I
ILOVERADIO2
Guest
Will adding 40ft to a FM antenna increase the 60dbu? Right now we are at 150ft and can go to 190ft. Will it help get the signal out?
-
Get involved.
We want your input!
Apply for Membership and join the conversations about everything related to broadcasting.
After we receive your registration, a moderator will review it. After your registration is approved, you will be permitted to post.
If you use a disposable or false email address, your registration will be rejected.
After your membership is approved, please take a minute to tell us a little bit about yourself.
https://www.radiodiscussions.com/forums/introduce-yourself.1088/
Thanks in advance and have fun!
RadioDiscussions Administrators
It will not help enough to make the trouble of doing it worth it. If you were talking 100+ feet, that would have more of an effect on it.
Also, bear in mind, if you raise it more than 10 feet, you need to file a change with the FCC.
Also, bear in mind, if you raise it more than 10 feet, you need to file a change with the FCC.
ILOVERADIO2 said:
I had to read your message a couple of times to figure out if you are the broadcaster talking about your transmit antenna, or the listener talking about the receive antenna. Then it came to me... you are the transmit end of the transaction.
I agree with Bengals Fan. You will theoretically get a slight (maybe unmeasureable) change in where your 60dbu curve falls. However, there is another factor. You didn't tell us whether you are in relatively flat country or if you are in a location with significant terrain features. I have done some studies on potential LPFM sites in the hills of North Georgia and when you use the Longley-Rice curves, it is amazing what a few feet of elevation change, or moving your site one mile or less horizontally can do to change your calculated, expected coverage area.
Also, we don't know the characteristics of your antenna. Single bay? Multiple bays? That too can be signficant in the hills and valleys country.
ILOVERADIO2 said:
Assuming this is an LPFM setup with 100 W ERP, and its radiation pattern is omnidirectional before and after the change (which are big assumptions), and the terrain is flat within a 5-mile radius of the transmit site, then the change will add about 0.58 miles* in the distance to the 60 dBu F(50,50) contour -- which amounts to an additional 16.8 square miles of coverage area (an area increase of 29%).
* from 4.3 miles to 4.88 miles
//
Nick said:
Almost. Just as long as it's not sticking up above the tower. I like to have a couple of feet between the top most bay and the top of the tower. I would rather lightning strike the top of the tower than the antenna. You are also limited by what your license says. The license will stipulate what your center of radiation is above ground. 10 feet either way of that, and you have to file for a change.
R. Fry said:
Again, hardly worth the time, effort and expense since I also doubt you have an extra 40 feet of feedline hanging off the tower.
Agree with Bengalsfan...it's not worth it unless you're doing a rebuild anyway. Of course, if you have to reduce power because of the increased height, even your extra 1/2 mile won't materialize.
I think the FCC spec on antenna height is +2/-4 meters. So you can probably go about 6.5' above the licensed figure without filing.
Consider this point as well...doubling the power (from 3KW to 6KW) at the same height adds a couple of miles or so to the coverage area of a Class A. Invariably the station owner's assessment is something similar to "yes, it's better...but nothing to shout about". Going from 150' to 190' at the same power level will yield a far less dramatic change than doubling the power on a Class A. I'm with the crowd on this one...unless you can do it for very minimal expense, don't bother with it.
I think the FCC spec on antenna height is +2/-4 meters. So you can probably go about 6.5' above the licensed figure without filing.
Consider this point as well...doubling the power (from 3KW to 6KW) at the same height adds a couple of miles or so to the coverage area of a Class A. Invariably the station owner's assessment is something similar to "yes, it's better...but nothing to shout about". Going from 150' to 190' at the same power level will yield a far less dramatic change than doubling the power on a Class A. I'm with the crowd on this one...unless you can do it for very minimal expense, don't bother with it.
R. Fry said:
By my rough and unofficial calculation, moving the electrical center of the antenna from 150' AAT to 190' would be roughly equivalent to increasing the ERP from 100W to 160W with no change in HAAT. In terms of percentage increase in coverage area, that ought to be like increasing an old Class A (3 kW @ 328') to 4.8 kW @328'. Not as good as increasing to 6 kW @328', but of some value. Like all generalizations, however, this one is dangerous because terrain (which we know nothing about) can have a big effect.
If that added 16 square miles of coverage has a lot of people in it, then it might be a very big improvement. If there's nobody in sight to listen, then it wouldn't be of much use. The bottom line is there is no single answer that covers every situation.
If you suspect it might help (like it could increase your audience, not just your coverage area) then why not hire somebody to run Longley-Rice predictions on the change? These reports can be coordinated with US Census data to give you an actual head count of what the change might mean to you, in your situation. It will cost money to do this, but it could also save you from making an expensive mistake. Conversely, you might find that making the change would be an excellent investment.
If you suspect it might help (like it could increase your audience, not just your coverage area) then why not hire somebody to run Longley-Rice predictions on the change? These reports can be coordinated with US Census data to give you an actual head count of what the change might mean to you, in your situation. It will cost money to do this, but it could also save you from making an expensive mistake. Conversely, you might find that making the change would be an excellent investment.
I
ILOVERADIO2
Guest
The FM is a translator used for a AM station. We are at 150ft with the ERP at 250watts. We now have free space on the tower at 190ft.
ILOVERADIO2 said:
Yea, you might want to get a consultant to do a study for you. You may have to reduce your power if you raised the antenna. Which would mean a wash. Actually, the only way that would help is if you had terrain issues. Raising the antenna may help there, but if you have to drop back to, say 200 watts, then it would not push your signal out any further.
I'm with Chuck, get a Longley-Rice study done to see exactly how this would effect your signal verses population.
Keep in mind that as an FM translator for an AM station you are limited to keeping the 60 dBu FM signal within 1: 2 mV/m daytime contour or 2: 25 mile radius of the tower, whichever is smaller. One can assume that the designer of your system built it to meet but not exceed these limitations unless there were mitigating circumstances we are unaware of. So you might be maxxed out exactly where you are on the tower with the power given. Going higher would surely result in reduced power. All of us armchair engineers have an opinion, but as pointed out, a consultant with the right software could plug in all the "what ifs" and spit out some pretty maps that the big boss could stare at and decide if he wants to open his wallet 
I
ILOVERADIO2
Guest
FM antenna is not on the AM tower and we have room to grow and still be within the 2.0mv
ILOVERADIO2 said:
But is the expense worth it. That's what we are trying to get you to see yet you don't seem to be understanding. The return on investment is going to be negligable.
Nick said:
You can't legally move the antenna or just boost the power. You're probably where you are for a reason of height vs. power. Just because someone moved off of the top doesn't mean you can just move the antenna up without also reducing power. What you're looking at is what lawyers call de minimus. You're pretty much looking at getting nothing for your trouble and money.
This is sorta like 20 questions. You posted a question looking for some free advice, and you have a bunch of talented engineers offering suggestions but without knowing the whole picture it is getting even more difficult to follow this thread. Unless it is a state secret why not share the name of the station and perhaps some of this brain trust can offer a more informed opinion. We can probably give you a go-no go opinion (the majority currently leaning toward no go) but in the end if you "go" you will need the services of a consulting engineer to make the maps, clear the interference contours, ect plus the actual cost of construction and perhaps new equipment. Again, as most posters have pointed out- cost vs benefit.
Another thing to think about is trees and buildings soaking up signal. At such low height you will probly see a greater increase than what is being calculated do to over shaowing the trees and buildings better, if you have them. If this is not going to present a problem with surrounding stations or translators 3 frequencies up or down the dail near you which I dought it would and your not going to have to pay an arm and a leg to get it done ie: tower climbing charge, more hardline, what have you and the FCC approves it then the decission is yours but make sure you check everything out good first before you jump in and make the move.
- Status