is allowed to become part of the
radiating antenna system, so unless the feedline is decoupled well, any
measurements you make will be subject to speculation as to whether the
radiation was from the antenna or the feedline.
It would also be interesting to use a simple current probe to sweep
along the feedline, and see what sorts of currents might be flowing
along the outside of the feedline, both with and without a good
common-mode choke or two in the feedline.

I agree. I think it's quite possible that even a decoupled feedline
will add enough radiation to make the antenna noticeably better
than it really is.
I'd prefer to see the source mounted *at* the antenna to make it
a decent comparison.
. . .

Even that might not be adequate, depending on the design of the
transmitter. Any power or other wires connected to the transmitter could
easily become part of the radiator, so they'd have to be decoupled. A
small, battery-powered transmitter with no connected wires would avoid
this potential problem.

Roy Lewallen, W7EL

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