What FM frequencies are more susceptible to multipath and interference?

[fordranger797]

I’m back with another question. I’m curious which FM frequencies (if any) are more susceptible to multipath interference, or interference in general.

Additionally, I’ve been under the impression that there may be a slight difference between a radio station broadcasting at 88.1 MHz and 107.9 MHz. I’ve always thought that the station at 88.1 would have a slight advantage, as the lower frequency may provide slightly better coverage to distant areas. Is that true, or would the difference be negligible?

 

[davideduardo]

Anecdote: When Bill Tanner and I arrived at KLVE in Los Angeles, the previous manager (who was being replaced) said that trying to do better at 107.5 was impossible because "those stations at the top of the FM dial did not do well".

Second book: #1 in LA total market.

Moral of this story: many of the observations and opinions about FM station coverage and success have to do with bad programming and management, not a bad signal.

Second anecdote: When we bought KSCA from the Gene Autry organization, the former manager met with Tanner, Richard Heftel and me at the old offices, appropriately located right down the street from the entrance to Forest Lawn. He told us that, due to the low-power signal, we should not expect to do any better than they had done in various formats: around a 1.0 to 1.2 share.

First book: mid 4 share. Second book, KLVE and KSCA were #1 and #2.

 

[Kelly A]

I’m back with another question. I’m curious which FM frequencies (if any) are more susceptible to multipath interference, or interference in general.

Multipath, as it relates to the VHF/UHF broadcast band is not frequency dependent. As I explained (or tried to in another thread) multipath is a reflection off of natural or man made surfaces and is a matter of physics at the point of reception.

Additionally, I’ve been under the impression that there may be a slight difference between a radio station broadcasting at 88.1 MHz and 107.9 MHz. I’ve always thought that the station at 88.1 would have a slight advantage, as the lower frequency may provide slightly better coverage to distant areas. Is that true, or would the difference be negligible?

It depends on the frequency and what surface is reflecting a signal that creates multiple signals to the receiver reception at different times. That's why it's called Multi-Path

 

[fordranger797]

Multipath, as it relates to the VHF/UHF broadcast band is not frequency dependent. As I explained (or tried to in another thread) multipath is a reflection off of natural or man made surfaces and is a matter of physics at the point of reception.

It depends on the frequency and what surface is reflecting a signal that creates multiple signals to the receiver reception at different times. That's why it's called Multi-Path

You explained it quite well, Kelly. Unfortunately some of us are not as well versed in broadcast engineering and science though 😂. What you are saying makes sense though. I recall seeing a graph of the electromagnetic spectrum. Analog TV took a sizeable chunk of the spectrum, with there being quite a bit of variation between channel 2 and 59 (for example). I would assume there would be some differences in that scenario, but FM frequencies are much closer together and thus don’t have a problem.

 

[fordranger797]

Anecdote: When Bill Tanner and I arrived at KLVE in Los Angeles, the previous manager (who was being replaced) said that trying to do better at 107.5 was impossible because "those stations at the top of the FM dial did not do well".

Second book: #1 in LA total market.

Moral of this story: many of the observations and opinions about FM station coverage and success have to do with bad programming and management, not a bad signal.

Second anecdote: When we bought KSCA from the Gene Autry organization, the former manager met with Tanner, Richard Heftel and me at the old offices, appropriately located right down the street from the entrance to Forest Lawn. He told us that, due to the low-power signal, we should not expect to do any better than they had done in various formats: around a 1.0 to 1.2 share.

First book: mid 4 share. Second book, KLVE and KSCA were #1 and #2.

Out of curiosity David, was there a stigma at the time because few stations were using those frequencies? It seems like FM didn’t really embrace the higher frequencies until the band started to get more crowded.

 

[Kelly A]

You explained it quite well, Kelly. Unfortunately some of us are not as well versed in broadcast engineering and science though 😂. What you are saying makes sense though. I recall seeing a graph of the electromagnetic spectrum. Analog TV took a sizeable chunk of the spectrum, with there being quite a bit of variation between channel 2 and 59 (for example). I would assume there would be some differences in that scenario, but FM frequencies are much closer together and thus don’t have a problem.

In essence, the spectrum affected by reflections depends on the reflective surface(s) between the distance of the point of transmission to the receiver. So, just like the physical frequency wavelength, the amount of reflection at that frequency is based on the size and shape of the natural or man-made surfaces reflecting as it causes reflections to the receiver. Again, multipath is not a transmission issue, but a reception issue based on the environment between the point of transmission and the receiver at a particular location. So, as a crude example; a bread truck parked 200ft away from you listening to an FM station transmitting at 88Mhz, may be more reflective at that frequency creating multipath conditions to your radio, than the same truck would reflect at say, 100Mhz in those same conditions.
In other words; there are no advantages from one end of the FM band to another, as it relates to multipath.
And this isn't a broadcast engineering-knowledge matter, but a physics one. I know a lot of broadcast technical folks who don't understand the physics of multipath.

 

[fybush]

Another way of looking at this: the wavelength of MW signals varies from over 500 meters at the low end of the dial to under 200 meters at the top, so a station on 540 has a wavelength almost three times as long as one on 1700.

A channel 2 TV signal is about 6 meters in wavelength, while channel 69 was barely a meter in wavelength.

By comparison, a signal on 88.1 is only a few centimeters longer in wavelength than one on 107.9.

The one area that matters for propagation is e-skip - it's common to have an opening that's great at 88 MHz but dies out before 100 MHz. I don't fully understand the physics behind the MUF (maximum usable frequency) of e-skip, but empirically, I gave seen it happen summer after summer, so I know it's real.

In general, though, there are a lot of factors (incoming co- and adjacent-channel interference, electrical noise) that will have much more effect on FM propagation than the slight variation in wavelength from one end of the dial to the other.

 

[davideduardo]

Out of curiosity David, was there a stigma at the time because few stations were using those frequencies? It seems like FM didn’t really embrace the higher frequencies until the band started to get more crowded.

The two LA examples were in 1995 and 1998. The band had been full for several decades.

 

[davideduardo]

I know a lot of broadcast technical folks who don't understand the physics of multipath.

When explaining it to non-technical managers and programmers, I use the example of reflected images in a house of mirrors at a county fair; distance and angle. And in FM, add elevation.

 

[K6JHU]

The world can correct me if I am wrong. Ionosphere is divided into layers. The ones we are interested in for propagation is D, E, F. The amount of ionization is determined by the Sun. How active (ionized) a layer is determines how much of the signal is reflected back. You can see this on SW. As the day goes on the MUF (Maximum Usable Frequency) goes up as the various layers ionize. Given an active Sun, the MUF can get as high as to affect the FM band. Since it is the E layer that will reflect the signal it is called 'E-Skip'. So E-Skip will affect the bottom of the FM band before it affects the top which is why he effect may disappear in the middle of the band.

 

[KeithE4]

Given an active Sun, the MUF can get as high as to affect the FM band. Since it is the E layer that will reflect the signal it is called 'E-Skip'. So E-Skip will affect the bottom of the FM band before it affects the top which is why he effect may disappear in the middle of the band.

Back in the summer of 1976 or '77 (too long ago to remember the exact or even approximate date), I was able to work Alabama on FM simplex via E-skip while driving in Wisconsin. This was on 2 meters (146.52 MHz), which is the highest frequency I've ever seen it. I've never heard of it making it into TV Channel 7 or higher, but I wouldn't rule it out.

 

[Kelly A]

The world can correct me if I am wrong. Ionosphere is divided into layers. The ones we are interested in for propagation is D, E, F. The amount of ionization is determined by the Sun. How active (ionized) a layer is determines how much of the signal is reflected back. You can see this on SW. As the day goes on the MUF (Maximum Usable Frequency) goes up as the various layers ionize. Given an active Sun, the MUF can get as high as to affect the FM band. Since it is the E layer that will reflect the signal it is called 'E-Skip'. So E-Skip will affect the bottom of the FM band before it affects the top which is why he effect may disappear in the middle of the band.

What does any of that have to do with VHF or UHF multipath?

 

[K6JHU]

One poster asked why the lower end of the FM band was prone to skip while the high end wasn't

 

[fybush]

Back in the summer of 1976 or '77 (too long ago to remember the exact or even approximate date), I was able to work Alabama on FM simplex via E-skip while driving in Wisconsin. This was on 2 meters (146.52 MHz), which is the highest frequency I've ever seen it. I've never heard of it making it into TV Channel 7 or higher, but I wouldn't rule it out.

E-skip into high VHF is extremely rare, but it has happened. I remember the WTFDA DX club list lighting up one July 4 weekend in the very early 2000s with reports of reception as high as channel 11 or thereabouts.

It could still happen in the DTV era, I suppose, but there are fewer DXers out looking for it now.

 

[KeithE4]

It could still happen in the DTV era, I suppose, but there are fewer DXers out looking for it now.

That, plus not being allowed to tune to channels that don't have a station, thanks to scanning. A couple of my early (2010-vintage) sets could be set manually, but not anything I have now. I have never seen any skip or tropo in the digital era other than once a few years back when all the Tucson UHFs came in after a monsoon storm. I was able to scan them before they faded out.

 

[pclover]

E-skip into high VHF is extremely rare, but it has happened. I remember the WTFDA DX club list lighting up one July 4 weekend in the very early 2000s with reports of reception as high as channel 11 or thereabouts.

It could still happen in the DTV era, I suppose, but there are fewer DXers out looking for it now.

Last year I had NOAA KJY97 162.525 MHz from Childress, TX here. A bit over 1200 Miles.

The 2004 I've read about and somebody blogged about it here 10 years later: A look back at the historic July 6, 2004 sporadic e-skip opening (audio and video)

 

[pclover]

The one area that matters for propagation is e-skip - it's common to have an opening that's great at 88 MHz but dies out before 100 MHz. I don't fully understand the physics behind the MUF (maximum usable frequency) of e-skip, but empirically, I gave seen it happen summer after summer, so I know it's real.

The MUF is important here because if you hit a frequency above the MUF it wont have a steep enough refraction angle to return to the earth. Also it can rapidly increase or decrease. Also it can have extremely sharp cutoffs.

An example of this is if you have like 1200 mile paths that stop at a certain frequency you likely hit the MUF there. If you decrease frequency you may notice your paths start to become shorter.

If you have short paths like under 600 Miles on a certain band 10M, 6M, FMBC there is a good chance that the MUF is very high and likely could support much higher frequency paths at longer distances.

 

[pclover]

The world can correct me if I am wrong. Ionosphere is divided into layers. The ones we are interested in for propagation is D, E, F. The amount of ionization is determined by the Sun. How active (ionized) a layer is determines how much of the signal is reflected back. You can see this on SW. As the day goes on the MUF (Maximum Usable Frequency) goes up as the various layers ionize. Given an active Sun, the MUF can get as high as to affect the FM band. Since it is the E layer that will reflect the signal it is called 'E-Skip'. So E-Skip will affect the bottom of the FM band before it affects the top which is why he effect may disappear in the middle of the band.

A LONG detailed post ahead.

If you want to break it down more the F layer during the day separates into a F1 and F2 layer. At night it sort of combines into a single F layer while the D layer disappears.

AFIK the highest the MUF for F2 layer will get to is around 60 MHz. This is during solar maximum.

The sun is a major factor on normal HF propagation but it's effect on Sporadic-E is unknown. Also the solar cycle seem to have no clear effect.

You can get a FMBC Sporadic-E opening in the middle of the night. The normal rules of of the D, E, and F layers don't apply to Sporadic-E. One of the many reasons that makes Sporadic-E mysterious because there is no clear trigger or cause just statistics of when it's more likely to occur.

Often, People will reference Sporadic-E as a separate layer (Es Layer) in the E region or sometimes as Es clouds. It's a special case because it can have extremely high ionization that is not found in other layers and doesn't follow normal ionospheric behavior.

As for FMBC and Sporadic-E it's more likely to occur at the lower end of the band because less ionization is required there. It's much more likely and frequent on the 10M and 6M ham bands during Late may into early august. It's more likely there because Again, Less ionization is required for lower frequencies.

As if this post isn't long enough you can also have more fun things happen with Sporadic-E if your lucky enough to get multi-hop that can really push the range out on things. Doing it on the FM broadcast band is super rare

 

[CentralFL]

I’m back with another question. I’m curious which FM frequencies (if any) are more susceptible to multipath interference, or interference in general.

Additionally, I’ve been under the impression that there may be a slight difference between a radio station broadcasting at 88.1 MHz and 107.9 MHz. I’ve always thought that the station at 88.1 would have a slight advantage, as the lower frequency may provide slightly better coverage to distant areas. Is that true, or would the difference be negligible?

I've wondered that myself and the answers are interesting to hear. As for interference, the stations at either end of the dial should be free of first-channel adjacents.

Especially for 107.9. There is nothing above it. It could have adjacent on 107.5, but not on 108.1.

88.1 wouldn't have an adjacent on 87.9, but there could be briefly trouble with those tiny transmitters for cars to link phones, mp3 players, add-on satellite receivers, etc.

 

[pclover]

I've wondered that myself and the answers are interesting to hear. As for interference, the stations at either end of the dial should be free of first-channel adjacents.

Especially for 107.9. There is nothing above it. It could have adjacent on 107.5, but not on 108.1.

88.1 wouldn't have an adjacent on 87.9, but there could be briefly trouble with those tiny transmitters for cars to link phones, mp3 players, add-on satellite receivers, etc.

Adjacent channel interference also depends on how good the front is on your radio. DSP Tuners do the best here due to the fact they can swap filter coefficients rapidly. Sometimes you'll call it selectivity. They can also do fun things like try to remove noise and other tricks to reduce the effects of multipath.

For single chip solutions NXP seems to do the best here and used in lots of things.

Of course the FCC has clear stated rules regarding adjacent channels but radio performance is a big factor of how big of an issue it will be to the listener.