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. 

[SteveShannon]

2 hours ago, WSBB368 said:

 

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 

Antennas are funny things. I completely agree with @tweiss3 that what you're suggesting with running a ground wire wouldn't be the same as a ground plane. He's absolutely correct. But I'm not 100% certain that your bike rack wouldn't satisfy the need for a ground plane. There are many ways to satisfy the need, with a counterpoise, radials, or a sheet of RF conductive material.  I would at least try it if you already have the parts.  If you don't have the parts already then I would go with what's tried and true, which is what he suggested.

But understand that I am a ham and I like trying different things with antennas even if they waste many hours.

[Lscott]

31 minutes ago, SteveShannon said:

But understand that I am a ham and I like trying different things with antennas even if they waste many hours.

And at times money. Oh well, its a hobby.

[SteveShannon]

2 hours ago, Lscott said:

And at ALL times money. O

Fixed it for you.

[WSBB368]

Antennas are funny things. I completely agree with [mention=3159]tweiss3[/mention] that what you're suggesting with running a ground wire wouldn't be the same as a ground plane. He's absolutely correct. But I'm not 100% certain that your bike rack wouldn't satisfy the need for a ground plane. There are many ways to satisfy the need, with a counterpoise, radials, or a sheet of RF conductive material.  I would at least try it if you already have the parts.  If you don't have the parts already then I would go with what's tried and true, which is what he suggested.
But understand that I am a ham and I like trying different things with antennas even if they waste many hours.

I’ll be using a Signal stick Signal stalk 20inch 1/4 wave on 2m, 3/4 wave on 70cm. Thanks for your help


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[kidphc]

I’ll be using a Signal stick Signal stalk 20inch 1/4 wave on 2m, 3/4 wave on 70cm. Thanks for your help


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Be warned ³/4 is iffy. I used one for a while and went back to a ⁵/8th. Just didn't get out too well due to the weird radiation pattern.

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[WSBB368]

Be warned ³/4 is iffy. I used one for a while and went back to a ⁵/8th. Just didn't get out too well due to the weird radiation pattern.

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Awesome … I have a swr meter to check it out will be interesting to see what difference if any from my Nagoya ut72


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[kidphc]

I was told by others that ³/4 antennas don't work well. Since the patten is different from 1/2 waves and 1/4. It is suppose to have a medium elevation lobes.. which for me i thought would work well with repeaters. But the opposite was true.

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[tweiss3]

3/4 gain have like a 30 degree up tilt, and negative gain at the horizon. It's why I have a 1/2 wave installed for UHF instead of using a diplexer to put both decks on the NMO 2/70sh

[kidphc]

3/4 gain have like a 30 degree up tilt, and negative gain at the horizon. It's why I have a 1/2 wave installed for UHF instead of using a diplexer to put both decks on the NMO 2/70sh

Yeah other hams i talked to at least when it came to mobile antennas. Recommend the following 5/8 better than 1/2 which is better than 1/4 for the reach on flat ground.

Most of my testing shows 5/8 over 5/8 was thr best for my area for reach and gain levels with mild terrain. So it is my preferred antenna style.

Too bad because I was so disappointed with my signal stalk on uhf. An antenna is still better than no antenna.

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[VETCOMMS]

Another cyclist, good to see.   That mounting to the aluminum Thule roof rack or the black roof rails that runs down the length of my Subaru Outback was what I was curious about.   I see a couple of mount ideas above but thought there was another mount that I've seen on an overlanding website that attached to the groove in the roof rack.  I even thought there was a fold over model for fitting in garages.  Thinking about that style of mount it wouldn't seem to be as sturdy as the two clamp models above.

Dumb question.   On the roof rack example some say to use a 1/4 wave antenna and others say 1/2 wave.   Can someone please educate me on when to choose one over the other?

[tweiss3]

If you have a ground plane (radials, through hole mount on roof of a car, cookie sheet, etc.), then you can use almost any antenna design, 1/4 wave, 1/2 wave, 3/4 wave, 5/8 wave.

If you do not have a ground plane, or your ground plane is too small, you must use a no-ground plane antenna, such as a 1/2 wave or 5/8 wave. Of course, both of those work better with a ground plane.

This is a good article to describe the radiation/gain patterns for the common types: https://practicalantennas.com/designs/verticals/5eights/

Of course, the benefit of 1/4 wave is its the smallest of the designs, and is very wideband, but the 1/2 wave is only 12" for UHF and can cover the entire UHF band (380-512) acceptably as well. 3/4 wave is 16-18" and is more narrow band (but can cover UHF ham and GMRS), and 5/8 or 5/8 over 5/8 is the largest. 

The ultimate decision depends on which compromises you are willing to take. Since I already have a 1/4" wave VHF antenna (at 18" tall), placing a 1/2 wave UHF (12" tall) 20" from it wasn't much of a compromise, I got gain, a better horizon radiation pattern, and didn't increase my clearance requirements. I do only use antennas with a base spring because he 18" VHF hits on parking garages I regularly have to park in.

[VETCOMMS]

Thank you for the explanation.  I appreciate the sharing of knowledge. 

[WSBB368]

Just got the Signal Stalk mounted and so far seems to be working well. Signals coming in strong both uhf and vhf and outbounds. Glad I moved to a nmo mount as the magnet on the nagoya ut72 left a ring on the paint but was able to polish it out. Now to test with swr meter 

 

[VETCOMMS]

How long is the Signal Stalk,  40"?  Keep us posted on swr please.   Thx for the pic,  can you share one with most of the vehicle just to see how it looks?

[wayoverthere]

13 hours ago, VETCOMMS said:

How long is the Signal Stalk,  40"?  Keep us posted on swr please.   Thx for the pic,  can you share one with most of the vehicle just to see how it looks?

looks like the small signal stalk (as the larger one has a bit more of a "salt shaker" base); just measured mine at 20 3/4" from the base of the plastic to the end of the whip.

[WSBB368]

So I have the 20inch Signal Stalk I just took some SWR readings and im pretty happy with what I have out the box no modifications. Don't think I'll be needing to make any adjustments So far so good. I like how its not too big of an antenna and it barley noticeable which I Like

[GreggInFL]

On 6/1/2024 at 9:32 AM, tweiss3 said:

If you do not have a ground plane, or your ground plane is too small, you must use a no-ground plane antenna, such as a 1/2 wave or 5/8 wave.

How small is too small?  Is there a height/diameter ratio that is optimal, or minimal?  What size pizza do I need for my 20" Nagoya?

[kidphc]

How small is too small?  Is there a height/diameter ratio that is optimal, or minimal?  What size pizza do I need for my 20" Nagoya?

Give or take about 1x1 foot square. Usually about 6" radius is enough for gmrs. For 2m 2.5x as much.

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[tweiss3]

Just now, GreggInFL said:

How small is too small?  Is there a height/diameter ratio that is optimal, or minimal?  What size pizza do I need for my 20" Nagoya?

1/4 wave in all directions from the antenna. For UHF that is approximately 12" diameter, for VHF that is approximately 36" diameter.

[GreggInFL]

Ah, thanks.

(Best bar in town. )

[dosw]

Nagoya / BTech shipped me an updated version of the GPK-01 to test.

 

The new version made the following changes: The NMO threads are longer, so that the antenna fits more tightly against the thread base. The ground plan rods don't have the crosshatch pattern, and have a thinner diameter where the crosshatch was before. This makes them a little harder to tighten, but they take less space. This also allows the antenna to bed more securely against the NMO threaded male side.

They didn't provide any kind of backing plate to match up to the O-ring on the back of NMO mounts. But at this point, with some diaelectric grease it should be a good enough waterproof connection to be acceptable for my needs. One may still want to caulk with a marine silicon or something like that. But it's going to be good enough now.