Highly variable SWR -- is this normal?

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Re: Highly variable SWR -- is this normal?

Postby m0lsx » Fri Jan 27, 2017 5:00 pm

lars wrote:Thanks. FWIW the antenna is a Thunderpole Orbitor 1.5m (http://www.thunderpole.co.uk/large-cb-a ... bitor.html). I spoke to the chap who designed it, and I sort-of got the impression that high SWRs in the upper channels were not entirely unusual. One of his colleagues said, essentially, if the SWR is < 3, just disconnect the bloody meter and stop obsessing about it. However, it bugs me, not knowing what the problem is..



What a genius he must be to deign a antenna that cant cover a few Khz of spectrum around 27.5 mhz that is capable of low SWR.
He sounds like someone who should not maybe designing antennas.
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Re: Highly variable SWR -- is this normal?

Postby lars » Sat Jan 28, 2017 10:19 am

It would certainly be nice to be able to blame the equipment, but I can't avoid a nagging suspicion that the real culprit is my incompetence.

But I've checked everything I can, within the limits of my tools (I only have a multimeter). There's a good ground cable from the antenna mount to the car roof (it's connected to one of the rear window bolts by about 18 inches of 50-amp cable). Nothing is loose or wobbly.

My lingering areas of uncertainty are...

1. The coax is pinched where it squeezes between the window and the car body. Could it be damaged in a way that affects RF, but not DC?
2. The antenna is 6" above the ground plane, and there is a small, grounded obstruction not far from it.
3. The coax is quite long for a car installation -- it's about 10m, and I used bog-standard cable, nothing special.

Is there any way I can test any of this? If I were to attach a dummy load where the antenna is now, would that help to identify problems in the wiring?
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Re: Highly variable SWR -- is this normal?

Postby m0lsx » Sat Jan 28, 2017 11:44 am

lars wrote: 1. The coax is pinched where it squeezes between the window and the car body. Could it be damaged in a way that affects RF, but not DC?
2. The antenna is 6" above the ground plane, and there is a small, grounded obstruction not far from it.
3. The coax is quite long for a car installation -- it's about 10m, and I used bog-standard cable, nothing special.

Is there any way I can test any of this? If I were to attach a dummy load where the antenna is now, would that help to identify problems in the wiring?



1..YES. The coax will have a plastic shield around the inner core & if the coax is bent in the wrong way this can become damaged, which would effect the impedance of the coax, as in part what defies the impedance is the distance between inner core & outer braid on the coax.
2..Yes. If it is even a few inches below the antenna then itis not a ground plane, but a close by metal object that will effect things. What is the obstruction? as that could make a big difference?
3. The coax if 50 ohms should make no difference. There are times when length matters, but this should not be it.

A dummy load should show 50 ohms, so it will possibly show if there is a problem in the coax.
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Re: Highly variable SWR -- is this normal?

Postby lars » Sat Jan 28, 2017 2:17 pm

Thanks. I guess I need to try a new antenna cable. Rather irritatingly, it's all one piece with the antenna mount, so I will probably have to replace the whole thing.

This is why I was wondering about a dummy load -- if the cable imedance is no longer 50 ohms because it is damaged, then (I'm guessing) I would still see poor SWR if I replaced the antenna with a 50 ohms dummy load, right? Or are things not as simple as that?
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Re: Highly variable SWR -- is this normal?

Postby m0lsx » Sat Jan 28, 2017 5:37 pm

lars wrote:Thanks. I guess I need to try a new antenna cable. Rather irritatingly, it's all one piece with the antenna mount, so I will probably have to replace the whole thing.

This is why I was wondering about a dummy load -- if the cable imedance is no longer 50 ohms because it is damaged, then (I'm guessing) I would still see poor SWR if I replaced the antenna with a 50 ohms dummy load, right? Or are things not as simple as that?


Yes, but you will need the relevant adaptors to connect the dummy load to the mag mount & don't forget a dummy needs to be a good one to take the load, so avoid anything CB.
A change in the impedance in the coax, would alter the SWR that you are seeing, but that antenna is clearly not very good if it's got a big disparity across just a few khz of spectrum at around 27.5 MHz.
What you are seeing would be normal at a much lower frequency, or across a big chunk of spectrum at around 27 MHz, but it should not be happening with your set up.
If you were trying to use say 26 to 28mhz & not seeing a difference in SWR, then the antenna would be a dummy load. But if you are just covering the few 100 khz of legal band then it should not be showing more than the smallest difference, or with a competently designed antenna.
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Re: Highly variable SWR -- is this normal?

Postby stanogs68 » Sat Jan 28, 2017 11:00 pm

can you not just stick it on the roof drill a hole through the roof with a decent size metal drill push the coax and connector through then use a tube of silica sealant on the hole ,job done :thumbup:
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Re: Highly variable SWR -- is this normal?

Postby Chris P » Sun Jan 29, 2017 12:18 am

I assume ( perhaps wrongly ) that the variation in SWR is noticeable when the vehicle is stationary as if its only when the vehicle is moving the variation could be due to the flexing of the "spring" loading coil , the inductance of which would change considerably if moving at high speed .

better designed mobile antennas loading coils are non flexible but even then the flexing of a whip can cause SWR variation
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Re: Highly variable SWR -- is this normal?

Postby lars » Sun Jan 29, 2017 3:45 pm

The mad thing is that the SWR varies from day to day, even hour to hour, when nothing that I can see has changed. Except the weather, perhaps. When I parked my car last night, I was seeing about 1.5 on ch. 1 and 2.5 on ch. 40. This morning I got in the car -- nothing changed except the time and weather -- it was 2.5-3 everywhere.

The SWR _doesn't_ vary when I drive, wiggle the connections, shake the antenna, thump the radio, anything. But it varies sometimes, by quite a bit, if I just wait.

I'm not sure that anything except a fault can cause this sort of variation, no? Or maybe water (rain, condensation) is getting into the antenna? It's driving me nuts, whatever it is :(
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Re: Highly variable SWR -- is this normal?

Postby m0lsx » Sun Jan 29, 2017 4:21 pm

Common mode current will cause that variation.
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Re: Highly variable SWR -- is this normal?

Postby lars » Mon Jan 30, 2017 11:01 am

Thanks. Are you able to explain in numpty language how to check that? Or provide some link?
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