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  #11  
Old 03-27-2009, 05:37 PM
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DaveInDenver DaveInDenver is offline
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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.
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  #12  
Old 03-27-2009, 05:44 PM
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Originally Posted by DaveInDenver View Post
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. 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!!
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  #13  
Old 04-14-2009, 08:51 AM
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good thread guys- very helpful to see what others have done (and wrong)
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  #14  
Old 04-14-2009, 09:55 AM
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Originally Posted by Crash View Post
I like your answer Dave, I just don't understand it. 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!!

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.
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