RF Interference from FM mast

[omega1]

Hi,

We share a tower with a local FM station and have been obviously running into problems with our equipment which is due to RF interference.

We are using an AP in client mode to receive from our base, and another AP in AP mode to 'rebroadcast'. During some tests we managed to get the FM station to turn their equipment off for a few seconds and noticed huge improvements in the system. There is clearly RF interference from this equipment. The equipment is on the 2.4Ghz band, and the interfering RF is from commercial FM equipment.

The questions are as follows:

What could we do, apart from separate the aerials as much as possible and move our equipment, to minimize this interference?

I have heard of filters that can be inserted to lower the RF interference, does anyone know any more info on these items?

Who could explain more on this type of interference and what problems we will encounter, maybe someone has experienced this before.

Thank you in advance.

[M/Q]

This is a very common and usually difficult issue to resolve. I assume that the FM station is primary on the tower? Also, if you want any advise at all, you will need to supply details, even ones that you consider insignificant. Just to start it of,

1. Frequency of FM station,
2. Channel you are using.
3. Location of your antennas
4. Physical relationship to FM antennas
5. Polarization of your antennas
6. Polarization of FM antennas (I assume it is vertical)
7. What type of antennas
8. What is the EIRP of the FM station?
9. What is your EIRP?

I would venture a guess that the FM station is on a near harmonic and it will be very difficult to improve this situation, without the use of filters. But filters are not always a good answer either, as they have a significant effect on the signal structure and tend to dampen all frequencies. Good ones are very expensive as well. I have only worked with this supplier a few times yet they were able to help.

http://www.wimo.de/cgi-bin/verteiler.pl?url=wlan-filter_e.htm

I still would exhaust all other options first.

[omega1]

Hi M/Q,

Thanks for your answer. I will try and answer all your questions in order for you (or anyone else) to possible help further. The FM is primary on the mast, it is the radio stations mast, we were hoping to rent space i¡on it. There are two FM stations on the mast, in amswer to your questions, one set of answers for each FM station:

1. Frequency of FM station,
91.10Mhz

2. Channel you are using.
Channel 3 (2.4095 GHz - 2.422 GHz)

3. Location of your antennas
In between the two FM antennas, approx 1 metre apart from either antenna

4. Physical relationship to FM antennas
unsure of question

5. Polarization of your antennas
24dbi grid dish

6. Polarization of FM antennas (I assume it is vertical)
vertical

7. What type of antennas
vertical ground plane

8. What is the EIRP of the FM station?
200w

9. What is your EIRP?
400mw

2nd radio station

1. Frequency of FM station,
96.20Mhz

2. Channel you are using.
Channel 3 (2.4095 GHz - 2.422 GHz)

3. Location of your antennas
In between the two FM antennas, approx 1 metre apart from either antenna

4. Physical relationship to FM antennas
unsure of question

5. Polarization of your antennas
24dbi grid dish

6. Polarization of FM antennas (I assume it is vertical)
circular

7. What type of antennas
circular polarization dipoles

8. What is the EIRP of the FM station?
250w

9. What is your EIRP?
400mw

Thanks.

[M/Q]

I have a few more questions.
1. Are both FM antennas vertical ground planes and at the same height?
2. Are you saying that your antennas are at the same height as well?
3. You mentioned two 802.11 devices, are both using dish antennas?
4. What is the physical relationship between the two 802.11 antennas?
5. Can you locate your antennas at a different height?
6. Do you have access to a quality spectrum analyzer?
7. Are both 802.11 devices using the same channel?

I would try to determine if there are any nulls in the FM antenna radiation pattern. It is difficult due to the close proximity, but doable. Since the FM devices are primary they get the top of the tower, I would move my equipment lower, to be out of the FM antenna's ground plane affect. Lastly I would be very picky as to the relationship between the two 802.11 antennas and certainly not use the same channel on both.

[omega1]

Hi, thanks for your answer, in response to your questions:

1. Are both FM antennas vertical ground planes and at the same height?
No, there is one vertical gorund plane at the very top of the mast, and then two circular polarized dipoles half way down, our only antenna, a 24dbi mesh dish is in between these two boradcast antennas

2. Are you saying that your antennas are at the same height as well?
as above, our wifi antenna is in between the top vertical ground plane and the top ciruclar pol. dipole.

3. You mentioned two 802.11 devices, are both using dish antennas?
no, sorry, my bad, we only have one

4. What is the physical relationship between the two 802.11 antennas?
as above, none

5. Can you locate your antennas at a different height?
yes, but only slightly further down either in between the two circular dipoles or right at the bottom, loosing LOS.

6. Do you have access to a quality spectrum analyzer?
afraid not.

7. Are both 802.11 devices using the same channel?
only one device.

thanks!