Have always wondered if a transmitter is custom-built for a certain frequency, or is that something that is changed with a setting?
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Random techie question
- Thread starter LITTLEBOYBLUE
- Start date
Modern transmitters are pretty much all frequency-agile, meaning they can be changed either on a front panel or gui to whatever frequency you'd need. Antennas should can and should be tuned to a frequency by the manufacturer. There are some broadband FM antennas that can do the whole range but they generally don't perform as well.
Old tube transmitters need a physical change to adjust frequency.
Old tube transmitters need a physical change to adjust frequency.
To clarify cc-mar's post. most FM transmitters are built to be frequency agile. On the other hand, AM transmitters are generally custom built to a specific frequency with many components that need to be changed if the transmitter moves to a frequency that is too far astray from the one it was built for.
That's not totally true. High power transmitters even solid state do have frequency dependent parts. The Exciters in most FM transmitters are frequency agile but when you get to the Power Amplifier they are not. The Harmonic filter on the output of an AM or FM transmitter is tuned to the frequency or Harmonic it is supposed to suppress. There may be other frequency dependent parts for combined PA amps to make the total transmitter output power.
Just to follow up on this: what makes an antenna tuned specifically for a frequency?
AM towers always seemed the same to me, whereas FM antennas come in a wide variety of styles and shapes.
AM towers always seemed the same to me, whereas FM antennas come in a wide variety of styles and shapes.
For AM it's two things, the height of the tower and the matching network at the base of the tower. Lower frequency AM stations do better with taller towers. Higher frequency AM stations use shorter towers. That's why you hear AM tower expressed in Wavelength. For a combined AM site it's picking (some times it not a choice) a tower height that will work for all stations and then the tuning network at the base of the tower to further match the tower to station frequency.
For FM it's how the bay is constructed. Arm length and spacing of the upper arm from the lower arm. Even the spacing of the elements from each other on a multi bay antenna. Some combined FM antennas have an upper section for one frequency range and a lower section for the other frequency range. That's why you some times see slightly different HAAT for stations using a combined antenna.
For FM it's how the bay is constructed. Arm length and spacing of the upper arm from the lower arm. Even the spacing of the elements from each other on a multi bay antenna. Some combined FM antennas have an upper section for one frequency range and a lower section for the other frequency range. That's why you some times see slightly different HAAT for stations using a combined antenna.
Let me start with why.
If an antenna is not tuned, it will reflect power back to the transmitter. It's a property of electromagnetics known as Jacobi's Law.
The transmitter has to dissipate any reflected power as heat. Too much reflected power will damage the transmitter, which is one reason FM transmitters sometimes shut down during icy weather. The ice changes the impedance of the antenna, which raises the reflected power. For that reason, you want your tuning in nominal conditions to be good, so that if you get a little ice, or other adverse condition, the transmitter won't fold back (reduce power) or shut down totally.
If you saw a multi-bay FM antenna for 90.1 and 104.9 from the same manufacturer on the ground, you'd probably notice the sizing difference, even though it would only be a few inches. But up on top of a 500 ft tower, they look pretty mostly the same.
I always pictured an FM antenna as a "controlled" short circuit - If the wave length of the antenna matches the frequency, the resulting waves are broadcast efficiently and very little energy returns to the source (transmitter). If the antenna length and frequency are mis-matched, then part of that radiation finds its way back to the source that could potentially damage the transmitter. In a true short circuit (e.g., defective lamp cord) 100% of the energy creates sparks, heat and finds its way back to the source (circuit breaker panel)
Amazing explanations from both @xmtrland and @PTBoardOp93 !
That definitely provides a lot of context on how these antennas function and how they are designed. Am I right to assume that antenna design has changed quite a bit over the years? I recall seeing older antenna models that looked a bit like different marine floats/buoys spaced across a pole that is mounted to the side of the tower. This design seems to have been replaced by metal elements that have taken the place of the float/buoy style design.
My assumption that mounting on the pole allows for beam tilt, though not every radio station uses beam tilt. I believe David Eduardo explained to me in a different thread that beam tilt is required in humid environments to account for changes in the weather environment between morning and afternoon.
Do the antenna elements combine together to radiate one single beam of “light” (or in this case, microwave energy)? Or do they each have an individual responsibility of targeting a specific degree angle from the tower to the target listeners?
Fascinating stuff here! I think we all can learn quite a bit from this post.
That definitely provides a lot of context on how these antennas function and how they are designed. Am I right to assume that antenna design has changed quite a bit over the years? I recall seeing older antenna models that looked a bit like different marine floats/buoys spaced across a pole that is mounted to the side of the tower. This design seems to have been replaced by metal elements that have taken the place of the float/buoy style design.
My assumption that mounting on the pole allows for beam tilt, though not every radio station uses beam tilt. I believe David Eduardo explained to me in a different thread that beam tilt is required in humid environments to account for changes in the weather environment between morning and afternoon.
Do the antenna elements combine together to radiate one single beam of “light” (or in this case, microwave energy)? Or do they each have an individual responsibility of targeting a specific degree angle from the tower to the target listeners?
Fascinating stuff here! I think we all can learn quite a bit from this post.
I suspect you're describing radomes, which are fiberglass covers for the antenna elements. These might be used if the material the antenna is made of is subject to rust/corrosion, or if significant icing is expected.
The design of FM antennas hasn't changed a whole lot in the last 40+ years. The ERI RotoTiller was patented in the late 70s, and the same antenna with some tweaks is commonly used today. One of the largest examples I've seen was in Birmingham, AL, where a 16-bay RotoTiller was used in a television application. Photos: NECRAT Picture V4.0
Beam tilt is not commonly used in the Great Lakes region where I have worked. I've understood it to be used in mountainous markets like Denver or ABQ, where the antenna is expected to be *much* higher than the audience.
This is a complicated question. The individual bays matter, especially when you start talking about directional antennas. The designer needs to model them. But the listener perceives them as a single coherent source due to the wavelength of FM (about 3 meters) being much smaller than the distance from the antenna to the listener. All of this is different on AM.
That must be it, fiberglass covers. Strangely, I haven't seen fiberglass covers on most modern transmitter/antenna setups. I was up at Mount Seymour in British Columbia and didn't see anyone using that design (despite the issue with winter icing). Maybe it's not necessary unless there is significant icing at locations further away from the coast.
Out of curiosity, why don't we see many examples of beam tilt being used on any of the Seattle FMs? Based on what you described, Cougar Mountain is probably too low to necessitate that type of design, but Tiger Mountain is pretty high compared to the listening audience. Is the distance from the market a mitigating factor? Where the audience is not position straight down, but spread out from the transmitter site?
I'll need to wrap my head around the antenna bay answer. If I understand correctly, each antenna element puts out a certain amount of power, and that power output would add up to be the ERP. I guess this makes sense if an antenna element can only have a certain amount of power pushed through it.
I have a couple random techie questions myself having little to do with the current discussion:
1. Does a transmitter and associated equipment plug into a standard outlet, or is there a different kind used?
2. At a shared site, who is in charge of keeping things running? For example, at the site that caught fire several years ago, you have Audacy, iHeart, Hubbard, and Classic Radio all sharing that antenna. So, whose job is it to keep the generator fueled in case of a power outage? I would imagine in the case of a power issue, only one engineer would be needed to go up there.
1. Does a transmitter and associated equipment plug into a standard outlet, or is there a different kind used?
2. At a shared site, who is in charge of keeping things running? For example, at the site that caught fire several years ago, you have Audacy, iHeart, Hubbard, and Classic Radio all sharing that antenna. So, whose job is it to keep the generator fueled in case of a power outage? I would imagine in the case of a power issue, only one engineer would be needed to go up there.
The transmitter would not. Much of the other equipment at a transmitter site wil use a standard AC plug.
The standard AC plug used in the USA is limited by the National Electric Code to 1.5kW, which is too low for most broadcast stations. Typically a transmitter is wired directly into the building power feed by an electrician.
This would be spelled out in the lease agreement between American Tower Co. and the various broadcasters who use the tower at West Tiger Mtn. I don't know what the arrangements are at West Tiger Mtn.
Where I've worked in the midwest, most commonly the tower owner provides only vertical space and anything required by regulation, such as tower lights. The broadcaster is responsible for all their equipment, including their own shack, generators, air conditioning, etc. But shared master antennas tend to be quite rare.
Really? I would have thought shared antennas would be somewhat common, particularly in larger markets. In Portland, Stonehenge is a major site, with multiple operators up there. Interestingly, all of the former CBS stations are at one site despite now being owned by two different companies. There's only one more site in Portland. As far as Cougar and Tiger, I'm guessing there are only five antennas for more than a dozen stations. I'm not sure if there's a third tower on Tiger that houses 97.3 or if that's on an antenna higher up on one of the existing towers, but even if it was on an existing tower, I would think that it would be easier to provide one generator for everyone. Perhaps Scott Fybush can provide some more information, but from reading his tower site columns, I found out that 94.7 in Portland does transmit from Stonehenge, but from a directional antenna higher up on the tower. To me, one generator would still be easier for that setup.
Shared antennas are pretty rare. Separate antennas on one tower are more common.
A couple large market examples of the latter:
- Sutro Tower in San Francisco is perhaps the greatest example. The three upper spires of Sutro Tower contain 11 separate antennas, and the lower levels contain many more. The lower levels are mostly aux facilities and microwave pickups, but also include some lower power stations and translators
- The masts atop Willis (Sears) tower in Chicago has a whole set of single station antennas for each of the FMs, all stacked neatly on the two masts..
The first master FM antenna wasn't installed until 1965-66 on the Empire State Building, and by the time that was proven tech, many FMs were already established. There wasn't a real incentive for stations to move in together.
Moving antennas is expensive. If it ain't broke...
About transmitter power-
Some small FM transmitters can run on 120 VAC 15 amp "house current". Such as 150 and 300 watt models frequently used for LPFM and FM Translator stations.
Indeed, you actually could have your LPFM or FM translator transmitter on a reasonably sized UPS, and stay on the air during the transfer to your back-up electric power generator.
Some small FM transmitters can run on 120 VAC 15 amp "house current". Such as 150 and 300 watt models frequently used for LPFM and FM Translator stations.
Indeed, you actually could have your LPFM or FM translator transmitter on a reasonably sized UPS, and stay on the air during the transfer to your back-up electric power generator.