Ground plane for magmount 800 Mhz analog cellular?

[Frank B]

We use a Motorola 3 watt analog cellular 'bag phone' when on the road.  I have both a 13 db yagi on a telescoping mast bolted to the side of the trailer, and a magmount 34" Maxrad whip antenna for the truck (double coil?).  I may need the magmount on the trailer, which has a rubber/plywood roof.  How much of a ground plane do I need for this magmount antenna?  I'm thinking I can put a plate or a bar on top of the plastic cover over the fridge vent, and put the magmount on that.  I already have a waterproof access hole on the side of the trailer for the coax to go through.

Thanks.

Frank.

 

[John From Detroit]

Well, let's see, 800 MHZ, 300/800 = 3/8 meter, 1 meter = about 39" 117/8= 14 (rounded) and 14 /2 = 7 inches

So a 7 inch circle should be plunty (Radius = 1/4 wavelength) or a square even, or a retangle (Note, you can use the diagnoal measurement if you have to) it's not at all fussy about shape

If I recall correctly the fridge vent is darn near a foot long, so if you capped it with something the magnet will stick to you'd be in great shape.

Does this help?

(I just finished building an antenna not 20 minutes ago, it's for a different frequency however)

NOTE: Not all antennas require a ground plane,  The one I just built, for example, does not, six DB gain on 145MHZ and no ground plane required

 

[Frank B]

John:

>Does this help?<

Yep! 

The cap is 22" x 9" so that ought to fit just fine.

I'm only assuming that this antenna needs a ground plane.  I don't know for sure.  However, a magmount would probably assume a metal plane under it.

Thanks.

Frank.

 

[Karl]

Frank,
A reasonably efficient ground plane antenna will have four horizontal 1/4 wave radials, equally spaced 90 degrees from each other and 90 degrees (perpendicular) to the vertical radiating element. The standard calculation for 1/4 wave antennas is 71.3232 divided by the frequency in MHz, or 71.3232/800 = .09 meters. In inches, that would be 3.54" for each of the horizontal radials and 3.54 inches for the vertical. As you are creating an artificial ground plane, the horizontal elements need not be actually grounded.