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Impedance Matching HF

Aug 1st 2013, 22:18

xof7fox

Joined: Jul 7th 2012, 16:27
Total Topics: 0
Total Posts: 0
My understanding is that typically a reasonably well placed and reasonably resonant HF Dipole has about a 72 Ohm Impedance. Given that this understanding is correct/accurate, why are not all HF Dipoles therefore equipped with a Balun that would transform the Dipole Impedance down to 50 Ohms, the Impedance typically being sent to the Dipole by the Transceiver?
Thanks - Fred K4XXK
Aug 2nd 2013, 20:27

KB0HAE

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
In general a balun is not needed for a resonant dipole. Many things can affect the actual impedence at the feedpoint. Such as height above the ground, nearby buildings/trees/objects etc... Also, it may not be possible for the antenna to be completely strait or level, and one or both legs may have to be bent to fit available space.

If the dipole is in an inverted V configuration, the feedpoint impedence is closer to 50 ohms. It is seldom possible to set up a perfect antenna...there are almost always compromises necesitated by available supports etc...
Aug 3rd 2013, 13:14

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
In practice, implementing that impedance transformation with a balun involving tranformers (coupled cores) actually reduces the overall system efficiency.

But, your idea has seen its appearance in web articles if you consider the "balun" as 1:1 balun with an automatic antenna tuner between the balun and the dipole. These days, most hams are looking for all band antenna solutions--they aren't looking for clever single band solutions--unless it is to get on 80 on 160M.

Zack Lau W1VT
ARRL Senior Lab Engineer

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