Rf Humming During Cell Phone Call

I have been working with mobile communication for sometime now and this is the first time i face a problem like this.

I fix a repeater for a room which had very weak network initially.After installation the network jumps up from -90db approx to -60db.

making and receiving calls get easier but when a call is made iof recieved there is a continuous hum in the background which the other person can also listen to this noise.

the moment i come out of the office and call from the place where i had fixed my pickup antenna,the hum is not there when i call or receive calls.

Initially i thought it is due to the cable running from near high voltage cables but then its a 900Mhz repeater and the other 2 operators on it dont face the same problem,only this particular network is having it which sadly belongs to the top boss working in that office

Not sure about hum, but is there a sporadic bzz-bzzt kind of sound when the phone is nearby? That's GSM modulation leaking in somewhere.

Can try ferrite beads, shielded cables, that sort of thing??

Tim

this sound is almost identical to the one we hear on landline phone when a cell phone next to landline is ringing.

But this problem is seen only on one network on 900gsm while others are not showing this problem.at the same time truth is other network doesnt need a repeater,even without repeater this other network works pretty good.

obviously i was using rg 58 which is not shielded well,i am going to replace is by rg 213u and see if this problem vanishes.

But at the same time i am doubting the network behaviour of that particular operator in that cell.Can that be the culprit?

RG58 at 900MHz? There ain't nothing left after ten meters! RG213 will do a heck of a lot better carrying those signals. Check the dB/length loss specs on your cable and figure against what distance it's going from box to antenna or whatever.

No guarantee about interference, but RG-58 is fairly leaky stuff. Could be.

I'd have to know a lot more about your setup to offer any more suggestions...

Tim

Got dat right, bruthuh!

These are good people to deal with
http://therfc.com/coax.htm

#1. Faulty repeater... Swap the repeater unit out for a known working one and test...

#2. Bad Antenna.. (just learning about these now) Check all antenna connections, Especially ground connections for it.. Swap out with known working unit...

#3. Bad Cables.. AFAIK, RG58 is not really suitable for GSM, and is more for Co-ax networks and AV... Replace with something like RG174, which is for GSM and RF...

RG-58 Is kinda crappy. I wouldn't use it for anything other than CB radio stuff.
Different brands also offer better quality than others.

Initially I used some old RG-58 I salvaged from an old token ring network in a school that was decommissioned. It was terrible with RF power applied to it. Leaky as all hell. Amplifiers, radios, TVs etc would pick up the signal or receive some sort of interference.

Bought some newer stuff with 98% shield and the problem was gone completely.
My Rig doesn't interfere with anything since the switch.
Still RG-58 cable, just much better quality.

RG-58 is lossy as hell, and not good for anything above, maybe 50Mhz. You can get away with it on 6m but the losses are pretty bad. Anything higher and well, its like using RG-59 for satellite TV reception. Too much loss at higher frequencies and not enough shielding to keep out the upper UHF interference.

Better coax may be the solution. Double shielded perhaps?

Also how is the antenna matching? I had a handheld CB rig plugged into a mag mount antenna, and it cause all kinds of humming and buzzing when transmitting due to RF being fed back into the transmitter. Better antenna with a closer match solved that.

"obviously i was using rg 58 which is not shielded well"
uhhh ... i need your address so i can send you the bill for a new keyboard ...
i blew coffee all over the old one when i read this .


depending on the situation at hand (including the environmental and physical conditions) , there are a few type of coaxial cable that would work .
this is a good place to start . a link to show the differences in coaxial cables :
http://en.wikipedia.org/wiki/Coaxial_cable

yes , that does sound like "rf overload" ... too much soup in the shack , or high reflected back into the port or rf from another source due to close proximity of the antennas (sometimes physical isolation is needed) .
what is the distance between the equipment and the antenna(s) ?

ps ... personally , i do not like pre-fabbed cables .
connectorizing is an art unto itself .
crappy craftsmanship in this area will result in nothing but grief .
another consideration is "minimum allowable radius bends" , these will be listed in a good spec sheet for the particular coax used .

As others have said,RG5x at 900Mhz,there won't even be a signal at the antenna end!

Edit:
I used some old networking coax (RG58) for some FM experiments. 10Ft cable,with 1W input,I had ~500mw at the antenna. HALF the power was lost,over 10 feet! And that was only ~100mhz.

That Radio Shack RG58 stuff doesn't even make good audio interconnects