[TheList] mag antenna advice

Sun Dec 14 22:17:30 AEDT 2014

Hi-ho…

Define high gain?  You’re not going to do much better than 3dB gain on a mobile VHF antenna unless it’s a really long stacked dipole / collinear arrangement which is going to cause more problems than it’s worth.

(You can buy mobile antennas that claim more gain on eBay etc, but if they’re not really long, they’re using more sales speak than technology, you can’t beat physics!)

A 5/8 whip for general VHF use is about 1m long, and gives ~3.5 dB gain, with a tail wind.  On most cars anything longer is going to give you all sorts or problems with trees, garage doors, carparks..  This assuming you put it on the roof?

With a mag-mount it can’t go on the boot/bonnet unless you take it off every time you open the boot/bonnet in which case you’re going to leave more marks on the car than the two or three small holes required to put a lip-mounted L bracket on there which you didn’t want to do in the first place. :-)

Anyway…  Jaycar sell a decent SO239 magmount with a PRS designed whip on them and you could put any whip you like on there.

http://jaycar.co.nz/productView.asp?ID=DC1064 <http://jaycar.co.nz/productView.asp?ID=DC1064>

For receive only VHF the PRS whip might be fine though though from memory it had a trap or some sort in the middle, I chucked mine out, just wanted the mag mount and couldn’t be bothered mail ordering one in.

I’ve got one of these and use it on the 4x4 to take a dual-band whip and it’s pretty solid, hangs onto the roof at highway speed and puts up with the 1.2m antenna being ‘twanged’ on trees etc.

Although it will damage the paint underneath if you have it on full time, or you trap a teeny little bit of grit under it and look at it the wrong way.

I had it on the hatchback that I commute in for about six months with a 1/4wave for APRS and it left a fine rusty ring on the roof where grit/water got under the edge even though it as a plastic face. I had to sand/prime it and pick up a matched spray-boom.

If I put APRS back in that car I’ll drill a few holes and put a proper 5/8 stud antenna mount in the middle rear of the roof or something I think.  I have a stainless SO259 ‘L’ bracket on the passenger side front which has the antenna for the mobile rig I also use to listen to other random stuff, mostly ATC when I’m driving near the airport.

YMMV, just my 2c worth etc.

Cheers, Chris H.

> On 14/12/2014, at 10:29 pm, Stephen <kiwis1 at slingshot.co.nz> wrote:
> 
> Can anyone please help with a high gain receiving only vhf mag antenna? If I have to get it from Oz or US so be it.
>  
> I’m not allowed to put any holes in our new car!, and my existing mag ants are deaf on vhf.
>  
> Thanks
>  
> Stephen
>  
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