emtunderwood
Member
- Joined
- Dec 8, 2007
- Messages
- 59
325ft of feedline and 2 10ft jumper on each end.
Antenna Help!!
- Thread starter emtunderwood
- Start date
- Status
Ubbe
Member
I normally see return losses around 25-30dB at best with 7/8 and 1 1/4 coaxes and any curve of the coax and tiewraps tighten too much make dents up in the 20dB level and if a connector of a jumper cable goes above 20dB it's usually a problem.
I once had a problem with a 400Mhz link that went dead between towers and when measure the SWR I had 5w out and 1w back from a 100ft RG214 coax. It was super cold like -20c and I climbed the tower to inspect the antenna connector and the center pin of the coax had retracted due to the cold. It was a N connector that could be screwed apart and reused but I couldn't melt the soldering of the center pin due to the cold. I had to use a crimp on connector. But the shear lenght of a lossy RG214 coax masked the SWR so it only indicated a small problem and not a total breakage of the signal to the antenna.
The lenght of your coax probably makes all readings look better than they are. I've never seen values as low as -50dB and -60dB and those vallyes down to -70dB looks strange. The antenna at -10dB to -20dB looks normal to me.
Are the instrument set to the correct coax type and calibrated just prior to the measurement?
/Ubbe
I once had a problem with a 400Mhz link that went dead between towers and when measure the SWR I had 5w out and 1w back from a 100ft RG214 coax. It was super cold like -20c and I climbed the tower to inspect the antenna connector and the center pin of the coax had retracted due to the cold. It was a N connector that could be screwed apart and reused but I couldn't melt the soldering of the center pin due to the cold. I had to use a crimp on connector. But the shear lenght of a lossy RG214 coax masked the SWR so it only indicated a small problem and not a total breakage of the signal to the antenna.
The lenght of your coax probably makes all readings look better than they are. I've never seen values as low as -50dB and -60dB and those vallyes down to -70dB looks strange. The antenna at -10dB to -20dB looks normal to me.
Are the instrument set to the correct coax type and calibrated just prior to the measurement?
/Ubbe
emtunderwood
Member
- Joined
- Dec 8, 2007
- Messages
- 59
yes, the sweep was calibrated and set to the coax prior to sweeping
emtunderwood
Member
- Joined
- Dec 8, 2007
- Messages
- 59
What should the readings look like?
Ubbe
Member
The last SWR picture are too waverly with too much SWR, expecially at the end of a 400ft coax. I usually see that when an antenna are covered in ice.
The second picture with the return loss usually are at a -30dB level and each tiewrap and curve of the coax in the tower would make a bump upwards and the connection between feeder and jumper cable always makes a bigger bump like 3-5dB. You have some strange bumps going the other direction downwards.
The analyzer sends out a pulse in the frequency range or sweeps over the whole frequency range depending of the mode and receives what are reflected back. So any strong RF signal in the location could desense or overload the receiver to make the readings unreliable. Also SWR would create some false readings in other modes.
/Ubbe
The second picture with the return loss usually are at a -30dB level and each tiewrap and curve of the coax in the tower would make a bump upwards and the connection between feeder and jumper cable always makes a bigger bump like 3-5dB. You have some strange bumps going the other direction downwards.
The analyzer sends out a pulse in the frequency range or sweeps over the whole frequency range depending of the mode and receives what are reflected back. So any strong RF signal in the location could desense or overload the receiver to make the readings unreliable. Also SWR would create some false readings in other modes.
/Ubbe
Exactly. Overload the spectrum analyzer or SWR meter.
- Status
