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Static Discharge of a Mobile Antenna

Discussion in 'Mobile Radio Systems' started by AA5ET, Mar 12, 2011.

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  1. AA5ET

    AA5ET Ham Member QRZ Page

    Hi all,

    I just installed a new 2-meter rig in my truck (FT-1900R). I don't do much with VHF or UHF and have a few questions about grounding my VHF mobile installation.

    Here's what I did so far:

    Since I already had a ball-mount on the truck and wasn't using it for HF anymore, instead of buying a factory antenna I cut a stainless steel whip down to 1/4-wave on 2 meters and mounted it to the ball mount. It works fine with less than 1.5:1 SWR across the whole 2-meter band.

    The antenna is fed with about ten feet of RG-213 and also I installed an LDG-SP200 surge protector inline which is grounded to the truck body. The surge protector is a gas discharge tube that acts as an insulator, but acts as a conductor when 250 volts is exceeded.

    My question is, since the surge protector will not ground the antenna unless 250 volts is exceeded, what happens to my rig below 250 volts? Will the rig safely shunt (static... dc) to ground below 250 volts or should I worry about it? I live in a dry area where static buildup is a problem.

    Can I get away with putting a high value resistor between the ground and antenna like I do on HF? On HF I will typically put a 100K or greater carbon resistor between the radiating element and ground to shunt static buildup to ground.

    Below is a picture of a resistor I was thinking of using. It's a 221K 4-watt non-inductive Caddock film resistor.

  2. WA4OTD

    WA4OTD Ham Member QRZ Page

    Generally any surface that can be touched by human hands should be tested to 2kv ESD minimium. Center conductor of an antenna would certainly fall into that category. To be sure I would call Yaesu customer service and confirm.

    Maybe my memory is bad, but 1.5:1 across the band seems better than it should be.
  3. AA5ET

    AA5ET Ham Member QRZ Page

    Guess I should have clarified - Less than 1.5:1 across the FM portion of the 2 M band. I'll check on the ESD specs. Thank you.
  4. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Yes you can put your resistor from antenna to ground to drain static charge.

    We had a lengthy discussion last year and several engineers agreed, with a general recommendation of about a 50kohm (or higher) non-inductive resistor of about 1, 2 or 3 watts being sufficient.

    I have never heard of anyone using gas discharge devices in mobile rigs. I don't see the point. What are you protecting against? Lightening? I strongly recommend removing it from your line.

    An 1.5 SWR across any band with any antenna is a probably a clue to losses and poor installation. Placing a 2m antenna on a trailer hitch is probably as poor an installation as you can get. A 1/4L needs a good metal ground plane immediately under it to be effective. That is the reason you see them on trunks, hoods, car tops. In fabric covered airplanes, they place copper strips inside glued to the fabric, equivalent to radials on an HF antenna.

    Alan has everything you need to know about mobile installs

    73, bill

    p.s. You can even add static 'wicks" to your vehicle to further reduce static build up. I did and it helps on HF. Again, Alan has an article on that.
  5. AA5ET

    AA5ET Ham Member QRZ Page

    The antenna is not mounted to a trailer hitch, it's mounted to a ball mount on the rear quarter panel of my truck (a ball mount is a type of permanent antenna mount). See the photograph below.

    It's a simple 1/4-wave vertical using the car body as a ground. It's about as good as one can get on a 1/4-wave mobile installation. Of course a 5/8-wave antenna would probably have a little flatter radiation pattern an a bit more gain, but I'm cheap and like to use what I have on hand. Checking the SWR again, and reactance with my trusty MFJ antenna analyizer, the SWR is 1.3 at 145 MHz and 1.4 at 148 MHz, and reactance is X=0. The antenna seems to work fine - full-quieting into repeaters 50 or more miles away. Maybe a 1/4-wave vertical is more broad-banded than something you're used to.

    Thanks for the info on using the resistor. I'm going to keep the surge suppressor in-line because there are a lot of thunderstorms here and would rather shunt the surge from a near strike to the body of my car, rather than through the radio. (But I don't think anything would protect my radio from a direct strike.)

  6. AA5ET

    AA5ET Ham Member QRZ Page

    Taking another look there shouldn't be a whole bunch of loss with my installation (unless I totally screwed up with the soldering).

    Ten feet of RG-213 has about 0.28 dB of loss at 1.4:1 SWR at 150 MHz

    The three PL-259 connectors (silver plated Amphenol I think) together should have about 0.15 dB of loss

    And the surge protector (according to the specs) has less than 0.1 dB of loss.

    Total loss = 0.53 dB

    Not perfect, but what is? I can live with it, and it works.

    By the way Bill, the ball mount pictured is the one that K0BG recommends on his website (for HF, although I'm now using it for VHF). I had an HF rig in this truck, but removed it.

    Last edited: Mar 13, 2011
  7. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    OK! my misunderstanding.

    You definitely have the most rugged quarterwave mount a fellow could have! :) Once you have that mount you have lots of flexibility for different ops.

    I think you are good to go then! 73, Bill
  8. AA5ET

    AA5ET Ham Member QRZ Page

    Thanks Bill, have a great evening.

  9. KB5HAB

    KB5HAB Ham Member QRZ Page

    I'm probably way off base here but then again I may get lucky. I can't see the top of the actual antenna but it looks like an old stainless steel CB whil that has been cut down. The very top end of the antenna used to have a ball on it I think. Most all auto antennas have a ball on the end and my old brain seems to remember it was for something called corona effect. What I can't remember is if this was also for static. I'm shooting in the dark but maybe that is giving you some headaches? Of course if you have a ball on the top end or have ground a round end on the antenna and sanded it nice and smooth just disregard me. Just trying to help and most of the guys on here have forgot more than I know.
  10. K0BG

    K0BG Ham Member QRZ Page

    I'm not surprised by the bandwidth at all. Considering that there is probably about 2 pF of loading by the ballmount itself, the large diameter of the ball, and the diameter of the whip, the bandwidth should indeed be more than a standard old 2 meter NMO-mounted 1/4 wave.
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