FM Broadcast Questions

[BinaryMode]

To those that may know. I have a few questions about FM broadcast radio.

There is an FM broadcast station in my area called Retro 102.5. According to Wikipedia they transmit 17,000 watts. Why wouldn't they broadcast 100,000 watts to increase their market?

How many watts of electrical power does it require to power a transmitter of 100,000 watts? Is it 120 or 240 or more volts?

Do these radio towers have a propane driven backup generator?

Is there redundancy with the feed to the tower either via POTS, fiber or microwave? By that I mean if the tower is fed via microwave, is there also a fiber connection or POTS connection for backup?

Thanks.

 

[prcguy]

Some broadcasters are power limited according to their intended market, area and transmitter location. Are they transmitting with 17,000 watts right out of the transmitter or is that ERP including antenna gain? A 100kW transmitter probably runs off 480V 3 phase and would consumer about 160,000 to 180,000 watts or about 375 amps at 480v or if its running on 220V (doubtful) it would consume about 818 amps at 220V.

Most broadcasters have some sort of backup and generator is very common. Many transmitters are at remote sites and programming is fed via fiber, Internet, microwave, satellite, etc.

 

[mmckenna]

I've got two FM Broadcast stations at one of my tower sites.

7,000 watts + antenna gain = 21,000 watts Effective radiated power.
It's on 220 volts and I think a 60 amp circuit.
Smaller station is 100 watts, community radio, small foot print. 110 volts.

Site has a propane backup generator that supports the FM stations as well as the cellular carriers.

To add to what prcguy said above, RF power depends on what you want to cover and what you can cover. While FM broadcast will bounce around pretty well, it tends to be limited by topography. So if a station is out on the plains and has a very tall tower, running more power may get more coverage. In the mountains, hills and valleys, the range may be limited by the surrounding topography, and not power. So burning more power may not be worth the cost.

 

[KevinC]

And they have awesome Bird watt meter slugs.

 

[prcguy]

If an FM broadcast station is actually transmitting 100kW out of the transmitter you will usually see coax like this 6 1/8" rigid line. With smaller transmitters up to 30kW you might see 3 1/8" rigid line. For up to about 5kW you can use 7/8" Heliax.

 

[jonsmth]

Year ago I worked at an FM with 100,000kw ERP. Essentially, we put out approx. 20kW out of the transmitter into the antenna. With the antenna gain and line losses, it was estimated to be an Effective Radiated Power of 100kW. (I think the effeciency was something in the neighborhood of 0.78. I recall the Voltage and Amperage readings of the transmitter being in the neighborhood of 8800V to 9200V and amperage around 2.85. This is back in the days (early 80s) when the operators had to take hourly transmitter readings and adjust the transmitter if it went out of limits. I'm not a Broadcast Engineer, but often times I saw the Engineer since it was my shift that signed the station off at midnight and the Engineer would show up to perform maintenance on the transmitter. I often assisted him. As for the limitation of the rated power of the station, that is controlled by the FCC so that the station will not interfere with another Broadcast FM operating on the same frequency or an adjacent frequency.

 

[BinaryMode]

That's some impressive coax! Is that brass? The welds look like TIG. I'm guessing the holes in the (dielectric?) are for ventilation as I'm assuming not only does the antenna get hot but so does the coax. .

As for the limitation of the rated power of the station, that is controlled by the FCC so that the station will not interfere with another Broadcast FM operating on the same frequency or an adjacent frequency.

I had thought that.

 

[AK9R]

That's some impressive coax! Is that brass? The welds look like TIG. I'm guessing the holes in the (dielectric?) are for ventilation as I'm assuming not only does the antenna get hot but so does the coax.

The outer tubing (shield) is probably copper with brass fittings welded onto the ends for coupling to other devices or feedline.

The center conductor is air-gapped from the shield. The white thing with the holes is a separator to keep the center conductor at a specified distance from the shield. In general terms, the distance between shield and center conductor determines the impedance of the coax and you want that impedance to remain constant. The gap between the shield and center conductor is usually filled with a gas, such as filtered and dried air, nitrogen, or argon, at a slight pressure above atmospheric to keep moisture from seeping into the coax. The gas is also a good indicator of breaks in the coax because the gas flow is monitored. If the gas flow increases, you've got a leak.

Most electric utilities in the U.S. can provide 4160 volts to industrial and commercial customers. In the days of tube power amplifiers at TV and radio broadcast transmitters, starting off at a higher voltage might have been an advantage. The TV transmitter that I'm most familiar with completely switched over to solid state amps 20-25 years ago. The amps are individual "drawers" that can be switched on and off with the outputs combined to provide the total output going up the tower. These solid state amps don't require as much voltage as a tube amp.

 

[a727469]

Does anyone know if there is an FM station with 100kw actual transmitter power, not ERP..?
maybe not in this country?

 

[AK9R]

The FCC has a classification system for FM broadcast stations: FM Broadcast Station Classes and Service Contours

I just did a search using the search tool on the FCC's website: FM Query Broadcast Station Search

702 Class C stations, though not all show 100 kW
365 Class C0 stations, though not all show 100 kW
1573 Class C1 stations, though not all show 100 kW

 

[dlwtrunked]

... Why wouldn't they broadcast 100,000 watts to increase their market?

....

Often increasing their market by increasing power would interfere with another stations market. In say that, keep in mind that there needs to be some distance between to provide reliable non-interference in the markets. A lot of work goes into predicting the coverage and coordinating so that interference is not a big issue.

 

[trimmerj]

Was fun when ducting took place and overtook the local signal. PD and GM are screaming "Can't you guys do anything?!"
Nope....

 

[prcguy]

That's some impressive coax! Is that brass? The welds look like TIG. I'm guessing the holes in the (dielectric?) are for ventilation as I'm assuming not only does the antenna get hot but so does the coax. .




I had thought that.

Yup, copper pipe for the shield and also copper pipe for the center conductor, Teflon insulators ever 4ft or so to hold and center the inner pipe. There are silver plated double male bullet things with fingers that slide into the mating center conductors to join them.

I have a job coming up soon to build out the transmission line for a backup UHF TV transmitter and it will probably be 3 1/8" rigid line. It comes prefabbed in about 6ft lengths buy you end up needing many shorter pieces so you measure precisely and use a metal cutting chop saw cutting the outer and inner conductor separately then reassemble the Teflon spacers and bullets. You gotta remove all burrs, debris and clean all inner surfaces with solvent before assembling as things can arc at very high power.

The 3 1/8" uses the equivalent of copper plumbing bends and straight couplings that are split and use hose clamps to assemble rather than a brass flange with bolts for most connections. Here is the last rigid line job I did, took 3 days to completely build the coax from two 10kW transmitters to filters, coax transfer switches, directional couplers and to the other side of the building where it exits to the outside coax run. It makes for a very busy day using dial calipers, plumb bobs and tape measures. And you have to think in three dimensions.

If anyone wants to upgrade their CB coax just call me!

 

[trimmerj]

Nice work!

 

[a727469]

The FCC has a classification system for FM broadcast stations: FM Broadcast Station Classes and Service Contours

I just did a search using the search tool on the FCC's website: FM Query Broadcast Station Search

702 Class C stations, though not all show 100 kW
365 Class C0 stations, though not all show 100 kW
1573 Class C1 stations, though not all show 100 kW

Thanks!..I was aware of the fm classifications but had not looked at numbers recently.

I guess they do not care about transmitter power on fm but just ERP, HAAT and db contour. I looked at a few C listings and unless I missed it, I see no transmitter power. I do wonder how many stations, if any are pushing out near 100kw because of very low haat. Obvious, very inefficient.

 

[WB5UOM]

ERP HAAT and db contour cause many big stations to pay to move distant smaller broadcasters that are on same freq
Its a big money thing...Years ago a Shreveport station paid to build a new 500ft tower several miles west of an existing stations tower in order they could increase power and coverage.
Did not cost the local station a dime.
(and I picked up the orignal 547ft tower for a song)

 

[W1KNE]

So to answer the original OP @BinaryMode s question, the reason why KTRR is not transmitting at 100kW is because it is a class C2 license, which is limited to 50kW max, up to 150 meters height above average terrain.

Now if they were to go to 100kW, they'd have to upgrade to a class C1, but this is where the FCC rules step in.

Radio stations in the United States operate under "allocations" and the FCC allocations are required to have minimum distances between each other, so as to not cause interference. The rules fall generally under 47 CFR 73.207. There are some (many) grandfathered stations that are shorter spaced, primarily in the Northeast, because they were short before the current spacing rules were set in 1964.

As for KTRR on 102.5C2, I ran the numbers for it as a "C1". And unfortunately, it would be too short to a few.

KHOC 102.5C. It is 264.38kM away. The FCC minimum is 270kM, so it is short. To KHOC, they would have to file for "73.215" spacing, which allows them to operate with a minimum distance of 249kM.
KQSE 102.5C2. It is 196.35kM away. The FCC minimum spacing is 224kM. So it is VERY short. The 73.215 spacing rules still require 211kM separation. So they would have to move.

However the separation to KPNY 1023C0, KBIQ 102.7C and KARS 102.9C1, all three of which have about 1kM of "wiggle room", would likely prevent that.

So KTRR is very very likely going to remain a C2, where it is at for the remainder of its days. Which is okay, if you check the ratings, KTRR is in the top 5 pretty regularly there, so the lack of being 100kW doesn't seem to be hurting them either.

--
Now with regards to the MAXIMUM allowed on FM. The maximum allotted power on FM is a "Full Class C". Which is 100kW at 600 meters (1968.6ft) above average terrain. There are several radio stations that are grandfathered and run well above this maximum. The station with the largest signal based on height and power is KRUZ 103.3, licensed to Santa Barbara, CA. It transmits 105kW at 905 meters. However half of that signal goes over water. The FM station with the largest land coverage is WHOM 94.9 on Mount Washington, NH, which is 50kW at 1141 meters.

The highest powered FM station in the whole country is WBCT 93.7 in Grand Rapids, MI, which transmits at a whopping 320kW. It is the only FM in the US over 300kW. (WLFP, the former WMC-FM 99.7 in Memphis, TN is at 300kW).

 

[nd5y]

One of the few radio related youtube channels not run by morons.
They explain all the hardware.

The forum software won't allow more than two media.
These two are different than the ones above.

- YouTube


- YouTube

 

[BinaryMode]

If anyone wants to upgrade their CB coax just call me!

HA! That made me smile.

So to answer the original OP @BinaryMode s question...

Very informative. Thanks.

One of the few radio related youtube channels not run by morons.
They explain all the hardware.

That was very informative. I've seen some of his videos in the past talking about Starlink.