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Author Topic: soldering coax connector, a few ohms ok?  (Read 506 times)
KE4CTX
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Posts: 9




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« on: June 07, 2007, 03:53:40 PM »

Hi All,

New general here. Getting ready to install a couple of antennas, I chose bury flex for VHF/UHF. For now I have a run of RG8X for the HF.

I'm new at soldering coax. What a learning experience! I've got to get some proper tools, including a coax cutter. I think I managed to muddle through the process with regular hand tools.

However, the RG8X  reads about 3 ohms when checking opposite ends after I installed the pl259s. I did not check it prior to installing the connectors. Is this normal, or a sign of a bad connector install?  

BTW, is it practical to unsolder and re-solder connectors, or should I just start with news ones?

Thanks
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N3JBH
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Posts: 2358




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« Reply #1 on: June 07, 2007, 04:07:31 PM »

do you mean your checking the say center pins on each end of the coax run? if so dont worry about it. what you need to worry about is the center pin to out side for short's etc. depending how long the coax cable is could very well produce a small amout of resistance.
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N3JBH
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« Reply #2 on: June 07, 2007, 04:09:31 PM »

BTW, is it practical to unsolder and re-solder connectors, or should I just start with news ones?
 practical as in cheap yes.  is it wise to do it No. use new ones you be better off.
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W5QDF
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Posts: 6




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« Reply #3 on: June 07, 2007, 04:10:24 PM »

Just cut the wire and put new PL259's on, it is much easier and quicker than trying to desolder the ones currently on there.  As for the 3 ohm measurement, it is probably because of the solder connections, but you will need to do a before and after reading to make sure your solder connection is not the problem.

My wife is my solder tech, I taught her and now she does a better job of soldering PL259's than I do.  When she boggles one and is not happy with the soldering she just cuts the wire and solders a new one instead of fighting with desoldering.  the cost is not such that you need to save every last one of your PL259's.

Have fun and hope to chat with you on the air.

David
W5QDF
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KG4QPQ
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Posts: 61




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« Reply #4 on: June 07, 2007, 04:59:43 PM »

You don't say how long your coax is or what kind of meter you are using. If you are using an analog meter, it is possible that you don't have it zeroed correctly or the battery is weak. If you are using a digital, it depends on the quality. Either way don't worry about a few ohms end to end. There is a certain amount of DC resistance in any cable and in your test leads.

I also don't recommend reusing connectors.

Good luck.
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KB9CRY
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« Reply #5 on: June 07, 2007, 05:55:11 PM »

I only check for continuity between the center and shield.  Open circuit, good to go.
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W7ETA
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Posts: 2527




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« Reply #6 on: June 07, 2007, 06:07:12 PM »

If you hold the meter probe onto the PL259 pin with you fingers, on each end of the coaxial cable, you are adding in your resistance to that of the center conductor.

Been there.  Done that a few times during the past two sin spot cycles.  ;-))

73
Bob
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KA1OS
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Posts: 44




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« Reply #7 on: June 07, 2007, 06:24:31 PM »

I just love soldering on a new PL259 perfectly and then realizing I hadn't put the screw collar on first.

Another reason not to hurry some things...
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KE3WD
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Posts: 5689




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« Reply #8 on: June 07, 2007, 09:31:33 PM »

Any reading below around 10 ohms and doesn't change with vibration is good.  

If you are using a DMM, you just learned about DC voltage drop and long runs of wire.  

That's right, the wire is not a perfect conductor, it exhibits some small amount of resistance per foot.  

As for reading the parallel resistance of your fingers with the ohm meter on the X1 ohm scale, *ahem*  Ain't happenin'.  Kirchoff says so.  


.
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KC8VWM
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Posts: 3119




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« Reply #9 on: June 07, 2007, 09:57:27 PM »

I just love soldering on a new PL259 perfectly and then realizing I hadn't put the screw collar on first.

---------

Curious, have you been watching me solder PL-259's in the past?  Smiley

73 de Charles - KC8VWM
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N6AJR
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Posts: 9908




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« Reply #10 on: June 07, 2007, 10:24:59 PM »

so lats month I was soldering up a couple of new cable and stuck the pl 259 on my lip for some reason to see if it was still hot ( duh!!) , BTW it was still hot...Smiley so thats how you burn your lip soldering  connectors..
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KE3WD
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Posts: 5689




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« Reply #11 on: June 07, 2007, 10:30:37 PM »

I just love soldering on a new PL259 perfectly and then realizing I hadn't put the screw collar on first.

---------

Everybody's done it, I'll bet.  

The way to avoid it is to never remove the collar until after the coax is stripped and dressed.  

Pick up the entire plug and when you unscrew the collar you will need a place to put it.  

Put it on the coax.  

Simple, but took me years to figure that one out for some reason.  


.
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NU0R
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Posts: 408




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« Reply #12 on: June 08, 2007, 04:44:03 AM »

I make a checklist of steps to installing the connectors-- using this method I have never left off the collar. Before this I forgot  it many times. An old Chinese proverb says---  "the dullest pencil is sharper than the sharpest mind! So true, So true!!! I also make check lists when disassembling anything mechanical. It sure comes in handy when it comes time to put it back together. A digital camera, or video of the circuit is also a lifesaver to confirm the layout. I also use tape to put on wires to identify where they go on reassembly. Just a few tricks for the mentally challenged like me. HI HI    Bruce
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AA4PB
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Posts: 12844




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« Reply #13 on: June 08, 2007, 05:43:56 AM »

What you are measuring is the DC resistance of the center conductor. That resistance depends on the size of the conductor (i.e. type of coax) and the length.

If you are measuring a length of 50 to 100 feet, 3 ohms seems pretty high - probably and indication of a cold solder joint on the connector.

The most important thing is that you measure infinity between the center conductor and the shield.
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K8AG
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Posts: 351




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« Reply #14 on: June 08, 2007, 10:01:53 AM »

3 ohms is a huge problem.  RG8X has a 16AWG center conductor.  16AWG wire has about 4.2 ohms of resistance per 1000 feet.  Yes connectors will insert some resistance, but 3 ohms is a lot unless you are running 500+ feet of the stuff.

Think of it this way, even if your antenna is 50 ohms exactly, through the coax your radio will see 53 ohms.  So roughly 6% of your power will be lost just in that connection.  And this is only DC.  RF will be higher.

I would check to see that all of your connections are not cold solder joints.  I use a soldering gun as it does less damage than an iron.  They get hotter and you can hit n' get without heating up the coax and connector too much.

My 2 cents

73, JP, K8AG
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