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I am trying to install a VHF/UHF antenna on my truck, and I have been studying everything I need to do that. I keep reading about chokes, in particular here: http://www.k0bg.com/choke.html
However, everything I find talking about it is in reference to 10m band and lower frequencies. Is choking necessary for the 2/70 bands?
The article you reference has to do with protecting the controller in a screwdriver antenna. Which you don't have at VHF or UHF.
The purpose of a screwdriver antenna is to lengthen or shorten the element to match the wavelength of your transmission band. VHF and UHF transmission bands are each wider than a single HF band but not wider than all of the HF bands grouped together. At VHF and UHF frequencies the ideal antenna length between the ends of the band is not that great.
A feedline choke can be installed on any antenna but unless you are planning on driving a wire brush you're not going to get much induced current in your feed line.
You don't mention what type of antenna you have or where on the truck you are mounting it. However as the linked article in the comment explains you will be fully unbalanced. Mobile installation is almost always fully unbalanced.
Trucks have some suboptimal mounting locations. They are not worse than motorcycles however. Accessories that you might have for your truck will change the below information. Which is for a regular Pickup. The main things that impact the information below would be a ladder rack or a metal cover.
Best: If you are planning on through hole mounting to the center of your roof deck, the ground plane will be physically established where the connector comes through the roof. If you are just planning on the one antenna you should try to get it close to the center front back left right. This would be an effort to maximize the amount of ground plane around the antenna, and to minimize the amount of directivity that the shape of your ground plane will provide your signal.
Good: If you are using a magmount, the ground plane is parasitic back to the radio chassis mount. A similar mounting point to the one above should be chosen. Route the cable in such a way that water won't drip in and it won't get smashed by the door as often.
Okay: Stake hole mounts I don't have a great deal of experience with. If you use the holes close to the cab these run into the same issues as a trunk mount car antenna. The metal structure of the cab drastically interferes with your radiation pattern and your match. If you have a full size pickup the rear stake holes will be more than a wavelength from the cab at 2m however you will still get Reflections from it.
Choking baluns are not typically used on VHF and UHF antenna systems in a vehicle.
Part of the reason is that the ground plane formed by the roof or trunk of the vehicle is quite effective in isolating the feedline from common mode currents (the currents that a choking balun is trying to reduce or eliminate) at VHF and UHF frequencies.
If you did try to build one from coiled coax or a parallel transmission line wrapped around a torroid, you would find that the strays at those frequencies would significantly limit their effectiveness. With careful attention to details, you could probably tune one to be effective but this is beyond the resources of the typical ham.
So I recommend carrying on without one just like the rest of us do.
The short answer to your question is no. I know this thread is a bit dated, but here are my two cents worth as an amateur and professional. Take whatever K0BG says with a grain of salt; he means well but does not understand modern vehicle electrical systems.
As already stated, K0GB refers to an HF antenna and has nothing to do with VHF and UHF applications. Any HF antenna made for a vehicle is a compromised design with poor efficiency. No way around it, as it is impossible to establish a proper ¼-wave ground plane and whip lengths. Therefore common-mode current is guaranteed to be an issue you can never eliminate. At best, you can make it somewhat usable. Without a proper ground plane, there will be an RF current on the coax shield and all the vehicle wiring. It has to go somewhere to balance out currents.
On the other hand, with VHF, it is possible to install an antenna without compromises, and UHF is straightforward. Once you move up to VHF and UHF frequencies, it opens doors to commercial antenna manufacturers that offer amateur band antennas. You are no longer limited to amateur-made products for HF. Mobile antennas for commercial LMR and Public Safety services are not compromised unless installed incorrectly in the wrong location. If installed in the wrong spot, the consequences are not as severe. The coax length between the radio and antenna is at a minimum ¼ wavelength long or longer. It will contain common-mode currents preventing it from backing up into the vehicle wiring, causing possible vehicle electronics malfunction, aka RFI.
The correct location is the roof of a vehicle. Dead center is perfect. The rooftop is the only flat horizontal surface on a vehicle, large and high enough to act as a ground plane. 2-meter antennas are the least flexible in mounting location because it has the largest ground plane requirement of 1-meter diameter. It does not give you much room to work with to move the antenna around off-center and maintain at least a ½-meter from any roof edge. UHF or 70 cm is more forgiving with roughly a 1-foot diameter ground plane requirement. So forgiving, you can get away with the center of the trunk lid on an automobile.
Location is crucial not only for proper operation but also for operator safety exposure to RF radiation. Located on the roof, you are shielded and not in the main lobe of horizontal radiation. Using a trunk lip mount requires a 3-wavelength distance from the vehicle's occupants. Easy to do with 70 cm, not likely at VHF, and forget HF.
My best advice to anyone doing a mobile install is to refrain from using amateur guides, websites, and especially amateur radio manufacturers' installation instructions. Most are outdated and dangerous. For example, how many amateur radio equipment manufacturers and hams recommend running the radios' negative battery wire directly to the battery term post? That practice is fire-hazardous and guarantees you will have RFI/EMI issues. It places your radio battery negative and coax shield in parallel with your vehicle's negative wiring. You have engine cranking current, headlights, fans, electric seat/window, wipers, electronics, and ignition current flowing through your radios ground plane—simple parallel circuit law. Easy to prove with a Clamp-On Amp Meter clamped to your negative battery radio lead. Turn off your radio and start the engine while monitoring the current. Surprise! With the engine running, start turning things on and monitor the current. Why do you have vehicle current flowing on your ground wire and coax shield with the radio turned off? Terminate the radio ground wire to the nearest chassis hard point, and all that goes away.
If you were to be a professional two-way radio installation technician doing any work for Public Safety, military, government, and utilities require certification and following Motorola R56 Appendix G, Mobile Installation Standards and Technique. Covers every aspect of a mobile installation.
73's
AI5DH
