Sponsored

Another Ground Plane Question.

Vanishing Point

Well-Known Member
First Name
Ernest
Joined
Aug 23, 2022
Threads
10
Messages
98
Reaction score
190
Location
Reno, NV
Vehicle(s)
2021 Jeep Gladiator
Occupation
Physician
So I think my math is okay but when I go to look for circular ground planes for sale for GMRS I see that folks (including some radio selling sites that I think are not in the consumer's best interest) selling 6" diameter discs as GMRS ground planes. I've even seen that mentioned on this site as being the correct size. That all you need is a 6" diameter disc. After doing the calculations I come up with needing about a 14" diameter disc.

GMRS frequency ~ 462 mhz
GMRS wavelength ~ 70cm
1/4 wavelength ~18cm
18 cm ~ 7 inches

So 7" should be the radius of the groundplane disc. I believe that you want the radius and not the diameter of your groundplane to be 1/4 wavelength. I kind of rounded up. I think if you do the math proper you can use a 6.5" radius disc.

Are my numbers wrong? I keep seeing the 6" diameter advice/disc for sale and keep wondering where I went wrong.

Sorry for posting this here, I don't belong to any of the HAM radio sites.
Sponsored

 

sarguy1941

Well-Known Member
Joined
Aug 20, 2019
Threads
23
Messages
529
Reaction score
330
Location
USA
Vehicle(s)
Jeep
GMRS is not 70 cm. 70 cm is ham 440mhz. 6" disc is all you need. Used the for years on our DOT trucks when mounting antenna on the mirrors mounts.
 
OP
OP

Vanishing Point

Well-Known Member
First Name
Ernest
Joined
Aug 23, 2022
Threads
10
Messages
98
Reaction score
190
Location
Reno, NV
Vehicle(s)
2021 Jeep Gladiator
Occupation
Physician
GMRS is not 70 cm. 70 cm is ham 440mhz. 6" disc is all you need. Used the for years on our DOT trucks when mounting antenna on the mirrors mounts.
'
It's technically about 64 to 65 cm (about 462 mhz). Which then the 1/4 wavelength would be 16 cm which is still 6.4 inches. And looking at the GMRS antennas it seems the 1/4 wave ones are about 6" long. And my reading/understanding is that the radius of the ground plane should be about 1/4 wavelength for a 1/4 wavelength antenna. So the diameter of the ground plane would need to be about 13 inches. So it still seems a 3" radius disc would be less than optimum. I'm sure a 3" radius disc would work but I have space for a bigger disc if that would work better.

Just trying to double check before I buy anything. 6" diameter is cheaper and easier than a 14" diameter. So if it's the okay thing I'd go that way.
 

Radio Guy

Well-Known Member
First Name
Mike
Joined
Feb 24, 2021
Threads
5
Messages
610
Reaction score
738
Location
So Cal
Vehicle(s)
2021 Jeep Gladiator Mojave
Occupation
Retired Broadcast Engineer
A ground plane or ground radials are ideally 1/4 wavelength minimum which is about 6” at GMRS frequencies. For a round disc that would be 6” radius or 12” diameter. A 6” diameter disc would not be adequate for a GMRS ground plane.
 
OP
OP

Vanishing Point

Well-Known Member
First Name
Ernest
Joined
Aug 23, 2022
Threads
10
Messages
98
Reaction score
190
Location
Reno, NV
Vehicle(s)
2021 Jeep Gladiator
Occupation
Physician
A ground plane or ground radials are ideally 1/4 wavelength minimum which is about 6” at GMRS frequencies. For a round disc that would be 6” radius or 12” diameter. A 6” diameter disc would not be adequate for a GMRS ground plane.
Thanks. That is what the maths says. The internet sometimes makes one doubt the maths. 🥴
 

Sponsored

kb5zcr

Well-Known Member
First Name
Tim
Joined
Sep 24, 2021
Threads
13
Messages
173
Reaction score
209
Location
Texas
Vehicle(s)
2023 Diesel Rubi Gladiator
Occupation
Retired
Howdy, a ground plane functions as the other half of a 1/2 wave dipole. The vertical element of the antenna is one half, and the ground plane is the other half. You can get the same performance using ground radials rather than a ground plane. This web link can help explain how a 1/2 wave antenna works and what you might need regarding using radials or a ground plain.

Depending on your install, using a counterpoise or radials may work easier, especially if installing your antenna on a rack above the bed or roof of your truck where a ground plain is not easy.

There is a lot of black magic in antenna theory.
Sometimes the best thing is to try the cheapest method using wire or an old pie pan ( if trying a ground plain for gmrs) and see what works and go from there.
A few 1/4 wavelength radials are usually easy, cheap, and work fine.

Check out this link for more info. This webpage is about hf freqs, but it all works the same for uhf freqs too.

https://www.onallbands.com/how-many-how-long-advice-for-installing-ground-or-above-ground-radials/
 

Radio Guy

Well-Known Member
First Name
Mike
Joined
Feb 24, 2021
Threads
5
Messages
610
Reaction score
738
Location
So Cal
Vehicle(s)
2021 Jeep Gladiator Mojave
Occupation
Retired Broadcast Engineer
Howdy, a ground plane functions as the other half of a 1/2 wave dipole. The vertical element of the antenna is one half, and the ground plane is the other half. You can get the same performance using ground radials rather than a ground plane. This web link can help explain how a 1/2 wave antenna works and what you might need regarding using radials or a ground plain.

Depending on your install, using a counterpoise or radials may work easier, especially if installing your antenna on a rack above the bed or roof of your truck where a ground plain is not easy.

There is a lot of black magic in antenna theory.
Sometimes the best thing is to try the cheapest method using wire or an old pie pan ( if trying a ground plain for gmrs) and see what works and go from there.
A few 1/4 wavelength radials are usually easy, cheap, and work fine.

Check out this link for more info. This webpage is about hf freqs, but it all works the same for uhf freqs too.

https://www.onallbands.com/how-many-how-long-advice-for-installing-ground-or-above-ground-radials/
The ground plane or ground radials are not really the other half of the antenna and unlike a 1/2 wave dipole the ground radials do not radiate. Therefore a ground plane has less gain than a 1/2 wave dipole.

Antenna books of the 1950s and 60s would show a ground plane having a mirror image of the vertical radiator under the ground plane suggesting it mimicked a dipole but that has been proven to be a wrong interpretation.
Sponsored

 
 



Top