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Author Topic: Static discharge from mobile antenna  (Read 1125 times)
KF5PGT
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Posts: 38




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« on: August 06, 2012, 06:20:18 AM »

This weekend, the family and I took a short trip to Florida. We took my wife's car (2012 Honda civic) and I stuck a cheapo mag mount 2 meter antenna to the center of the roof. I connected it via BNC to an HT and earbud headphones. This setup worked fine for the most part. However, around Mobile Bay we ran into some very heavy thunderstorms. No problems until we crossed a longish bridge over a bay in Florida. I was switching between monitoring the NOAA weather radio broadcast and 146.52 when I started hearing a fast, rhythmic popping in my headphones. I pulled them out of my ear and felt the static electricity discharge thru the headphones. I quickly turned off the radio and disconnected the antenna. If I touched the antenna BNC, I got a small static jolt. I'm guessing that the antenna was getting the static from the charged particles in the thunderstorm, and being over a decently sized body of water amplified the problem. Once we got off the bridge everything went back to normal

So here's my question. How can I pervent this from happening again? Add a ground wire to the frame of the car?
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K1CJS
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« Reply #1 on: August 06, 2012, 07:45:50 AM »

It probably wasn't the water.  It was the metalwork in the bridge that had attracted the charge, and your antenna was also attracting it since your car was 'inside' the bridge metalwork with an electrostatic difference between the antenna, the car and the bridge.  Could be that the bridge was about to become the receiver of a good lightning strike.

How to prevent it?  Begin by using a good antenna that is actually attached mechanically to the car body or by taking in the magmount when you come near a thunderstorm.  Other than that, it's hard to say.
« Last Edit: August 06, 2012, 07:50:38 AM by K1CJS » Logged
AA4PB
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« Reply #2 on: August 06, 2012, 07:50:39 AM »

Connect the coax shield to the vehicle chassis. This will keep it from using you as the conduction path! The HT probably has DC conduction between center conductor and shield so when the shield is grounded, the static charge on the antenna will probably never build up to the point of causing any problems.
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K4SAV
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Posts: 1840




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« Reply #3 on: August 06, 2012, 05:00:04 PM »

Welcome to Florida, home of weird lightning events.  Having lived there for 38 years and working HF mobile while driving to work, I experienced this many times on HF.  I'm pretty sure it is due to charged raindrops.  It doesn't happen every time it rains, but only occasionally.  When this happens it's interesting to watch the S meter and change speeds.  S meter reading seems to be a direct function of vehicle speed.  I have seen 70 mph produce s9+40 dB on 20 and 40 meters.  At other times when it is raining, vehicle speed doesn't do anything.  I have never received a shock but my antenna was always well grounded.

However I have never experienced the same thing on 2 meters while mobile.  I have experienced a somewhat similar condition on 2 meters several times.  It occurs after a lightning strike, which I usually see in the distance.  I hear the pop on the radio then 15 to 30 seconds of static.  Then the static slowly dissipates.  I'm guessing that this somehow changes the charge in the ground near my vehicle and causes the antenna to generate a corona as the ground equalizes the charge.  Just a guess as to the cause, but it's definitely triggered by the lightning strike.  It doesn't happen every time and it doesn't have to be raining.  This is relatively rare.

Another event, which is even more exciting, is to be waiting for a traffic light and have a lightning strike hit a power pole 5 ft from your car.  Survived that one OK with no damage to the rig, but I was worried.

Jerry, K4SAV
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K1CJS
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« Reply #4 on: August 07, 2012, 04:50:07 AM »

What you experienced was similar to the stories of 'St. Elmo's fire' on the rigging of sailing ships.
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AA4PB
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« Reply #5 on: August 07, 2012, 05:14:35 AM »

... and the reason they put static wicks on the trailing edge of surfaces on aircraft. My first avionics job was to inspect and trim all the static wicks  Cheesy
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W5DXP
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« Reply #6 on: August 07, 2012, 05:31:11 AM »

This happens all the time on dipoles where one dipole element is DC isolated from the other. It can happen with rain, snow, and/or dust in certain areas of the country. In a mobile setup, it happens when the monopole does not have a DC path to the coax braid. If the mobile antenna is a 50 ohm feedpoint impedance, a 1k carbon resistor from the monopole feedpoint to the coax braid will usually bleed off the static charge. With a mag-mount, one may need to ensure a path from the coax braid to chassis ground, e.g. using an HT with no attached vehicle chassis reference.

I once rode my bicycle in an AZ thunderstorm. As I was riding along at night, continuous one-inch arcs were formed on each side of my metal bicycle rims, across the rubber tires, to the pavement.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K7KBN
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Posts: 2802




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« Reply #7 on: August 07, 2012, 09:55:35 AM »

(Spoken softly to self):  Keep pedaling!  Don't put your feet down... Grin
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73
Pat K7KBN
CWO4 USNR Ret.
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