Mobile antenna mounts

[cdosrun]

Hello All,

I have a question that I am sure many of you will have useful suggestions for.

I am very new to playing with radios, I am booked to take my Amateur course (foundation level) in the New Year but in the interim, I have bought a cheap 11m CB transceiver off EBay to give myself the opportunity to play with antennae and a VSWR meter. I know there seem to be disparaging comments directed at CB and vice versa but it seemed like a simple and legal way to have a go.

Anyway, I intend to install an antenna on my car but as I am going to have strip the headlining out to do it, I want to make sure I use a suitable mount and feeder to enable me to switch to 2m/70cm in due course without too much more work.

A lot of the mounts seem to use a SO239 coupling to the antenna, which I thought posed a problem of inconsistent impedance, causing high VSWR at UHF frequencies (something I would like to avoid for obvious reasons). I have read about NMO mounts, but haven't seen much in the UK online Amateur shops with that style of mount and I don't know how difficult it would be to convert to SO239 for a CB aerial for the intervening period.

What would you recommend I do/use? Should I just put one mount in now and change it later?

I hope I haven't offended anyone by mentioning CB here but I thought it would probably be more productive than asking my question in the CB forum. I meant to take the Amateur exam years ago, I revised for it back in 2002 and was passing all the online tests I could find with decent scores but didn't do it quickly enough when I moved back to the UK, so I have to go through the three different classes sequentially now (hence the procrastination)!

Thank you,

Andrew

 

[Captain Kilowatt]

Don't worry about going with an antenna that has an SO-239 mount. The non-constant impedance thing you are talking about is refering to the normal mating PL-259 connector. There are many VHF/UHF antennas that are made with the SO-239 type of mount.I use one for 2m/70 cm and it is not an issue. These mounts are made to more exact standards than the typical PL-259/SO-239 connectors.NMO mounts are great but there is nothing wrong with an SO-239 mount either.

 

[hookedon6]

if you decide to drill a hole, the important thing is to make sure that you have a good ground. a little strip of paint will have to be removed around the outside of the hole, it won't show after the mount is placed in the hole. good luck on getting your foundation license. have fun too.

 

[Moleculo]

I hope I haven't offended anyone by mentioning CB here but I thought it would probably be more productive than asking my question in the CB forum.

Don't worry about it...we're not that uptight

Welcome to the forum.

 

[cdosrun]

Thank you for the help. I have now bought an SO239 through-hole mount and I will strike up the courage to install it over Christmas. Obviously it is necessary to keep everything water tight so I presume I can use some silicone sealant around the outside of the external parts once they are tightened down (obviously not between the mount and car so as to isolate the ground).

Another question for you helpful people before I do start drilling holes (if you wouldn't mind):

I understand that ideally, the antenna should be in the very centre of the roof for best omnidirectional performance. However, the roof on my car is reasonably flimsy and I am not too keen on this massive antenna flexing it until it fatigues so would it be terrible to situate it nearer the back where there is a little less flex in the roof panel? Alternatively, I can put it nearer the front which would be closer to the middle as there appears to be a strengthening member around that area but it will put it very close to the FM aerial for the car radio. Am I likely to induce enough voltage into the FM aerial to cause any damage to the car radio? I know that at 11m, the coupling will be poor and easily filtered by the radio but I am thinking more about when/if I get around to switching to 2m.

Thanks again,

Andrew

 

[Captain Kilowatt]

Depending on the radio 2m may swamp it anyway but it should not cause any damage at all. If you want you can always install a large piece of metal under the roof on the inside and mount the antenna mount thru it and the roof. This will add a lot of strength to the roof by distributing the force over a wide area. I have heard of a few people even using an old circular saw blade and mounting it under the roof above the headliner. Any piece of metal there would reinforce it a lot.

 

[W5LZ]

Andrew,
While the center of the roof seems ideal, it may not always be for some reason. It isn't really so terrible to move that mounting spot to a more convenient position. At worse, it changes the radiation pattern to something not so symmetrical. Not very directional, just sort of 'lumpy' maybe, and not all that noticeable, if at all.
Weather proofing the mount, not a bad idea! Most antenna manufacturers give recommendations on how to do that, supply a sealing gasket, or something of that nature. I would use them. A very light application of some kind of sealing 'goo' might be used, but pay attention to the electrical contact points of the antenna, maybe not there, or -after- those contacts are made. (Hmm... that may not make much sense, just not sure how to say it. You probably get the idea though.)
'QRN' is right about using a 'backing-plate' if the mounting point is very flexible. Some of the larger mounts come with them, most don't. The 'trick' is finding something that isn't too thick that the mount can't be used. That's more of a 'trick' than you might think! A better idea is to find a spot that doesn't flex much, and that's 'flat', no curves.
Reasonable care in choosing a mounting point is all that's required unless you have a very unusual vehicle. And since all you guys have the steering wheel on the wrong side anyway, I have no idea what else would make it -that- unusual. Have fun.
- 'Doc