VHF/UHF mobile antenna ground plane question...

[thethinginthewoods]

Having just recieved my first technician HAM license (KF7TBF), I managed to acquire a Kenwood TM-D710a dual band 2m/70cm radio and a Larsen NMO 2/70B antenna as an early Christmas gift and I plan on installing them on my 2000 Jeep Cherokee (XJ). I have done a lot of research regarding ground planes and mounting options for CB and HAM antennas, but still have a few questions...

The dilemma I am faced with is trying to work around my roof rack. I know mobile antennas and roof racks don't play nicely together, but I am not willing to part with my roof rack. I currently have a basic yakima rack, consisting of an arrangement of tubular powder-coated bars that attach to the factory cross-bars with padded plastic clamps. I am not concerned about antenna height, as I have another Jeep for my serious trail-work in the forest.

I would prefer to mount the antenna with a mount similar to this:

However, this means the antenna would be raised about 3-6" off the roof, reducing the effectiveness of the metal roof acting as the counterpoise. Would a simple ground strap from the base of the mount to a metal part of the roof solve this issue? If not, would mounting via an NMO through the roof and having the antenna stick up between the bars of the roof rack cause problems/interference with the radiating waveform from the antenna?

I've also considered constructing a mesh floor for the rack to work as a groundplane...

Suggestions? Comments?

Any help is greatly appreciated!

 

[brussum]

First and foremost, congratulations!

I feel your pain...I had to deal with the sunroof and roof rack (BajaRack) on my roof. I was able to squeeze the antenna at the tail end of the roof between the rack and back lift gate. Not ideal, but better than some alternatives. You've invested in some great equipment with the radio and antenna. It'd be a shame to lose all that performance with a clamp-on mount. Is there any chance you can do a NMO permanent mount? I don't think you're going to get much, if any, advantage from a ground strap, although having a good ground throughout the system is critical to top performance.

 

[brussum]

Also, check out http://k0bg.com/. Probably the best site out there for mobile HF/VHF/UHF radios. The site's owner, Alan, is also active in QRZ.com.

 

[gary in ohio]

For VHF and UHF stations a metal roof rack is more than enough counterpoise... I know some folks who have picked up some of the marine copper tape and attached that to the top of the roof rack for additional help, but its not needed.

 

[4x4mike]

I've had my license for a year now and have gone through a couple antennas and mounts in that year. I would suggest getting something solid and something you plan on keeping for a while. I ran an NMO mag mount for a while. Mostly because I wanted the flexibility antennas and the mag mount allowed me to move everything around or take it off if need be.

Recently I've ditched the mag mount and drill holes for permanent NMO mounts. They are super clean and in an area I tested with the mag mount. I still have the flexibility in antenna choices as well.

 

[Phreak480]

Congrats on the license (K2ZMB here) i had an 89 cherokee and I occasionally used my roofrack so i just permanent mounted and NMO towards the rear of the roof and had zero issues with the roof rack interfering. Like any other install try and find a local ham who can help and teach you how to tune the antenna for minimum SWR and off you go. Or if the rear of the roof would interfere with your roof rack usage then go for the front of the roof.

 

[thethinginthewoods]

Thanks for all the suggestions everyone! I am also an advocate of a true through-roof NMO mount, but my only concern was the positioning of the antenna should I decide to upgrade to a larger rack in the future, such as a full size ARB expedition mesh-floor roof rack. I really hate doing anything to the body of a vehicle unless I know for sure I am going to be permanently satisfied with the work or location. I suppose I am going to have to bite the bullet and just do it. I can always patch the hole professionally down the road if need be.

Another question I have after doing more research on the specific antenna I purchased was about the fact that it is a 1/2 wave antenna (just on the VHF 2m, as far as I can tell). I understand that 1/2 wave antennas need no ground plane, so that would greatly increase my options for positioning, but what about the UHF 70cm bandwidth? Do I need a proper ground plane when transmitting on the UHF bands with that antenna?

I already have a good quality SWR meter capable of taking readings in the wattage range of the TM-D710a and I have previous experience tuning many CB antennas. The Larsen NMO 2m/70cm antenna is supposed to be pre-tuned and ready to go, but you know how that goes... always better to check rather than be sorry when I cook a $500 radio transmitter!

Lastly, when people make portable "go-box" HAM stations out of mobile radios, Pelican cases, batteries and other related equipment, how do they normally ground their antennas when in the field? Without a large metal vehicle to act as a ground plane, do they just run a conductor into the ground, or...?

 

[Phreak480]

Definitely figure the layout of a future rack upgrade into your plan and place the nmo accordingly, and yeah you know better on that tuned out of the box bs. It's usable out of the box, but will be better with tuning. When people do go box's if they do them correct they either use a mag mount onto a big chunk of metal such as a file cabinet or air conditioner if in a building or have ground radials for antennas that must stand alone, or some even use dipoles or j-poles.

 

[4x4mike]

I hear you on changing things around on the roof, that's why I had the mag mount for awhile. I wanted something a little cleaner and I didn't want the coax messing up my paint any more. I have a Yakima rack on the roof year round. After I built my roof basket I pretty much knew where things were going to be so I drilled the hole.

I just sold my Larsen 2m/70cm to a member here. My main reason was because I only have a 2m rig so I didn't have the need for a dual band. I run a 1/4 wave and can swap out for a 5/8 wave. I ran the dual band on the mag mount probably 4 times and on the NMO once. I couldn't tell any difference in performance and out of the box it was always good to go. I know others that have the same antenna and they use all different mounts (albeit all were NMO). I've seen it on tire racks, magnets, trunk lips, etc. I assume they all work just fine for the user as I never heard anything otherwise.

As for portable go boxes I see all kinds of things. I've seen mag mounts stuck to mail boxes, car hoods, file cabinets and even cookie sheets. Actually cookie sheets seem to be the most frequent. They are flat and easy to pack away. I guess I've seen equal numbers of J poles and the like.

 

[Tennmogger]

.......Lastly, when people make portable "go-box" HAM stations out of mobile radios, Pelican cases, batteries and other related equipment, how do they normally ground their antennas when in the field? Without a large metal vehicle to act as a ground plane, do they just run a conductor into the ground, or...?

There are two kinds of grounds. One is part of the antenna and the other is for safety. A vehicle acts as the other half of an antenna system, the "ground" half. For example, a ground plane/counterpoise is needed for a quarter-wave or 5/8-wave antenna. Without the vehicle the antenna would not be complete. A more descriptive name for this kind of 'ground' is counterpoise.

A half wave is a complete antenna without needing an extra counterpoise so connection to the vehicle is not so important. Many antennas used for portable are complete balanced antennas by themselves, like dipoles, G5RV antennas, Doublets, or dual Buddypoles, etc. Others, like verticals, or random wires, need the counterpoise and must be operated against a ground, counterpoise, or a vehicle (acting as a counterpoise) and tuned with a tuner.

Seldom will an antenna used in the field be satisfied by a ground stake. The earth is a hard thing to attach to! Better to lay a conductor across the ground and use that as the counter poise.

A safety ground is a totally different animal. A ground rod (with same difficulty of attachment to earth) can prevent electrical shock from leaking power from a generator, or from static buildup from weather, etc. Even a ground rod is no protection from lightning, but might help guide it to earth instead of to the radio operator.

As for your antenna for the vehicle, why not install it on a large aluminum cookie tin, or similar, and fasten that to the rack. It can be moved if you need to move it. To change racks, just move the antenna and plate assembly.

Bob WB4ETT