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Matching impedance with coax

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November 5th 09, 12:06 AM posted to alt.internet.wireless,rec.radio.amateur.antenna

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Matching impedance with coax

On Tue, 3 Nov 2009 18:24:19 -0800, (Dave Platt)
wrote:

In article ,
amdx wrote:

I'm trying to get an understanding of the MFJ-1800 wifi antenna.
The antenna has a folded loop as the active element.
Is this considered to have a 300 ohm output impedance?

Not necessarily.
(snip)

The presence of these "parasitic" elements will greatly change the
feedpoint impedance of the FD... typically, to a lower value. Close
enough spacing of the parasitics can reduce the feedpoint impedance by
quite a lot.

I suspect that the design of the MFJ antenna was done in a way which
places the parasitic elements close enough to reduce the folded
dipole's impedance to somewhere in the neighborhood of 50 ohms. All
that would be necessary, then, to allow a direct feed from a 50-ohm
coax, would be a choke balun (to convert the unbalanced coax feed to a
balanced drive to the folded dipole, without altering the impedance).

The presense and spacing of the parasitic elements isn't going to
change the feedpoint impedance that much.

Mike needs to check out - and understand - how a side-mount folded
dipole is matched to a 50 ohm line. I'm sure this yagi will simply
use a similar series coax balun.

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