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Is an ERP of 100 watts REALLY that much different than 1000 watts?

R

radiofish

Situation:

We want to install an FM antenna about half-way up on a wireless tower. The ERP will be 1000 watts.

The tower currently has several wireless carries and public safety antennas near the top of the tower. The tower owner is concerned that the proposed "high-powered" 1000 watt FM signal is excessive and will harm the other antennas on the tower.

However, on a similar tower that this tower owner owns, a 100 watt FM antenna is allowed very close to the top of the tower near the public safety antennas without concern.

Question:

At what distance is the field strength from a 1000 watt antenna equivalent to that of a 100 watt antenna? Certainly the vertical distance isolation of a 1000 watt antenna many meters lower on a tower mitigates the difference in power level relative to the public safety antennas at the top?

Can anyone suggest a simple argument that can be made to the tower owner in defense of the proposed 1000 watt antenna?

(note we're not even considering the effect of multiple bays on the upward/downward radiation, or the potential harmonic frequencies, etc.)
 
Sounds like a no-win situation. Any problem, or defection with his tenants will be blamed on your operation.

Simple answer to your question: 10 db
 
Tom, what distance in meters separation is 100 watts the same effect as 1000 watts on a tower?

My argument will be that my 1000 watt antenna at X meters down lower on the tower is much better than that 100 watt antenna at closer proximity to the "important" antennas.
 
You need to find another tower. Any problems the tower owner experiences will be your fault regardless. It will be one long hassle dealing with him.
 
RadioFish said:
Can anyone suggest a simple argument that can be made to the tower owner in defense of the proposed 1000 watt antenna?
Click to expand...

For the same az/el radiation patterns, the fields in all directions from an FM antenna radiating 1000 watts ERP are 3.16X greater than when radiating 100 watts ERP.

The free-space far field varies inversely with distance. So for these conditions, in theory the radiation center of the 1000 watt antenna would need to be 3.16X times further away than the 100 watt antenna for the two systems to produce the same field at a given point along the vertical dimension of the tower.

If the tower owner is technically oriented, he might be more willing to accept the use of an FM antenna with an even number of bays spaced vertically at 1/2-wave intervals.

In theory such an array in free space radiates nearly zero field at elevation angles of +/-90 degrees, no matter what its ERP in the horizontal plane.
 
Another vote for "you need to find another tower". Anything that goes wrong can be pinned on you, and your station would have to pay for it to be fixed. If it can't be remedied, then guess what? You could be kicked off the tower anyway!

There is a BIG difference between 1kW and 100 watts. That's why there's no 1kW LPFM stations.

More importantly, what's the HAAT of your proposed station?
 
All calculations and configurations aside, what is it you are trying to do? What size area are you serving? We have a 100 Watt translator in Las Vegas located about 1000 feet above the main town on a mountain that provides city grade coverage over most of the metro area. Building penetration is not great but not bad, mobile coverage is fine well outside the city grade limits, and in general over most of the area it performs about as well as considerably higher powered stations. If you are only interested in serving a limited area, it probably won't make much difference. It won't win DX awards or get way out into the Rural areas. I will second the concerns others have that you will become responsible for everything that goes wrong with everyone else up there, even at the 100 watt level.
 
As a guy who spent many years in the two way radio industry installing systems on shared towers, trust me, any de-sense or front end overload on any of their receivers, real or perceived, will be blamed on YOU with the "everything was alright until that big transmitter was installed" argument! Depending on how many users are on the tower, what freqs, etc, the problem will be multiplied. I recently inquired to a tower operator about locating a 250 watt translator on his tower and I could hear in his voice that broadcast equipment was outside his comfort level.
 
Not specified is the number and spacing of bays. You shouldn't have a problem if you use half-wave spacing and even number of bays. Your upward and downward radiation will be zilch.

I've got a 10 kw on a mixed tower with no issues.
 
5KW FM with a pile of antennas 15' above it...zero issues. What gets me...what kind of tower owner sends good money walking in this economy?
 
Thanks for all of the replies. Great advice. Unfortunately no real good options on the tower. This particular tower is government owned, American Tower managed (bad, and bad) the height would hopefully be about 200'. 2-bay but that might change. It is a place with tower limitations and this NCE-FM is very tightly spaced with little choice of towers. The choices are 40' roof-top antenna at 1kW, the 200' height on the preferred tower, or else the next tower is 5+ miles away and require at least 5-10 kW and $3000/mo rent to get coverage over some population.

The best I can do is point out that the 1kW with enough separation and multiple bays 1/2 wave spaced is no worse of an impact than the LPFM.

Thanks again for your suggestions.
 
Get a professional pattern study from your antenna manufacturer. You may need to go 4 bays 1/2 wave spaced instead of 2 bays to reduce the downward (and upward) radiation.

You also may need to put some bandpass filters on their equipment to keep your station out of their front ends.

Is there any advantage to the 5-10kW site 5 miles away? As in, if you license it with those parameters, will it keep other NCE's from encroaching on your coverage? How rural a spot is this, or maybe I should say, how crowded of a dial is in your location?
 
If the 100W station is using a single bay but yours is using multi-bay half-wave or 0.8wave spaced bays, your field at the other antennas probably will be lower than that of the 100W station. Figure out the field from the 100W station at the closest antenna, then check with your antenna manufacturer to predict your field at the same antenna. If yours is lower, then you should have no issues. I'm on at least one site where there are 2 250W translators in close proximity to cellular and 2-way with no problems.
 
I've heard so many stories of finicky tower owners and people trying to prevent new towers when cell towers pop up all over with no problem at all getting approved. I've heard of tower owners just switching FM BC transmitters off if any interference problems at all came up. I've heard of zoning officials, neighbors of the tower, city officials, FAA officials, and even relatives of FCC engineers bird dogging the approval and construction process, trying to find something or anything wrong that they could find to block, stop, or delay a new station or relocation. Sometimes it's competitors behind the whole thing, even NCE FM competitors. I know of no other industry so open to sabotaging and openly blocking competitors, often through legal though questionable means. I used to do some technical consulting, but I all but left because of the disillusionment about these tactics. Everything I proposed ended up being blocked and challenged at every level. Practically every fax I sent ended up in the wrong hands of competitors, and alternative station buying and upgrade plans were stolen outright and built by competitors who often ran into zero opposition. One proposed TL change ended up within 15 seconds latitude and longitude of what was on the fax.
 
Back to the how much difference between 1000 watts and 100 watts question. Relatively speaking of course, put 10 watts of audio into a speaker aimed out your window. Walk as far as you can while still considering the sound to be audible above the surrounding noise. Stand still as someone turns the volume down to 1 watt. There's your difference in laymen's terms.
 
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