Transmitter power

[kc0ltv]

How much power does the transmitter in a typical 100,000 watt ERP station output?

How about a 25,000, 6,000, or 1,000 watt ERP station?

I know ERP varies depending on the antenna used and cable length, but how about on average?

 

[techie2]

I've heard of 100kW stations using 40kW output transmitters that usually consist of paired 20kW units.

The rest have more options since it depends on the licensed ERP and how much gain is in the antenna, but for most it's probably something like this...
50kW ERP --> 20kW TPO
25kW ERP --> 10kW TPO
6kW ERP --> 5 kW TPO (maybe a 2.5 or 3.5 kW box depending on the actual licensed ERP)
1kW --> ?? (lots more options with a lower power facility...)

 

[littlejohn]

The question is kind of like asking "How big is a fish?"
Normally, a 40KW transmitter is sufficient to make 100KW ERP off a six bay antenna. A very common combination.
With the advent of 35KW boxes, some opt to take the savings and run an eight bay antenna.
The rule of thumb is, a single circularly polarized bay will radiate about half the input power in the horizontal plane. (Actually in multibay arrays, there's a bit more gain) consequently a 6-bay will do about 3.3 power gain, less line losses. This for full wave spaced bays. For half wave spacong (an increasingly common topology) the gain will be somewhat less. Half wave soaciing however allows better control both of downward radiation and the azimuthal pattern. Consequently they are often used in directional antennas.

 

[busyradioguy]

And don't forget the transmitter's efficiency rating. That plays a part in the equation.

In one case, a station I know of used a 35 kW TPO for an ERP around 60 kW. One year their antenna was torched by lightning. The replacement antenna by itself was more efficient. So said station was able to drop the TPO down to about 26 kW, and still puts out 60 kW of ERP.

 

[Studio1]

I worked for a station here in the early 90s that ran two Harris 10 kW units into a combiner then up to the antennas for an EIRP of 50 kW.

 

[kyscott]

And on top of all that, you have stations that are 'derated'. That is the FCC has a benchmark in antenna height, I believe it's 100 meters HAAT. If your antenna is higher than 100 meters HAAT, your ERP will be reduced so that your station will only cover what a station would normally cover in your class at 100 meters.

For example, WDJX in Louisville is a class B FM station. at 100 meters HAAT, Class Bs are allowed 50kW, but since WDJX sits up over twice than that (218 meters) they are derated to 24kW needing only 17 kW TPO to achieve their ERP. Then you factor in all the antenna parameters like has been mentioned before. They have a three bay, half wave spaced roto-tiller and 400 feet of 3" heliax. Antenna or feedline changes and transmitter power changes.

 

[littlejohn]

This thread can cover a lot of information. One thing let's clear up though - transmitter efficiency has not to do with radiated power. For example - a 40KW RCA transmitter ran about 60 - 65% efficiency. A Collins 40KW ran about 72 - 75% efficiency. However, both made 40KW of RF. The RCA ran the power and airconditioning bill up higher is about all.

And while we're at it, let's put 'efficiency' in perspective. I had a seller try mightily to sell me an MW-1 when we ran a Bauer 707. His pitch was savings from efficiency. Which savings was about a 100 Watt lightbulb. Conversely, replace an RCA BTA-50F with a Harris DX-50, and you can buy a fairly nice house on the savings in electricity burn, in a reasonably short period of time.

We're seeing this situation recur as we add HD to our FM systems. The HD signal is non-sinusoidal, hence it wants a fairly linear transmitter. Which is a fairly inefficient transmitter compared to analog FM which is sinusoidal and for which a good class C amplifier works well and delivers conversion efficiencies (DC in to RF out) in the 80 % range. Even worse, the ineficiency of the linear box shows up as heat load for the A/C system. When we jack the digital carriwer levels up, this is only going to get worse.

We have enough challenges coming along to keep the job interesting for the near future, looks as if.

 

[more kilowatts]

On the other side of the coin, look how 1 watt ERP is arrived at with 16 watts TPO for a low power FM station in Greenville, SC:

http://fjallfoss.fcc.gov/prod/cdbs/forms/prod/getattachment_exh.cgi?exhibit_id=127066

 

[BobOnTheJob]

And on top of all that, you have stations that are 'derated'. That is the FCC has a benchmark in antenna height, I believe it's 100 meters HAAT. If your antenna is higher than 100 meters HAAT, your ERP will be reduced so that your station will only cover what a station would normally cover in your class at 100 meters.

For example, WDJX in Louisville is a class B FM station. at 100 meters HAAT, Class Bs are allowed 50kW, but since WDJX sits up over twice than that (218 meters) they are derated to 24kW needing only 17 kW TPO to achieve their ERP. Then you factor in all the antenna parameters like has been mentioned before. They have a three bay, half wave spaced roto-tiller and 400 feet of 3" heliax. Antenna or feedline changes and transmitter power changes.

Not to quibble over 50 meters, but on this side of the river, a B begins derating at 150 meters or 492'. In the pre-metric days, we got 500'. A's here do indeed start to derate at 100 meters as I believe they do there as well. Guess I'll never fully understand why any of the Lexington stations or WAMZ let the full Class C rating slip away. Is WSGS 101.1 the only full C in KY?

 

[fm-engineer]

I believe class B and C's are 500' HAAT, class A's are 300' or 100m. Many class C's are way over the HAAT limits, therefore, they reduce TPO the higher they go. I know of two class C's in VA that are 1600' and 1900' HAAT. Combine the reduced TPO with antenna gain, and you could get 100 kW with 30 kW or so. I'm not sure if the grandfathered "superpower" class C's have HAAT limits.

 

[BobOnTheJob]

I believe class B and C's are 500' HAAT, class A's are 300' or 100m. Many class C's are way over the HAAT limits, therefore, they reduce TPO the higher they go. I know of two class C's in VA that are 1600' and 1900' HAAT. Combine the reduced TPO with antenna gain, and you could get 100 kW with 30 kW or so. I'm not sure if the grandfathered "superpower" class C's have HAAT limits.

There are more than one class of C stations. From memory, C3 is 25KW/328', C2 is 50KW/492', C1 is 100KW at 984', C is 100KW at the metric version of 2000' (1968'??). The VA stations are probably C1's. There was talk about a C0 class that was to specify something like 1400' at 100KW. Not sure if that ever came to pass or not. Living 90 air miles from the nearest C1, I can only drool at flame throwers like them. I know...keep the drool at least several inches from the 10+KV it takes to power those babies.

 

[First Ticket]

One of the problems I have confronted for years is the question from the sales department...."I have a client who wants to know, how much power does station XXXX put out?"

So I tell them that the station is a class A with an ERP of 115 watts with a single bay antenna and a TPO of 300 watts. Then comes the explanation that the station covers 5 counties with that little bit of power because the antenna is 500 meters above average terrain.

Then it's up to the AE to try and explain that to the client!

 

[MW_FM_DT_DXer]

C1 is 100KW at 984'
There was talk about a C0 class that was to specify something like 1400' at 100KW. Not sure if that ever came to pass or not.

C1 is 100 kW at 981', not 984'.
Class C0 did come to pass, and it specifies 100 kW at 1476'.

 

[kyscott]

Not to quibble over 50 meters, but on this side of the river, a B begins derating at 150 meters or 492'. In the pre-metric days, we got 500'. A's here do indeed start to derate at 100 meters as I believe they do there as well. Guess I'll never fully understand why any of the Lexington stations or WAMZ let the full Class C rating slip away. Is WSGS 101.1 the only full C in KY?

You Hoosiers always gotta be different!

I went back and looked it up, indeed it's 150m on that side of the river. Either way, WAMZ folks should have taken the derating from WHAS' TV tower. That would truely have been a monster signal. Because they sit lower than the Knobs down on Brook's Hill, I think WDJX really as the better signal of all the FMs in Louisville.

 

[OKCRadioGuy]

We've got a combined system here in the City that has 4 full C's blasting at 1545 feet above the ground along with one C1 and one soon-to-be C1. The only bad thing is that the antenna is about the equivelent of a 12 bay and there are some areas closer to the stick that the null-fill isn't all that great. All of the stations out of that site certainly have the range though! ;D

 

[kyscott]

We've got a combined system here in the City that has 4 full C's blasting at 1545 feet above the ground along with one C1 and one soon-to-be C1. The only bad thing is that the antenna is about the equivelent of a 12 bay and there are some areas closer to the stick that the null-fill isn't all that great. All of the stations out of that site certainly have the range though! ;D

considering how flat Oklahoma is, I bet those signals cover over 100 miles out from the site. Having 12 bays, I can see where near field null fill could be a problem.

 

[radiorob2.0]

And on top of all that, you have stations that are 'derated'. That is the FCC has a benchmark in antenna height, I believe it's 100 meters HAAT. If your antenna is higher than 100 meters HAAT, your ERP will be reduced so that your station will only cover what a station would normally cover in your class at 100 meters.

For example, WDJX in Louisville is a class B FM station. at 100 meters HAAT, Class Bs are allowed 50kW, but since WDJX sits up over twice than that (218 meters) they are derated to 24kW needing only 17 kW TPO to achieve their ERP. Then you factor in all the antenna parameters like has been mentioned before. They have a three bay, half wave spaced roto-tiller and 400 feet of 3" heliax. Antenna or feedline changes and transmitter power changes.

Not to quibble over 50 meters, but on this side of the river, a B begins derating at 150 meters or 492'. In the pre-metric days, we got 500'. A's here do indeed start to derate at 100 meters as I believe they do there as well. Guess I'll never fully understand why any of the Lexington stations or WAMZ let the full Class C rating slip away. Is WSGS 101.1 the only full C in KY?

96.1 WSTO and 92.5 WBKR in Owensboro are both Class C's. In the case of Lexington they did let it slip away. There had been discussion for years for a "Super Tower" to place the FM's and TV's but it never happened.

 

[w9wi]

I believe class B and C's are 500' HAAT, class A's are 300' or 100m. Many class C's are way over the HAAT limits, therefore, they reduce TPO the higher they go. I know of two class C's in VA that are 1600' and 1900' HAAT. Combine the reduced TPO with antenna gain, and you could get 100 kW with 30 kW or so. I'm not sure if the grandfathered "superpower" class C's have HAAT limits.

The "real dope" on class limits from 47CFR73.211:

Class D: no new stations.
No minimum; no maximum (or minimum) HAAT. It's my understanding transmitter power is limited to 10 watts but higher ERPs are possible due to antenna gain.

Class A: Minimum power: 100 watts
Maximum power: 6,000 watts
Maximum HAAT: 100m
Class contour distance: 28km

Class B1: Minimum power: 6,000 watts
Maximum power: 25,000 watts
Maximum HAAT: 100m
Class contour distance: 39km

Class B: Minimum power: 25,000 watts
Maximum power: 50,000 watts
Maximum HAAT: 150m
Class contour distance: 52km

Class C3: same as B1

Class C2: same as B

Class C1: Minimum power: 50,000 watts
Maximum power: 100,000 watts
Maximum HAAT: 299m
Class contour distance: 72km

Class C0: (educated guess, this class was created after my copy of the rules was printed)
Minimum power: 100,000 watts
Maximum power: 100,000 watts
Maximum HAAT: 450m
Class contour distance: 82km?

Class C: Minimum power: 100,000 watts
Maximum power: 100,000 watts
Maximum HAAT: 600m
Minimum HAAT: 450m
Class contour distance: 92km

The "Class contour distance" is the distance to the 1mV/m (60dBu) contour on the F(50,50) curves in 73.333.

A station's HAAT may exceed the cited maximum if the power is reduced to limit the 60dBu contour to the class contour distance. (however, the station may not exceed the listed maximum power if its antenna is lower than the maximum HAAT. Stations will not be authorized with ERPs of less than 100 watts, unless the antenna is high enough that a reduction below 100 watts is necessary to keep the 60dBu contour below 28km. (or the station goes for a LPFM or translator license... there are also some existing Class D stations that don't meet this minimum)

Class B/B1 stations are only authorized in Zone I; class C/C0/C1/C2/C3 stations only in Zone II.

The class differences also result in differences in protected contour (how weak a station's signal can be and still be protected from interference from a newly-authorized station) and in required allocation distances. (how far apart channel allotments can be on the same and nearby frequencies, where there are stations authorized on those allotments or not)