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Old 03-06-2018, 2:51 AM
paulears paulears is offline
Join Date: Oct 2015
Location: Lowestoft - UK
Posts: 332

You use a balun at the radio connector? Are you using flat twin feeder right back to the radio?

I~f you are using a J-Pole - just for receive, they work surprisingly well when the thing is badly tuned on receive, but the transmission tuning is much more typical. Using your meter, the most useful feature is simply to see the VSWR curve, because this shows you where it's resonant, and where it isn't, the gain suffers. With a J-Pole, you have to cut the long out and back element, but then tune the matching stub where it attaches to the vertical - so in effect, the length of the radiating (or receiving) component has to be right, and then when it is, the matching section enables as much as possible to flow down the cable. I think most people cut the radiator with maths, and tune the matching section for minimum VSWR. Zx is the one that I pay attention to - Impedance gets messed up by the phase errors that capacitive and inductive reactance contribute - but the practical upshot for my level of antenna work is that if the impedance is as close to 50 as you can get, the other parameters fall into place.

VSWR - as close to 1:1 as I can get - frankly the one measurement that really seems to matter.

There's no need for the balun if you are feeding it with ordinary unbalanced coax. It's presence in the circuit will distort your readings - so you could cut the antenna WRONG. What you need is to dump the balun and fit a common mode choke - a ferrite ring, or a few of them just to stop your feeder becoming part of the antenna - these I would put at the top, near to then antenna - because otherwise, the antenna cable could (not always will) radiate, and all your choke will do is stop the last few feet! There's no need for a balun - because the balanced part is the 1/4 wave at the top, and the feed point sorts this out.

The fella who invented the Slim-Jim, the variant of the J-Pole lived near me back in the 70s, when I was the young one - and all the older hams had one. I tried one and hated it. The J-Pole has hardly any measurable gain over a simple wave ground plane, which is so much easier to make - and apart from trimming the length, they just work. J-poles need two sections to be adjusted, 3 if you include moving the feed point. So three adjustments that can restore VSWR to 1:1 or close. However - as they interact, how can you be sure that the theoretical gain (which isn't anything special) is being achieved. You could have a wrong length, and therefore inefficient radiator electrically tuned back to 1:1 but with gain less than a dipole - I'm guessing this would be shown in the other test results the meter show - but as for interpreting them? Not sure I could use them to categorically say the top section is too short, or too long.

J-pole have a tiny amount of gain and it's made up from a bit more in one direction and a bit less in another. The polar pattern in the E plane is not symmetrical, and again, this impacts on dimples in the Omni-directional pattern too. Dipoles are fully omni, and symmetrical in the E plane.
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