Problem with MFJ 259B

[KC0MS]

Ok, I am trying to tune a 20m hamstick mobile antenna which is mounted on the lip of the trunk lid by the rear window.

When connected to the MFJ 259B, I can get the SWR reading down low.....however, when I move my hand away from the MFJ259B....the reading changes drastically (like from 1.2:1, up to 2.0:1 for example).

As a matter of fact, I can just get my hand close to the analyzer and the meter readings change. This no matter if the MFJ is sitting on the car, the car seat, or outside the car on the ground.

All the metal parts of the car are bonded (and verified with a continuity meter)....there is no "leakage" between the center conductor and the shield anywhere.

And the really goofy thing is no matter what the MFJ indicates, the SWR indicated on the mobile rig (Kenwood TS 480Sat) is 3.0:1 all across the 20 meter band...yet the internal antenna tuner will bring the swr right down "flat".

I am completely baffled as to just what is going on here. Any help would be greatly appreciated.

[KC7YRA]

Need to get rid of some of that common mode on the coax. Essentially, the MFJ is part of the antenna system, which means the coax is also radiating.

Brad

[W5DXP]

Ground the MFJ-259B to the vehicle chassis with a very short wire and see what happens.

[KB4QAA]

1. Ensure that your batteries are up to full operating voltage. MFJ analyzers are very prone to misreading when voltage sags.
2. Attach a short ground jumper between the analyzer and antenna base.
3. You may need to put an RF choke at the base of the antenna.

[K0RGR]

I see bizarre stuff like this on the lower bands, but on 20? The symptoms just scream 'grounding' don't they? Sometimes, no matter how well we think we've bonded something, we don't really have a good RF ground. They use too much plastic in modern cars, too.

How is the antenna bonded to the trunk lid, and how is the trunk lid bonded to the car? If you don't have a grounding strap from the lid to the car body and/or frame, you could have a DC connection that's not very useful at RF. I also don't trust the mounting screws on my trunk lip mount to make decent connection, though they will if you carefully remove the paint under them. I do that and also add a few feet of copper tape under the edge - that eliminated some weirdness that I was experiencing. I don't use my lip mount on HF, but I have used it on 10 meters in the past.

[KC0MS]

I see bizarre stuff like this on the lower bands, but on 20? The symptoms just scream 'grounding' don't they? Sometimes, no matter how well we think we've bonded something, we don't really have a good RF ground. They use too much plastic in modern cars, too.

How is the antenna bonded to the trunk lid, and how is the trunk lid bonded to the car? If you don't have a grounding strap from the lid to the car body and/or frame, you could have a DC connection that's not very useful at RF. I also don't trust the mounting screws on my trunk lip mount to make decent connection, though they will if you carefully remove the paint under them. I do that and also add a few feet of copper tape under the edge - that eliminated some weirdness that I was experiencing. I don't use my lip mount on HF, but I have used it on 10 meters in the past.

It suddenly occured to me, that the car is isolated from "real" ground...and the transceiver and antenna are, shall I say, "tuned" to that particular enviornment. I was outside the car when handling the MFJ analyzer, and therefore was creating a "different" path to ground for the antenna.

As it turned out, I WAS the problem. And to "cure" it, I simply got inside the car with the MFJ analyzer and voila, the "problem" disappeared.

Your remark about the "RF ground", is what caught my eye and got me thinking. Thanks....