[Antennas] measuring SWR

Sat, 10 May 2003 13:03:45 -0500

Mike,

I've built around a dozen j-poles and variants in the past 10 years and 
what you've got is an impedance mismatch. Your feedline is 50 ohm 
unbalanced and your antenna is probably around 300 ohms unbalanced.

This impedance mismatch is causing current to flow on the outside of your 
feedline coax, making it part of the antenna. When you change the length of 
the feedline, you change the characteristics of the antenna.

Now one solution could be to adjust the antenna feedpoint for minimum SWR 
while the antenna is in the desired mounting location. However, since the 
feedline is now part of the antenna, the feedline is now radiating RF. This 
can cause the radiation pattern to behave unpredictably.

Another option is to coil the feedline into a small, multi-turn loop (5-6 
turns should be enough) near the antenna. This will act as a choke to the 
RF current on the shield of the coax and will prevent the feedline from 
becoming part of the antenna. You need to do this when you adjust the 
antenna and then use the exact same coil when you install the antenna. The 
downside is, this doesn't really solve the mismatch problem, it just blocks 
the RF on the outside of the coax from getting back to the feedline. This 
blocked RF will be converted to heat in the choke, which results in less 
power to the antenna.

A better choice is to correct the mismatch with a balun.

Most j-pole designs have a characteristic impedance around 300 ohms. This 
is 6 times the impedance of the coax and your radio. So a 6:1 balun would 
be best.  But in practice, a 4:1 balun will work well enough you won't 
notice a difference. The impedance of your antenna can be approximately 
calculated with the formula:

Z = 267*log(2*S/D)

where
S is the spacing between the elements, center to center
D is the diameter of the conductors.
You can use either centimeters or inches for this calculation as long as 
you use the same units for S and D.

I've used a 1/2 wavelength section of the same type of coax as the feedline 
to make a 4:1 impedance transformer. The ARRL handbook has designs for 
these in the transmission line section.

You can also match the impedance with a series-section transformer. I've 
tried these as well but found that the length of the series sections end up 
being larger than the 1/2 wave 4:1 balun. These are also covered in the 
ARRL handbook transmission line section and are a bit more work to design.

If you're up to some heavier reading on jpole design, you might want to go 
to http://snow.prohosting.com/~w0rcy/Jpole/jpole.html
and read the article by N3GO.

BTW, wherever possible, you should measure the SWR as close to the antenna 
as you can. Losses in the feedline will make the SWR at the radio measure 
lower than the actual SWR occurring at the antenna.

Good luck & 73
Doug, K0ELR

At 01:38 AM 5/10/2003, Mike Miller wrote:
>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
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