Completely irrational SWR, please help

[morgz84]

So having bought a "new" car recently, I am desperately trying to get a radio set up. The radio is a Superstar 6900N and the antenna is a Sirio 145 on the 145PL mount. The cable from the mag mount goes down into the bonnet and into the glove box area, and no further, the leads on these mag mounts are only 3.6M. No problem, at least I thought because I want to put an amp inline. There is a separate patch lead going from the glove box to the radio. I have been trying to tune the antenna and been getting nowhere.

On most blocks it's around 2.7, some higher. I have since redone the PL259 on the antenna lead. I had already chopped and replaced the factory connector as I needed to thread the cable into the cabin. When I did this and plugged the rig straight into the antenna it was showing much more sensible result - about 1.2 on the lower bands and 2.0 on the higher, suggesting that all I need to do is shorten the whip slightly. However as soon as I extend the lead, either via external SWR meter or female to female joiner, we are back to 2.7 +. I made a brand new patch lead (about 75cm), with new coax and high quality connectors, still 2.7 - 3.3 across the board. I tested the patch lead on my home base, it increased the SWR ever so slightly from 1.2 to 1.4 but completely acceptable. My regular patch leads on the home base are RG213, so perhaps the thinner coax would explain that. I have tried one of my known good RG213 leads from the home base which improves SWR marginally, but it is still not acceptable. I have even tried another Superstar 6900N that I have, same.

I am absolutely at my wits end with this. It seems to be beyond logic and just don't know what I am supposed to do to get this set up working properly.

[The Collector]

I think certain mag-mounted aerials need the co-ax a certain length or the SWR goes up. I think K40's were the same iirc.

If you can't get it any shorter, try it longer and reroute it to take up the slack to see if it helps at all. Coiling it can affect the swr too, so you could try and see if it improves it whereas it used to make it go higher whenever I tried years ago!

[108EH002]

Is the antenna sitting in the middle of the roof ?,have you tried it at the back of the roof at the furthest position possible ? = Have had a few motors that I had difficulty obtaining a good swr.

[The Collector]

You could also try running a separate ground wire from the outer of the co-ax to an earthing point on the car in case the magnet's not having the effect it should.

[morgz84]

Thanks everyone for these suggestions. I have been reading all about coax lengths as a result of these suggestion and opinion certainly seems to be divided. However, I've created a new patch lead to bring the total run to 5.5 metres, and it definitely seems to have improved matters, albeit only tested on the drive so far as I've been drinking. I have been getting circa 1.4. It does still swing up and down a bit when I move the cable about though. I think this is due to the new pl259 I've put on. I thought I'd be clever and solder the earth through the little "port holes". I understand that this can be problematic on thin coax like rg58 as it's easy to end up melting the insulation on the centre core. Had enough for today but in the daylight tomorrow I will redo the connector yet again, but I'll do it the twisting the braid into the plug thread way as I've done the patch lead. I did have a K40 once, but never had any issue with a similar setup other than having to chop it down slightly. It was a different car, a Ford Mondeo. The car I am trying to get this lot working with is a 2004 Toyota Avensis estate.

[ch25]

When you move coax and swr is changing it means, that antenna have bad/not enough counterpoise. Simple as that.
Cable acts as counterpoise and changing it's length or position have influence on swr.
Just another bad mobile antenna installation.
Chris

[dt307]

ive have the same antenna over 20 years a ml145 and mag as you have.its been my main antenna for years
i have replaced the coax a few times over the years with what ever length i guessed to be long enough for my needs with no ill effect.
its quite a narrow antenna and wont cover from mid band to 10m
i set my swr to be 1.1 on uk 19 and its around 1.4 on 555 and 1.8 on 28.5 which is workable for me.
the only time i got issues and is the shield connection on the coax on the mag remove it and scrape it all spotless and add a star washer.
as been said if the coax is making a huge difference to the swr its acting as a counterpoise and not a feed line.
i also cut a disk of foil and fitted under the rubber boot which helped with swr .

[morgz84]

When you move coax and swr is changing it means, that antenna have bad/not enough counterpoise. Simple as that.
Cable acts as counterpoise and changing it's length or position have influence on swr.
Just another bad mobile antenna installation.
Chris

Thankyou. How very helpful and what a pleasant human being you are.

[The Collector]

Chris also works for The Samaritans of a weekend

[morgz84]

Chris also works for The Samaritans of a weekend

Not entirely certain what your commment means but mne was a diplomatic reply to a rather narcissistic comment.

[The Collector]

I meant his comment may not have used the kindest or most sympathetic wording (not a quality a Samaritan would have ).

Bigpimp used to use the same short shrift replies and often it wound people up the wrong way, like this did yourself.

[morgz84]

I meant his comment may not have used the kindest or most sympathetic wording (not a quality a Samaritan would have ).

Bigpimp used to use the same short shrift replies and often it wound people up the wrong way, like this did yourself.

It's just completely unnecessary. We are all here to help and learn from each other. Everyone does things "wrong" from time to time and we can learn from our mistakes and the support of others makes it an easier process. Actually, his post makes an interesting point, there was just no need for that snide comment at the end.

I am looking into improving the grounding. I would like to do this as non destructively as I can. I wonder if I could carefully strip the outer insulation of the coax close to the mount and run a cable from there?

[morgz84]

ive have the same antenna over 20 years a ml145 and mag as you have.its been my main antenna for years
i have replaced the coax a few times over the years with what ever length i guessed to be long enough for my needs with no ill effect.
its quite a narrow antenna and wont cover from mid band to 10m
i set my swr to be 1.1 on uk 19 and its around 1.4 on 555 and 1.8 on 28.5 which is workable for me.
the only time i got issues and is the shield connection on the coax on the mag remove it and scrape it all spotless and add a star washer.
as been said if the coax is making a huge difference to the swr its acting as a counterpoise and not a feed line.
i also cut a disk of foil and fitted under the rubber boot which helped with swr .

Ok, perhaps a daft question, but when you say under the rubber boot, do you mean between the rubber and the car or inside the base.

[dt307]

lift the magmount off your car and remove the rubber boot/cover that covers the mag ,cut a disk of foil and insert it and refit boot

[The Collector]

Or leave the coax/mag as it is and fit a temporary earthing wire from a bolt on the car's body somewhere, to the outer fixings of the radio's pl socket and see it thst makes a difference. If it does, either make it permanent, or easily removable if you take the radio out of the car after use.