how small of a space for AM transmitter site possible?

[pianoplayer88key]

Ok first of all I'm not sure whether to post this in here, or in the tech tips forum... but since my chance of getting my own licensed AM station is worse than that of the FCC saying "Pirates! Welcome to the Airwaves! Broadcast ALL YOU WANT!", I chose not to put it in "tech tips".

I've noticed that most AM transmitter sites cover quite a large geographical area - several acres usually. This could be either because of the ground system, or the guy wires. I've noticed some sites eliminate one, OR the other, for example using a Franklin antenna (so no ground system is needed) or using a self-supporting tower (so no guy wires are needed).

I have yet to see a station combine the two, though - a self-supporting Franklin antenna. Is that possible? Also, how small of a space would it be possible to have such a transmitter site? (I've seen towers with a fence surrounding them (for protection from high field strength to prevent people getting too close) fairly close - often with no more than 50 feet from one side to the other, and (I'm guessing, though) possibly somewhat less.)

So... CAN an AM station's transmitter site (we'll assume it's a single-tower non-directional operation) fit into a very small space, like 1/8th of an acre, by using a self-supporting Franklin antenna?

 

[Goat Rodeo Cowboy]

I will defer to the A.M. R-F experts to respond to your questions about what a Franklin antenna can do in the situation you describe.

Let's address the NON-RF aspects of your scenario. So you come up with some way to keep your site down to 1/8th of an acre. What is going to be the use of the land that "hems you in?" Are people going to have houses there? Are businesses going to operate there? Will you be surrounded by a boat storage yard? An automobile salvage yard? A city park maybe?

Will those who occupy the land beyond your fence constantly be on your case about the station audio they hear in their telephone and maybe every other piece of electronics they own?

As you investigate the mechanics of a Franklin antenna, inquire about the long term maintenance aspects. Will there come a day when you need to hire a rigging company to come in with a mobile crane to facilitate changing out mid-point insulators? When you have the tower painted, will the dripping of paint splatter on your nearby neighbors? Will falling ice in the winter reach a drop zone the is outside your 1/8th acre fence?

You have asked a theoretical question. Don't overlook the practical questions.

 

[OKCRadioGuy]

I don't belive the FCC will allow any new Franklin systems, only grandfathered stations. With that being said in theory a person could have less of a grounding system and still probably get enough efficency with the Franklin, although all fo them I have seen have had a normal AM grounding system in addition to the tower. I am not sure if the FCC would still require it anyway. From a practical standpoint it isn't a very good idea in that interference issues to neighbors and the liablity of the tower falling on them (insurance) would pretty much make buying property instead a better idea in most all cases. (who knows in some place like L.A. or N.Y.C, but in most all other places where land is somewhat affordable) If a guy was wanting to save grounding space and even not have to worry about zoning problems as much there are some emerging technologies like the Valcom antenna that may work. In this day and age I wouldn't think anyone would try the Franklin approach as a plausable fix. Along with all the other negitives for the theoretical application the cost of building one and the pain of maintaning one probably would rule it out. (WOAI and others have discontinued the use of theirs)

 

[DanStrassberg]

If a guy was wanting to save grounding space and even not have to worry about zoning problems as much there are some emerging technologies like the Valcom antenna that may work. In this day and age I wouldn't think anyone would try the Franklin approach as a plausable fix. Along with all the other negitives for the theoretical application the cost of building one and the pain of maintaning one probably would rule it out. (WOAI and others have discontinued the use of theirs)

AFAIK, Valcom whips meet FCC minimum Class B efficiency requirements only at 1180 kHz and above AND they require a standard ground system (120 1/4-wavelength radials or the equivalent). Also, they are limited in power-handling capabilities to ~1 kW.

True that the majority of operators have given up Franklins as too costly to maintain, but you never hear about the ones that apparently haven't proven too costly to maintain. I'm thinking of KSTP (days only) and KFBK (which uses a pair in a DA). I've been told that the KFBK Franklins have no ground systems because none are necessary. Anyhow, these are the shortest of the US installations, and that could have something to do with the lack of maintenance issues.

 

[grich]

Apparently WHO in Des Moines still likes their Franklin...they spent big $$$ rebuilding the system a few years ago...new RF components, etc.

 

[Goat Rodeo Cowboy]

Going back to the original question. You have not given us any indications why you would be interested in having an AM transmitter/tower on such a postage stamp size plot.

If it is to try and keep costs down by not purchasing additional land, the idea will not fly. The cost of designing and installing a Franklin will eat you alive. Engineers with strong A.M. skills are getting to fewer and fewer in number. A lot of small home-town A.M. stations get by without an engineer, picking up the phone to call in some one on a contract basis now and then. The cost of keeping a relationship with a engineer who understands and can maintain your Franklin will cost you more money over a five year period than will 3 extra acres of land......

EXCEPT​

if you are in a dense, highly urban area where you may have to drive 100 miles before you find 3 or 4 acres of bare land. Then comes the question: do you want to invest a "Franklin Mint" full of money to establish an A.M. signal in that kind of community?

 

[PTBoardOp94]

I once worked at a 500 watt daytimer station whose tower was situated in a fenced in area so small that I could probably have stood with one arm holding the fence and the other holding the tower base. Never tried it, for obvious reasons. Standard half-wave antenna if memory serves. Guyed tower, but probably on an acre of land.

Stations like KFI (Los Angeles) have grounding systems but do not own the land on which they are built. Would "renting" (probably leasing) your grounding system be a feasible idea? I'm not sure exactly how large KFI's owned tower site is, but I suspect it is about half an acre.

 

[bossbill]

Well, with a requirement to run radials out 1/4 wave, that works out to over 2 acres at the high end of the AM band, and 20 acres at the low end...

 

[LA_Guy]

If a guy was wanting to save grounding space and even not have to worry about zoning problems as much there are some emerging technologies like the Valcom antenna that may work. In this day and age I wouldn't think anyone would try the Franklin approach as a plausable fix. Along with all the other negitives for the theoretical application the cost of building one and the pain of maintaning one probably would rule it out. (WOAI and others have discontinued the use of theirs)

AFAIK, Valcom whips meet FCC minimum Class B efficiency requirements only at 1180 kHz and above AND they require a standard ground system (120 1/4-wavelength radials or the equivalent). Also, they are limited in power-handling capabilities to ~1 kW.

True that the majority of operators have given up Franklins as too costly to maintain, but you never hear about the ones that apparently haven't proven too costly to maintain. I'm thinking of KSTP (days only) and KFBK (which uses a pair in a DA). I've been told that the KFBK Franklins have no ground systems because none are necessary. Anyhow, these are the shortest of the US installations, and that could have something to do with the lack of maintenance issues.

Actually, Valcom antenna systems make the required FCC efficency at 1200 and above with radials 150 feet long. This is what was specified in the FCC release. This is a circular area slightly less then 2 acres. Also, Valcoms are limited to 5 kW, not a kilowatt.

 

[The Beave]

I once worked at a 500 watt daytimer station whose tower was situated in a fenced in area so small that I could probably have stood with one arm holding the fence and the other holding the tower base. Never tried it, for obvious reasons. Standard half-wave antenna if memory serves. Guyed tower, but probably on an acre of land.

Stations like KFI (Los Angeles) have grounding systems but do not own the land on which they are built. Would "renting" (probably leasing) your grounding system be a feasible idea? I'm not sure exactly how large KFI's owned tower site is, but I suspect it is about half an acre.

I certainly pray that you were not touching the tower while it was on... even at 500 watts you would be toast.

Our 50kw "twin Towers" takes up in the neighborhood of 4 acres but I have no clue how deep the or far out the radials go.
The fences are approximately 15 feet from the tower bases.

 

[radioguybroadcasting]

I certainly pray that you were not touching the tower while it was on... even at 500 watts you would be toast.

Our 50kw "twin Towers" takes up in the neighborhood of 4 acres but I have no clue how deep the or far out the radials go.
The fences are approximately 15 feet from the tower bases.

Naw, you wouldn't be toast at 500 Watts.. just quite badly burned.

 

[KyDXIn]

I certainly pray that you were not touching the tower while it was on... even at 500 watts you would be toast.

Our 50kw "twin Towers" takes up in the neighborhood of 4 acres but I have no clue how deep the or far out the radials go.
The fences are approximately 15 feet from the tower bases.

Naw, you wouldn't be toast at 500 Watts.. just quite badly burned.

Do I smell BACON??

 

[The Beave]

Do I smell BACON??

You have a good nose and there is no wind today.

 

[tomservo]

I once worked at a 500 watt daytimer station whose tower was situated in a fenced in area so small that I could probably have stood with one arm holding the fence and the other holding the tower base. Never tried it, for obvious reasons. Standard half-wave antenna if memory serves. Guyed tower, but probably on an acre of land.

We have a similar setup in my town for the 1kw AM that has been silent for years. The tower still stands on a small plot of land in the residential section of town, and the fence is so close one plucky daredevil could easily climb up, reach over and touch the tower. It can't be more than 60 feet to the road/property line in any direction, so I have no idea what kind of radial system they have.

The radials at 1400 kHz would be what... 175 feet?

 

[RadeoEngineer]

Naw, you wouldn't be toast at 500 Watts.. just quite badly burned.

I had a shunt fed tower running 489 watts with the feeds about 5'6" above the ground. I'm 5'11". One day walking around poking at funny looking holes in the ground beneath the tower I found myself sitting on the ground slightly dazed. I had a 4" burn mark across my forehead. The feeds got moved up to 10' above ground shortly after.

 

[PTBoardOp94]

I had a shunt fed tower running 489 watts with the feeds about 5'6" above the ground. I'm 5'11". One day walking around poking at funny looking holes in the ground beneath the tower I found myself sitting on the ground slightly dazed. I had a 4" burn mark across my forehead. The feeds got moved up to 10' above ground shortly after.

Harry Potter!

To clarify, no I didn't try touching a live AM tower. Just trying to illustrate how small the fenced in area was.

 

[The Beave]

I once worked at a 500 watt daytimer station whose tower was situated in a fenced in area so small that I could probably have stood with one arm holding the fence and the other holding the tower base. Never tried it, for obvious reasons. Standard half-wave antenna if memory serves. Guyed tower, but probably on an acre of land.

We have a similar setup in my town for the 1kw AM that has been silent for years. The tower still stands on a small plot of land in the residential section of town, and the fence is so close one plucky daredevil could easily climb up, reach over and touch the tower. It can't be more than 60 feet to the road/property line in any direction, so I have no idea what kind of radial system they have.

The radials at 1400 kHz would be what... 175 feet?

It depends on the antenna system - in 1993 I worked on that same frequency in Tucson a non directional... that i think was in the neighborhood of 180'. This is where the consulting engineers come in.

 

[bossbill]

It is based on the frequency. Take the number 300, divide by the frequency in MHz, convert to feet, divide by 4, and you get the quarter wavelength. That's the radius. Example, 1000 kHz= 1 MHz. That gives you 300 x 3.28= 984 ft, divide by 4, get 246 ft. To get a radius of 246 ft, you need a piece of ground 492 x 492. That's about 5 1/2 acres.

At 1500 kHz, it works out to ab out 2.5 acres.

Like I said in an earlier post, about 2 acres at the high end and 20 acres at the low end.

Often, the area around the tower gets built-up. Theoretically, a ground system still exists under the roads and building that might be in the vicinity.