How critical is a ground plane?

[Smitty74]

I have a dual band comet antenna meant for automotive use. Its like 4 foot tall.  If I put it on a "lip mount" base on the edge of my car hood, is the fact that I dont have a good ground plane going to cause me problems? I tried hooking the antenna up to my SWR meter with it just sitting in the middle of my driveway by itself, and I was getting a 1.15 reading. How critical is a good ground plane? am I losing signal? am I going to damage my equipment? It seems like it doesnt need it, and I definitely see lots of other people using these kinds of mounts.

[SteveShannon]

2 minutes ago, Smitty74 said:

I have a dual band comet antenna meant for automotive use. Its like 4 foot tall.  If I put it on a "lip mount" base on the edge of my car hood, is the fact that I dont have a good ground plane going to cause me problems? I tried hooking the antenna up to my SWR meter with it just sitting in the middle of my driveway by itself, and I was getting a 1.15 reading. How critical is a good ground plane? am I losing signal? am I going to damage my equipment? It seems like it doesnt need it, and I definitely see lots of other people using these kinds of mounts.

You didn’t give us enough information.  Does the antenna say “ground plane required?”

A ground plane is important for an antenna that is designed to need it.  Not all antennas are. But, that lip mount almost certainly provides a connection to your hood, which will serve as the ground plane.  You won’t damage your equipment.

I would just hook it up and try.

[Lscott]

1 hour ago, Smitty74 said:

I have a dual band comet antenna meant for automotive use. Its like 4 foot tall.  If I put it on a "lip mount" base on the edge of my car hood, is the fact that I dont have a good ground plane going to cause me problems? I tried hooking the antenna up to my SWR meter with it just sitting in the middle of my driveway by itself, and I was getting a 1.15 reading. How critical is a good ground plane? am I losing signal? am I going to damage my equipment? It seems like it doesnt need it, and I definitely see lots of other people using these kinds of mounts.

If the SWR looks good you most likely won't damage your radio. The ground plane will influence the direction of maximum signal. That's usually in the direction of the largest area, metal, around the antenna. With a lip mount the signal strength will be higher towards the side of the car with the hood. How much difference that makes is hard to say. 

If you really want to know just do some tests. Have a buddy a few miles away monitor your signal level on his radio, not the audio, with the signal strength indicator on his radio. Most radios have one, usually a simple bar graph type. Then turn your car so its in a different orientation relative to you buddy's radio, transmit and have them check the signal strength. This would be easy to do in a parking lot. Doing this while driving around introduces too many other variables that affect the signal strength. If there isn't much or any discernible change then don't worry about it.   

[marcspaz]

I agree with the others.  The lip mount should give you a decent ground plane.  At these frequencies, electro-magnetic coupling is fine.  Just make sure the screws are tight enough to provide support that the antenna doesn't wave around, fall off or bend the sheet metal, but you don't have to worry about puncturing the paint/etc. to get a good ground.

 

 

 

[Smitty74]

1 hour ago, Lscott said:

If the SWR looks good you most likely won't damage your radio.

That was my thinking. Honestly When the SWR came back at a 1.15 with no ground plane of any kind (just sitting on the ground in my driveway) I was a little shocked.

Thanks for all the suggestions to test it and see how it works out. Im encouraged.....

[WRUU653]

21 hours ago, Smitty74 said:

with no ground plane of any kind (just sitting on the ground in my driveway)

That is literally providing a ground plane. 

[muggz]

I put a spare (Diamond) 1/2 wave 2m/70cm mobile antenna on my Chevy Bolt with a hood lip mount and was surprised and happy to get ~1.3 SWR on GMRS as measured by my fars-o-meter 2000.  The way I see it, a mobile antenna with directionality is way better than a rubber ducky antenna inside the car.

[marcspaz]

^^^  True story!

[Lscott]

On 3/17/2023 at 10:47 AM, muggz said:

I put a spare (Diamond) 1/2 wave 2m/70cm mobile antenna on my Chevy Bolt with a hood lip mount and was surprised and happy to get ~1.3 SWR on GMRS as measured by my fars-o-meter 2000. 

What model was that? I'm still looking for a mobile dual band antenna, 1/2 wave - ground independent, that's usable on MURS and GMRS as well as the Ham bands.

I thought the Diamond SG7500A would, but it isn't going to work based on my tests. On the Ham bands it looks OK.

https://forums.mygmrs.com/gallery/image/283-sg7500a-swr-scans-2jpg/

 

[DONE]

First question is do you understand what function the ground plane has regarding antenna placement?

Then you need to figure out what you want to accomplish.  Which is typically to talk 360 degrees if you are referring to a mobile antenna. 

And yes, there are antenna's that REQUIRE no ground plane that are mobile antenna's but they are typically low or no gain designs that generate a 'ground plane' by their design.  These are also referred to as di-pole antenna's where there are two radiators that are in line vertically and the lower element is 180 degrees out of phase of the upper element creating a radiation pattern without a conductive plane below the upper element.  These are for specific applications however and are not the norm. 

Now there is something else that needs to be considered with a mobile antenna and how it's used.  That being it's designed frequency range.  A ham antenna will almost NEVER work well on GMRS.  Not only will the SWR be high but if it's a gain antenna, where there are multiple elements in phase creating gain due to pattern manipulation, the antenna being operated outside of the designed frequency range will result in poor performance of the antenna. 

But specific to a ground plane and mobile operations.  You want the ground plane to be equal in all directions if possible and the minimum distance from the point of mounting to be at least the distance of the first radiating element of the antenna.  If the antenna is 1/4 wave, then the ground plane needs to be 1/2 in diameter. If it's a 5/8 then the ground plane needs to be 5/8 radius (radius is 1/2 the diameter).   If that is not achieved then the resulting antenna pattern is NOT going to be equal in all directions around the vehicle.

 

[muggz]

2 hours ago, Lscott said:

What model was that? I'm still looking for a mobile dual band antenna, 1/2 wave - ground independent, that's usable on MURS and GMRS as well as the Ham bands.

I thought the Diamond SG7500A would, but it isn't going to work based on my tests. On the Ham bands it looks OK.

https://forums.mygmrs.com/gallery/image/283-sg7500a-swr-scans-2jpg/

 

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