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Old 10-13-2012, 02:31 PM   #1
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Multimeter through the roof when cb turned on

Hi Guys
I have been getting assistance on my post about high swr readings (Cant get an SWR reading below 3) and I think I may have come up with something that is causing the problem.

I have a 3' Firestik Firefly on a terraflex spare tire mount antenna bracket, with a pl259 connector on each end of my coax.
When I test my grounds ( I have done everything but a ground strap ) I get great readings at my multi meter. Usually around .1 to .3 This is not only at my antenna mount, but at the base of the stud/coax collar.
I still need to swap the coax to see if that is the problem, but I just tried something. When I turned on my cb, the coupling that screws into the swr meter from the antenna when from a .1 to 22.7 on the multi meter.

What in the world would cause that? Is that normal? I did a voltage drop test and it was .001

Have I stumbled on something that may help diagnose my problem?

Dave

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Old 10-13-2012, 07:51 PM   #2
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If you were testing resistance then it isn't an accuracte reading. You cannot test Ohms on a live circuit. It must be an isolated, de-energized circuit or wire.

A voltage drop test is used on a live circuit to determine, amongst other things, if the circuit needs to be tested for excessive resistance.

Make sense?

Have you tried adding a spring or disconnect to get the shaft taller?

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Old 10-13-2012, 08:49 PM   #3
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No, it doesn't make sense, but I trust your comments. I won't worry about those numbers.

Yes I did add a spring with zero changes. I was still getting a 3 on my SWR meter.

However... A little while ago I decided to hack up am old magnetic antenna to get a chunk of different coax. WellSonOfaBitch! My SWR changed from 3 to 2. That tells me that if I get a decent piece of coax I may just get some decent numbers. FINALLY.
I'll let you know when I find some new coax.

Dave
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Old 10-13-2012, 09:14 PM   #4
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You can never buy too nice of coax. Ill specify about the meter readings when I have a few more minutes.

Keep us posted on your results.
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Old 10-14-2012, 10:15 AM   #5
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Ok, let me try to explain what that meter is doing under particular tests, it may help.

Basics of electricity: voltage is pressure of electricity, resistance is restrictive force (think corrosion), and amps are actual power flow. In a water hose, amps would be how much water is coming out. Volts would be how much pressure is pushing the flow, and resistance would be how much mud & junk is on the inside of the hose. So if you turned the hose on, and very little water came out, you would have low amps caused by high ohms OR low pressure - time to test for voltage drop to see which. If voltage is normal, test the circuit for resistance, because you have good pressure but not flow - it must be restricted somewhere. If it was flowing normally, and you put your thumb over the end, you would be increasing pressure (voltage), but decreasing total flow (amps).
An open circuit is a "broken" wire, a closed circuit is a solid wire. One has power flow, the other does not.
Watts are a way to look at total power consumption without reference to specific amp/voltage. A circuit of 12V, 15A, is equal to a circuit of 120V, 1.5A. They would both use equal watts, 180. (W=A x V). Watt is just a measure of power used, like a Joule(?) in physics.

When the setting is on resistance, or Ohms, the meter tests the conductivity of a circuit or wire. The way that it does this is by sending a tiny pulse of electricity out the pos lead, which in this case you had attached say to the coax or mount. Then, when you place the neg lead on the frame, say for example, the meter takes a reading of the remaining pulse that it sent (having travelled from the mount through the tailgate and hinge, down to the frame and into the neg lead), and determines from how much energy made it back on that neg lead how much resistance is in that circuit. If you did this to a live circuit with a load, like a light bulb, and tested both sides of the light, the power for the light would have an easier path going through the meter than that light, so your meter becomes the light instead. If the light uses enough power, you will smoke your meter doing this. It is the exact same as taking a bare piece of wire and connecting it on both sides. This is why the circuit must be off. MUST BE. With house voltage, getting this wrong can kill you, and at all voltages will damage meters, componants, and circuitry.

Relavent to you - With your meter hooked up and the radio on, instead of testing the conductivity of the mount to frame ground path, you are providing an alternative path (a ground strap) through your meter. The reading is how much energy is travelling through those leads to find ground. Because it is a ground circuit, you have'nt bypassed any loads, so the meter displays instead of frying. BUT there is other energy in the meter aside from the pulse it sent out, so it just gets confused on whats happening.

So how to test a live circuit for conductivity?

A voltage drop test. While set on voltage (must be ac for ac, dc for dc, but they work the same), the meter does something totally different. Instead of being a closed circuit, it is open, not allowing power to flow through the meter as it does in a resistance test. The meter looks at each lead independently, and displays the difference in voltage between each two. So for your light bulb test, with the light on, you would connect one lead in front of the light (pos) and the other behind it (neg). It would take a voltage reading before the light, then the light would use whatever power it wanted, then the other lead would show the meter how much voltage is present after the light, and it would display the difference of those two for you to see the consumption of that load (light). Wih this test you can actually stick one of each lead into an electrical socket (plug), just like mama told ya not to, and be fine. Try it with an ohm test and you'll loose your eyebrows when you start to glow, and will not be fine.

Relavent to you - When you put a lead on the coax collar (sheilding) with the meter on voltage, the pos lead will look at how much pressure is there. Then with the neg lead on frame or batt ground, it will look at the pressure there. If your path to ground is good, the pressure will equalize, like a hose with water layed out straight. If your path to ground is not good, you just folded that hose in half. Now pressure on one side is high, while it is low on the other. The result? No water, or very little, coming out the hose, because of a choke point. Then we can start ohm testing small areas to localize the "hose fold" or choke point.

Make any more sense? Not sure how good of an explination that is, it took me more than a conversation to get it. I still know auto techs that are lost with a multimeter, they are not easy to understand. Nor is electricity.
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Old 10-29-2012, 09:58 PM   #6
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That was will written and you put it in words that a fire fighter operator can understand.
Thanks for taking the time to help.

Dave
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Old 10-30-2012, 07:59 PM   #7
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Quote:
Originally Posted by Fireslayer
That was will written and you put it in words that a fire fighter operator can understand.
Thanks for taking the time to help.

Dave
My pleasure.

Thanks for running into the smoke.

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