FM Static in San Mateo
 

Here is my FM Fool Report,

http://imgur.com/6kAvp0S

I am having static trouble with 107.7 and 105.3 even while in Mono mode. I have tried a custom folded dipole tuned to 105.3 and I am now using my RCA ANT751 on my roof, which I split before it goes to the TVs but I am still getting static. Any helpful tips on the source of the static? Thanks.

FM is received best with an antenna designed for FM frequencies, 88 to 108MHz. The RCA ANT751 is designed for TV RF channels 7 to 13 and 14 to 51, 174 to 216MHz and 470 to 698MHz. The elements on the ANT751 are too short to offer any gain receiving FM stations. An antenna like the FM6 on a rotator would be better for FM radio. If you wanted an omnidirectional antenna, a FMSS would work, but it would have trouble with stations that have a co channel warning flag, because it would not offer any rejection for signals from unwanted directions.

Also, from the FM fool image you linked to, there are two strong signals on 107.7 within 66 degrees of each other. The first is KSAN, the second is KSAN-FM1. KSAN-FM1 is an on channel booster for KSAN, depending on how they are synchronized it is possible they could be adding destructively at your location.

Both stations have on-channel boosters on Mt Diablo and might be causing the interference as suggested. A directional antenna with sharp null in the direction of Diablo when aimed at San Francisco would be needed. You'd be looking at two fairly large antennas to do that, though.

The other possibility is that, since you're so close to the numerous TV and FM transmitters on San Bruno and Sutro, is that the front end of your receiver is simply getting swamped. I'd try some attenuation (10-20 dB) at the receiver input to see if you can cool things off. Such attenuation would also knock the on-channel signals from Diablo down and would remove them as a possibility.

The other possibility is simply that you put the antenna in a multi-path prone location. Relocating the antenna, of course, is the remedy.

Yeah, it was a shot in the dark tapping my ANT751 but it wasn't difficult. Previously I made a custom folded dipole optimized for 105.3 but it still had static. This was mounted indoors on the wall at the top of the ceiling and I was pretty disappointed when it did not work. The ANT751 test was just to see what the signals were like on my roof but since the static remains I think I've co-channel interference.

I think the Omni would make things worse. I thought about putting my custom folded dipole on the roof and aiming its null right at 51/43 degrees where the co-channels broadcast.

I am working on a plan-b, or I could be has high as plan-e or f, but now that it is (possibly) co-channel interference I going to think about how to compensate for that. Thanks for the reply.

I missed the co-channel for 105.3 but you are right both stations are dual broadcast and I must be getting interference from the other tower.

This is a great idea and worth a try. I'll stack in some splitters until I start to lose stations and see how it goes. At the very least it will be a good test.

Can't move the antenna...

I had good FM reception using a Y5-7-13 mounted outside. Some times having an antenna outside high up, even off band, is better than having a properly tuned antenna inside.

You should try a vertically polarized FM antenna. KSAN has 8.9 kW ERP horizontal and 8.9 kW ERP vertical. KSAN-FM1 has 0.185 kW ERP horizontal and 0 kW ERP vertical according to the FCC database. Other than signal cross polarized by bouncing off stuff, the booster should not be doing anything vertically polarized.

Channelmaster has an FM antenna with a vertical mount on it. http://www.channelmasterstore.com/Ma..._p/cm-3026.htm If you mount a yagi vertically, a fiberglass mast (or something strong non conducting) would be good to prevent a metal mast from changing the pattern.

KITS is a bit more of a mess. The FCC database shows 4 boosters for it. Of those, 1, 2 and 3 are all only horizontally polarized. KITS-FM4 is vertically polarized. KITS-FM4 does not show up on your plot, and looking at the power spectrum at the bottom of the plot, it is probably below -100 dBm at your location.

So, if I were you, I would get the channelmaster vertical FM Yagi and a section of fiberglass mast, put it up on the roof, point it at 337 degrees True, and see what happens.

I think this is my strategy now.

But similar to KSAN, KITS main tower broadcasts both H & V polarizations and all the booster stations are H polarization except FM-4 out of Antioch which is V polarized and not showing up on my FM Fool report so probably not a source of interference.

http://www.hamuniverse.com/vertbazooka.html

I think I might build something like this Bazooka antenna as a test. I can tune the lengths to 106.5 MHz (half way between my problem stations) and see if a vertical antenna cuts the static. The design uses RG-8U cable but I only have RG-6 cable do you think that would impact the design much? Also, the guy says this has 55-60 ohms impedance (assuming RG-8U) so if I made it out of RG-6 would that make the impedance worse or better for the 75 ohm down lead cable? I am assuming an impedance miss match of 60 ohm into 75 Ohm would just impact the efficiency and there is plenty of signal in my area so it won't kill the reception. Also, the input to the tuner is 300 Ohm so I have a balun on the receiver end already.

You really need some type of directional antenna to reduce /eliminate the co-channel interference. An omni will pick up both signals & more than likely not offer any improvement.

Do you have room for either of the suggested antennas?

I want to run a test to see if the polarization makes a difference. Making that Bazooka antenna costs me about an hour of my time and a piece of coax so its not expensive and I am curious if it would help with the static. According to the math a vertical antenna should only pickup the local tower. I am not sure what it will do to the rest of the FM stations.

It is a good suggestion and I do have room but I'd want to put it on the roof and I don't want to spend a bunch of money to find out that it does not work. I'd be using the same location and down lead of my TV antenna so I'd need a FM/TV diplexer, non-conducting mast, etc. so its a much bigger project than the Bazooka antenna. Plus I'd have to clear it with the landlord and it is raining so no going on the roof for now. Also, I am reluctant to mess with the TV antenna because I have it dialed in and getting (almost) all my stations. If the Bazooka antenna helps and demonstrates that polarization can help then I might proceed with a setup as you suggest.

See my reply to ADTech and his suggestion about receiver overload, it looks like there might be more than one cause at hand here. Thanks for all your help!

So I had a chance to test this idea. I didn't have any attenuation couplers so I used a DishTV diplexer. I says 900 to 2125 MHz so I figured it has more than 20db loss at FM frequencies. It knocked out a number of stations completely including one of the stations I am trying to fix, 107.7. However, I was still able to get 105.3 and it seemed to be better. Before if I had it in stereo mode it would flicker in and out of mono/stereo. Behind the Sat. Diplexer it stayed on stereo with out dropping down to mono. The static was greatly improved too, still there but much less noticeable.

One question I had regarding receiver amp overload is the first station on my FM Fool report, KCSM 91.1. I am only 1.4 miles from their towers and the Fool report says its at -9.9 dbm. Is it possible that this broadcast is overloading my receiver? I thought I read any thing under -10 dbm could cause trouble, either amp overload or harmonics.

I am also going to test a vertically polarized antenna to see if that impacts co-channel interference. See my replies to No Static At All. Thanks for you help, BTW.

Do you have access to the attic? Attics generally work well for FM reception.

Update
 

I think the main cause of the static is interference from KCSM 91.1 that seems to overload the amp in my receiver. To summarize, I split a coax line off my ANT751 TV antenna on the roof then into a balun then into the 300 ohm FM antenna input of my receiver as a test since I had static after trying various indoor folded dipole FM (T) antennas.

The key tip was from ADTech to put in 10-20db loss to rule out an overloaded amp. I didn't have an attenuator handy so I installed a Dish TV Satellite 950-2150 MHz diplexer that was lying around. I actually have it installed backwards with the antenna plugged into the output port and the balun plugged into the Sat1 and the Sat2 port capped with a 75 ohm terminator. This arrangement knocked out most of my stations, at least on auto tune, except 91.1 KCSM and one of the stations I was troubleshooting, KITS 105.3 which was dramatically improved in static level and stereo lock (used to flip mono/stereo before).

This made me suspect KCSM/91.1 as the cause. So after a fruitless search on the internet to find a notch filter or highpass filter that would do the trick, I found this PDF;

http://www.ic.gc.ca/eic/site/smt-gst.nsf/vwapj/solution.pdf/$FILE/solution.pdf

In Appendix 4 it described how to make a notch filter using an open antenna cable stub. I added a stub of twin lead cut to the correct length to the 300 ohm side of my balun and the results were dramatic. Immediately almost all my stations came back even with the high loss Sat. Diplexer in place, many of them are now rock solid in stereo, no hiss or static so it seems like my troubles are solved.

Thanks to all for your help.