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?

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    $\begingroup$ You might be interested in ham.stackexchange.com/q/7225/8717 $\endgroup$ – Mike Waters Apr 7 '17 at 21:32
  • $\begingroup$ There are many kinds and applications for chokes. In some places, "choke" can mean something as broad as inductor, and certainly those are essential at VHF. You might want to be more specific about what kind you mean. $\endgroup$ – Phil Frost - W8II Apr 8 '17 at 3:07
  • $\begingroup$ friendly reminder: please accept an answer or clarify what is not satisfactory about the answers you've gotten. This site stops working if askers don't give feedback!! $\endgroup$ – Marcus Müller Apr 15 '17 at 10:22

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.

  • $\begingroup$ I notice when I turn on my vehicle with my handheld inside I go from relatively clean copy to more static and noise. Will mounting the antenna outside completely solve that EMI, or would I still need to choke the feed line? $\endgroup$ – SandPiper Apr 12 '17 at 12:53
  • $\begingroup$ Also, in the article it mentions the choke adds 2.2K to 2.7K ohm impedance to the feedline... does that not cause an impedance mismatch with the antenna? $\endgroup$ – SandPiper Apr 12 '17 at 12:54
  • $\begingroup$ In that case you could definitely choke the feed line if you are powering the radio off the vehicle, then a large rated capacitor across the feed line will help filter AC off the DC feed. You could be seeing interference from the spark plug wires or even an alternator with marginal diodes. You can confirm that is a source by setting a meter to ac volts and checking your power feed with the engine on and off. that interference can be mitigated by a large capacitor across the feedline. More... $\endgroup$ – Rowan Hawkins Apr 15 '17 at 5:34
  • $\begingroup$ The other issues you run into running an HT inside of a vehicle with an HT rubber ducky is that you are essentially inside of a faraday cage even though there's some really big holes in it. Because of that having the antenna external to the vehicle will help your noise reception as a matter of course. $\endgroup$ – Rowan Hawkins Apr 15 '17 at 5:37

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.


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