Dual band ground plane radials

[KG5YWM]

Good morning,

I've got a question for the antenna gurus. I have a dual band uhf/vhf mobile antenna. It worked very well on my truck but it's just too durn big so I went back to my shorter dual band that is good enough for what I need. I would like to use the longer antenna as a base station antenna and I'm looking at making my own ground plane.

My question is, what length of radials do you think would give decent performance on both 2M and 70cm? Should I just split the difference or set the radials longer like I'm setting up just for 2M?

Thanks!

 

[nd5y]

Ground plane radials should be at least 1/4 wave on 2m (19"). Shorter ones will not work well on 2m.

 

[fyrfyter33]

You are probably going to waste more time trying to get the mobile antenna to work as a fixed ground plane antenna, than just building one.

I built one of these about 10 years ago and did it in less than an hour. This works perfect in an attic. I used it for several years on both 2m & 70cm without any problems.

http://www.hamuniverse.com/2metergp.html

 

[AI7PM]

What brand/model is the antenna?

 

[mmckenna]

If it's an NMO mount antenna, the NMO base kits work well enough. Excellent way to put the antenna to good use.

1/4 wave or longer on the lowest frequency for the ground radials.

 

[KG5YWM]

What brand/model is the antenna?

https://www.gigaparts.com/comet-antennas-csb-790a.html

I found this. I think I'll give it a try.

https://www.gigaparts.com/mfj-1401.html

 

[KG5YWM]

BTW, thanks everyone for your inputs!

 

[Golay]

Radial lengths

Make the radials 5% more than a 1/4 wave on the lowest frequency of the dual band antenna.
Will work just fine for both bands.

 

[cmdrwill]

Hope that MFJ mount has radials and they are at least 20" long.

https://www.gigaparts.com/comet-antennas-csb-790a.html

WOW a real tree trimmer. Had one on a Sheriffs SAR truck, it went to the junk pile, never did work right. But boy could it find trees to trim.

 

[KG5YWM]

Hope that MFJ mount has radials and they are at least 20" long.

https://www.gigaparts.com/comet-antennas-csb-790a.html

WOW a real tree trimmer. Had one on a Sheriffs SAR truck, it went to the junk pile, never did work right. But boy could it find trees to trim.

Yes, they are 20.5". The limb whacker worked great on the center of the roof of my truck although the SWR on 70cm was a little high, almost 2:1. I could have tuned it in a little better, but the area I'm in has a lot of low hanging trees so it just wasn't gonna work.

 

[kk4obi]

Only two opposing radials are needed to make a omnidirectional ground plane. They should be identical. These radials can be tuned by adjusting length and angle to get a near perfect SWR.

In your case of a trapped vertical, a second pair of radials can also be perfectly tuned in the same manner to any desired frequency In the second band.

This is an extremely simple way to make a dual band, trapped ground plane for a base station.

About the only refinement would be to make three radials per band. This is minor technical improvement that would not be detected in routine operation.

If you are interested in the details of radial-to-vertical lengths vs. radial angle, here is the antenna modeling information:

The characteristics of a vertical mono-pole antenna with 1,2,3,4,6 or 8-Elevated Radials

 

[lmrtek]

RadIals should ALWAYS be more than 1\4 wavelength at the lowest frequency you will use.
............
If ground planes worked like some hams believe, we would all have to drive 12 inch wide cars to make our 460Mhz antennas work.
............

 

[nd5y]

RadIals should ALWAYS be more than 1\4 wavelength at the lowest frequency you will use.

Explain why.

 

[prcguy]

A ground plane made of individual radials is tuned and should be near resonant 1/4 wavelength and their resonant lengths will change slightly depending on how many you have. Making ground radials 5 or 10% longer than a resonant 1/4 wavelength is old and incorrect info. A solid sheet metal ground plane like a car roof is a different animal and follows different rules.

There is a lot of misconceptions about individual radials and a bunch of short ones like 20 but only 1/8 wavelength long can be better than three that are 1/4 wavelength each. This has some to do with the spacing at the ends, which can be more important than the overall lengths and there is also more RF currents at the feedpoint end of the ground radials compared to the ends.

Otherwise when going for 1/4 wavelength radials, ideally they are resonant so the RF currents will have an exact 1/2 wavelength round trip from the feedpoint to the end then back to the feedpoint where they will mimic the feedpoint impedance. Then you can trim the vertical element and or bend radials downward to achieve the best match.

If the radials are not a resonant 1/4 wavelength they can skew the feedpoint impedance some and you also have the capacitive effects between the vertical element and ground plane that will change when lengthening or shortening radials. There is a lot of stuff going on that interacts making antennas and ground planes a very complicated subject.

RadIals should ALWAYS be more than 1\4 wavelength at the lowest frequency you will use.
............
If ground planes worked like some hams believe, we would all have to drive 12 inch wide cars to make our 460Mhz antennas work.
............

 

[smason]

If ground planes worked like some hams believe, we would all have to drive 12 inch wide cars to make our 460Mhz antennas work.
............

Brilliant, love it!!