900 MHz Antennas and Vertical Seperation

[Sloan1]

Ive got a question in regards to two identical radio systems at one site, interfering with each other.

We have temporarily added a Point to Multi point radio at the same location as another. Both are in the 900 MHz range and only .0125 MHz separates them. Unfortunately due to space, the antennas are only about 6 foot apart and at the same height. Both radios are low power (5 watts). Each radio system does have bandpass filters but that is really just to filter out the Paging systems on nearby frequencies and cant be tuned down enough to filter out something only .0125 MHz away.

Upon turning on the second unit and keying it up, we saw some significant degradation in signal levels on the original site and its remotes.

Is it the fact that the antennas are too close together that is the root cause, no vertical separation, or that they are so close together in frequency?

I have suggested that we try and raise the height of one antenna by several feet, hoping this could smooth things out. Some had mentioned adding a notch filter to one of the radios to filter out the other radios transmitting frequency.

What are your thoughts? Appreciate any responses.

 

[SCPD]

stack

Ive got a question in regards to two identical radio systems at one site, interfering with each other.

We have temporarily added a Point to Multi point radio at the same location as another. Both are in the 900 MHz range and only .0125 MHz separates them. Unfortunately due to space, the antennas are only about 6 foot apart and at the same height. Both radios are low power (5 watts). Each radio system does have bandpass filters but that is really just to filter out the Paging systems on nearby frequencies and cant be tuned down enough to filter out something only .0125 MHz away.

Upon turning on the second unit and keying it up, we saw some significant degradation in signal levels on the original site and its remotes.

Is it the fact that the antennas are too close together that is the root cause, no vertical separation, or that they are so close together in frequency?

I have suggested that we try and raise the height of one antenna by several feet, hoping this could smooth things out. Some had mentioned adding a notch filter to one of the radios to filter out the other radios transmitting frequency.

What are your thoughts? Appreciate any responses.

I would try putting one vertical directly under the other on the same mast, try to keep them at least a wavelength apart. I'm not sure this may work, because of the feedline from the top may get induced by the antenna on the bottom, but this would be the first thing I would try.

 

[davidgcet]

are these licensed band? if so your coordinator should be shot. if unlicensed, try changing channels to further seperation.

i have 2 900mhz ptp links on licensed band with grid antennas back to back at the same height. the coordinator got it so both TX at say 957 and rx at 954(not the exact freqs i can't remember them right now) and never had an issue. but if they have you as close as you say then they screwed up the license.

edit: is is possible to set one to horizontal polarization and leave the other vertical? this will also add some padding to the interferencing signals.

 

[Sloan1]

Frequencies are licensed. The thing is, for the opertating area, they only had a handful of frequencies. Unfortunately, these two are too close together, one is .88125 and the other is .89375.

There isnt a lot of room where these two antennas are so there dont seem to be a lot of options. Im thinking, even if the frequencies were a bit futher a part in frequency but still that close together, that may still cause a problem but Im not sure.

 

[RKG]

RFS Celwave offers a free application ("CelCalc") that includes a routine for calculating the isolation between two collinear vertically separated antennas. Unfortunately, it isn't on the laptop I have with me, so I can't give you a figure; from memory, however, 6 feet isn't much.

 

[freddaniel]

Is each point-to-multipoint radio using a duplexer on their respective antennas? If so, then the solution is to simply use one of the two duplexers, and one of the two antennas. Combine the two transmitters with a hybrid combiner (3 dB loss) and the output goes to the TX side of the duplexer. the receivers can be combined with a splitter (and preamp if you want to recover the 3 dB splitter loss) and connected to the Rec. port on the duplexer. This will work because BOTH radios are within the bandpass of the duplexer.
If you really need more performance, eliminate the duplexer and use separate antennas, with at least 10 ft vertical separation, in place of the duplexer.

 

[louisiana_jeeper]

My gut feeling and technical opinion is freqencies are to close for a common site. I also do not have confidence in cans of any sort curing. Now if push came to shove combine the suggestions here use cans and cross polarize antennas. Memory says 20+ db rejection (Number in my head is really 27db) on cross polarized but my memory is not always correct so look it up.

If you want to try for an experiment hook one radio to a mobile antenna at ground level and retest system. That would give more seperation of antennas and also would be a lot easier than messing with things in the air.

 

[freddaniel]

I guess I should have said the problem appears to be blocking, due to antenna proximity.

Since they are point-to-multipoint, these frequencies would fall under FCC Part 101.147(b)(3).

Also, these must be PAIRED DUPLEX frequencies, not simplex. Sadly, the author just did not say.

If this is a MASTER (REPEATER) site for both radios, Radio A would be required to TX at 959.88125 and Rec at 928.88125, while Radio B would TX at 959.89375 and Rec at 928.89375 MHz. If both radios are REMOTE sites, then the frequencies are reversed on both radios. If one is master and the other is a remote, then it is really a tough fix, but it can be done with phase cancellation.

Complete and accurate information is always necessary to start with.