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Foxhunter
Junior Member
Username: Foxhunter
Post Number: 13
Registered: 4-2008
| | Posted on Sunday, June 01, 2008 - 11:05 pm: | 
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This is a problem that has bothered me for some time. I'm hoping for some additional input from anyone that has an informed and accurate answer. I have read "mentionings" from several sources in the past that seem to really CONFLICT. I have read that the ONLY place to get an accurate SWR reading is at the feedpoint of the antenna. Often this is done incorporating a SWR with a remote head to be placed where the feedline terminates and the antenna begins. After reading this on a number of sites I then began to notice some of the higher quality meters that are sold have this remote-head unit.
Now, to the contrary I've read, again from several sources, that mounting a remote-head at the feedpoint will only show the SWR of the antenna. It won't detect any standing-waves or problems ALONG the feedline that may affect SWR.
I am "one of those guys" who like reliable and accurate readings. Why have instrumentation if it does not display proper data? This "split" in opinion does vex me enough that I am unsure what is really the "proper" mounting location for an SWR meter. Should I consider having an SWR meter AT BOTH ENDS(?).
I have a radio--->3ft jumper--->small amp--->3ft jumper--->SWR/Wattmeter--->18ft Belden mini-8x--->Radiator/Antenna.
I am aware (somewhat) that placement of the SWR meter along the coax is critical for accuracy. Now an additional "problem I have is also the combined length of all my coax----24ft total. I am using an 11-Meter radio and I originally went with the "standard 18ft" length. Then I added a SWR meter. Then I added a small amp. I went with using the "muliples of 3" theory. I guess those are some of my initial mistakes because I didn't subtract the additional 6ft of jumpers from the final 18ft length after the SWR meter. Then I didn't consider "velocity factor", correct? With the Belden mini-8x jumpers, and considering velocity factor and 11-meters, what would your opinions of the length of the final section of coax be? Someone please give me a number, and why? Again, I am caught between measuring either 468 OR 495/Frequency. Which formula # would you use and why? What should be the total lengths? Should perhaps the jumpers THEMSELVES be shorter/longer?
May I quote (from an older Copper Forum topic) concerning my first and original question at the start of my post tonight also? Quote:
"...the reason you get different swr readings with different cable lengths is simple... draw a graph of a sine wave, a straight zero line with the wave going above and below it (just like in math class in school)
The zero line is your coax, the sine wave is the swr voltage. The higher the swr, the higher the peaks of the wave. But, at the points where the wave crosses the zero line, you will measure no voltage no matter how high the swr. So what happens is, if you cut your coax to the right length, your meter will end up on one of these zero points and will be fooled into thinking the swr is flat. In theory, you should be able to take 2 jumpers, one say 3 feet long, and another say 6 feet. put the meter in the middle of them and hook them up, short one to the radio, long one to the amp. note the swr. now swap them.. you'll still have the exact same jumper length, but the meter will be in a different spot and the reading should change, showing it's not the length of the cable that matters, but the meter placement."
The above quote is how I discovered the Copper Forum originally some time ago. I've told many other drivers I talk with how good the Forum is and the amount of information contained and the helpfulness of it's techs and members. Will some of you please address these questions? People have told me "don't worry about it" (and the like) elsewhere but I'm hoping someone will provide real information for my particular set-up as mentioned/described above, and also the discrepancy over actual meter-placement location. I don't post unless I have a real question that I'm really unsure about. Thank you for your help and patience. Foxhunter. |
Captian_radio
Intermediate Member
Username: Captian_radio
Post Number: 400
Registered: 9-2004
| | Posted on Monday, June 02, 2008 - 6:05 am: |
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If your swr changes when you make the feedline longer or shorter then there is something wrong with the antenna system and as for the swr meter, place it after the radio,amp etc for a proper reading.I never add to remove any of the length of the coax,I always adjust the antenna to make it resonant,I don't try and fool the meter,it will work better in the long run.
Bob VE1CZ/CEF451
Robert L. Spicer The days of radio are just beginning!
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Foxhunter
Junior Member
Username: Foxhunter
Post Number: 14
Registered: 4-2008
| | Posted on Monday, June 02, 2008 - 9:08 pm: |
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Thank you Captain for your reply. Well to shortly rephrase: The Meter is currently located 6ft from the Radio (with an Amp in-between) then a final additional 18ft of coax to the antenna. "Fooling the meter" is EXACTLY what I am trying to avoid. My use of 3ft jumpers between the Radio & Amp and then between the Amp & Meter totals 6ft. How would I know the correct jumper length in-between the components? Maybe they should be 1ft and not three feet each? I am just concerned that my SWR meter's placement (at 6ft from the radio) is poor. And is it really true that SWR Meter placement is to be done at the coax/antenna junction? |
Joey_migs
New member
Username: Joey_migs
Post Number: 2
Registered: 9-2009
| | Posted on Tuesday, September 08, 2009 - 5:42 pm: |
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SWR is totally unaffected by length of transmission line. The apparent change in SWR is due to the properties of the SWR meter and not the SWR itself. It is only when you use a current-(ISWR) or voltage-based (VSWR) SWR meter (which most are) that the SWR seems to be affected by line length. SWR is the ratio of impedance not the ratio of Voltage or Current. True SWR is Load impedance/Surge impedance or surge impedance/load impedance depending on which is the greater value. |
Stacy_adams
New member
Username: Stacy_adams
Post Number: 4
Registered: 12-2009
| | Posted on Thursday, February 04, 2010 - 1:33 pm: |
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In reading many posts to get an understanding of this issue, I thought I'd ask my questions in this post.
Background:
If I place an amp and a LP filter after my swr meter, I tend get a higher SWR reading than I do when I measure after the amp and LP filter; specifically, when I move the meter directly after the devices directly to the antenna feedline it comes back down flat as it was with no devices in line.
My specific questions are:
Does the higher SWR betwen the radio and the amp make any difference to the efficiency of the broadcast chain if the SWR reading at the feedline is flat?
Is there merit in trying to lower the swr between a radio and an amp with coax length? I believe all one is doing is changing the SWR reading and not the impedence. Is this an accurate statement?
Many thanks... |
Tech833
Intermediate Member
Username: Tech833
Post Number: 136
Registered: 12-2001
| | Posted on Friday, February 05, 2010 - 11:10 am: |
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Two separate issues here.
First, if you measure SWR between the radio and the amp, you are measuring the input matching network of your amp, not your antenna.
Second, coax length DOES make an SWR difference if your antenna has some reactance at the feedpoint. Changing the length of the coax changes the rotation (phase angle) at the feedpoint and can mask a slightly inductive or capacitive feedpoint. If your meter sees a lower SWR, then so does your radio. Where that SWR went? Coax loss and/or heat loss.
Your radio 'Mythbuster' since 1998
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Stacy_adams
New member
Username: Stacy_adams
Post Number: 5
Registered: 12-2009
| | Posted on Friday, February 05, 2010 - 1:24 pm: |
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Ah, ok, thanks. So does it make a difference - if the match between the radio and the amp is higher than the overall efficiency of the transmission line given the match between the amp and the antenna is low? |
Tech833
Intermediate Member
Username: Tech833
Post Number: 137
Registered: 12-2001
| | Posted on Friday, February 05, 2010 - 9:13 pm: |
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Nope. Your amp has an impedance matching section at its RF input. It matches the input impedance of the amp to 50 ohms j0 so your radio will be happy. It has almost nothing to do with your antenna, unless the amp is bypassed, of course.
If you measure SWR between the transceiver and the amp, you are only measuring the amp's matching network.
Your radio 'Mythbuster' since 1998
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Stacy_adams
New member
Username: Stacy_adams
Post Number: 6
Registered: 12-2009
| | Posted on Saturday, February 06, 2010 - 2:43 pm: |
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Many thanks - I got it now.
Regards... |
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