Magmount advice

Hi, I've recently been playing with an old Mocomo 59s 23 channel CB and my equally old PAMA magmount / DV27 type antenna from approximately 1982.

I can't help noticing that the middle of the roof of my Octavia 2 estate doesn't appear nearly as robust as the roof of my old MK1 Escort was,  there is an awful lot of flexing and wobbling going on.   Is this likely to cause any permanent damage to the roof?  Is it worth placing the magmount near to the rear edge of the roof where it appears to be much stronger, or will this make the antenna too directional?   I need to improve the antenna situation anyway, with an swr of about 1.5 my son couldn't receive an intelligable signal from more than about 500m away on our Realistic pro26 scanner.    .


I do have a Sirio ML 145 magmount on order, that might be better from a roof flexing point of view in having less weight high up than the dv27, what do people think?   Am I overthinking this issue?  
Are there perhaps better antenna solutions these days that fix to the roof bars (the roof is gutterless)?

Many thanks

J

You're right, roofs are a lot thinner than they used to be. I take it drilling a hole in the middle of the roof is out?

The Sirio magmount is a fair bit larger so spreads the load and will be a whole lot better. Whilst a lot of people fix to the roof bars they provide a very poor RF ground no matter what you read with the DC multimeter. Do be careful about pinching the coax where you bring it into the car as that alters the impedance of the coax when you pinch it and therefore the SWR. It can alter it to radio damaging levels even though you can test it with a multimeter and see no short.

I'd not be over concerned about the scanner unless you'd hooked it up to a CB antenna. Every time you halve the length from a quarter wave you reduce that efficiency by 75%. A 4ft antenna has just 25% of the efficiency of a 1/4 wave, a CB 2ft antenna has just 6% of the efficiency of a quarter wave. Carrying on the 6 inch one of the scanner will have an efficiency of less than half a percent of a full size quarter wave antenna. It has at least -24dB gain compared to a quarter wave on 27MHz. That means it will receive a signal more than 8 S points lower. Something that will be a nice strong S9 on a CB using a 102" whip will be barely S1 on the scanner. Chuck in a bit of background noise and you could be barely noticeable on the scanner.

If you want to get really serious about the mobile installation pop over to www.k0bg.com and read the sections on mounting, grounding and bonding.

That's a great reply Northern Crusader, thank you very much. And thanks for the point about pinching the coax, I think to avoid this I'm going to have to bring it in through one of the rear windows. The doors and tailgate all fit very tightly and very well, there's no obvious easy route in, at least that I've found.

Hi. John.

What I did in that situation was put my K40 with mag-mount in the
center of the roof with SO239 H/duty roof mt. Then no cables get
trapped also it makes it possible to use a 3/8 adapter to use my
80/40/20 whips when static mobile. Good luck/ DX 73's Mike.

Thanks Vendee, I'm not sure how you are routing the cable though unless it's through the roof, in which case why the magmount?

Hi. John.

The K40 only works with its own mag-mount. Also I use the roof
mount for my 80/40/20m. Ham whips via 3/8 adapter. Most of
the Ham whips/Tank whips for 11m. are 3/8. 73's Mike.