Do AM radio tower sites create more RF interference than FM?

[fordranger797]

This is something that I’ve been curious about for a while. I’ve had the opportunity to visit a few tower sites over the years, but they’ve all been FM. Regardless of the ERP, it seems like FM tower sites seem to create very little interference for other devices in close proximity (or at least none that I’ve noticed). On the other hand, I had a different experience while being close to a high-powered AM site. While I couldn’t get close with my car radio to determine if the strong signal was wiping out other stations, I did notice that my cellular service was entirely absent from my cell phone.

Do AM radio sites create significantly more interference than their FM counterparts?

 

[Kelly A]

Do AM radio sites create significantly more interference than their FM counterparts?

Depends on what you mean by "interference". Close proximity to an AM station definitely can interfere with some vulnerable consumer and industrial electronic devices like: land line phones, some older radio receivers, electronic organs/keyboards, alarm systems, or devices with LCD touchscreen displays. AM is more disruptive to these devices due to the longer wavelength and higher field strength closer to ground level.
That doesn't mean that FM sites can't create interference with some devices too. Consolidated FM facilities with several high power FM stations have been known to cause RFI to some consumer devices. As mentioned through, the main RF field is by design higher off the ground than an AM facility would be, so it seems like the interference potential is less.

 

[secondchoice]

There was a rumor going around middle Tennessee radio stations in the early 1970’s that WLAC had a florescent light that stayed “on” at night in the transmitter building when they were directional at night.

FM could interfere with the old analog TV sometimes. I worked at WIKI when they were on 100.1. I rented a place about 50 yards from the studio and transmitter site.* Channel 11 Louisville (the old analog) was impossible to watch because 100.1 times 2 equals 200.2.

* It was nice to walk to work.

 

[gr8oldies]

RFI can be a constant headache at a radio station co-located with an AM transmitter facility.

 

[Kelly A]

RFI can be a constant headache at a radio station co-located with an AM transmitter facility.

I wouldn't say a constant headache. If you build a new AM site, wrangling, then solving the majority RFI complaints just takes time patience, and knowing what to do. Once the initial RFI complaints are mitigated, things can remain pretty quiet for years.

 

[kwthom]

<...> While I couldn’t get close with my car radio to determine if the strong signal was wiping out other stations, I did notice that my cellular service was entirely absent from my cell phone.

Do AM radio sites create significantly more interference than their FM counterparts?

You didn't need to be 'close with your car radio', but splatter from being 100 meters or so from a radiating AM tower probably wiped out a significant portion of your AM radio selectivity. Same thing could potentially happen if you were to be next to a 160m amateur station, running at full (1kw) power as well.

Cell phone? Not out of the realm of possibility, but...

As others mentioned, field strength of many FM facilities are designed to minimize radiation to personnel on the ground, but in proximity to the site.

 

[Greg Strickland]

I agree with Kelly A's comments. Adding that RF from FM and RF from AM can have different effects on equipment. In the case of a studio facility that is co-located with a transmitting antenna, competent engineering and adequate funds can resolve RF interference if it occurs. FCC rules address blanketing interference from AM and FM facilities and establish procedures for compliance with FCC rules.

 

[fordranger797]

Thank you for the responses! It makes sense why AM would be particularly problematic in certain situations. At the moment, I live in an area where many of the local AM towers are located for my current city. While I’m not close enough to any of the towers to experience any interference at my apartment, I do notice that many of the signals are reflected on different frequencies around the AM band. For instance, I’ve heard a local station on 1470 on multiple AM frequencies. I know that reflections happen with FM as well, but is it more common with AM?

I was a little surprised by how close some neighborhoods are to some of the AM tower sites. It seems like many AM stations try to maintain some sort of buffer space between the tower and residential properties.

 

[davideduardo]

I was a little surprised by how close some neighborhoods are to some of the AM tower sites. It seems like many AM stations try to maintain some sort of buffer space between the tower and residential properties.

What has happened is that stations that located in earlier eras in locations where they could get a big hunk of land cheap (and in a location that favored their signal, most often directional, being focused on the population center.

As cities grew, the land stations did not own was sold for developments or stores or factories. The stations had little control over that.

 

[Kelly A]

While I’m not close enough to any of the towers to experience any interference at my apartment, I do notice that many of the signals are reflected on different frequencies around the AM band. For instance, I’ve heard a local station on 1470 on multiple AM frequencies. I know that reflections happen with FM as well, but is it more common with AM?

Hearing an AM station on multiple frequencies is typically caused by receiver front end overload. Years ago I had some DX'er file a complaint that one of my stations was creating interference to several HF ham bands, but only at night. I usually avoid making field visits, but she was being quite the squeaky wheel, threatening to go to the Commission, blah..blah. So, I packed up my Potomac Instruments AM field strength meter and HP spectrum analyzer, and met the complainant at her home.
I visually spotted the problem right away. From her back yard, I could literally see the two tower lights of my station. She was right in the night directional main field, with a horizontal 'longwire' antenna that was cut to almost exactly 1/4 wavelength at the station frequency. I believe her receiver was a wide/multi band Sony device. I connected my field set to her outdoor antenna, and proceeded to show her how the station didn't actually appear on anything other that the stations primary frequency. I then did the same with my spectrum analyzer, showing her a visual representation of the same. Of course then she started to make casual inferences that my employer should buy a better receiver for her. My response was; "ain't gonna happen".