Strange Signal Behaviour

We initially built a tower site with (2) 900 Mhz 60 degree sectors to feed 3 SM’s close by (2-3 miles). After being dissapointed with the poor signal levels -85- 82 -87 we then changed the sector antenna’s to a Omni. Immediately upon installation the levels dropped to -50’s and -60’s where we would expect. After running at this level for a period I logged back in one day to find the signal level once again down in the mud. We have since then tried a different Omni (no change) moved the antenna from the top of the tower to the side (stand off), and found no improvement. We have tried different coax and use LMR400. Any ideas?

Just curious what is the length of the coax?

Do you use a watt/swr meter to measure the power output at the antenna, and SWR at the radio?

What’s the spectrum like in the area? Do you see any other Canopy gear in Alignment mode?

Check the AC voltage - make sure it’s 110VAC. Low AC voltage will make the AP’s act erratically.

The coax length would be approx 40 ft. I am not familuar with the SWR / watt meter. Definately willing to try anything. Do you connect this meter at the radio and record and compare the output to the connection going into the antenna?
As for the spectrum, we did switch the AP to an SM to enable the analizer and we see on average the max in the 900’s range around -80. Is this what you mean in “alignment mode”? I dont suspect any other gear in this area as it is very remote??
And regarding the power we are powering the Canopy radio over ethernet? I thought this was low voltage dc? It’s all new to me and thanks for your thoughts!

40’ of LMR400 with 2 connectors is about 3dB of loss which would not explain the 20 to 30dB drop you describe.

Sealing connectors is critical. If you did not seal well it is possible that water has seeped into the connector. This would easily explain the symptoms you describe.

The POE injector gets power from the wall…Measure the voltage at the plug that the POE injector is plugged into. If it’s less than 110 it could indicate a problem with the AC circuit.

Strangely enough however because we had panel antenna’s originally we do have two separate runs of coax, (just installed in 07) and have tried the other coax.
I will indeed check the ac power going into our poe equipment.

If the site is so remote, is there any reason why you are using 40+ feet of coax to go from the radio to the antenna? Just wondering… i’m in a remote area and always mount radios next to the antenna and just run cat5 up the tower from a weatherproof box below. Guess we’re not worried about theft.


Your issue sounds like interference in the 900 spectrum. It also sounds intermittent. What sort of signal levels do you see at the SM side as opposed to the AP side? It’s always good to look at both ends. A solid readout at the SM doesn’t always mean a solid readout at the AP and vice-versa.

I would run more spectrum analysis at the AP and really take note of fluctuating levels in any freqs more than 4 up and 4 down from whatever freq you have the AP set at. example would be you have AP set at 919 and you’d expect jumps from 915-923 but anything out of that range would be noise/usage of another freq and could be the cause of your issues.

Also, did you check for any 900 related devices at customer ends? Mainly old cordless phones? Custs will say they do not have, but they do. Unplug and remove batteries from all 900 phones.

You can always try to use one the hidden 900 freqs (like 900-906 ?) to see if the problems go away. You can enable these by telneting into your AP and SM. Type ‘help’ … there is a flag you can set.

My two cents


As Jerry noted 40ft of LMR400 with connectors in line is probably between 2 and 3db loss…or between 1/3 and 1/2 power loss to the antenna.

SWR is “standing wave ratio”, or VSWR "Voltage standing wave ratio"

In simple terms … SWR compares the maximum amount of voltage or current that can be delivered by the radio to the minimum voltage or current that actually leaves via the transmission line and antenna.

At 900mhz you must keep the SWR as low as possible. Not over 1.5/1

40ft of coax, and probably a SWR of 2.0/1 only 1/3 of the power output of the radio would be transmitted by the antenna.

I appreciate the feedback very much. We will move the radio and polyphaser to the top of the tower…check our voltages and do some more testing with the spectrum analizer tomorrow.

Have you always had polyphasers in there?

Try it without to see if there is a change.

Yes we have had the polyphasers installed since day one, however we have tried removing them…??

Yesterday we moved the radio to the top of the tower-eliminating all coax. Antenna was also moved to a new mount which give us an additional 2 ft of height.
Tested voltages input to the RT (POE Device) 123 vAC.
After running the internal spectrum analizer we moved the freq to 920 which was best (averaging -87).
The three subscriber radios prior to the work performed yesterday were running signal levels of -80 -76 -86.
After all work was completed those same readings went to -84 -68 -86.
(pulling hair out)
Today we did try to go in to see in telnet how you change the freq to 900. According to the analizer this would gain us approx 1-2 db which we know isn’t enough, but we are trying to get the best gain we can? What command in telnet enables the 900 to 906 on both AP’s and SM’s?
Any Ideas?

Just a thought when reading another users questions today. What does this Omni antenna do for us as a vertical polorazation? Help? Hurt? I see that someone else recommends MTI sectors and to “Make sure they are horizontal”. Do you all see our omni and vertical as a bad thing?

Who is a good vendor to buy the MTI 900 90degree sector antenna?

We purchase all of our MTI products through wincomm. They take a while to ship them but have always been reliable and a good price.

What types of antennas are at the customer end? How far away is each customer? What kind of tree cover (density, tree type)? What is the elevation difference between the tower and customers?

I have found that when we have had to move a 900 antenna on a tower even a few feet that it can have drastic effects on a customers signal level (dBm). Also signal level is rarely affected by interference noise. Interference will have a bigger effect on jitter. To see a change of 20 dBm you have to have a LOT of physical interference change, like dense trees getting wet or something like a vehicle or building in the way.

Integrated SM’s are Horizontal.

If your AP antenna is Vertical you are going to see a -20dB drop in signal level.

We do have vertical polarity on our Omni. However today we installed a yagi antenna and replaced the 12 dbi panel antenna and gained almost 30db. :slight_smile: FINALLY Thanks for your input