What Scenario Would a Broadcast Engineer Prefer?

[fordranger797]

I thought of this question that relates to power output versus height, so I thought I would pass it along to the experts. In my current market (Vancouver BC), all of the local FM and TV broadcasters use the same mountain for transmitting. However, there are multiple towers spaced at various heights of the mountain.

Some stations transmit from the tower site located at the very top of the mountain access road, which provides an approximate HAAT of 2250 feet. Stations that use this tower may have some variance in their precise HAAT, but all transmit with approximately 75kw. At the same time, some broadcasters using this site are also running a lower power output (10kw).

Make your way slightly down the mountain access road, and you'll run into another tower site. This site is slightly lower (at 2,026 feet HAAT), but broadcasters using this tower operate at 100kw.

Continue down the mountain, and there's one remaining tower site. This site is located somewhere around 1,200 feet HAAT, and stations using the tower operate at 100kw.


With all of this information in mind, I was curious about which scenario is most "ideal" to someone who is attempting to engineer the best possible configuration for their station. Granted, all of these sites seem to work just fine in the core listening area, and I'm really not considering the fact that the "best site" is always the one that is most affordable to the owners and most accessible to the engineer. For simplicity, I'll list my questions:

1. Is the best scenario being at the top of the mountain with a slightly lower ERP? Or would you rather be at the next site down with max ERP for superior building penetration? Would being slightly lower (while running 100kw) actually create some multipath interference, which could be avoided at the higher site?

2. Is the lowest site (located at 1,000 feet below the tallest site) somewhere that most engineers would want to avoid to prevent multipath interference?

3. If your equipment is located on a tower on the side of a mountain, will the engineer mount the antenna on the side of the tower (facing the metropolitan area) without vertical polarization? What is the effect for those who happen to live near the mountain transmission site (if the beam has a horizontal polarization to better serve communities further away from the tower)?

Thanks in advance for any information!

 

[Kelly A]

There really isn't a combination that's superior for every scenario. Terrain surrounding the market served plays a huge role. Boise Idaho is a great example where neither being high with less ERP, or down lower with more ERP is an advantage over the other. The reason is the Boise market consists of communites that are located in (for the lack of another term) gulches blocked by medium sized hills. In that case, one might go up on a higher hill, but doing so requires a significant amount of antenna beam tilt to get signal into the Boise metro. Then the hills blocking other market communities become high multipath regions. Something similar in the Denver market.

1. Is the best scenario being at the top of the mountain with a slightly lower ERP? Or would you rather be at the next site down with max ERP for superior building penetration? Would being slightly lower (while running 100kw) actually create some multipath interference, which could be avoided at the higher site?

Again it depends on the surrounding terrain, large buildings in urban areas, etc. Being close to a higher power station means the receiver will mainly decode the primary signal. Multipath happens when the primary signal reflects off of manmade or natural surfaces and arrives at the receiver at roughly the same field strength, but slightly out of time. If a receiver can lock onto the main carrier as the strongest, it ignores the potentially lower-strength reflections.

3. If your equipment is located on a tower on the side of a mountain, will the engineer mount the antenna on the side of the tower (facing the metropolitan area) without vertical polarization?

Purely horizontal polarization went away years ago. It was sort of a leftover from the early VHF analog TV days when TV antennas were the classic H-pol yagis. When vehicle antennas went all pretty much vertical polarity, FM stations started using circular, elliptical, or in rare cases, vertical-only polarization. It isn't unusual for stations, and much preferred, with antennas side-mounted to a guyed or self-supported tower, to have the antenna face the city of license to avoid needless reradiation caused by the tower structure.

What is the effect for those who happen to live near the mountain transmission site (if the beam has a horizontal polarization to better serve communities further away from the tower)?

You can order antennas with 60-40 split elliptical polarization, but not many do. Again, if your listeners primarily use vertical antennas, you're potentially cheating yourself by emitting too much H-Pol.

In general; it's always best to run with the herd where possible. That way you create a controlled signal environment for your listeners. With signals coming from the majority of the stations in the same direction, you give the station signal parity with your competitors.

 

[Lazy J]

At those power levels, (75kw and 100kw) on basically the same mountain, you're not going to notice much difference in building penetration. I'd go for the maximum HAAT to get the most coverage.

 

[fordranger797]

Again it depends on the surrounding terrain, large buildings in urban areas, etc. Being close to a higher power station means the receiver will mainly decode the primary signal. Multipath happens when the primary signal reflects off of manmade or natural surfaces and arrives at the receiver at roughly the same field strength, but slightly out of time. If a receiver can lock onto the main carrier as the strongest, it ignores the potentially lower-strength reflections.

Purely horizontal polarization went away years ago. It was sort of a leftover from the early VHF analog TV days when TV antennas were the classic H-pol yagis. When vehicle antennas went all pretty much vertical polarity, FM stations started using circular, elliptical, or in rare cases, vertical-only polarization. It isn't unusual for stations, and much preferred, with antennas side-mounted to a guyed or self-supported tower, to have the antenna face the city of license to avoid needless reradiation caused by the tower structure.

You can order antennas with 60-40 split elliptical polarization, but not many do. Again, if your listeners primarily use vertical antennas, you're potentially cheating yourself by emitting too much H-Pol.

In general; it's always best to run with the herd where possible. That way you create a controlled signal environment for your listeners. With signals coming from the majority of the stations in the same direction, you give the station signal parity with your competitors.

Thanks, Kelly. As always, that’s a very helpful explanation. This explains why radio stations that use cougar mountain in Seattle don’t struggle with multipath interference. What scenario would cause multipath to occur? Perhaps if you’re trying to pick up a station that transmits at 100kw, but you’re far from the tower site (thus not giving the radio the opportunity to lock onto the strongest signal)?

I’m trying to picture the modern antennas that you’re describing. When I had the opportunity to visit a transmitter site for a station I previously worked for, the antenna looked quite a bit different from what you’re describing (probably due to the age of the facility). I would say that the antenna looked like five large buoys mounted to a pole on the side of a tower. I’m not entirely sure why five were required, or if that has anything to do with polarity. I haven’t seen this style of antenna in use anywhere else. Rather, most shared broadcasting towers seem to have multiple yagi-like antennas sticking out of the side (I’ll include an image below of what I’m thinking of). If I understand what you’re saying correctly, with this modern equipment, the antenna can be much smaller and more effective. I also assume that only one antenna is required, which saves space on the tower.

 

[fordranger797]

At those power levels, (75kw and 100kw) on basically the same mountain, you're not going to notice much difference in building penetration. I'd go for the maximum HAAT to get the most coverage.

Thank you for your comment. I’ve noticed that the 75kw stations at the top of the mountain do seem to be superior over long distances. That’s probably where I’d want to be as well. What are your thoughts on a lower powered station that uses a relatively high HAAT? In the example I listed, there are a few stations at or below 10kw that are using the highest tower on the mountain. Surprisingly, they get out pretty well. It seems like HAAT does matter more.

 

[PTBoardOp94]

Because we're talking mountaintop site, how much snow occurs in winter at both sites matters. If I have to hire a helicopter to get to the top of the mountain in January, that could make the lower site attractive. If both sites are inaccessible in winter, then you might have to procure an aux site off the mountain to ensure service.

What scenario would cause multipath to occur? Perhaps if you’re trying to pick up a station that transmits at 100kw, but you’re far from the tower site (thus not giving the radio the opportunity to lock onto the strongest signal)?

Given the scenario here, suppose you elect for a lower antenna on the northern face of the mountain. If you are trying to receive the station to the south of the mountain, you will often get multipath, because the mountain scatters all possible first paths.

 

[Kelly A]

Thanks, Kelly. As always, that’s a very helpful explanation. This explains why radio stations that use cougar mountain in Seattle don’t struggle with multipath interference. What scenario would cause multipath to occur? Perhaps if you’re trying to pick up a station that transmits at 100kw, but you’re far from the tower site (thus not giving the radio the opportunity to lock onto the strongest signal)?

Getting into the weeds a little bit more but; technically multipath is a pure physics phenomenon caused when a radio signal is received by a receiver combined with the receiver seeing reflection(s) of similar level but at different time(s). You've probably heard more extreme examples of multipath where reflections to the main signal completely cancel each other out, making reception stop. At the receiving end, you alter the reflected signal level as seen by the receiver by moving a couple of feet one way or the other, and reception returns. Now, some technical folks will claim that multipath can be reduced or eliminated by certain audio processing, transmitter 'tuning', etc. It's true that the apparent audible effects of multipath are enhanced when a station runs stereo versus mono, but that's just because the receiver is demodulating a wider bandwidth, so the natural effect of multipath is enhanced while in stereo. The multipath to the receiver still exists, stereo just makes it more apparent to the listener. Stations that used to use audio processing to 'widen' the stereo effect, increased the L-R level of their carrier, and doing so sometimes would cause noticeably 'louder' multipath. Again, it doesn't mean the station is creating multipath, because it can't. It's that when multipath occurs at the receiver due to reflections off natural and manmade surfaces around the receiver, the effect is more pronounced.
Something that sounds like multipath but isn't that can be caused in the transmission process, is amplitude-phase distortion:
The Interpretation of Amplitude and Phase Distortion in Terms of Paired Echoes
For broadcast transmitters, this is usually caused by a bandwidth restriction in an amplifier stage. For example; I ran across a station that had a partially failed antenna that represented a poor load to the transmitter at one sideband. The transmitter seemed to run okay, but there seemed to be a hiss, or multipath as one drove around. Older tube transmitters would have a similar problem if not maintained properly and tuned for minimum AM noise.

I’m trying to picture the modern antennas that you’re describing. When I had the opportunity to visit a transmitter site for a station I previously worked for, the antenna looked quite a bit different from what you’re describing (probably due to the age of the facility). I would say that the antenna looked like five large buoys mounted to a pole on the side of a tower. I’m not entirely sure why five were required, or if that has anything to do with polarity.

You've likely heard that stations broadcast at 75kW ERP, 100kW ERP, whatever. In most of those cases the actual transmitter input power to the antenna is much lower than 75 or 100kW. As an example, a 100kW FM with a 'six bay' antenna, would likely have a 30kW transmitter providing power to the antenna. As you add antenna radiating elements, or we call them bays, the amount of power toward to horizon is more focused. So, the Effective Radiated Power to the horizon, is 75 or 100kW, depending on the number of antenna elements or bays. In the photo you posted, there's something like 8-elements/bays, which creates that gain focusing the signal toward the horizon.
Regarding polarization of the antenna; if you look at the photo you included the arrow-shaped things are the actual radiating elements for each bay. You'll see the copper elements of the arrow have 'points' going at angles from each other. Each of those points that make up the arrow shape radiates at that angle, together radiating a signal that's both horizontally and vertically polarized at the same time.

I haven’t seen this style of antenna in use anywhere else. Rather, most shared broadcasting towers seem to have multiple yagi-like antennas sticking out of the side (I’ll include an image below of what I’m thinking of). If I understand what you’re saying correctly, with this modern equipment, the antenna can be much smaller and more effective. I also assume that only one antenna is required, which saves space on the tower.

The example in that photo looks like a Kathrein brand panel antenna. The flat grid panels behind the arrows in each bay act as reflectors.

 

[fordranger797]

Getting into the weeds a little bit more but; technically multipath is a pure physics phenomenon caused when a radio signal is received by a receiver combined with the receiver seeing reflection(s) of similar level but at different time(s). You've probably heard more extreme examples of multipath where reflections to the main signal completely cancel each other out, making reception stop. At the receiving end, you alter the reflected signal level as seen by the receiver by moving a couple of feet one way or the other, and reception returns. Now, some technical folks will claim that multipath can be reduced or eliminated by certain audio processing, transmitter 'tuning', etc. It's true that the apparent audible effects of multipath are enhanced when a station runs stereo versus mono, but that's just because the receiver is demodulating a wider bandwidth, so the natural effect of multipath is enhanced while in stereo. The multipath to the receiver still exists, stereo just makes it more apparent to the listener. Stations that used to use audio processing to 'widen' the stereo effect, increased the L-R level of their carrier, and doing so sometimes would cause noticeably 'louder' multipath. Again, it doesn't mean the station is creating multipath, because it can't. It's that when multipath occurs at the receiver due to reflections off natural and manmade surfaces around the receiver, the effect is more pronounced.
Something that sounds like multipath but isn't that can be caused in the transmission process, is amplitude-phase distortion:
The Interpretation of Amplitude and Phase Distortion in Terms of Paired Echoes
For broadcast transmitters, this is usually caused by a bandwidth restriction in an amplifier stage. For example; I ran across a station that had a partially failed antenna that represented a poor load to the transmitter at one sideband. The transmitter seemed to run okay, but there seemed to be a hiss, or multipath as one drove around. Older tube transmitters would have a similar problem if not maintained properly and tuned for minimum AM noise.

You've likely heard that stations broadcast at 75kW ERP, 100kW ERP, whatever. In most of those cases the actual transmitter input power to the antenna is much lower than 75 or 100kW. As an example, a 100kW FM with a 'six bay' antenna, would likely have a 30kW transmitter providing power to the antenna. As you add antenna radiating elements, or we call them bays, the amount of power toward to horizon is more focused. So, the Effective Radiated Power to the horizon, is 75 or 100kW, depending on the number of antenna elements or bays. In the photo you posted, there's something like 8-elements/bays, which creates that gain focusing the signal toward the horizon.
Regarding polarization of the antenna; if you look at the photo you included the arrow-shaped things are the actual radiating elements for each bay. You'll see the copper elements of the arrow have 'points' going at angles from each other. Each of those points that make up the arrow shape radiates at that angle, together radiating a signal that's both horizontally and vertically polarized at the same time.

The example in that photo looks like a Kathrein brand panel antenna. The flat grid panels behind the arrows in each bay act as reflectors.

Thanks for the explanation, Kelly. So if my understanding is correct, running a higher ERP doesn't automatically make a station more susceptible to multipath interference. My previous understanding was that multipath was a more common phenomena among stations that used a higher ERP (but this is not necessarily the case, as it's more receiver dependent). If you're in a large metropolitan area with many manmade structures surrounding you (which can generate multipath interference), will a higher ERP help you avoid reception issues? From what you've wrote, it sounds like a high ERP will give you a better chance at making sure the radio/receiver picks up the strongest signal and ignores interference. I'm also wondering if there's a noticeable difference between stations that max out ERP, versus stations that at a higher elevation that dial back the ERP a bit.

Using the Seattle market as an example, you told me before that Cougar Mountain is arguably a better transmissions site (for a multitude of reasons). But more specific to engineering, you mentioned that Cougar stations have an edge over Tiger Mountain because they tend to have more optimal building penetration compared to their Tiger Mountain counterparts. I would assume that these stations may technically have less multipath in the metro area compared to stations that are maxed out at 60kw. Personally, I don't really notice a difference between a station on Cougar versus Tiger (leaving me to assume that at high power levels, the difference is negligible), but it's noticeable when you compare a station with a high ERP (such as 95.7) to a station at the same site with a low ERP (104.5).

Back to the antenna conversation, I definitely was aware that the transmitter itself doesn't power up to the maximum ERP, but I didn't know that the number of antenna elements factored into the equation. Using my photo as a guide, is it likely that all of those antenna elements belong to the same station, and all are emitting approximately 9~10kw? This is where my understanding really derails, as I have traditionally assumed that each element may have to be oriented a certain way to provide the best coverage for the metro area. For example, perhaps the bottom elements tilts down slightly (toward listeners close to the tower site), while elements near the top emit signal more directly toward the horizon. This is an entirely inaccurate oversimplification, I'm sure. If I understand what you're saying, these elements are not configured with any sort of tilt, and the elements act as one singular antenna (since it's horizontally and vertically oriented at the same time).

I'm guessing the antenna we used at the old transmissions site was old technology compared to these modern Kathrein antennas. Looking at photos from some of the broadcast towers in major markets, everyone is using Kathrein antennas (or antennas that look similar).

Sorry for the long post, but this is a very interesting topic to me. Not to mention, I think very few people (outside of broadcast engineers) understand how all of this truly works.

 

[fordranger797]

Because we're talking mountaintop site, how much snow occurs in winter at both sites matters. If I have to hire a helicopter to get to the top of the mountain in January, that could make the lower site attractive. If both sites are inaccessible in winter, then you might have to procure an aux site off the mountain to ensure service.


Given the scenario here, suppose you elect for a lower antenna on the northern face of the mountain. If you are trying to receive the station to the south of the mountain, you will often get multipath, because the mountain scatters all possible first paths.

That definitely makes sense. In many markets with mountain transmitter sites, it seems like the antennas are only located on one “face” of the mountain if there’s no population to serve on the other side. Otherwise, the setup wouldn’t work (I assume).

 

[Kelly A]

So if my understanding is correct, running a higher ERP doesn't automatically make a station more susceptible to multipath interference.

Remember that multipath is not created by the station, but based on signal reflections caused by natural or manmade objects between the transmission site and the receiver-antenna. The ERP of a station doesn't create a multipath condition, reflective surfaces near the receiver antenna do. As another example using DTV: An old piece of coax connected to a roof antenna with poor shielding can create multipath to the DTV receiver because there are multiple points of signal entry: The main signal collected from the antenna connected to the coax, and reflected signals leaking into the coax through a failed shield. Let's say you're sitting at a stop in your vehicle listening to a local FM, and a large truck pulls up two lanes over. Sections of the truck happen to be a good reflectors at the station frequency, so now your radio is receiving the primary signal, and one reflecting a millisecond after the primary signal. The answer to your question is there is no guaranteed correlation between ERP and receiving distance from the station as to where multipath, or how much occurs. Multipath is a receiving issue, not a transmission one.

My previous understanding was that multipath was a more common phenomena among stations that used a higher ERP (but this is not necessarily the case, as it's more receiver dependent). If you're in a large metropolitan area with many manmade structures surrounding you (which can generate multipath interference), will a higher ERP help you avoid reception issues?

But multipath can vary because the reflective surfaces reflect better at a certain frequency. That's why one station may be worse for multipath at a particular intersection, and another station further up or down the dial, may be clean.

From what you've wrote, it sounds like a high ERP will give you a better chance at making sure the radio/receiver picks up the strongest signal and ignores interference. I'm also wondering if there's a noticeable difference between stations that max out ERP, versus stations that at a higher elevation that dial back the ERP a bit.

Stations are licensed for their ERP depending on a lot of factors. One applies to the Commission for a Construction Permit based on the amount of antenna gain to arrive at an ERP. For example; I can buy a two-bay antenna for a 100kW FM station with technically no antenna gain, and dump a ton of transmitter power into it. Now my antenna has no focus toward the horizon, so all that power is going up, down, and in all directions equally. Obviously, this would be a very expensive design, but technically would be valid. That's why nobody doing full-class stations would do that sort of thing. Many "rimshot" FMs will use a ton of antenna gain to focus the signal toward the more distant community they're trying to reach. That's assuming they don't have a co-or adjacent channel problem with another station. Again, none of this has anything to do with multipath.

Using the Seattle market as an example, you told me before that Cougar Mountain is arguably a better transmissions site (for a multitude of reasons). But more specific to engineering, you mentioned that Cougar stations have an edge over Tiger Mountain because they tend to have more optimal building penetration compared to their Tiger Mountain counterparts.

If you looked at a topographic map of the Seattle-Tacoma market, it's a relatively narrow corridor between mountain ranges running North/South. Cougar Mt. is around eight miles closer to the market population base and with a clear view of I-5 and I-405, and a good look angle to parts of downtown Seattle than WTM. Building penetration is better than WTM simply because of closer proximity combined with field strength. Think of it as looking at a flashlight beam at night. You can see the light much more intensely ten feet away, vs. twenty feet away. You can still see the light from both distances, it's just that one is more intense.

Back to the antenna conversation, I definitely was aware that the transmitter itself doesn't power up to the maximum ERP, but I didn't know that the number of antenna elements factored into the equation. Using my photo as a guide, is it likely that all of those antenna elements belong to the same station, and all are emitting approximately 9~10kw?

Sure, power is evenly distributed to all the antenna elements (bays). The bays are physically spaced (generally) 1 wavelength apart at the station frequency.

This is where my understanding really derails, as I have traditionally assumed that each element may have to be oriented a certain way to provide the best coverage for the metro area. For example, perhaps the bottom elements tilts down slightly (toward listeners close to the tower site), while elements near the top emit signal more directly toward the horizon. This is an entirely inaccurate oversimplification, I'm sure. If I understand what you're saying, these elements are not configured with any sort of tilt, and the elements act as one singular antenna (since it's horizontally and vertically oriented at the same time).

What you're describing is called 'beam tilt'. It's exactly what it says. The signal toward the horizon is angled down using electrical or mechanical means, to make sure the right amount of signal reaches the market. Some geographical locations suffer from pretty severe temperature inversions, where rising warm air can literally deflect a signal away from the intended target. Some stations use a slight amount of beam tilt to counteract expected inversions in the late afternoon hours.

I'm guessing the antenna we used at the old transmissions site was old technology compared to these modern Kathrein antennas. Looking at photos from some of the broadcast towers in major markets, everyone is using Kathrein antennas (or antennas that look similar).

There haven't been significant antenna design changes in decades. Since physics won't let you cheat, there are only so many ways one can design an FM dipole antenna. Different manufacturers have their own unique shapes or looks, but they all do pretty much the same thing.

 

[Kelly A]

Because we're talking mountaintop site, how much snow occurs in winter at both sites matters. If I have to hire a helicopter to get to the top of the mountain in January, that could make the lower site attractive. If both sites are inaccessible in winter, then you might have to procure an aux site off the mountain to ensure service.

Examples of that in the Seattle market were Three Sisters Mt South of Enumclaw and to an extent; West Tiger Mt. Three sisters is over 3,000ft ASL, and the logging road to the site frequently became impassible in the Winter. Because the old KMTT used to be powered by a generator 24/7, weekly maintenance required a trip up in a helicopter., weather permitting.
There have been a few times where WTM has been so snowed-in, that even a snowcat can't get up the mountain. That combined with 70+ MPH winds and blowing snow, and you're not getting up the mountain until the weather clears.
That's why most of the WTM stations have their backup sites on Cougar. It's easier to get at year-round with full market coverage.

Given the scenario here, suppose you elect for a lower antenna on the northern face of the mountain. If you are trying to receive the station to the south of the mountain, you will often get multipath, because the mountain scatters all possible first paths.

Has nothing to do with multipath. The mountain (terrain) simply blocks (attenuates) the signal.

 

[Greg Strickland]

I admire Kelly A for taking the time to reply in detail.

 

[Kelly A]

Here's another way to look at multipath: Imagine in your mind's eye standing in the middle of your hometown. Instead of FM radio signals coming from a tower site on a nearby hill, imagine instead a giant light source aimed at the town from the broadcast tower. As you look around, you can see light coming from the tower directly between buildings and the sky above, but you also see that same light reflecting off of certain surfaces near you, like glass, polished concrete, or even a painted wall. You walk twenty feet in one direction, and now you see more light from the tower reflected off of surfaces at different angles than you can see directly from the tower. The light is still there, it's just coming from reflections, not as apparent directly from where you stand. That image is similar to what a radio receiver sees from its perspective; some signals are direct, and many are reflected. As you move around, your eyes see the number of direct vs. reflected signals vary, yet the radio signal (in this example light source) is constant. If you moved the light source up or down toward the horizon, all you do is change the geographical size of the area being illuminated, but the prior effects of direct light versus reflections remain. This is why I keep insisting that multipath occurs at the receiving end, not transmission.

 

[ContinuousWave]

At those power levels, (75kw and 100kw) on basically the same mountain, you're not going to notice much difference in building penetration. I'd go for the maximum HAAT to get the most coverage.

On mountain tops, you really need null fill.. otherwise your signal is going to go right over the building tops 😁

 

[Greg Strickland]

ContinuousWave- I suggest working out the specific details.

 

[Kelly A]

On mountain tops, you really need null fill.. otherwise your signal is going to go right over the building tops 😁

As Greg mentioned; it depends on the geography and in some cases, average year round climate conditions.

 

[fordranger797]

Remember that multipath is not created by the station, but based on signal reflections caused by natural or manmade objects between the transmission site and the receiver-antenna. The ERP of a station doesn't create a multipath condition, reflective surfaces near the receiver antenna do. As another example using DTV: An old piece of coax connected to a roof antenna with poor shielding can create multipath to the DTV receiver because there are multiple points of signal entry: The main signal collected from the antenna connected to the coax, and reflected signals leaking into the coax through a failed shield. Let's say you're sitting at a stop in your vehicle listening to a local FM, and a large truck pulls up two lanes over. Sections of the truck happen to be a good reflectors at the station frequency, so now your radio is receiving the primary signal, and one reflecting a millisecond after the primary signal. The answer to your question is there is no guaranteed correlation between ERP and receiving distance from the station as to where multipath, or how much occurs. Multipath is a receiving issue, not a transmission one.

But multipath can vary because the reflective surfaces reflect better at a certain frequency. That's why one station may be worse for multipath at a particular intersection, and another station further up or down the dial, may be clean.

Stations are licensed for their ERP depending on a lot of factors. One applies to the Commission for a Construction Permit based on the amount of antenna gain to arrive at an ERP. For example; I can buy a two-bay antenna for a 100kW FM station with technically no antenna gain, and dump a ton of transmitter power into it. Now my antenna has no focus toward the horizon, so all that power is going up, down, and in all directions equally. Obviously, this would be a very expensive design, but technically would be valid. That's why nobody doing full-class stations would do that sort of thing. Many "rimshot" FMs will use a ton of antenna gain to focus the signal toward the more distant community they're trying to reach. That's assuming they don't have a co-or adjacent channel problem with another station. Again, none of this has anything to do with multipath.

If you looked at a topographic map of the Seattle-Tacoma market, it's a relatively narrow corridor between mountain ranges running North/South. Cougar Mt. is around eight miles closer to the market population base and with a clear view of I-5 and I-405, and a good look angle to parts of downtown Seattle than WTM. Building penetration is better than WTM simply because of closer proximity combined with field strength. Think of it as looking at a flashlight beam at night. You can see the light much more intensely ten feet away, vs. twenty feet away. You can still see the light from both distances, it's just that one is more intense.

Sure, power is evenly distributed to all the antenna elements (bays). The bays are physically spaced (generally) 1 wavelength apart at the station frequency.

What you're describing is called 'beam tilt'. It's exactly what it says. The signal toward the horizon is angled down using electrical or mechanical means, to make sure the right amount of signal reaches the market. Some geographical locations suffer from pretty severe temperature inversions, where rising warm air can literally deflect a signal away from the intended target. Some stations use a slight amount of beam tilt to counteract expected inversions in the late afternoon hours.

There haven't been significant antenna design changes in decades. Since physics won't let you cheat, there are only so many ways one can design an FM dipole antenna. Different manufacturers have their own unique shapes or looks, but they all do pretty much the same thing.

Wow, Kelley. This is insanely helpful! Thank you for your explanation. I definitely was not aware that there were mechanisms that could shift the "beam" depending on temperature inversions. Would you normally need this type of setup in a humid environment with rising heat?

The information you provided regarding the antenna bays is extremely helpful as well. It would seem that most major market radio stations would be careful about how the signal is focused. Using Seattle as an example once more, a broadcaster would probably want to ensure that power is focused S, SW, W, NW, and N where most of the population is located. And from what I've seen, the antennas are usually mounted on the west side of the the tower. Even with this type of setup, it seems like a good amount of signal also is transmitted eastward as well. I'm assuming that the way these antennas are installed in Seattle differ from the photo example I used earlier (in Vancouver), where there is literally no use at all for the signal to the north side of the mountain, and reflectors are installed on the tower behind the antenna. I would also assume that this strategy is similar to what the engineers at KTDD 104.9 would do, where there appears to be little to no signal behind the tower (to the south), and as much signal as possible being aimed to the north.

If you're an engineer on Tiger Mountain, would there ever be a reason to install the antenna bays slightly different directions on the tower to provide optimal coverage in all directions, or is it sufficient to orient each bay in the same direction? Based on my understanding at this point, this would be pointless because the antenna bays don't simply transmit signal in one specific direction.

 

[fordranger797]

I admire Kelly A for taking the time to reply in detail.

He's a god among men for putting up with engineering newbies like myself. It's a topic I've always found interesting, but never have had the opportunity to learn about in a classroom setting. I've been lucky to work in a few stations where the broadcast engineer has been around to bug with questions, but it's incredible that Radio Discussions has an engineering-specific forum where these topics can be discussed.

 

[fordranger797]

Here's another way to look at multipath: Imagine in your mind's eye standing in the middle of your hometown. Instead of FM radio signals coming from a tower site on a nearby hill, imagine instead a giant light source aimed at the town from the broadcast tower. As you look around, you can see light coming from the tower directly between buildings and the sky above, but you also see that same light reflecting off of certain surfaces near you, like glass, polished concrete, or even a painted wall. You walk twenty feet in one direction, and now you see more light from the tower reflected off of surfaces at different angles than you can see directly from the tower. The light is still there, it's just coming from reflections, not as apparent directly from where you stand. That image is similar to what a radio receiver sees from its perspective; some signals are direct, and many are reflected. As you move around, your eyes see the number of direct vs. reflected signals vary, yet the radio signal (in this example light source) is constant. If you moved the light source up or down toward the horizon, all you do is change the geographical size of the area being illuminated, but the prior effects of direct light versus reflections remain. This is why I keep insisting that multipath occurs at the receiving end, not transmission.

This scenario makes a ton of sense. It also demonstrates the importance of HAAT, because it seems like the multipath effect would diminish somewhat as the tower is raised (and a better line of sight is established).

 

[Kelly A]

Wow, Kelley. This is insanely helpful! Thank you for your explanation. I definitely was not aware that there were mechanisms that could shift the "beam" depending on temperature inversions. Would you normally need this type of setup in a humid environment with rising heat?

Temperature inversion layers commonly form in the later afternoon and are more common over bodies of water, dense cities where heat from the pavement and buildings rise in a column. For VHF-UHF transmission sites located close, or right in a dense city, beam tilt is commonly used to keep a certain amount of the signal from being deflected away during afternoon drive times.

The information you provided regarding the antenna bays is extremely helpful as well. It would seem that most major market radio stations would be careful about how the signal is focused.

It's not rocket science actually. The antenna is placed in a way that best covers the city of license and community/market. At broadcast FM frequencies, the radiated signal is by no means a pinpoint.

If you're an engineer on Tiger Mountain, would there ever be a reason to install the antenna bays slightly different directions on the tower to provide optimal coverage in all directions, or is it sufficient to orient each bay in the same direction?

Remember that adding antenna elements/bays focuses the signal toward the horizon. If one intentionally skewed the stacked elements, the antenna gain would be reduced, essentially reducing the effectiveness of the radiation pattern. Manufacturers typically use tuned reflecting pieces called 'parasitic elements' in various places on the antenna to direct the signal in a particular direction. As in your photo, sometimes the element is a screen, or depending on the design could be a piece of copper tubing of a certain length spaced at a precise distance from the radiating elements (bays).
There are other 'panel' style antennas that place one or more elements backed by a reflector next to each other at an angle (credit ERI):