Why would a base antenna check good with a SWR bridge 3 feet under the antenna followed there after with 75ft of coax to the radio ,but then check in the red at the radio ? Hmmm
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SWR HAPPENINGS ?
- Thread starter Switch Kit
- Start date
G
Guest
Guest
So you are reading the SWR at the radio end and it is bad, but if you move the meter out to within 3' of the antenna the same SWR meter reads good?
so all your doing is moving the meter and adding a three foot jumper to read SWR at the antenna?
so all your doing is moving the meter and adding a three foot jumper to read SWR at the antenna?
thats correct Marconi , but I can't believe 3 foot jumper on a IMAX 2000 makes it go from 3.0 at the radio to a 1.3 under the antenna 30 ft up in the sky......it's somewhat strange yes ? The radio is still talking and talking well. I suppose I've seen some strange things in my time .....coax checks negitive for shorts and is basically brand new.....is this a Honest question ? .......You bet'cha !! ....if the coax was shorted on either end (which I checked ) How could it not be shorted period ? ......I suppose further investagations could warrent a solution ? .....thanks
1. Well I have purchased runs of coax before that had a factory mended patch in the coax, and there was an SWR bump in the line when running RF thru it. In this case is was pretty easy to find, because there was a modest bump in the coax that you can feel by hand. I returned the coax for replacement and that was it. Most reputable dealers will check a run for just such occurances, because manufactures do make mends when the run is short or gets damaged somehow during production.
2. If you can't find a physical bump then check the line for thru put, using a good dummy load and a meter at the far end. Depending on the quality of the line, at 75 feet you probably should show with 100 watts in maybe 90+ watts out if RG/8, and maybe 80-90 watts thru if RG8X. Smaller line will show less, but it really depends on the quality of the line.
Personally I doubt it is no 2, because whenever I have found a line that showed little thru put, it ordinarily would show very little SWR as well, when checked for SWR with RF thru the line. Your line is not showing that, (attenuation.) In these cases it was always deterioration due to long time exposure to water in the coax that had started the copper shield to oxidize and turn green or black. In setups like this the resistance was increased for the whole line and made the antenna appear unusually broad banded as well when it should not show this tendency.
It could also be a transformer affect due to the line that just so happens to show such a SWR at the end of 75'. You can check that by adding about 10' to the line length using a barrel connector and a 10' jumper. Then check SWR and see if it changes with the line connected to your antenna. Don’t use a dummy load in this case, because the condition is usually magnified, as the match in the antenna system gets worse, farther away from resonance.
2. If you can't find a physical bump then check the line for thru put, using a good dummy load and a meter at the far end. Depending on the quality of the line, at 75 feet you probably should show with 100 watts in maybe 90+ watts out if RG/8, and maybe 80-90 watts thru if RG8X. Smaller line will show less, but it really depends on the quality of the line.
Personally I doubt it is no 2, because whenever I have found a line that showed little thru put, it ordinarily would show very little SWR as well, when checked for SWR with RF thru the line. Your line is not showing that, (attenuation.) In these cases it was always deterioration due to long time exposure to water in the coax that had started the copper shield to oxidize and turn green or black. In setups like this the resistance was increased for the whole line and made the antenna appear unusually broad banded as well when it should not show this tendency.
It could also be a transformer affect due to the line that just so happens to show such a SWR at the end of 75'. You can check that by adding about 10' to the line length using a barrel connector and a 10' jumper. Then check SWR and see if it changes with the line connected to your antenna. Don’t use a dummy load in this case, because the condition is usually magnified, as the match in the antenna system gets worse, farther away from resonance.
Thank you very much for your time Marconi , much appreciated. DAMN !!! IM just going to have to get my fat a$$ back up on that 30ft roof again to the chimminy (IM getting to old for this $hit !!) This is my 3rd IMAX 2000 I have dealt with and never had this problem before , as soon as I hear IMAX ....I know that 99.9% of the time it's a piece of cake ....I even put those in trees camoflaged from anyone seeing them some 20ft up the center of the tree with a 1.1 match there after. When it's a house with 3 different layers of roof to get where you need to be on a major slaint .....these 46 year old legs can get a little bit shakey )-: .......It's really bothering my conscience all the same ....because I know things need to be right. As I said ,the radio is talking very well , from 12 miles away from base station to motor car the base hits me 5 to 7 with loud and proud audio ,IM guessing if I were a base antenna that would be 7 to 9, .....Even with a short somewhere ? ......could an antenna still work that well ? When getting radio checks off the base I was doing that 20 miles away as a crow flys with great radio checks back.......I will do some more checking as you suggested Marconi )-: It's always been a habit for me to always check the antennas from the bottom with a SWR bridge once I have then up ,or at least up as far as I can reach before they go up any further......this situation is surely a first for me , and as far as coax goes , and what I might be up against ? ...I always check it for continuity / shorts before I even get started , especially when the install can be life threatening , the less I have to move up or down ,the better off I am. (-: Thanks again Marconi
What it sounds like is that the feed line is 'adding' enough reactance to change the SWR reading. That means that the antenna and feed line are not of the same impedance, and the 'usual' methog of 'curing' that is to retune the antenna's matching system (if it has one) or the antenna it's self. Is the feed line/coax bad? Not necessarily but certainly possible.
- 'Doc
- 'Doc
Sounds like lossy or bad cable. What percentage of braid coverage is it?
CW, if it were just very lossy cable and was due to deterioration or contamination, then I believe Switch Kit would be seeing a very good SWR at the end of his 75' of coax instead of 3:1. Of course that has to rule out a physical break in the integrity, like crushing or a cut.
I had an antenna where the line was saturated with water over a long period of time until it started to discolor inside the clear coax cover and I could see it going on. In that case a check using a dummy load with a meter and a 10 watts input only showed about 1 watt getting to the dummy load. Prior to its removal the antenna worked find, but with low output signal. It showed no SWR, and the bandwidth curve for the antenna appeared to be exceptionally broad, like from 20 to 10 meters. Just as a matter of record the ears on that antenna system was unusually quite relative due to ambient noise and I seem to never have any problem reading a weak signal. In fact at times my station would hear things that even a close neighbor or two could not. I would have never suspected that in such a condition either. I think this type of lossy cable will likely react that way again and I do not believe this the kind of problem Switch Kit has. So the term lossy will have to be specified as to reason or type of loss I would think.
Just my opinion.
I had an antenna where the line was saturated with water over a long period of time until it started to discolor inside the clear coax cover and I could see it going on. In that case a check using a dummy load with a meter and a 10 watts input only showed about 1 watt getting to the dummy load. Prior to its removal the antenna worked find, but with low output signal. It showed no SWR, and the bandwidth curve for the antenna appeared to be exceptionally broad, like from 20 to 10 meters. Just as a matter of record the ears on that antenna system was unusually quite relative due to ambient noise and I seem to never have any problem reading a weak signal. In fact at times my station would hear things that even a close neighbor or two could not. I would have never suspected that in such a condition either. I think this type of lossy cable will likely react that way again and I do not believe this the kind of problem Switch Kit has. So the term lossy will have to be specified as to reason or type of loss I would think.
Just my opinion.
CWM,
You're right, it could be because of deterioration, but that's a very 'long shot'. Unless the stuff was lots of years old, or it was abused in some way, or was some really odd brand/style, it's just not something that happens very often. Even with abused coax, it's only an occasional thing. Of course, theres some really 'odd' stuff being sold so I won't say it's not possible...
- 'Doc
You're right, it could be because of deterioration, but that's a very 'long shot'. Unless the stuff was lots of years old, or it was abused in some way, or was some really odd brand/style, it's just not something that happens very often. Even with abused coax, it's only an occasional thing. Of course, theres some really 'odd' stuff being sold so I won't say it's not possible...
- 'Doc
Marconi,
What you said about what happened with your coax, "and the bandwidth curve for the antenna appeared to be exceptionally broad", is also one of the reasons some antennas seem to tune like a 'breeze', and work from day light to dark... resistance. Also, the attenuation (both directions) is why some antennas seem to be able to hear snails hicup at humongus distances times. Not that attenuation is always a good thing... just sometimes.
- 'Doc
PS - I never like using an attenuator or turning the RF gain down, but them things are usually on a radio for a good reason.
What you said about what happened with your coax, "and the bandwidth curve for the antenna appeared to be exceptionally broad", is also one of the reasons some antennas seem to tune like a 'breeze', and work from day light to dark... resistance. Also, the attenuation (both directions) is why some antennas seem to be able to hear snails hicup at humongus distances times. Not that attenuation is always a good thing... just sometimes.
- 'Doc
PS - I never like using an attenuator or turning the RF gain down, but them things are usually on a radio for a good reason.
Yup 'Doc, I believe this idea needs to be understood a little better by a lot of folks. You are right, that got me to really thinking about what is involved with losses. I saw a reality that really seemed to fly in the face of common wisdom of BS which ever it was.
To have a lossy run of coax not conjur up all the worst things one could imagine about a bad line was like a paradox to me. I just could not figure that one out in light of what I would likely hear about such an incident, so back to studying. I sure wasn't about to forget about that one until I understood it.
Simply stated, in this business if you get things more better than they should be, then you likely have losses as a buddy.
To have a lossy run of coax not conjur up all the worst things one could imagine about a bad line was like a paradox to me. I just could not figure that one out in light of what I would likely hear about such an incident, so back to studying. I sure wasn't about to forget about that one until I understood it.
Simply stated, in this business if you get things more better than they should be, then you likely have losses as a buddy.