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Coax ?
- Thread starter brash
- Start date
I'm not an expert on antennas or coax, but myself, I use the same grade or better coax just to be on the safe side.
73s
73s
As long as the impedance is 50 ohms it does matter if you mix and match coax cable. i use LDF4 for my main runs with pigtails of Belden 8214 (RG8) on each end with jumpers in the shack of RG-8X. It's all 50 ohm cable. I note that you mentioned using 59 which i assume you mean RG-59. You cannot use that. It is 75 ohm cable and will cause a mismatch. You need to stay with 50 ohm cable.
typo, i ment 58, i know its all 50 ohm. just wondered with the diff in quality
A timely article in this month's Radcom investigated coax. They bought some really cheap RG58 from Ebay and the impedance was 100 Ohms....
I wonder what the impedance transformation would be using two feet of it as a meter jumper. As for my comment about "even the worst quality cable is fine." was referring to something like RG58 versus RG-8 versus RG-213 etc. Manufacturers is a different story. Belden and Amphenol or Times is great. Unknown Ebay specials are no so much.
Unfortunately it is impossible to predict the change in X and R without knowing what they are to begin with. However, if you are familiar with using the phase angle and the magnitude of Z for impedance measurements as well, these are the phase shifts you should expect to see...
Channel 1, a phase shift of 29.89 degrees.
Channel 19, a phase shift of 30.14 degrees.
Channel 40, a phase shift of 30.38 degrees.
If you are going towards the source (radio) from where the measurement was taken it will be clockwise, if you are going towards the antenna it will be counterclockwise.
So if you started with a reading of R = 50 and X = 50, with a 2 foot jumper connecting your analyzer to the antenna, your antenna would be very close to having an actual R = 35 and = X 35.
If I went the other way on the feedline, say I knew my antennas impedance and wanted to know the impedance on the other side of the jumper, then you would get a reading close to R = 90 and an X = 55.
Just in case you are wondering, all three examples would have an SWR of 3.4:1...
These figures are assuming an exact 2 foot section of coax with a velocity factor of .66.
The DB
EDIT: My phase changes are correct, but the X and R values calculated later are not, I missed a step... I used 50 ohm coax for the adjustments instead of 100 ohm... If I get time I'll recalculate later today...
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The author used a metre, 3.3ft, as a patch lead. It was enough to cause the transceiver to fold back the power from 100W to 50W even when connected to a known good dummy load and in the article he states the cable was getting hot.
"A measurement on the remaining 99m of cable showed a braid DC resistance of 20 Ohms and the core was a whopping 98 Ohms."
Connected to an AIM 4170 set to TDR it had a surge impedance of over 100 Ohms
I don't know why I was trying to use 50 ohm cable above... anyway...
If you have a 50 ohm source, and then you use 2 feet of 100 ohm coax with a velocity factor of .66 you will have a complex impedance of 43 - j20 (R = 43 X = -20) ohms on the other side of the jumper. If you had an antenna who's impedance was closer to 43 + j20 then you would actually have a better match than a direct feed...
The DB
If you have a 50 ohm source, and then you use 2 feet of 100 ohm coax with a velocity factor of .66 you will have a complex impedance of 43 - j20 (R = 43 X = -20) ohms on the other side of the jumper. If you had an antenna who's impedance was closer to 43 + j20 then you would actually have a better match than a direct feed...
The DB