Indoor FM broadcast antenna? I may have no choice...

[Radio-X]

Looking at starting up an LPFM whenever (FCC and University) give approval which honestly could take awhile...

But...here's the kicker: I can't put up a tower. Too much cash, County Planning Commission is run by hippies, and University doesn't want a 100' pole up in the air for drunk kids to do...what they do.

So, with a proper 100w transmitter and a fairly simple 3dB gain antenna, I have two options: Either place it at about 10' HAAT in the "weather station" fenced-in area near a 20' brick building or I can mount the setup in the spire of the main university building (About 30-40' HAAT and in a wood and glass enclosure.)

I know WMMO in Orlando did this in the 90's when they signed on...but they were on a 350-400' building. I'm not sure if FCC will make you jump through hoops to do this or if there are current legit examples of this. My big concern is what type of signal loss can I expect from putting it in the spire? At that height would I be better off putting it on a 10' weather pole?

I'm guessing just in my head I'd get 2 mi coverage at best 10' with that power, but would putting it indoors kill the signal as it does with a receiving antenna? There are several dorms in local apartment complexes 3-4 miles away so that's the magic number I'm shooting for in reasonable range.

Thanks!

Radio-X

 

[DudeFan]

10 feet HAAT should be a non-starter, for a number of reasons, not including RF safety running 100watts.

Yes, putting your antenna inside a wooden and brick structure will cause problems.

Is there a taller structure on campus with limited roof access. Two 20 foot sections of Rohn (or its equivalent) with house brackets fastened to an elevator dog house would easily do the job and help tremendously.

Is there a cellular tower nearby? If they have aperture available, it might not be cheap, but would be a good solution.

Is there a central heating plant chimney nearby that you can do a side-mount?

 

[joep]

First off, let's not get HAAT (above AVERAGE terrain) confused with HAGL (above ground level).

It's possible to have an antenna mounted 100 feet AGL and yet have the HAAT be 10 feet. I once had an FM antenna for a Class A mounted at 200 feet on a tower, and the HAAT was nearly zero. (No, it didn't work well. That's why it was moved) A quick and dirty way to calculate if you are within exposure guidelines is to fill in the RF exposure worksheet from an FCC 303S form.

All of Dudefan's suggestions are good ones. It's probably a lot easier to find an existing structure to mount it on. A mast attached to a penthouse works well, and 100 watts with a 10 foot mast on a rooftop should be well within exposure limits.

Good luck with your project.

 

[Radio-X]

10 feet HAAT should be a non-starter, for a number of reasons, not including RF safety running 100watts.

Is there a taller structure on campus with limited roof access. Two 20 foot sections of Rohn (or its equivalent) with house brackets fastened to an elevator dog house would easily do the job and help tremendously.

Is there a cellular tower nearby? If they have aperture available, it might not be cheap, but would be a good solution.

Is there a central heating plant chimney nearby that you can do a side-mount?

This is a "modern" campus...built within the past 10 years, so the tallest thing on campus is the main building's spire. It's at least 30' up and we're on flat coastal plain with 20-40' pines all around us.

Like I said, planning commission and the university does not want to see the antenna (esp. on the main building)

Only other options are: Clear Channel has a class C1 750' FM tower about 3 miles away. You can see it from the main campus. Mounting it half way up with even just 5w would be AMAZING...but that means leases and money. There's a 130' cell tower next to it, but no line-of-sight from the campus so I'd imagine that's a non-starter too.

I was very wary of putting the antenna in the "weather garden" anyway...buildings and grounds suggested it but even though its fenced in I'd be scared of someone tampering with it...didn't think of the RF issues with that low of a power, but good lookin' out!

Assuming it has to go into the spire, are we talking about a significant loss of signal? I know this is a pretty darned rare subject since I only know of two actual licenced FM's that have had a fully-enclosed antenna...and I know one of them moved away very quick! I'm guessing the signal will be concentrated in the area of the windows (luckily there are 4 on all sides) so it'd be a really weird coverage area.

Radio-X

 

[Goat Rodeo Cowboy]

Assuming it has to go into the spire, are we talking about a significant loss of signal? I know this is a pretty darned rare subject since I only know of two actual licenced FM's that have had a fully-enclosed antenna...and I know one of them moved away very quick!

This "spire" you speak of: what is the nature of it's construction? Is it frame (wood) construction.... is it steel frame construction? What is the external covering? Brick? Stone? shingles? Metal?

What is the shape and the dimensions of this spire? If it has 4 windows to the side, it must not be a pointy little thing like some church spires.

The cell phone industry has resorted to "cloistering" antennas in church spires when the zoning gods were not otherwise cooperative. Spires come is a variety of shapes, sizes, and construction styles.

Here is an alternate thought-starter. Another refugee from the cell-phone culture wars. In that world it is a given that ALL cell tower structures are evil and ugly... even gleaming mono-poles. Out in California a cell-phone guy noted that there were these water towers scattered through the rolling hills without a big zoning fight. Not water towers up on legs, but the kind that look like their parents was farmer's silos.... only smaller in diameter. The cell-phone guy found a tank manufacturer who could construct a "tank" out of fiberglass material and they used non-RF-reflecting paint to make the phony water tank blend into the landscape like all the real water towers and everybody lived happily ever-after. If your phony water tank had no external ladder or other things that could be used by adventuresome would-be-mountain-climber students, you might get it past he campus decor-police. I wouldn't expect it to be cheap however.

 

[OKCRadioGuy]

I'd go for the CC tower. Tell te hippies that you'll need a small budget to help keep their campus all pretty by NOT transmitting from there, but giving their school funding credits and the publicity exposure on the LPFM...

 

[OKCRadioGuy]

Tell them you're gonna "stick it to the man" on his own tower. You just need a little bread to get it all done...

 

[frankberry]

Speak with someone at CC. They may be willing to donate the tower space as a tax writeoff.
It never hurts to ask.

 

[DudeFan]

From what you've described, you've got to get up over the 40' pines.

If you can get 30 meters HAAT up on the cell tower, you would probably be good to go.

Also scout around the campus and neighborhoods for a monopole of some sort. Are there large power tranmission towers (yes, there will be ancilliary issues, but the cell companies have done it).

The CC tower is probably your best bet. 30 meters up (+/- for the ground level about sea level) and you've got a pretty decent signal with campus building penetration, assuming CC has available aperture.

 

[Fieldtech1]

When all else fails, how about mounted to one of the tall trees?

Case and point......

http://www.necrat.us/wlpvlp_pro.html

 

[Radio-X]

Assuming it has to go into the spire, are we talking about a significant loss of signal? I know this is a pretty darned rare subject since I only know of two actual licenced FM's that have had a fully-enclosed antenna...and I know one of them moved away very quick!

This "spire" you speak of: what is the nature of it's construction? Is it frame (wood) construction.... is it steel frame construction? What is the external covering? Brick? Stone? shingles? Metal?

What is the shape and the dimensions of this spire? If it has 4 windows to the side, it must not be a pointy little thing like some church spires.

The cell phone industry has resorted to "cloistering" antennas in church spires when the zoning gods were not otherwise cooperative. Spires come is a variety of shapes, sizes, and construction styles.

The spire is wooden frame construction. Dimensions are probably 6'x6'. Need a locked pulldown attic ladder to get up there. It's about 15' tall, with a sloped "house"-styled tin roof, LARGE windows on all 4 sides about 4'x6' and the remainder is brick exterior construction. It is like a friggin' oven up there from April-October, so it's safe to say only the antenna should be up there.

I'm thinking since time is on my side here, I may talk to CC about mounting it on their tower at about the 200-300' range. It means probably 10 watts or less (too lazy to calculate it on the net right now). The issue is even though the CC tower is within eyeshot of us, it primarily broadcasts to the larger Arbitron rated market rather than us (old school early 80's B to C move-in to cover both markets). I know they are required to "serve" their community of license so kindly pointing that out along with the possible tax breaks they could get would be a good starter.

Plus this is market 150-ish...one of CC's last small markets! I reckon if you combined the two markets, we're still talking over market 100...and it ain't like I'm asking to borrow a spot on the Empire State Building's master antenna, so I feel quite hopeful. CC is the only user of the tower as well and I'm almost 100% sure they own and maintain it.

If not, then I'm thinking its gotta be the spire. The next closest tower is a 400' tower owned by the statewide PBS station...but thats 5 miles away in the OTHER direction. No-go for a LP100 and a sprawling campus.

The tree idea is ingenious! Problem is, all of these pines were planted at the same time as lumber trees...so there's really no "tall" trees to mount on.

We will keep our fingers crossed for the CC solution...

Radio-X

 

[WNTIRadio]

Well, you just said "tin roof". That right there rules out putting the antenna inside the spire on the building. Large windows or not... and that depends upon whether or not they are wood or steel framed, it's not going to work with sheets of tin around 1/2 wavelength from the antenna (or farther or closer for that matter). Rule that possibility out now and save yourself the headaches.

Look up the ASRN of the tower that the CC station is on and see if they indeed own it. There's a good chance that it may be American Tower or some other tower co. It should be easy to find by searching the FM Query with the calls of the CC station and then getting the ASRN.

Could you place a single bay small Shively on top of the spire? On a pole on a non-penetrating rooftop mount on one of the other buildings?

Another possibility is to put up a telephone pole. They come in all sorts of sizes, from 20' to 80' and up. Since it's an LPFM, the wind-loading isn't an issue. They're also pretty cheap. Put it in with the trees and make it 20' higher and chances are nobody would notice it.

 

[wkbam1690]

The tree idea is ingenious! Problem is, all of these pines were planted at the same time as lumber trees...so there's really no "tall" trees to mount on.

We will keep our fingers crossed for the CC solution...

Radio-X

Plus if you tree-mount it you'll have to file a license modification every couple of years to increase the HAAT ;D

 

[DudeFan]

If you could use the CC tower, get the antenna lower, so you are at most 50-60m HAAT. 10watts or less is probably not going to get you enough power density in your desired coverage area. If you haven't talked to antenna folks yet, get the best one you can get. Our SWR is ok. The ERI that we have is much much better. I know you can't change the laws of physics, but the ERI sure does seem to have some magic dust or something that throws the signal out there really well.

 

[ChiefEngineer]

Power Poles, Power lines, anything tall. Big issue in Florida though.

Look at hiding something in plain sight. A 2 story bldg is 22 feet. Add not a monopole but an aluminum pole. Special aircraft aluminum poles of longer length can be used, and painted. Paint the FM antenna. For those on the ground locate thepole in teh center of the roof of a large building and it WILL be visible farther away but not below. Especially of there is landscaping.

 

[bossbill]

On the tree install, you also need to add enough coax to accommodate the HAAT changes over a period of time. And accept small signal changes in heavy winds!

 

[frankberry]

If you mount the antenna on a tree, expect a good deal of signal absorption by the tree. The coverage pattern will be skewed.

 

[PTBoardOp94]

I imagine 40' would be a reasonable height to tree-mount, if it comes to that. Just pick one of those 40' pines. That would give you line of sight of about 6 miles, and a decent signal for about 2 miles around. That's really only a guess, though, because I don't want to manually calculate field strengths on a Sunday, and the FCC's calculator assumes a minimum height of 30 meters.

For what its worth, FCC says that 100 watts at 30 meters gives a 54 dBu F(50,50) radius of 8 km

 

[DanStrassberg]

Well, you just said "tin roof". That right there rules out putting the antenna inside the spire on the building.

I took "tin roof" to mean that the tin part would be above his antenna--not surrounding it. If that's what he meant, depending on the vertical radiation pattern, there might be very little radiation toward the metal. Still, I don't think you'd want the roof to be ungrounded, and there might be a huge safety problem if it were grounded. The grounding would have to carry a lightning strike without setting the whole building on fire! Presumably, the National Electrical Code has something to say about that, even if the local code does not. But I'd say that there is a good chance that the local code does cover grounding of metal roofs on wood-frame structures.

 

[WNTIRadio]

Don't put it on the tree, "plant" a 60' telephone pole in with the 40' pines. It will blend better than those silly cell-trees, especially because it's real wood. Make sure you run a good ground wire down the side of the thing and ground it well at the bottom.

I've used telephone poles to support STL dishes and they've worked very well. If it can hold up that pole pig and a streetlamp, plus the wires attached to it, a paraflector or a single bay is nothing.