Highly variable SWR -- is this normal?

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Highly variable SWR -- is this normal?

Postby lars » Thu Jan 26, 2017 8:16 pm

Hi folks

I'm installing a 27MHz FM CB radio in my car, to use when marshalling charity races. I'm not very familiar with this kind of installation, so I don't know if what I see is normal.

The antenna is mounted on the roof rack, and the coax enters the car in the gap between the back door and the frame. It's a tight fit, but not vice-like. When I measure the SWR, the results are very sensitive indeed to the position of the coax, particularly where it enters the cab. A couple of inches of movement of the cable in this area can make a difference (on a specific channel) between an SWR < 1.2 and an SWR > 2.5. Fiddling with the coax elsewhere does not have this effect.

I have a suspicion that the coax is damaged, perhaps by being squeezed between two pieces of metal. However, the DC conductivity is not interrupted. I telephoned the vendor, and was told that CB antenna set-ups are very sensitive to cable routing, and that I shouldn't be too concerned about this. Apparently passing coax into a vehicle in the gap between the door and the frame is common practice, and not usually troublesome.

I'm expecting to have to tune the antenna, but it's going to be difficult if such small changes to the set-up can have such marked effects.

So -- is it really the case the position of the antenna coax can have such a profound effect on SWR? Is there any way to test the integrity of the coax if its DC resistance seems fine?

Thanks
Lars
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Re: Highly variable SWR -- is this normal?

Postby m0lsx » Thu Jan 26, 2017 9:04 pm

It sounds like it potentially could be damage some where.
BUT, at 27mhz you will need a good contact to Chassis to get an efficient RF radiating system. So as unless the roof rack has a good metal to metal connection with the car body, you may find you have issues due to that.
Also make sure the mounting bracket at the roof rack has good metal to metal contact.
I had a hatch back mount on my car for years with no issues & I know plenty of others who have coax passing through the back door with no issue. So I do wonder if you are getting common mode current on the outside of your coax. Which is a real potential if you are not getting the earth the antenna needs.
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Re: Highly variable SWR -- is this normal?

Postby lars » Thu Jan 26, 2017 10:07 pm

Thanks.

To be honest, I was prepared for the SWR to be generally crap -- it was the enormous variability that took me by surprise.

The antenna mount has good metal-to-metal contact with the bar of the roof rack, but that bar is not connected to anything other than rubber bushings. So the ground plane -- if that is even the right word -- is a 4-foot long horizontal steel tube. I could probably find a way to connect it to the car bodywork, if you think that would help? Not sure where, exactly, though.
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Re: Highly variable SWR -- is this normal?

Postby m0lsx » Thu Jan 26, 2017 10:50 pm

It sounds more like a roof bar than a roof rack, which is not good. As not only is it too small to be effective, but it will effect the radiation pattern of your antenna.
For HF in a hatch back, I used braided cable to the bolts on the back door to get a good earth. I also made sure when using a hatchback mount, that my mount had a connection to metal, not painted metal.
Plenty of CB'ers seem to get away with some badly put together mobile stations, but plenty probably also have issues they lack the knowledge to realise they have. As 27 MHz needs a good earth & will not perform well without it.
One thing worth remembering is that it's not how your station performs during a lift that matters, as a wet length of string will work the world on the right day. It's how your station performs on a poor day that counts.
Take a look at the following link. It's how someone set his car up to operate from. It took him many many hours to get right. But he works the world
https://www.flickr.com/photos/32626524@N08/sets/72157623142510771/



Operating at 27 Mhz you do not need to got to the same lengths as he does for lower frequencies. But it shows what can be needed.
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Re: Highly variable SWR -- is this normal?

Postby lars » Fri Jan 27, 2017 9:50 am

Yeah, I hear you. Given the sub-optimality of the set-up, I doubt I would have even started were it not for the fact that I only have to achieve a range of a few hundred yards. So I'm hoping that if I can keep the SWR out of the red zone, it will do for what I need. However, I will try improving the grounding if the temperature ever gets back above freezing.

I wonder how people get away with mag-mount antennas? I don't imagine that their users scrape the paint off their car roofs to improve coupling and, in fact, the ones I have seen have a rubber base to prevent scratching. So they can only work -- if they work at all -- by the miniscule capacitive coupling between the mount and the roof. I could switch my bar mount for a mag-mount if I thought it would be better, but from a plain physics perspective I don't see how it can be.
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Re: Highly variable SWR -- is this normal?

Postby m0lsx » Fri Jan 27, 2017 10:41 am

I use a triple mag & don't go below 20 meters. & as a whole operate 17 & 10 meters, even when using proper set up.
At 27 mhz you can get away with a mag mount, but not always & the bigger & stronger the magnet the better the system works.
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Re: Highly variable SWR -- is this normal?

Postby lars » Fri Jan 27, 2017 1:15 pm

Thanks. I've connected the base of the antenna mount to the car roof via a short length of heavy cable. The SWR is still crap, but at least its consistently crap. I guess I need to adjust the antenna now. SWR is about 1.5 on channel 1, and 2.8 on channel 40, and conventional wisdom says that in that situation I need to shorten the antenna. If you, or anyone else, has any gut instinct about how much to shorten it, I would appreciate it. Right now I don't know whether I'm looking at feet or fractions of an inch. But I'm hoping the latter, because otherwise it will involve a saw.
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Re: Highly variable SWR -- is this normal?

Postby lars » Fri Jan 27, 2017 2:09 pm

FWIW I tried shortening the antenna in 5mm increments, and it made no difference -- SWR is still ~3 on ch. 40 and < 1.5 on ch. 1. I'm reluctant to keep sawing pieces off it, especially as it was sold as a "pre-tuned" 27MHz CB antenna. However, I tried replacing the antenna with 2 ft of copper mains conductor, and that definitely has the SWR much worse at the ch. 1 end. So that sort-of suggests that there is some correct length for this antenna. But I'm reluctant to saw six inches off it, because I can't put it back again.
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Re: Highly variable SWR -- is this normal?

Postby m0lsx » Fri Jan 27, 2017 3:26 pm

That is one of a hell of a variation across a few khz of spectrum.
I have never really used a CB much, other than when we were broadcasting & then having legal radio comms was handy. But from that limited experience years ago, my guess would be that this may be down to the small earth provided by that roof bar added together with the fact that a lot & maybe most CB antennas are very poorly made.
How is the SWR changing across the whole spectrum? Knowing what it is at the edges is one thing, but it must be a VERY hi Q antenna to peak from 1.5:1 to 2.8:1 across a few khz. I would expect that where the antenna is exceptionally short, such as top band or 80 meters, but not at 27mhz.
A VSWR 1.5 is good & anything under 3 is far from a disaster. However if it has a lossy loading coil to give that pre-tuned experience then I would expect the SWR be >2:1.

The real issue is that without being there & being able to see what the antenna looks like on a HF VSWR analyser, it's hard to make a judgement.
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Re: Highly variable SWR -- is this normal?

Postby lars » Fri Jan 27, 2017 4:36 pm

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.

The SWR varies more-or-less linearly across the channel range, so 1.5 on ch.1,then 2.0 on ch. 20, ~3 on ch. 40. An adjustment of an inch or so in length does not affect this pattern significantly. The designer said that if removing an inch from the length does not help, it probably wouldn't help to shorten it any more. However, I can get an SWR or 1.5 on ch. 40 by removing the whip from the coil, and replacing it with two feet of copper wire. So my gut feeling is that the proper antenna could produce better SWR in the higher channels if it wanted to -- but, of course, that might well be at the expense of worse SWR elsewhere.

I've checked for obvious faults -- there is DC continuity everywhere there should be, and nowhere there shouldn't. Arguably powering the radio from a cigar lighter socket is non-ideal, but it's designed to be used that way.

If the roof bar isn't much of a ground plane, then the nearest potential ground plane is the car roof, which is about 6" below the antenna coil, and 12" below the whip. Maybe this is non-ideal? There's a six-inch tall grounded metal stalk very close the antenna coil -- it is where my emergency beacon attaches. I accept that this might also be a problem, although for various reasons I can't do much about it.

If I've just got to live with this situation, then fair enough. What worries me, though, is that something might actually be faulty, and I just can't figure out what.
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