144/440 Mobile antenna counterpoise Question

[W2KWA]

Hello all im using a comet SBB5 mobile antenna on a window guard in my house, it works well but would like to add a counterpoise to the antenna what is the reccommended length for the counterpoise for the 144 and 440mhz freq range.. I have antenna space restriction, Thanks for your help...

W2MLH

 

[wyomingmedic]

The general idea is to get a larger image plane (ground plane) under the antenna.

On 2 meters, a little more than 19 inches in all directions from the antenna base will make a decent GP (38 inches total).

On 70cm, a little more than 6 inches in all directions (12 inches total) makes a decent GP.

Since you are using a dual band antenna, you will want a GP for the lowest frequency (2 meters in this case).

What I would recommend is something the size of a large cookie sheet. Should make a usable GP area. Or you could use an actual cookie sheet. Generally works well.

WM

 

[W2KWA]

Thanks for the reply but it's mounted on a window guard so a cookie sheet is not going to work so i'll have to drape the 19" wire downwards or bring it into the window should i use 2 19" lengths or more???

 

[wyomingmedic]

There are some odd idiosyncrasies when it comes to building a GP (groundplane). If you have a 1/4 wave antenna (which you do not), the design is for the radials to come off at a 45 degree angle from the base. Changing the angle can change the impedance of the antenna and therefore the SWR.

Now, a 5/8ths wave (closer to what you have) generally is built to have the radials come out at 90 degrees to the base (flat). This is a little more difficult for you in your situation.

If I were you, I would put 4 radial wires on the base and drape 2 along the length of the window sill, put one inside the window and the other outside, just hanging. This will be less than optimal, but it WILL work.

The best method would be to have the antenna mounted in the center of a solid, flat piece of metal at least 38 inches across. 1 radial wire does help things. 2 is better, 3 is better still and 4 is great. IMO, anything more than 4 radials is overkill unless you can mount to a solid piece of metal.

WM

 

[reedeb]

Right now my mobiles are on mag mount antennas on a cookie sheet on a stand in my room [I live in an apartment complex and no outside antennas allowed]

I live in No East Dallas and can hit a repeaters in other counties so I love it [YES wish I could get up outside and up but I live with what I got]