measuring SWR and fluctuations in measurement

[hbscanner1]

Hi All! just bought a Rig Experts antennae analyzer and it works great! Question I had is this : have a 20 meter hamstick and tuning it to resonance around 14.3 Mhz. When I get the metal whip inserted into the ferule, I can get no lower than about 2.4:1, but with even a 1 or 2 mm difference in insertion length into the fiberglass portion of the antennae, the SWR can go to 10:1 or greater! would one expect this great a variation in SWR with so minute a change in insertion depth of whip? and if so, it could be difficult to use the set screw and tighten it without moving the whip even one iota!!! any thoughts/comments on this from your all experience would be greatly appreciated!!

 

[mmckenna]

Are you touching the antenna while you are testing? Sounds kind of like you are detuning it.

 

[W5GX]

Silly question, but where are you mounting it?

I had an old Dakota and mounted a 2m/70cm antenna on an in-the-bed tool box, real close to the cab. We couldn't get consistent SWR readings at all. I moved the mount to the side of the tool box, separating the antenna from the cab, and readings were better.

You may have it mounted in a good location, so this may not help.

In my experience with 2m/70cm, I've seen 1/8" make a difference, but not 1 or 2 mm. Are you measuring it the same way every time?

 

[RBMTS]

Also are you sure that the grounding or ground strap is totally secure and tight?

 

[WA0CBW]

Also is the whip being inserted so far that it engages the top coil of the mast?
BB

 

[ko6jw_2]

You give no information on how the antenna is mounted. A short vertical like this needs a good ground whether it is a vehicle body or a counterpoise if not mounted on a vehicle. This will cause extreme swings in SWR. Also, I use a 20-30 foot piece of cable between the antenna and the analyzer to avoid interfering with the antenna. I would also suggest setting the rod at some arbitrary length and doing a sweep to find where it is resonant. Then try to minimize the SWR and then do the final tuning. You may find more than one resonance. Make sure you're using the correct one or you will get weird results. Also, check the coax. You could have a problem with connectors etc.

How to adjust the set screw. Take an arbitrary measurement as above with the set screw tightened enough to hold the whip. If you are above the desired frequency, lengthen the whip a little, tighten again and make another measurement. If below reverse the procedure. Repeat until you are under 1.5:1 or so. You may never get to 1:1. Don't obsess about SWR. Anything below 2:1 will be good. Also, short verticals tune sharply and have narrow bandwidth. You won't likely cover the whole 20 meter band. Move well away from the antenna as mentioned above.