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Radio Frequency vs. Power

R

RadioFreeAtlanta

Hope this is the topic board to post this....

I have a question concerning frequency and wattage/power.

If a 100,000 watt station is at 88.1, will it reach a longer/further distance than one at 107.9 with the same wattage? I have always wondered if a station's frequency matters in terms of power and strength.

I appreciate your kind replies.
 
Power and height above average terrain (and the terrain) will always be the major factors in FM coverage.
The advantages/disadvantages seen in AM at opposite ends of the dial do not come into play for 88-108 Mhz.
That said, as frequencies go up, they behave "more and more" like optical beams, with line of sight behavior.
Theorectically, 88 could "curve" over the horizon at bit more than 108, but not enough so's you'd ever be able to measure or notice.
 
Tom Wells said:
Power and height above average terrain (and the terrain) will always be the major factors in FM coverage.
The advantages/disadvantages seen in AM at opposite ends of the dial do not come into play for 88-108 Mhz.
That said, as frequencies go up, they behave "more and more" like optical beams, with line of sight behavior.
Theorectically, 88 could "curve" over the horizon at bit more than 108, but not enough so's you'd ever be able to measure or notice.
Click to expand...
To add to what Tom stated so well, the difference in frequnecies between 88 and 108 mhz is a factor of 1.22:1...pretty insignificant. Whereas on AM, the ratio is over 3:1...a huge difference. Same thing on TV, where the spacing from channel 2 to the upper UHF channels have a factor of over 11:1. While no frequency related power level differences are applied at AM or FM frequnecies, channel 2 TV is allowed 100,000 watts of visual power, whereas the UHF stations can operate with up to 5,000,000 watts in a fairly effective effort to level the playing field. Good question to be sure, but in the grand scheme of things, it's like the houses on one end of the block being worth more because the ground is 10' higher on that end.
 
OK.. Here's a fun subjective question sort of on the same subject path. If you had two class A's sitting at 300 feet and 6k would you crank them up to 500 feet, 700 feet, 900 feet, or higher with the required power reduction being considered? I am in Oklahoma City where it's fairly flat but we are pretty spread out too. The signal in OKC is located in the NE side of town, within basically the edge of the population. My other tower is located about 10 miles from it's intended target. Both the town and the tower at this station is in a bit of a hole. I have been told by several people that going lower than 1.5k isn't a great idea due to signal penitration limitations. What's the concensus?
 
In a relatively flat market like Oklahoma City, you're probably better off at 100 meters with 6 kW. The radio horizon at this height, assuming no intervening hills, falls at a radius of 35 km or about 21 miles. See: http://en.wikipedia.org/wiki/Radio_horizon

If you have a significant audience beyond this distance or a major obstruction, then you might consider going higher.

As far as building penetration is concerned, the "best" combination of height and ERP all depends on distance to the buildings and the obstructions in between. But keep in mind, on an unobstructed path, the inverse square law (6 dB reduction each time the distance is doubled) applies regardless of how much power you start with.

Several years ago, I built a Class A on top of the tallest building in the central business district of a large city, where the HAAT of 275 meters (900 feet) allows an ERP of only 780 watts for 6 kW equivalence. However, most of the office buildings are within a mile, so the signal penetrates without much trouble -- in fact, it sounds fine even in subway stations and underground parking garages. The low power made it practical to use a single bay antenna, and the high elevation provides line-of-sight coverage across most of the city, which does have some terrain issues.

Needless to say, if I had chosen to mount the antenna on a shorter building, or at a rental site several miles away, we could have had the full 6 kW (and a bigger power bill), but nearby tall buildings would have cast large shadows and worsened multipath.
 
Play Freebird said:
In a relatively flat market like Oklahoma City, you're probably better off at 100 meters with 6 kW. The radio horizon at this height, assuming no intervening hills, falls at a radius of 35 km or about 21 miles. See: http://en.wikipedia.org/wiki/Radio_horizon
Click to expand...

I'm gonna have to disagree with you here, Freebird. Always take height over power. If you had a Class A at 700 feet, derated, it would still put out a pretty sweet signal. I'll take height any day.
 
In the OKC version of my two signals the downtown district is about 5 miles away. The transmitter site is NE of that downtown area. The sprawl of town extends at least 15-20 miles if the extended suburbs are included. My gut thoughts on the subject are about 600 feet up would be good, but I'm open to ideas. Thanks for the input so far!
 
FM stations with HAATs higher than their class maximum must reduce their ERP so that their 1 mV/m, f(50,50) coverage radius is the same from the higher HAAT as if they were at the class maximum.

All but one (WFMT) of the Class B FMs on Sears Tower in Chicago must use an ERP of ~4.5 kW, which is more than 10 dB below the ERP they could radiate from 150 m HAAT.

This reduction does have an affect on "building penetration" in the metro area as compared to 50 kW from 150 m, but FM coverage from Sears Tower still is quite good.
//
 
There are always exceptions, depending on the local terrain situation. A few stations can take advantage of the flawed FCC coverage prediction method, and are using "apparently low" sites on bluffs or sides of valleys that don't require de-rating but work very well in the desired directions.

One of the best examples is KQBT near Albuquerque on the west face of the Sandia Mountains. KQBT is Class A with 3 kW at only 30 m HAAT (the antenna is just 18 m AGL) and licensed "nondirectional", but look at their coverage across town:

http://www.fcc.gov/fcc-bin/FMTV-service-area?x=FM87509.html

Because of terrain averaging, this site provides a 1500 foot elevation above the city with no need for derating. And I'd be willing to bet the antenna is highly "optimized" to throw the power west, so the actual ERP is probably 6 kW towards the valley.

Compare this with the specs of Z-100's Empire State Building facility (6 kW at 1360 ft) and it's clear that this little 3 kW Class A is really a "Class C2" where it matters.

Of course, economics also come into play. If a Class A station serves a small town in a rural area and can achieve 100 m HAAT with less than a 200 foot tower (on a hilltop) with no major obstructions in the way of the population centers, there's probably no reason to spend the money to build a taller structure which would require more land and the ongoing expense of obstruction marking.
 
Mr. Fry makes a good point woith reference to building penetration... empirically there's a 'tip point' above which the signal will be useable, below which it won't. However, also empirically, the height always trumps the higher ERP. I suspect this may be because the bulk of listenership is in mostly wood houses and automobiles rather than office buildings which show a higher RF attenuation.

I haven't been in The City for several years, but I'd worry about building shading if I got too low. KKNG isn't still on that interelaved abortion is it?
 
No... They just moved to a new stick (where ch 34 used to be) for their main with their sister stations about a year ago. Now they are on a new ERI combined thing (it's own version of a abortion actually!). That Shievly interleaved thing on the Oklahoma Tower NEVER worked worth a damn compared to it's sister station at the same height that was using just a ERI rototiller on the side of 25s tower. Certainly a comprimise from the day it was installed wasn't it?
 
I really think my little station DOES have a lot of shading issues. Looking at some line-of-sight software online (I realize radio will see past that of course) and looking at some real prediction maps that I have access to shows some pretty nasty holes in the coverage, at least secondary where one would normally expect primary coverage. I live on the south part of the city and the mapping software is pretty consistant with what I get even in the car. I bet I do have some shading that a bit more height might cure. Being that the city center is only 5 miles away I'd guess penitration to the downtown buildings would work even at lower power levels, at least to a point. Thanks for the input!
 
We are on a tall tower and there are several others of course in the area. Although we certainly haven't figured out if we can go higher on the existing tower or elsewhere, I would assume something could be worked out after HDTV conversion/analog sundown. We are currently at 300 feet. There isn't much more on the tower other than considerably further up. A good guess of heights might be 500 feet 700 feet and maybe even 900 feet. Anything over 900 would start getting into too little power. We are limited to 2k less than a full 6k class A due to short spacing. If we moved south a bit that would be fixed. At this point we are stictly dreaming about how to make things better, but at some point the budget might allow us to act on it. Thanks for the info!
 
If you're out in the tower farm, I'd run the thing up as high as I could get it. We put KKNG on there so it wouldn't overlap KWEN when we moved it to Sand Springs. Knowing we were selling KKNG. When it first went up there, the rigger got one of the bays upsidedown... you couldn't hear the thing in the parking lot hardly. It did improve a bit when he corrected it.
ERI put something you dint like? I've always had good luck with their stuff. Including their panels and cavity backed stuff.
There used to be a family of skunks who lived up there, we often watched Mama lead the kids around the towers. From a distance.
I'll be in TheCity (well, Blanchasrd anyway) in a couple weeks, and then over to Tulsa, gonna see some of the old radio types from Back When.
 
"I have several additional studies that I need to prepare this evening. If it's OK with you, I'll post the information tonight and post a link to it here before morning." That would be excellent! I'd love to see it. THANK YOU!
 
littlejohn:
"We put KKNG on there so it wouldn't overlap KWEN when we moved it to Sand Springs. Knowing we were selling KKNG." I forgot about that. The old FCC rule that certain countours couldn't overlap.. "When it first went up there, the rigger got one of the bays upsidedown... you couldn't hear the thing in the parking lot hardly. It did improve a bit when he corrected it." I heard about that deal. Sometimes I wonder if we had the same upside down issue with that antenna even after we went back with it after a burn up (had to stay with the Shievely deal because KJ was still on there and having both antennas on the same pole was an issue) One time I listened to 92.5 from OKC just outside of DALLAS up in a plane once we got a little out of town! I think it worked better up there than on the ground. I tried other stations from OKC and didn't have the the same results right after that.
"ERI put something you dint like? I've always had good luck with their stuff. Including their panels and cavity backed stuff."
Normally I'm a big ERI fan. I just don't think this new thing they put up is 100 percent, mainly because of nulls closer to the tower. It's supposed to have null-fill but I can find several places that it's not to perfect. The old rototillers that 104.1 and 107.7 used to my ears worked better in town. Out of town the new stick, because it's up so high, works better of course.
"There used to be a family of skunks who lived up there, we often watched Mama lead the kids around the towers. From a distance." I heard stories about the scunks but luckily I never ran into the LaPew family out there. That site sure brought other wierd things out including abandoned cars and some other wierd things. We got much better about securing the gate after a stolen car appeared one time.
"I'll be in TheCity (well, Blanchasrd anyway) in a couple weeks, and then over to Tulsa, gonna see some of the old radio types from Back When." Excellent! Always good to catch up with radio folks. A lot of the some guys are still around in the market. I guess we're pretty lucky in that respect. Most of the CEs around here have been at it for 25 plus years.
 
I would take more height over more power. We have had this discussion many times before. Frequency doesn't affect how far a station goes, assuming no interference. Theoretically, a higher frequency wave would have more energy, but the difference between 88 and 108 is insignificant. A bird won't die if it lands on the antenna of a 50000 watt FM, but it will die inside a 1000 watt microwave oven.
 
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