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Crash
03-26-2009, 09:38 AM
Trying to run down a problem with my CB that has been going on now for a couple of years. I know, I know, get a ham radio. :o Just had the Uniden Pro 520XL bench tested at the shop I bought it a in 2002 and it checked out fine, according to the tech. Problem is an intermittent screaching when keying the mike and now, after the Moab pre-run, send and receive distance seem to be very short - 3-400 yards in direct sight seems to be max. Robbie suggested running a separate ground wire from the antenna base to the truck's frame but if the connection between the base and the bumper ohm out to zero, doesn't that indicate a perfectly good ground? Simple enough to try Robbie's suggestion but I thought the laws of physics were immutable. In testing the antenna leads - I have both front and rear locations on the truck with different leads - I shorted out one end of each and tested for a closed circuit at the other ends. Meter indicates an open circuit on both leading me to believe that you can't test antenna leads this way. Is that right? Maybe it is just time to start over and, no, I ain't going ham. :hill:

DaveInDenver
03-26-2009, 12:29 PM
Oh man, where to start?

My first guess was a broken or partially broken coax.

Remember that DC ground with an ohm-meter is not the same as RF ground. You might read a dead short with a DC meter but that might not be a short to RF. It depends on what you expect, a short might actually indicate a short (if there is a capacitor it will read open with a DC meter but will appear like a connection to AC).

You have a Firestik IIRC, that should NOT measure short to ground on the center of the mount (i.e. the threaded part of the antenna). The mount itself will have a nylon ring that insulates the 3/8" stud from the metal it's mounted in.

If there is no antenna on the mount, just a coax feed and a mount, then yes you should measure an open and a DC meter is fine for that. With an antenna in the mount, then things change. No antenna mounted and the end shorted at the mount, then a small ohm value at the connector will verify that you have no breaks in the center conductor or shield. Put the meter on beep and go along the length, moving it to check.

But other than a hard failure (i.e., shorted coax or cut insulation to ground) the DC won't tell you the condition of the coax. It is possible (likely?) that you have a nick or kink in the coax and only a RF meter will help you there. An SWR meter for example.

Most people use really crappy coax (no offense, I didn't spend much on my CB coax either). Age, temp cycles, vibration, all that is easy to break them. I would pull new coax and clean up the connections, I'll bet your issue goes away.

treerootCO
03-26-2009, 12:59 PM
I have new coax you can wire up as a test as well as another handful of radios and an SWR meter I stole from John. Should be easy enough to prove out. The cool kids run both because a mix of ham and no ham is equal to or worse than the guys with no communication in their rigs.

Crash
03-26-2009, 01:02 PM
Should have been a little more specific about my test. Testing only coax in the previously mentioned scenario, no antenna or radio connected. I was looking for a short but seemed to have found open circuits on both leads I'm testing. Again, I shorted one end of the individual coax leads and tested the other end for continuity, got no beeps suggesting an open circuit in each of the leads - if coax can be tested like that. Testing with an SWR showed one of the leads, with Firestik installed, to give the desired results. Dave, your suggestion of buying a new, crappy:hill:, lead is probably the next step.

Groucho
03-26-2009, 04:25 PM
The shield, or outer ring (the thing you spin to thread the coax body onto the radio/antenna) is one end, and the center, or straw-like thing is the other. If neither end is connected to a radio or antenna, and you place a meter across one end or the other (one test lead to the shield or outer ring and the other test lead to the inner or straw-like part) while testing for continuity should show open. If it shows closed, your problem is coax. Even if it shows open, if the coax is routed around things that may move while in motion it may be causing an intermittent short as Dave suggested. If you shorted one end and got no beep at the other, you have a break, hence no signal.

I say get a new piece of coax, and before running it underneath anything, check it out.

Just remember, in theory CB and Amateur Radio are the same, just different privileges. If you use your CB, you are simply transmitting and receiving radio waves, just like Amateur Radio. Subtle differences in the method make the two not alike.

Crash
03-26-2009, 06:09 PM
I have new coax you can wire up as a test as well as another handful of radios and an SWR meter I stole from John. Should be easy enough to prove out. The cool kids run both because a mix of ham and no ham is equal to or worse than the guys with no communication in their rigs.

Root, Groucho's response gave me the answer needed to try a new piece of coax and I probably should go ahead a buy some. I go to the shop not far from you on 44th near Harlan. Thanks and thanks to you, too, Nathaniel.
I sure could use a pair of the resistors for the temp gauge fix on the 80, though. Got any? :) It would mean :beer::beer::beer::beer::beer::beer:

Crash
03-27-2009, 11:34 AM
Uh, :doh: I may have the answer to the "problem" raised in this thread. Remember how it was mentioned that there are two leads to the radio, one from the ARB bumper in the front and the other from the Slee bumper in the rear? Antenna was connected to one or the other while the unused lead had a plug installed in the connector at the bumper to keep road junk out of it and when tested, both leads showed open circuit? Is the answer starting to become apparant to you yet? I pulled the shorter front lead out of the truck and, uh, guess what? The connector at the radio that I thought was going to be the other end of the pulled lead turned out to not be that one. Who needs to mark leads as to which is which? Not me!! :rolleyes: Now that I have warmed back up after being in the cold garage, it's back out to test the radio with my handheld unit and the Firestik on the other end of the lead THAT IS CONNECTED TO THE RADIO. Want to bet all problems have been miraculously cured? Oh, by the way Dave, I checked the labeing on the pulled lead and it reads "Platinum" Truckers Series. How could anything labeled as such be crappy? :lmao:

corsair23
03-27-2009, 12:05 PM
:lmao:

Steve, that is right up there with me wondering why my burned CDs woouldn't play in the CD changer...it helps to have them inserted into the changer right side up :doh:

nakman
03-27-2009, 04:16 PM
Good one, Steve! Your radio will also not work if you have the antenna disconnected form the mount. amhik... :rolleyes:

and I've done the CD thing too, Jeff. :brick: :)

Crash
03-27-2009, 04:22 PM
Good one, Steve! Your radio will also not work if you have the antenna disconnected form the mount. amhik... :rolleyes:

and I've done the CD thing too, Jeff. :brick: :)

At least there are three of us that will admit it Tim!! Good for us being secure with our foibles. I did have about a 300 yd. range as long as it was line of sight. It does beg the question, though, of why the CB shop's SWR meter said the antenna was reading 1, or whatever "perfect" is when the lead they were testing had a broken 90* bend type mount serving as a plug in the other end. I am so not into electronics.

DaveInDenver
03-27-2009, 05:37 PM
At least there are three of us that will admit it Tim!! Good for us being secure with our foibles. I did have about a 300 yd. range as long as it was line of sight. It does beg the question, though, of why the CB shop's SWR meter said the antenna was reading 1, or whatever "perfect" is when the lead they were testing had a broken 90* bend type mount serving as a plug in the other end. I am so not into electronics.
This is the reason why you should never 100% trust a SWR reading. This is just the ratio of forward power compared to reflected power. If you happen to have the end connected to a dummy load (which is just a big resistor), you will get zero reflection and so an exact SWR of 1:1. But a resistor will only convert 100% of the energy into heat and produces absolutely zero RF energy. So while you have a perfect 1:1 SWR, you will also have zero chance of talking to anyone on the air.

Also if your coax happens to be exactly some multiple of 1/2 wavelength long, it could hit the unterminated end of the cable at a voltage null, which would mean the reflected wave would be a voltage null at the feed end (at the radio). Depending on the quality of the meter, that would appear like there is zero reflected energy and again an SWR of 1:1. Since most cheap meters are just VSWR meters, this could easily be the problem. The 'V' part of VSWR is voltage standing wave ratio, so if both wave voltages are in phase and at the same amplitude, the ratio is zero. This is why I really, really hate Firestik's recommendation of using an 18' long cable. The wavelength of a CB carrier is about 36', so 18' is 1/2 wavelength. They tell you to do this so that you always have a low SWR but it also masks any feedline issues you have. Let me be perfectly clear here, if you have a solid, clean installation and a well tuned antenna, the length of coax you use between the radio and antenna simply does not matter.

Crash
03-27-2009, 05:44 PM
This is the reason why you should never 100% trust a SWR reading. This is just the ratio of forward power compared to reflected power. If you happen to have the end connected to a dummy load (which is just a big resistor), you will get zero reflection and so an exact SWR of 1:1. But a resistor will only convert 100% of the energy into heat and produces absolutely zero RF energy. So while you have a perfect 1:1 SWR, you will also have zero chance of talking to anyone on the air.

Also if your coax happens to be exactly some multiple of 1/2 wavelength long, it could hit the unterminated end of the cable at a voltage null, which would mean the reflected wave would be a voltage null at the feed end (at the radio). Depending on the quality of the meter, that would appear like there is zero reflected energy and again an SWR of 1:1. Since most cheap meters are just VSWR meters, this could easily be the problem. The 'V' part of VSWR is voltage standing wave ratio, so if both wave voltages are in phase and at the same amplitude, the ratio is zero. This is why I really, really hate Firestik's recommendation of using an 18' long cable. The wavelength of a CB carrier is about 36', so 18' is 1/2 wavelength. They tell you to do this so that you always have a low SWR but it also masks any feedline issues you have. Let me be perfectly clear here, if you have a solid, clean installation and a well tuned antenna, the length of coax you use between the radio and antenna simply does not matter.

I like your answer Dave, I just don't understand it. :lmao: I do remember an on air discussion that I had with Jeff Zepp regarding the subject of cable lenth on Slaughterhouse one day. Actually, I do understand the part about having a SOLID connection, meaning having an antenna connected to the lead that connects to the radio. 10-4 good buddy!! :thumb:

Ron Helmuth
04-14-2009, 08:51 AM
good thread guys- very helpful to see what others have done (and wrong)

Groucho
04-14-2009, 09:55 AM
I like your answer Dave, I just don't understand it. :lmao: I do remember an on air discussion that I had with Jeff Zepp regarding the subject of cable lenth on Slaughterhouse one day. Actually, I do understand the part about having a SOLID connection, meaning having an antenna connected to the lead that connects to the radio. 10-4 good buddy!! :thumb:


Just to be anal...

What Dave is referring to is that in some rare cases if you have a piece of Coax that is just the right length it will appear to be an antenna to the radio and the VSWR meter. That is what Dave said will falsely give you a good SWR reading--Simply that the coax is acting as the antenna itself and the results are that the forward and reflected power give a ratio of 1:1 or close, but the coax is by no means radiating any signal to the air.

In the example of the garden hose representing the coax and the nozzle representing the antenna with water flow being the energy trying to get into the air (electrical or water), in this rare case there is almost no water going out of the hose because the hose diameter is enormous and doesn't fit around the nozzle, so it leaks everything out. Sure, there is a little water running through it, but it won't go very far from the end of the hose. And even if you were to throw massive amounts of pressure (more watts of power for the radio's signal), it still wouldn't do much. Imagine a tube the size of a subway trying to (without reducing) put water into a regular garden nozzle sprayer. There would be so much loss it ain't worth it.

Just another anomaly that pertains to all RF and coax, CB and HAM alike.