Ground Plane? & Other Questions - Pictures

[nickdigg]

I use these antennas for receiving only. I have a dual band, tri band larsen, 800mhz, and cb antenna that I swap out depending on what I plant to listen to. Do I have any sort of ground plane with this configuration? Is a proper ground plane necessary for receiving? How can I improve my reception? Is antenna tuning necessary for receiving and how can I do that without a transmitter?

I plan to install a CB radio and Dual band ham in the near future. I plan to install those antennas on the roof. What should my spacing be between antennas? Should I run them front to back or horizontally? At the distance apart that you suggest can I damage any equipment when transmitting on one of the radios?

What differences will I see from using a mag mount vs nmo drilled mount?

I have spent time searching these topics but haven't found an answer to my specific setup questions. Any help and opinions would be appreciated!







'94 Toyota Pickup

 

[W6KRU]

You have no ground plane which means half of your antenna is missing. See: Ground plane - Wikipedia, the free encyclopedia

 

[ko6jw_2]

That's a lot of questions. The antennas appear to be mounted to your metal rack that is attached to the truck. This may be sufficient as a ground plane. I have several ham friends who use this type of arrangement and it seems to work. Tuning antennas for reception is not as critical as for transmitting, but never hurts. To tune without a transmitter requires an antenna analyzer which is, in effect, a very low power transmitter that reads SWR and impedance. They aren't cheap, but maybe you can find a ham in your area that has one. Another point to consider is the type of antenna. Half wave antennas don't require a ground plane, but quarter waves and 5/8 waves etc. do need ground planes. A permanent mount is better than a mag mount anytime.

As for spacing there is no exact guideline. If I said a quarter wave length, then you wouldn't have enough room on the roof at CB frequencies. As far apart as possible given the available space is as good as I can say.

Side to side or front to back? Antennas mounted at the sides might be more directional so front to back might be better. Mobile antennas tend to be directional to some extent and there's not a lot you can do.

The question of damage is also difficult to answer. Ham and commercial radios are designed to survive exposure to strong RF fields, but scanners are usually not. I had to send a 396T back to Uniden after it got zapped by a 6 meter transmitter. I avoid transmitting in the car when my scanner is on (mostly). So far so good. There is no amount of separation that can be achieved that would give protection on a truck roof.

Hope this helps. Remember there's no absolute right way - so experiment.

 

[prcguy]

The setup in the pics typically does not work very well do to lack of ground plane and the antennas will not achieve a decent match. I've measured dozens of similar setups with the same outcome.

You can probably add some radials pointing forward, back and to the inside but roof mounting will fix all the problems.
prcguy

 

[profiledescent]

"I use these antennas for receiving only." Then what you have will work fine.

"Matching", VSWR, antenna type, etc. are not critical elements when compared to transmitting. Certain multi band antennas or "Gain" antennas will tend to reduce receive capability away from their given band specifications.

For scanning, I've used a 19 inch whip mounted to the truck bed (about over the wheelwell) and inside my topper. Worked fine. I never analyzed the setup, so have no idea what it would have presented for transmit. It worked well for scanning from 900mhz down through airband.

My transmit antennas go on the roof, permanent mount, no screwing around.

 

[nickdigg]

Thanks for all of the replies. I figured that this setup wasn't ideal and I haven't been too impressed with my reception but I haven't had anything to compare it to.

I think the new plan is to drill and mount two antennas on the roof. One for CB and one for a dual band ham radio. Those will be used for both receiving and transmitting. I'll put them front to back with as much separation as I can while still having some metal on all sides. I will then mount my tri band Larsen scanner antenna with a hood mount. It seems like that will give me the best possible setup.

This should separate the scanner a good distance from the transmitting antennas.

Any recommended hood mounting options? I have an l bracket for a nmo mount that I can screw into the side of the fender in the lip between the fender and hood. Would this be my best bet? Should I keep it on the opposite side of the FM radio antenna?

Thanks for the help. I'll try and post some new pics once the job is done.

 

[profiledescent]

" I have an l bracket for a nmo mount that I can screw into the side of the fender in the lip between the fender and hood."

Those work well. Rigid, and you get a good ground connection.

 

[cpfinlay]

Mounting antennas that need a good ground plane on a surface that is not large enough to provide a good ground plane for the frequency can be worse than simply using a "no ground plane" antenna. A decent ground plane should be 1/4 wave.

For your circumstance, I would recommend mounting only the CB antenna (by itself) in the *middle* of the roof to give it as much and as even a ground plane as possible. It will be up high, so be sure to have a flexible antenna and/or a spring mount. Read the entire page here, but especially note the 9 square feet part: Do I need a no-ground-plane antenna?

Then replace your 2m/70cm antenna with a "NGP" model, such as the Diamond NR770HBNMO. That antenna works very well mounted on the vehicle side like your existing mounts.

The scanner antenna is not a big deal as long as you have the appropriate type for the desired bands and your terrain. Leave it where it is.

My 2c based on my research and 30 years in ham radio.

 

[profiledescent]

......

Then replace your 2m/70cm antenna with a "NGP" model, such as the Diamond NR770HBNMO. That antenna works very well mounted on the vehicle side like your existing mounts.

The scanner antenna is not a big deal as long as you have the appropriate type for the desired bands and your terrain. Leave it where it is.

My 2c based on my research and 30 years in ham radio.

I second his suggestion on the NR770. I've seen that antenna in various odd circumstances still provide a low VSWR and normal radiation patterns. (Anritsu analyzer results) Myself and a couple of other hams have them on magmounts, which we stick to a cookie sheet and set on top of our travel trailers. It's a quick set up and take down for overnight or field day weekend. It's the only antenna I'll mag mount, and only for fixed station use.

What ever you do, don't mount it, or any antenna, right behind the cab pillar on the bedrail. Even the NR770 will have pattern issues in that spot. You'll see this done a lot, and my guess is they haven't seen the results on an analyzer.

 

[nickdigg]

I appreciate the additional information on NGP antennas. Perhaps that is a better route for me to take. The roof would be a little cramped with two antennas.

It looks like the Diamond is half wave for VHF, which doesn't need a ground plane? But 5/8 wave for UHF, how do they accomplish NGP for UHF? Do I need special coax or will my current mount be fine?

I'm assuming that for NGP CB antennas its the coax that makes it possible (saw something about it in that article you linked to)? Since half wave for CB would be huge. I'm assuming that is why you suggest using the available ground plane for CB and not HAM? I currently have a Browning BR-140.

Will this setup give me the best compromise between CB and 2m/70cm? or would using a NGP antenna on CB give me better results for the dual band? Just wondering if I need to decide which is more important for range.

Will my tri band Larsen scanner antenna be better off in its current position with no ground plane but high in the air, or on the hood where it will have a ground plane to one side but lower in the sky, with the cab partially blocking it? I'd have to guess it current position would be better. Is there a better antenna for my circumstances?

Thanks for all the help. I'll be out of school for winter break and am planning on optimizing my setup and permanently installing a CB and dual band radio. Looking forward to having a proper setup and continuing to learn about the wonderful radio hobby.

 

[Project25_MASTR]

For a CB, in your particular case the best spot is a NMO on the roof. I don't think you can get no ground plane nmo low band antennas. Is try a fender mount for the CB. Your headache rack has plenty of ground plane for above 100MHz. NGP antennas are often tall and expensive so think about that. Also, most imports from the 80s and 90s don't have grounded beds. So for bed based antennas you should be aware of that (or even headache rack based). I've done CB off of headache racks on flatbeds and personal trucks, but the racks were built much differently and had better ties to the bed and ground compared to the average headache rack.

 

[cpfinlay]

In addition to being 1/2 wave on VHF, the Diamond antenna is actually a 5/8 *over* a 5/8, with a phasing coil in between for UHF. This collinear configuration requires no ground plane. This antenna does not use the coax to provide the counterpoise.

For the CB antenna, I would recommend drilling a hole in the middle of the roof for a sturdy mount and securely grounding it that way. It’s not an ideal ground plane, but it will work better than no ground. I am not familiar enough with the NGP models for CB (Firestik, for example), which use the shielded part of the cable for a counterpoise. I do not think I would want a radiating cable in the cab with me; especially right over my head, even though 4 watts @ 27mhz may not be that big of a deal...

The scanner antenna is fine where it is at. I don’t see the need to move it.