"Dave Platt" wrote in message
...
I've built a VHF/UHF Log Periodic for monitoring. It works quite well,
but I've been curious about one thing... Using an analyzer, I get a
good SWR on the majority of the antenna (below 1.5), but there's some
areas that are fairly high. Well above 3.0. Obviously this isn't
crucial for just monitoring, but I use that as a guide as far as
seeing how well the antenna is designed. The thing that I don't
understand is that the SWR readings will change if the cable is moved.
The cable comes off the front end of the boom, travels back to the
middle, then comes off across a horizontal mast (the antenna is
vertical) and down the vertical mast. All that part of the cable
doesn't move... But if I pick up the cable on the ground and change
the location a bit, the SWR will change quite a bit... Areas that
before had a poor SWR now have a good one, and areas that were good
change to poor. Why would this happen? Also, would using a longer
cable possibly make the SWR better? Typically I have tested with a 20'
RG8X cable. I also usually test with the antenna about 8' off the
ground. Would it improve with better elevation off the ground as well?

Hi Dave

What type balun did you use?

'Tis a good question!

If an antenna's measured SWR changes when the doax is moved, it tends
to mean one of two things: either there's something physically loose
which is causing an intermittent electrical connnection, or you have a
significant amount of RF current flowing back down the outside of the
coax, and the feedline is acting as part of the antenna.

I suspect that your high-SWR points are at frequencies where the
outside of the feedline is close to a multiple of 1/2 electrical
wavelength, and appears as a relatively low RF impedance to ground.
Changing the feedline position or length could shift the effect of
this unwanted RF pathway and move the high-SWR frequencies around.

It's also possible that your SWR meter isn't actually measuring what
you think it's measuring. If there's a strong UHF/VHF signal being
transmitted in your area, the feedline or antenna might be picking it
up and it might be confusing your analyzer (MFJ analyzers are
notoriously subject to confusion on the lower HF frequencies for this
reason, and you might be having a similar problem at VHF/UHF).

Using a good balun (with the correct impedance-transformation ratio)
at the feedpoint may help matters, if you aren't doing this already.

Equally likely to help would be adding some choking impedance to the
feedline, in the form of one-or-a-few snap-on two-part ferrite
"interference suppressor" chokes. I'd suggest installing one just
below the point at which the feedline leaves the antenna boom.

I suspect that choking the feedline will make the wonky SWR
measurement settle down.

And, yes, adding a longer feedline will probably reduce the SWR
weirdness (or at least move it around to different frequencies), but
it's not a particularly good solution to the problem... you'd probably
have to add a lot of coax (and thus a lot of unwanted losses) in order
to reduce the SWR excursions by much.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Hi Dave Platt

I suspect the original Dave has mounted the antenna so the dipoles are
parallel with the mast. And it wasnt clear that he'd taken into account
the need for some sort of a device to allow a balanced antenna to an
unbalanced line. It seemed like a good 'starting question'.
By my analysis, the LPDA is a set of dipoles fed from one end so it tends
to create a pattern null in the direction toward the dipoles from the feed
end. Those dipoles which are too short or too long to be well matched to
the line wont radiate enough to upset the null depth. If the feed line and
tower are coupled to only one half of the dipoles some unbalance is
expected.

Jerry