| Author |
Message |
    
Ohiobiker
| | Posted on Monday, April 08, 2002 - 9:36 pm: | 
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Ok guys here is what I want/am going to do...and it is becoming a hassle to see wattage change due to coax lenght. I want to run a switch box allowing me to switch between 3 different transcievers. I am running a Yaesu FT101Z, a Cobra 2000, and a President HR-2510. I understand the "flow of things as how to set this up....But what specific lenghts of coax should I use to achive the most reliable and honest wattage readings?... Do the lenghts of the coax need to be the same lenght from each transciever to the switchbox? The set up is as follows....All Three radios are in to the switchbox then the switchbox goes to my Palomar300A, from the amp to the PDC600LP meters, from the meters to my Antron-99. All my grounds are in place and everything is set up ready to go...I just want to know what lenghts of jumpers to use for most reliable meter readings? In the past I have heard and used 9ft lenghts. But this seems like a lot of coax to have behind my desk. Also if I use a "wire loom" since Only one radio will be transmitting at a time would this be a problem having them all that close together in one cable loom? and of course the coax to and from the amp would not be included in the "wire loom" of the transcievers. The Palomar is not close to the rest of the set up...it is consealed in a small cabinet away from the rest of the transmitters. I have it this way for several reasons...mainly to keep RF feed back away from the rest of the equipment. The amps tubes are old and tired. Will this in anyway affect the wattage reading of the transmitters when it is not being used or turned on? I know there will be many different opinions on this. And I will evaulate them all and then make my attempt to get it right on the 1st try.(hate making jumpers only to have to unsoldier and re-make them)...Well Gentlemen, Do we all have our thinking Caps on?...LOL....Thanks for your help and time...OHIOBIKER... |
Tech181
| | Posted on Monday, April 08, 2002 - 11:59 pm: |
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Ohiobiker,
Well you're destined to get 30 different responses to this question, so here's mine.
I have always used 3 foot jumpers, and I myself have found my metering to be accurate. They seem to work well for me.
I also use RG-8X for the jumpers because it's more flexible than using the 213 or what have you, and easier to use in close quarters.
Steve
Tech181
Tech181@copperelectronics.com |
USA194
| | Posted on Tuesday, April 09, 2002 - 1:16 am: |
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The coax length fools your SWR meter into thinking that your SWR is different to what it actually is.
The ONLY way to know what your SWR actually is, is to use a correct length of coax for the operating frequency
In order to get the TRUE SWR of the system, you have to throw away that 3-foot jumper cable for now.
Here is how to figure out your true 1/2-wave:
492 x (Velocity Factor) / Frequency (MHz)
For example, I want to figure out the true half wave coax length for RG-59/U (foam) on my home channel (ch. 33 - 27.335):
492 x .79 / 27.335 = 14.219 feet
Now add 13.52 to itself to determine your 1/2 wave multiples. Remember to use every other number. See the example below:
14.219 feet 1/2-wave multiple, 28.438 feet 1-wave multiple, 42.657 feet 1/2-wave multiple, 56.876 feet 1-wave multiple, 71.095 feet 1/2-wave multiple, 85.314 feet 1-wave multiple
Use only the lengths that fall on the 1/2-wave multiples and you will be all set.
The SWR meter has to fall on a 1/2-wave point on the coax run. Using the example above, you need a 14.219 foot jumper from the radio to the SWR meter, and a 1/2-wave multiple length from the SWR meter to the antenna. If my antenna is 65 feet away from my radio, I need a 14.219 foot jumper from the radio to the SWR meter, and a 71.095 foot length between the SWR meter and the antenna.
Now, you have to ask yourself one thing... Is it worth all this trouble to find the TRUE SWR? |
bruce
| | Posted on Tuesday, April 09, 2002 - 7:37 am: |
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i agree with 181 3 foot jumper should do the trick and like him i use rg-8x good coax for hf/vhf... USE GOOD COAX A MUST ! |
bruce
| | Posted on Tuesday, April 09, 2002 - 11:09 am: |
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usa194 you are correct however very short jumper length almost always work just fine the reason that 1/2 wave mult are needed is because a VSWR meter works on VOLTAGE and VOLTAGE is zero at 1/2 and a all mult of 1/2 wave but thats a 12 foot hunk of non foam coax now at vhf/uhf even the 3 foot wire can make readings useless |
Znut
| | Posted on Tuesday, April 09, 2002 - 6:22 pm: |
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Good Evening Folks!
Would we even be pondering this issue if UHF connectors had a specific impedance? (by the way, they don't) If it was a true 50 ohm interface, we wouldn't have to consider cutting the coax to 0 or 180 degrees in phase. I don't worry too much about it anymore because I might use the same coax on different bands.
I've seen folks change the jacks on their rigs to "N" type connectors, which are somewhat similar in size (exact same mounting for the 4 hole jack that goes in the radio) and reasonably priced. You can get them to fit any coax (no, "N" type and "UHF" type conn.s WILL NOT MATE).
Maybe the manufacturers should consider leaving the stone age and install 50 ohm jacks (N types?) at the factory in equipment. Some new rigs by Icom, Yeasu and Kenwood have "N" type jacks for the 440mhz and 1200mhz bands. I've seen VHF, UHF, and even HF antennas with them!
Just wanted to chime in with that. Take care guys.
Randy |
bruce
| | Posted on Tuesday, April 09, 2002 - 9:10 pm: |
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not a bad idea |
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