[Antennas] measuring SWR

2003年5月10日 08:48:18 -0500

Mike, it appears that you are feeding a balanced load - the 1/4-wave
matching section - with unbalanced coax. Unless you use an effective balun
at the feedpoint, the outer braid of the coax becomes a radiating portion of
the actual antenna. Thus, the feedline length, position, etc. affect the SWR
and the actual performance of the antenna. In addition the presence of
common-mode current on the outer braid will upset the operation of the SWR
meter and May cause it to give erroneous readings.
The only way to eliminate the feedline transformer action is to terminate it
in its Zo of around 50+j0 ohms. With careful adjust of the tap position on
the matching section and an effective balun, you should be able to achieve
good operation.
You can fiddle with the feedline length, but again without a balun, the
outer braid is part of the radiating system so the length will have effects
above and beyond merely altering the input impedance of the line.
Use whatever feedline length is convenient to reach from the antenna to the
rig or tuner, and add a proper balun, and you should obtain satisfactory
performance.
73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:[email protected]>
----- Original Message -----
From: "Mike Miller" <[email protected]>
To: <[email protected]>
Sent: Saturday, May 10, 2003 1:38 AM
Subject: [Antennas] measuring SWR
> I'm trying to match my 2m copper pipe j-pole to the feed line.
> When I had it on the ground I could adjust the feed point and
> get a very low swr across the band. After I put it up I couldn't
> get a good match at the lower frequencies but could at the high
> end of the band. This was measured at the transmitter end of the
> feedline. Now I've changed the feedline length and the apparent
> swr has changed again.
>> How can I eliminate the transmission line transformer effect
> while trying to adjust the feed point taps to a 50 Ohm match?
> Will using a 1/2 wave length line between the swr meter and
> antenna show me the true swr?
>> TIA
>> Mike KC9DOA
> - - -
>> Your moderator for this list is:
> Larry Wilson KE1HZ [email protected]
> _______________________________________________
> Antennas mailing list
> [email protected]
> http://mailman.qth.net/mailman/listinfo/antennas