GMRS antenna question

[Radio Guy]

Radio Guy, I have a question regarding how much ground plane is best VS how much is fine and wouldn't make much of a difference.
The photo of the antenna above on the red crossbar has plenty of counterpoise (as long as its grounded to the crossbar) for a 1/4 wave gmrs antenna.
There are many 1/4 wave whips where the operator attaches a 1/4 wave counterpoise and it works fine. I think in the mobile environment it can't make much difference.
What do you think?

Thanks and 73, Tim

That’s a complex question and will vary with different antennas and mounting locations. This will probably go long so lets start with a perfect flat infinite size ground plane made of copper. I’m talking covering your entire city in copper sheet then plant your antenna in the middle. That will pull the radiation pattern down towards the horizon the most with equal signal in all directions around a compass. It will also give the lowest impedance at the feedpoint which is about 35 ohms for a 1/4 wave whip and the most capacitance between the whip and ground plane.

Now make the ground plane 1/2 wavelength across or 1/4 wavelength out from the antenna in all directions and the radiation pattern will point up above the horizon a bit and with equal signal around a compass. The feedpoint impedance will be slightly higher but still less than 50 ohms with less capacitance between the whip and ground plane.

A solid 1/2 wavelength across solid ground plane with the whip in the middle is the gold standard, 1 for being the smallest size to provide a near perfect ground plane and 2 for providing 1/2 wave round trip for RF on the ground, 1/4 wavelength out from the whip then reflecting off the edge and back to the feedpoint mimicking the feedpoint impedance and cancelling radiation in the ground plane. The latter being more applicable to wire or rod radials vs a solid metal sheet with the solid sheet being more forgiving and the radials having the ability to be individually tuned or de-tuned, which can be good or bad.

Now move the whip to the edge of the gold standard 1/2 wavelength across solid ground plane, which would be similar to a roof mount at the edge of a roof and now half the whip is sitting over an “adequate“ ground plane and the other half is hanging in “free space”. The radiation pattern towards the ground plane will be pulled down towards the horizon, not exactly at it but slightly upwards and the pattern away from the ground plane will point significantly upwards with much less signal at the horizon. As you drive the vehicle around in a circle and transmit there will be a very noticeable change in signal level which can be maybe 6dB or more.

In this example the capacitance between the whip and ground plane is reduced a lot which will raise the feedpoint impedance causing you to lengthen the whip to get back some capacitance but detuning the antenna in the process. Finding the best match will be a compromise and some antennas may
achieve an acceptable match and others may not and things like being a full 1/4 wave whip vs shortened with a loading coil, location along the whip of the loading coil and other factors will determine the potential match.

A CB antenna “wanting” a 1/2 wavelength across solid ground plane of 18ft diameter is much more of a challenge than a UHF GMRS antenna only “wanting” a 12in dia ground plane. A CB antenna in the middle of an extended van roof seems like overkill but it’s actually a fraction of the gold standard 18ft diameter that it wants, but any CB antenna will work great on a metal van roof.

Put the Same CB antenna on the edge of a Gladiator hood (NMO trunk lip mount) and you are starving it for ground plane. The hood is a fraction of the needed ground plane at 27MHz and the radiation pattern will be above the horizon in all directions and especially looking towards the rear and side away from the hood with a significant loss of gain at the horizon. The feedpoint impedance of this hood mounted CB antenna will be well above 50 ohms and you will be compromising the tuning to get an acceptable match, which may not be possible with some antennas. Adding some conductive tape under the plastic cowl pieces to extend the ground plane slightly towards the rear as I did on my Mojave will have little effect at 27MHz CB frequencies due to its wavelength of about 36ft.

Swap the edge of hood mounted CB antenna for a GMRS antenna and now the hood is way bigger than the gold standard of 12” dia needed for UHF. Problem is the mount is at the very edge of the hood and one side of the antenna is nearly floating in air with a plastic cowl behind the hood. The radiation pattern will be great in the direction of the most hood mass and will be lacking in the direction of no ground plane. Since there is a lot of ground plane at UHF in some directions I think any UHF antenna will tune up and provide a good match. In this case adding a half square foot of conductive tape under the plastic cowl piece to extend the ground plane will help with the radiation pattern to the rear of the truck because a full wavelength at UHF is about 24in and even a small mount of metal can make an improvement.

Lets move the antenna mount to a 1” X 6ft long cross bar sitting 2” above the roof. The roof is fiberglass providing no ground plane and even if it were placing the feedpoint away from the ground plane increases impedance and adds ground loses which equate to loss of antenna efficiency. So you have this 1in wide by 6ft long metal rod for a ground plane. At UHF it’s plenty long in two very narrow directions but totally lacking everywhere else. Radiation pattern will be up towards the clouds in most directions and a little lower off to the sides it will come down a little but nowhere near what a hood mounted antenna might do. The antenna is high and in the clear compared to a hood mount but it’s not seeing the horizon where all your buddies radios live.

Replace the UHF GMRS antenna with a CB antenna needing about 17X more ground plane and it’s a disaster. There just isn’t enough ground plane to do anything for the radiation pattern at the horizon and a huge loss of capacitance between the whip and ground plane and it will be nearly impossible to get any CB antenna to match sitting on a 1in X 6ft long metal rod.

If you could extend the metal rod to about 9ft long with the CB antenna mounted at one end you have made a 1/2 wave inverted V dipole which is a real antenna and can perform quite well in two directions broadside to the dipole. But both halves of the dipole, the whip and 9ft cross bar are the hot radiating parts of the antenna and with the cross bar being horizontal this dipole will radiate on a 45deg slant if it were in free space causing about a 3dB degradation to vertical mounted whips around you and less off the non broadside. Plus the crossbar is sitting horizontal above a metal truck which will completely interfere with its radiation pattern and capacitance between whip and counterpoise or ground radial, the other half of the antenna. Bottom line is you will never get any performance out of a CB antenna mounted to a cross bar above a roof.

I‘ll stop here but there is a lot more to antennas, ground planes and grounding in general. Antennas are very complex and I am no expert, although I‘ve worked with them all my career In addition to installing and tuning about 1,000 of them at one of my first jobs. I hope my rant has answered at least one of the questions.

Sponsored

 

[kb5zcr]

Radio Guy,
Thanks for the response. Since the wavelength at gmrs freqs is so short compared to the 27 MHz of CB, perhaps just making a vertical dipole, it would only be about a foot or so tall, I just don't know how it would do being so close to the truck.

 

[bd100]

Interested in whether or not this passes muster? I made this for the little magnetic mount antenna that came with my Midland radio.

How bad is it?? ?

Probably better to use the bolt closer to the hood. Better still to use the little stick-on metal disc they provide to mount it actually on the hood, a few inches forward from the back edge and a few inches away from the side edge. That's what I did. It gives you grounded metal hood in most directions. It may be useful to add a grounded wire in that slot going from the hood to the windshield along the cowl, thus adding a grounded radial in the backwards direction. Or add grounded sheet metal under the cowl.

Bonus points if you buy a connector and tools and shorten that thin 20 foot cable down to the length you actually need. That cable eats up a lot of the signal for both transmit and receive. Even better if you get an adapter to change from the thin coax to RG58 for most of the run, and leave the thin coax for the last six inches or so next to the antenna. This is what I did, but it's a pain to work with that thin coax no matter how you do it. So it's much easier to buy a small-ish mag mount with RG58 from the start, and a suitable disc to stick on the hood, such as:

Adhesive mag mount disc

Edit: ... and if I were to do this over I'd probably choose:

Laird TE Connectivity GB8PI NMO mount
and
Laird TE Connectivity QWB450 antenna

That gives you a magnetic NMO mount with only 12 feet of cable (instead of 20 or such) for fewer losses, and a small antenna for general use and better angles in changing elevations, but you can also swap in a high gain antenna for use in flat terrain. Stick the disc forward a bit on the hood so it has metal in all directions. The antenna is a bit in your view, but the short and thin quarter wave whip doesn't stick up too high. And you can put it on the passenger side of the hood so it's not in front of the driver all day long.

ANOTHER EDIT: Measure or make a template to see if a 3.5" disc would actually fit where you want it, since the hood has some not-so-level surfaces near the side.

 

[Radio Guy]

Radio Guy,
Thanks for the response. Since the wavelength at gmrs freqs is so short compared to the 27 MHz of CB, perhaps just making a vertical dipole, it would only be about a foot or so tall, I just don't know how it would do being so close to the truck.

You could make a 1/2 wave center fed dipole for GMRS out of wire, about 12” tall as you mentioned and stick it to the inside back window with clear packing tape. Just bring the coax out the side for a foot or two sideways before heading downwards so it doesn’t mess up the radiation pattern too much. No grounding or ground plane required. That should easily outperform a short pud antenna on a bracket that sticks out the side of the hood and it would be nearly invisible.

 

[kb5zcr]

Yup, I've made many 2 meter antennas the same way using just stiff wire. In fact my HF antenna here at the house is 130 ft or so of wire.

 

[Alpine Warthog]

You could make a 1/2 wave center fed dipole for GMRS out of wire, about 12” tall as you mentioned and stick it to the inside back window with clear packing tape. Just bring the coax out the side for a foot or two sideways before heading downwards so it doesn’t mess up the radiation pattern too much. No grounding or ground plane required. That should easily outperform a short pud antenna on a bracket that sticks out the side of the hood and it would be nearly invisible.

How would a fiberglass camper shell affect this?

Also what gauge wire? Totally down with doing a home built antenna on the back glass.

Struggling to get an antenna that's highly affective but also won't impede my ability to remove the camper shell and the Jeep top. The more I read you and @kb5zcr the more I realize my little ghost antenna on the roof light bar bracket is stupid

 

[Radio Guy]

How would a fiberglass camper shell affect this?

Also what gauge wire? Totally down with doing a home built antenna on the back glass.

Struggling to get an antenna that's highly affective but also won't impede my ability to remove the camper shell and the Jeep top. The more I read you and @kb5zcr the more I realize my little ghost antenna on the roof light bar bracket is stupid

I’ve made UHF dipoles to stick to the inside of windows from about 24ga wire but thicker like 14 or 12ga would be better for a 50watt radio and would be a little wider bandwidth. This type of antenna should work great inside a fiberglass camper as long as the paint is non metallic.

I usually make the dipole with coax attached then lay out a strip of clear packing tape and lay the dipole neatly across the center of the tape before sticking to a window. For the inside of a camper I would use gaffers tape or gorilla tape.

 

[JTR178]

This looks real clean. Can you provide a link to the bracke you used? Thanks!

Midland universal lip mount

 

[zxd9]

I'll give you a real-world experience. I have a 50w radio. I mounted my Laird BB4505CNS antenna off the side of the cowl just like post #9. On the White Rim Road in Utah which has many elevation changes, turns and large rock obstructions, I was able to keep in touch with another group of travelers miles behind me that I did not have line of sight with over two days. Most users aren't going for "longest" distance awards, we just need to keep in touch with a relatively close group of users.

I can receive a bit of a choppy signal from a repeater 67 miles away on this antenna while driving. I've communicated 30 miles to another repeater on the move. So while not a perfect mounting location, it does work.

 

[bd100]

In Utah you may be bouncing signals off canyon walls and such. Fun stuff.

Sponsored