Highly variable SWR -- is this normal?

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Re: Highly variable SWR -- is this normal?

Postby m0lsx » Mon Jan 30, 2017 11:51 am

I have a MFJ-854, which will check for common mode. But as a whole most people cannot check for common mode current. They design it out of the system.
Common mode will always be a potential issue, but especially so with small antennas that do not have good earthing systems. As in it's simplest form, common mode is caused when a antenna does not radiate efficiently & throws some of the current back down the outside of the coax.
There are times when stopping the common mode on the coax is desirable, for example in Windom antennas where a properly designed Windom, uses a section of the coax as part of the antenna. But that is using a measured length of coax before the ferrites.
As a whole putting ferrites or a balun at the feed point or before the shack is due to an inefficient antenna.
So the best way to sort common mode is to use a decent antenna & a good counterpoise & earthing system, but that is not always possible or on longer wavelengths achievable.
At 27 MHz you should be able to avoid too many problems.

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Re: Highly variable SWR -- is this normal?

Postby lars » Thu Feb 02, 2017 3:08 pm

Hey-ho. So, after replacing the antenna, mount, and coax, I finally figured out why my SWR is so variable. But I'm darned if I can figure out what to do about it.

There is about four feet of excess cable in the coax run between the antenna mount and the radio. What I do with that surplus cable has an enormous effect on the SWR. If I eliminate it altogether, by using a shorter cable, the SWR is crap -- > 2.5 everywhere. If I wind it up into a coil and strap it down, it's shockingly crap -- > 3 everywhere. After several hours of experiment I find that I need to coil it loosely into a 1-foot loops,and lay it on the floor in the boot. Then, depending on _exactly_ where I lie it, I can get the SWR < 1.5 everywhere.

But here's the problem -- a movement of any part of this coil by more than an inch or so sends the SWR back to the > 2.5 region. So I think what's been driving me nuts is that every time I drive the car, this bundle of surplus coax moves a little, and that affects the antenna matching.

But WHY? Does this make any kind of sense? It seems like I have to make a sort of common-mode choke in the coax, and I've certainly seen that done elsewhere. I guess that if I really _do_ need a common-mode choke, I should put it on the outside of the car, not in the boot.

But why would I need to do this in the first place? Does this indicate some other kind of ground fault somewhere? The antenna mount is on a roof bar, which doesn't amount to much of a ground plane. But if I connect it to the car body using a foot-long ground strap, it really doesn't make the situation any better. Without buying a different vehicle, I doubt I can make the ground plane any better.

I'm beginning to think that my car just isn't suitable for an antenna; but I can't see why -- it's not as if it's got a plastic roof or anything.
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Re: Highly variable SWR -- is this normal?

Postby m0lsx » Thu Feb 02, 2017 3:19 pm

The coax is showing symptoms created by the poorly mounted & in all probability poorly designed antenna. The cause is not excess coax & removing it or re routing it is not the solution.
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Re: Highly variable SWR -- is this normal?

Postby lars » Thu Feb 02, 2017 3:50 pm

Fair enough; but it is what it is. I have the same coil-loaded antenna that everybody else uses for off-road applications -- not because of any particular RF performance, but because they will tolerate being banged into low-hanging branches and what-not. And I've tried every way of mounting the bloody thing that I can, that doesn't involve welding.

I guess I shall just have to do what the vendor suggested all along -- disconnect the meter and hope for the best :/

Thanks.
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