Ground plane vs no ground plane mobile antennas

[VETCOMMS]

Am I correct in my thought process that ground plane antennas are ones mounted directly to a vehicle hood/ roof and said vehicle doesn't need to be metal but the plane refers to the horizontal surface the RF is designed to spread out horizontally along?

Whereas a non ground plane antenna is one used on truck hood lip mounts or SUV cargo roof racks because the antenna is basically hovering above any horizontal surface of the vehicle. 

Is this thinking accurate?  I ask because I have a Laird nmo ground plane antenna on my truck's hood lip mount and wonder if performance would be better if I changed to a non ground plane antenna. 

[kidphc]

All antennas require a ground plane. It's the other half of the antenna in a mobile enviromment. It's the counterpoise for the element.

To answer your question.

Yes, a no ground plane antenna will work better in the situation you describe, were there is not much metal underneath to work with as a ground plane. It works even better with an adequate ground plane.

Most no ground plane antennas ended up using the coax as the counterpoise. Just can't beat the laws of physics. Doesn't matter if it is UHF or HF.

But the difference might be minimal to not noticable.



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[WRKC935]

24 minutes ago, VETCOMMS said:

Am I correct in my thought process that ground plane antennas are ones mounted directly to a vehicle hood/ roof and said vehicle doesn't need to be metal but the plane refers to the horizontal surface the RF is designed to spread out horizontally along?

Whereas a non ground plane antenna is one used on truck hood lip mounts or SUV cargo roof racks because the antenna is basically hovering above any horizontal surface of the vehicle. 

Is this thinking accurate?  I ask because I have a Laird nmo ground plane antenna on my truck's hood lip mount and wonder if performance would be better if I changed to a non ground plane antenna. 

The lip mount (at least the ones with attachment screws) are grounded and would not require a 'no ground plane' type of antenna. 

Mounting an antenna on a car or truck is going to have the pattern effected by where it's mounted to some degree.  The 'ground plane' of the antenna (car body) helps to bring in an or direct the signal out of an antenna.  So if you have an antenna on the right side of a vehicle, the pattern will skew to the left to some degree.  If it's not mounted at the highest point on the vehicle, some parts of the vehicle that are higher than the antenna will block some part of the signal to and from the antenna. 

But, eliminating all those things required a hole in your car for an antenna mount.  And some folks can't bring themselves to drill that hole.  Some are driving lease vehicles that they can't drill the hole.  Others are limited by the fact they trade their car's in every 70,000 miles and feel that it brings down the value of the car.  Then there is the 'wife' variable.  The "You're not gonna put that shit on our car" which typically applies to ANY antenna on a vehicle (I know all about this one).

But if you are running around in a vehicle that you are gonna keep until it's reached the 'beater with a heater' status and not traded in with the idea that if you keep it the value will go down, or any of that, I would say just drill that hole. 

 

Now what a 'no ground plane' antenna is good for is situations or vehicles that don't have a good ground plane to begin with. 

Vehicles with fiberglass roof's and panels that are not going to generate a good ground plane.  Boats are another application for this type of antenna.

But you need to understand that a no ground plane antenna has a significant limitation, that being gain.  That type of antenna is designed to create a ground plane.  This is done by the antenna being a dipole design.  Meaning there is a bottom element and a top element.  And that is where you loos the ability to have significant gain.  Dipole antenna designs where one element is sort of positive and the other element is negative (the actual voltages change due to it being an AC voltage) stacking elements for the sake of more gain is not possible in a mobile antenna.  Any 'gain' listed in the antenna spec will be dBi and not dBd. The dBi gain number is based on an isotropic radiator which is a 'paper' antenna that doesn't really exist.  And dBd is gain over a dipole.  Of course a dipole antenna can't have gain over that type of antenna.  And ALL dipoles will exhibit some gain over an isotropic radiator due to the pattern difference in the antenna designs.  I realize that is getting sort of technical.  But it's something that you can google and research for a better understanding of what I am referring to, and you might even learn something about antenna's.

 

 

 

[kidphc]

The lip mount (at least the ones with attachment screws) are grounded and would not require a 'no ground plane' type of antenna. 
Mounting an antenna on a car or truck is going to have the pattern effected by where it's mounted to some degree.  The 'ground plane' of the antenna (car body) helps to bring in an or direct the signal out of an antenna.  So if you have an antenna on the right side of a vehicle, the pattern will skew to the left to some degree.  If it's not mounted at the highest point on the vehicle, some parts of the vehicle that are higher than the antenna will block some part of the signal to and from the antenna. 
But, eliminating all those things required a hole in your car for an antenna mount.  And some folks can't bring themselves to drill that hole.  Some are driving lease vehicles that they can't drill the hole.  Others are limited by the fact they trade their car's in every 70,000 miles and feel that it brings down the value of the car.  Then there is the 'wife' variable.  The "You're not gonna put that shit on our car" which typically applies to ANY antenna on a vehicle (I know all about this one).
But if you are running around in a vehicle that you are gonna keep until it's reached the 'beater with a heater' status and not traded in with the idea that if you keep it the value will go down, or any of that, I would say just drill that hole. 
 
Now what a 'no ground plane' antenna is good for is situations or vehicles that don't have a good ground plane to begin with. 
Vehicles with fiberglass roof's and panels that are not going to generate a good ground plane.  Boats are another application for this type of antenna.
But you need to understand that a no ground plane antenna has a significant limitation, that being gain.  That type of antenna is designed to create a ground plane.  This is done by the antenna being a dipole design.  Meaning there is a bottom element and a top element.  And that is where you loos the ability to have significant gain.  Dipole antenna designs where one element is sort of positive and the other element is negative (the actual voltages change due to it being an AC voltage) stacking elements for the sake of more gain is not possible in a mobile antenna.  Any 'gain' listed in the antenna spec will be dBi and not dBd. The dBi gain number is based on an isotropic radiator which is a 'paper' antenna that doesn't really exist.  And dBd is gain over a dipole.  Of course a dipole antenna can't have gain over that type of antenna.  And ALL dipoles will exhibit some gain over an isotropic radiator due to the pattern difference in the antenna designs.  I realize that is getting sort of technical.  But it's something that you can google and research for a better understanding of what I am referring to, and you might even learn something about antenna's.
 
 
 

I always try to explain antennas as a dipole. You have one end that radiates and the other as being the reflector. With mobiles you snip the reflector portion off, and need the ground plane.

When you increase gain or remove the ground plane you add more compromises. So of it can be good ( like gain, which everyone see as benefit, but neglect pattern a lot of time) and some of it bad (introduces things you have to make up for).

Think of antennas as a slide bar, you take from one end to gain something but lose something in return.

BTW, love your explanation.


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[WSBB368]

It’s funny I just added a ground plane to my Nagoya UT 72 mag mount that I’m using for my house. I had it laying around and decided to use it for my mobile unit when I’m home until I can get a proper antenna . I went to Lowe’s and got a 24x24 sheet of metal for 15$ mounted the antenna and popped it up on the roof used it all day yesterday seemed to work well


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[BoxCar]

18 minutes ago, WSBB368 said:

It’s funny I just added a ground plane to my Nagoya UT 72 mag mount that I’m using for my house. I had it laying around and decided to use it for my mobile unit when I’m home until I can get a proper antenna . I went to Lowe’s and got a 24x24 sheet of metal for 15$ mounted the antenna and popped it up on the roof used it all day yesterday seemed to work well


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Been using a sheet of steel like that for my base as well for over 2 years now.

[WRYZ926]

A lot of people have used cookie sheets under a mag mount mobile antenna for their base stations with good results. I did just that until I could get an actual base antenna installed.

[SteveShannon]

2 hours ago, VETCOMMS said:

Am I correct in my thought process that ground plane antennas are ones mounted directly to a vehicle hood/ roof and said vehicle doesn't need to be metal but the plane refers to the horizontal surface the RF is designed to spread out horizontally along?

Whereas a non ground plane antenna is one used on truck hood lip mounts or SUV cargo roof racks because the antenna is basically hovering above any horizontal surface of the vehicle. 

Is this thinking accurate?  I ask because I have a Laird nmo ground plane antenna on my truck's hood lip mount and wonder if performance would be better if I changed to a non ground plane antenna. 

 

Your Laird antenna coupled with your lip mount should be just fine. Even though the mount increases the distance above the metal, it still RF couples to the metal to allow the metal to act as a ground plane.

[VETCOMMS]

Thx for the replies.   I'm just curious what makes the two antenna styles different.  Does a no ground plane antenna radiate less in a downward direction?

[BoxCar]

1 minute ago, VETCOMMS said:

Thx for the replies.   I'm just curious what makes the two antenna styles different.  Does a no ground plane antenna radiate less in a downward direction?

To partially answer your question, visualize an AC wave. How often that wave cycles is its frequency. As the frequency increases, the waves become much closer together. At 465 million cycles per second (465 MHz) the distance the wave needs to complete one complete swing is quite short (measured in meters). Now think of that wave riding on a wire. The full wave starts at zero (the wire) then swings to an amplitude before swinging back to zero. That's one half of a full wave. The half wave continues on to repeat in the opposite direction (polarity) to complete the full wave. In radio we use the distance a wave needs to move on a wire as an antenna. We feed the antenna at the point where the wave crosses the zero point and cut the wire to the length needed. The full wave still needs to happen with the parts of the wave not on the wire being found in (on) the metal the radiating wire is connected to. The metal forms the ground plane and can be any shape as long as there's enough of ti.

[VETCOMMS]

Thx for the education.   I ask these questions because I have another supplication.   It's an aluminum roof rack on an Outback.  

I took this picture for several reasons. 

1.  The rack is 5.5 inches above the vehicle roof. 

2.  You can see the low clearance under the garage door. 

3.  I've put a mag mount antenna just in front of the stock shark fin radio antenna with ok results but thought a rack mount antenna would work more effectively. 

So does one use a ground plane antenna on the rack because it's close enough to the roof of the vehicle?

[SteveShannon]

1 hour ago, VETCOMMS said:

Thx for the education.   I ask these questions because I have another supplication.   It's an aluminum roof rack on an Outback.  

I took this picture for several reasons. 

1.  The rack is 5.5 inches above the vehicle roof. 

2.  You can see the low clearance under the garage door. 

3.  I've put a mag mount antenna just in front of the stock shark fin radio antenna with ok results but thought a rack mount antenna would work more effectively. 

So does one use a ground plane antenna on the rack because it's close enough to the roof of the vehicle?

Try it. Have a friend give you reception reports. 
I think you’ll discover that being on the roof will work about the same as being on the roof rack. 

[dosw]

I bought a Nagoya GPK-01 ground plane kit, and discovered that it isn't weatherproof. Pictures attached...

 

With the antenna fully screwed down to the NMO threads, the ground plane rods cannot be connected. With the ground plane rods connected, you cannot fully tighten an antenna on the NMO threads. In either case, the O-ring on NMO antennas is greater diameter than the NMO backing on the GPK-01 ground plane kit. I've tried this with an HYS SDN1-T, a Midland MXTA25, and a Midland MXTA26. Each one had an o-ring of greater diameter than the backing of the NMO mount, and each one extended low enough when fully threaded that the ground plane rods couldn't be attached.

 

I reached out to Btech before returning the product because I wanted to make sure that I was not mistaken. Btech responded within a day to say that they were forwarding the pictures and description of the problem for further review. A day later they responded to say that they were going to be addressing this issue "at manufacturing", and would send me a replacement sample for evaluation after the corrections have been made.

 

It'll probably be a little too late for my needs; I will just get a no-ground-plane antenna to put on the sailboat mast, and don't intend to lower the mast again for a few years. But I'm really happy with the outcome of the email to support, since it helped them to recognize a design deficiency, and because they do intend to correct it. Once I've received the replacement I'll post here about it.

As I look at the pictures, it seems like the part that the rods screw into could be manufactured so that it's below the bracket, and the antenna could mate against the bracket itself.

 

[marcspaz]

Based on the pictures, the o-ring wouldn't touch the mount to seal it, even if the radials aren't installed.

[dosw]

7 minutes ago, marcspaz said:

Based on the pictures, the o-ring wouldn't touch the mount to seal it, even if the radials aren't installed.

Correct. They would have to move the entire screw-in-rods section to below the bracket, and lower the height of the NMO threads so that the bracket's plate could become the backing plate against which the antenna's o-ring seats.

 

Who knows if they'll get it right. Doesn't seem like that hard of a concept but people manufacture what the blueprints say without asking questions, and the blueprints were probably drafted by someone who was just delivering what was asked for, but without fully understanding the problem being solved. Again, the purpose got lost in the several hops between good intentions and implementation.

[SteveShannon]

5 minutes ago, WRQW589 said:

Correct. They would have to move the entire screw-in-rods section to below the bracket, and lower the height of the NMO threads so that the bracket's plate could become the backing plate against which the antenna's o-ring seats.

 

Who knows if they'll get it right. Doesn't seem like that hard of a concept but people manufacture what the blueprints say without asking questions, and the blueprints were probably drafted by someone who was just delivering what was asked for, but without fully understanding the problem being solved. Again, the purpose got lost in the several hops between good intentions and implementation.

All they really need to do is increase the diameter of the ground plane adapter to the same diameter as the antenna at its base. 

[GreggInFL]

Sorry to go slightly off topic, but it's a related question not quite worthy of its own thread: If I place a mag-mount antenna on top of an Airstream trailer, would the trailer's aluminum shell function as a ground plane?  I'm thinking yes, but I'm new here.

TIA.

[kidphc]

Sorry to go slightly off topic, but it's a related question not quite worthy of its own thread: If I place a mag-mount antenna on top of an Airstream trailer, would the trailer's aluminum shell function as a ground plane?  I'm thinking yes, but I'm new here.
TIA.

Trick question.

1. Yes, aluminum will work for a ground plane, like when using a drill through nmo. Many antennas are made with aluminum.

2. No, it will not work with a mag mount. Aluminum is non-ferrous, so a magnet will not stick to it or make capitance with it.

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[SteveShannon]

42 minutes ago, kidphc said:

Trick question.

1. Yes, aluminum will work for a ground plane, like when using a drill through nmo. Many antennas are made with aluminum.

2. No, it will not work with a mag mount. Aluminum is non-ferrous, so a magnet will not stick to it or make capitance with it.

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It might not stick magnetically, but it certainly will capacitively couple.  All that’s required for a capacitor is two conductors separated by a dielectric material and aluminum is a very good conductor.

Stick-on steel disks are available to attach magnetic mount antenna mounts to aluminum bodies.

[WRYZ926]

4 hours ago, SteveShannon said:

All they really need to do is increase the diameter of the ground plane adapter to the same diameter as the antenna at its base. 

That is the easiest and fasted way to correct the issue. It would not cost much at all in the cost of redesign or materials to do that.

[Lscott]

I was at my typical Thursday night radio coffee group get together last night. One of the guys had a good setup for a ground plane for a mobile antenna mounted on the rack behind the truck cab. I took a couple of photos to share on the group here. Might give a few people some ideas to try out.

[tweiss3]

On 5/26/2024 at 4:19 PM, VETCOMMS said:

Thx for the education.   I ask these questions because I have another supplication.   It's an aluminum roof rack on an Outback.  

I took this picture for several reasons. 

1.  The rack is 5.5 inches above the vehicle roof. 

2.  You can see the low clearance under the garage door. 

3.  I've put a mag mount antenna just in front of the stock shark fin radio antenna with ok results but thought a rack mount antenna would work more effectively. 

So does one use a ground plane antenna on the rack because it's close enough to the roof of the vehicle?

This is an old photo, as I now have the 1/2 wave UHF 19" to the passenger side from this antenna, but I have my VHF antenna (Larsen NMO2/70SH) about 12" in front of the aluminum roof rack cross brace. I haven't noticed any issues since it's been there for 4 years.

16 minutes ago, Lscott said:

I was at my typical Thursday night radio coffee group get together last night. One of the guys had a good setup for a ground plane for a mobile antenna mounted on the rack behind the truck cab. I took a couple of photos to share on the group here. Might give a few people some ideas to try out.

This is a great alternative. I've even seen a 56" x 36" rectangular plate, closer to the roof level, that could hold 2 antennas and a light. As long as it's 18-19" in all directions for VHF and 6-8" for UHF, it should be a sufficient ground plane. 

Antennas noted as no ground plane will work without one (such as 1/2 wave), but with a ground plane the work better. My UHF 1/2 wave is noted as omnidirectional without a ground plane but with a ground plane it adds 4db gain. 

[WSBB368]

2 hours ago, Lscott said:

I was at my typical Thursday night radio coffee group get together last night. One of the guys had a good setup for a ground plane for a mobile antenna mounted on the rack behind the truck cab. I took a couple of photos to share on the group here. Might give a few people some ideas to try out.

So I’m getting ready for a setup similar to this on my Toyota Avalon but mine will be attached to bike rack rail across the roof. The antenna will be over my roof “not attached” so technically I won’t need a ground plane since I have the roof correct ? 

 

 

 

 

 

[tweiss3]

Just now, WSBB368 said:

So I’m getting ready for a setup similar to this on my Toyota Avalon but mine will be attached to bike rack rail across the roof. The antenna will be over my roof “not attached” so technically I won’t need a ground plane since I have the roof correct ? 

Incorrect. The NMO mount takes the shield of the coax cable and creates the ground plane with the roof of the car. When you have it above the the roof on a roof rack, the shield doesn't get to roof to make the ground plane. You can use a 1/2" wave on a roof rack mount https://www.dxengineering.com/parts/cma-rs-660u. You can also use a ground plane disc if you wanted better performance, or to use a 1/4 wave whip https://theantennafarm.com/shop-by-categories/antennas-mounts/antenna-accessories/misc-antenna-parts/11713-taf-disc346-detail

[WSBB368]

On 7/26/2021 at 11:23 PM, BoxCar said:

The place with the helpful hardware folks sells small sheets of sheet steel. I picked up a piece 2' x2' for less than $15 there.

 

4 minutes ago, tweiss3 said:

Incorrect. The NMO mount takes the shield of the coax cable and creates the ground plane with the roof of the car. When you have it above the the roof on a roof rack, the shield doesn't get to roof to make the ground plane. You can use a 1/2" wave on a roof rack mount https://www.dxengineering.com/parts/cma-rs-660u. You can also use a ground plane disc if you wanted better performance, or to use a 1/4 wave whip https://theantennafarm.com/shop-by-categories/antennas-mounts/antenna-accessories/misc-antenna-parts/11713-taf-disc346-detail

So I bought this mount for the rack but could I just add a ground wire from the mount run it along the rail and ground it to the car somewhere 

[tweiss3]

No, RF ground and voltage ground are two different things. 

You can still use the mount, but use a 1/2 wave antenna, it will be about 12" in length. I am currently using this antenna on my car https://www.arcantenna.com/products/bmwu4002s