Part 15 AM antennas

[Nostalgia]

Please pardon if these questions have been asked/answered here before:

A new local part 15 am operator has asked for some help installing his antennas. He was told that he could install TWO Rangemaster transmitters on his roof and get more gain. HMMMMM. Knowing what I know of two tower AM broadcast directional arrays, this seems a little too simplistic. I Googled this and did not find anything or anybody doing this. Does anyone know if this has been done and if so how and where and most importantly did it work?

I was given a link for a RangeStar1000 antenna, but that link linked me to the Erickson site where I didnt see much info. I plan on calling them tomorrow for more info, but again if any of you out there have used this antenna I would love to hear your real world comments.

And lastly, his site is on a flat roof of a 4 story office building. What do you think is the best mounting for the Rangemaster: low on the roof over a field of 30 or so 10' radials or elevating it maybe 10' on a pole with a few guy wires (radials). Look forward to your comments.

 

[RadioMan203]

While I'm sure Keith hamilton can give you what he has experienced with his product in a multiple transmitter configuration, I can tell you that using any antenna other than the 102" whip will void the unit's FCC certification. It seems somewhat pointless to purchase a certified transmitter to only void the certification by using a different antenna.

 

[rfry]

... He was told that he could install TWO Rangemaster transmitters on his roof and get more gain. ... Does anyone know if this has been done and if so how and where and most importantly did it work?

The FCC's view of this may be a problem (see http://part15.us/node/1954 ). If the mutual coupling between the two antennas was known, then technically it could be made to work if the two transmitters used phase-locked r-f carriers, and their modulation levels were identical in amplitude and phase. Probably this is impractical for users of Part 15 setups.

And lastly, his site is on a flat roof of a 4 story office building. What do you think is the best mounting for the Rangemaster: low on the roof over a field of 30 or so 10' radials or elevating it maybe 10' on a pole with a few guy wires (radials).

Caveat: Such elevated installations can radiate more from the "lightning ground" or the steel frame of the building than from the ~3-meter whip considered to be the antenna. The increased coverage area might result in FCC attention. The link below gives more detail about this.

http://boards.radio-info.com/smf/index.php?topic=117278.0

Sorry that this info probably is not what you were hoping for, but maybe it is better to know up front.

//

 

[Nostalgia]

Bill, good point. Hadnt thought of that. Actually I was happy the customer had decided on the Rangemaster because of the certification. The only reason I brought up the RangeStar1000 was a friend of mine had send me that link and I had never heard of it before so I was just curious. Stock antenna it shall be.

RFry, I think somebody had dreamed up the idea of getting a figure 8 or elliptical type of pattern with some north-south gain out of this particular arrangement but I personally dont see that happening with the equipment available. PERHAPS, and I say that with all caps (based on a broadcast model), that with a single transmitter w/ 50 ohm outputs, phasing lines/base ATU's and sufficient real estate to space the antennas I could see a directional antenna array, albiet extremely illegal! I do agree with your assessment of the elevated antenna. I am going to see the roof today and I am hoping to see some lightning rods and large guage wire strung around it. My other two scenarios were in case I dont find what I am hoping for up there.

Thank you all for your help, all input is appreciated.

 

[WCWalker]

The FCC's view on clustering or synching transmitters like the Rangemaster follows:

http://fjallfoss.fcc.gov/oetcf/kdb/forms/FTSSearchResultPage.cfm?id=20973&switch=P

 

[Nostalgia]

[Caveat: Such elevated installations can radiate more from the "lightning ground" or the steel frame of the building than from the ~3-meter whip considered to be the antenna. The increased coverage area might result in FCC attention. The link below gives more detail about this.
//

It was as I expected, a typical commercial building roof with lighting rods and a large guage wire going 4 stories down to a ground rod. I plan to go with NEC recommendations for antenna lightning protection and attach the short ground lead to the buildings ground system, just like every TV antenna mast ever sold from Radio Shack advises you to do. I may be a bit naive here but I see nothing in part 15 that forbids you to increase the efficiency of your antenna as long as you meet the power input limits and 3 meter antenna length. So with your calculations Mr Fry this thing should work pretty well up there.

 

[rfry]

... I may be a bit naive here but I see nothing in part 15 that forbids you to increase the efficiency of your antenna as long as you meet the power input limits and 3 meter antenna length.

True, but connecting the ground terminal of an elevated Part 15 AM transmitter to a long conducting path to a buried ground rod in fact does add the entire length of those conductors to the length of the ~3-m whip attached to the antenna output connector of the transmitter -- making the antenna system functionally non-compliant with 15.219(b). This is simply a fact of physics (proof on request).

Some operators, and even some manufacturers of Part 15 AM transmitters claim that only a short "ground lead" connecting an elevated transmitter to another conductor leading to a ground rod needs to be included in the 3-m length given in 15.219(b). However nothing exists at the junction of the short ground lead and the conductors it connects to, to prevent radiation from those conductors. And, as those conductors almost always are longer than the short ground lead and ~3-m whip, the entire path from the elevated transmitter to an r-f ground produces most of the radiation from such an installation.

Of course everyone is free to install whatever configuration they wish. However there have been a few cases where the FCC has cited operators based on 15.219(b), so this info might be useful in this situation.

//

 

[Nostalgia]

Ah yes, but the "ground lead" must be connected to a "ground system" which is left undefined by part 15. This can be a copper ground rod driven in the ground, a full set of 120 radials, a few ground plane radials spread around, the AC power ground, or as in our case an existing lighting ground system for the building. I am aware from other postings that different FCC offices have taken different views of this but going back to my earlier post, I was referring to the National Electric Code instead of the antenna modeling NEC, and that suggests, actually requires the antenna system to be grounded. And most diagrams supplied from manufacturers of towers, TV antennas, ect even show a single ground lead going from the antenna or support structure going to a ground rod driven in the dirt. Would any reasonable FCC engineer suggest that the short ground lead (3 meters=9.8425', 102" whip=8.51', 9.8425-8.51=1.3425') be left floating?? By definition it must be connected to something, and virtually anything would increase its physical length, even if it were connected to the ground lead of the power outlet as some suggest. And as far as the lighting vs RF ground argument, the FCC makes no distinction that I can see. At a broadcast station the full ground system under each tower serves as both the RF counterpoise and lightning dissipation system should the tower take a hit.

I do welcome your input and would like to see the "proof" which you could post here or PM me if you want to take this off the board. I have read several old threads on this subject and can see where it can be beat to death. I would like to hear from the operators of some part 15 stations out there with their real world experiences.

 

[rfry]

Ah yes, but the "ground lead" must be connected to a "ground system" which is left undefined by part 15. This can be a copper ground rod driven in the ground, a full set of 120 radials, a few ground plane radials spread around, the AC power ground, or as in our case an existing lighting ground system for the building.

The FCC doesn't define what an r-f ground consists of, but by definition, an r-f ground cannot radiate. Otherwise it is an antenna, or part of one.

A ground rod or any number of buried radials comprise a true r-f ground, because such components by themselves do not contribute far-field radiation to that produced by a MW monopole.

On the other hand, the conducting path leading from an elevated Part 15 AM transmitter to a buried r-f ground carries r-f current that does produce radiation into the far field -- more than that from the 3-m whip and short "ground lead," combined.

Below are links to two graphic output calculations from Numerical Electromagnetics Code to show this.

1. A plot of the r-f current distribution along the "ground" conductors and the 3-m whip of a Part 15 AM transmitter system elevated 15 feet above the surface of the earth. Note that the r-f current is maximized, and uniform along the "ground" conductor, and tapers from that value to near zero at the top of the 3-m whip.

http://i62.photobucket.com/albums/h85/rfry-100/CurrentDistributionOnPt15AMSystemWi.gif

2. A plot comparing the elevation patterns and gains of a 3-m monopole at earth level, an elevated 3-m monopole using a 6 meter conductor to a buried r-f ground, and a reference 1/4-wave monopole used by AM broadcast stations. Note the higher gain for the elevated 3-m system than for the one at earth level.

http://i62.photobucket.com/albums/h85/rfry-100/Part15AMAntennaElevationPatternComp.gif

Both of these graphics illustrate the point that the effective radiating lengths of elevated systems are functionally non-compliant with Part 15.219(b).

//

 

[stormy01]

LOL - I still find it amusing that some people are still trying to find ways around the rules when there are better approaches like getting some investors together and building a real radio station. There is not enough spectrum available for every hobby "broadcaster" to send a signal for miles, what part of that don't you guys get?

 

[LibertyNT]

LOL - I still find it amusing that some people are still trying to find ways around the rules when there are better approaches like getting some investors together and building a real radio station. There is not enough spectrum available for every hobby "broadcaster" to send a signal for miles, what part of that don't you guys get?

well lets see...
buying a station is very expensive. And there is Plenty of spectrum available to go miles...
so to you i say good day.

I would like to hear from the operators of some part 15 stations out there with their real world experiences.

Well My station is about a year old now. a nice antenna on the ground attached to my outside wall has done just fine for me out here. A good ground probably is what makes my signal so good. i just shoved an old rabbit ear dipole into the ground and hooked up a wire to that. It's served me well.

A better example would be KLSR 1650 in Rowlett, TX. he broadcasts from a 30 foot pole. and im assuming his ground wire is that long. he has been operating for a few years now and he has never been cited. His range is about 15 miles or so. But hes by a lake.

 

[Nostalgia]

LOL - I still find it amusing that some people are still trying to find ways around the rules when there are better approaches like getting some investors together and building a real radio station. There is not enough spectrum available for every hobby "broadcaster" to send a signal for miles, what part of that don't you guys get?

well lets see...
buying a station is very expensive. And there is Plenty of spectrum available to go miles...
so to you i say good day.

Actually he said BUILDING a radio station which is an even more ridiculous statement. The costs notwithstanding, maybe stormy01 hasn't heard that the FCC hasn't had an AM filing window for who knows how long. And to use his own words against him, in urban areas like mine there is no available spectrum to build a new station with 3rd adjacent channel protections but I can shoehorn in a part 15 AM to serve our small area. Perhaps he should put his headphones back on and listen to the DX while others actually DO radio, even on a small scale.

Thanks for jumping into the fray LibertyNT, and I will check out KSLR.

 

[rfry]

Well My station is about a year old now. ... A better example would be KLSR 1650 in Rowlett, TX. he broadcasts from a 30 foot pole. and im assuming his ground wire is that long. he has been operating for a few years now and he has never been cited. His range is about 15 miles or so. But hes by a lake.

For perspective on this...

The link below leads to plots of field intensity vs. distance from the transmit antenna for three "Part 15" AM systems. These fields were calculated for a perfect r-f ground, which is much better than can be provided even by salt water conditions (let alone an inland lake).

The blue curve there shows the field intensity from a typical Part 15 system elevated about 33 feet above the surface of the earth, using a "ground" conductor of about that length.

Note that the field intensity for the blue curve at 2 km (about 1.24 miles) is less than 400 microvolts/meter. Even at 5 miles for these conditions, it would be 24.8% of that, or about 100 microvolts/meter -- which could not provide very useful reception by a typical indoor AM radio.

Two considerations apply:

- A "Part 15" AM system with a long ground conductor installed at this elevation is non-compliant with 15.219(b), and

- Propagation loss from such a system due to earth conductivity over a 10- to 15-mile terrain path would reduce the groundwave field at the receiver to a value well below that which would be listenable -- and maybe even detectable by that receiver.

http://i62.photobucket.com/albums/h85/rfry-100/Part15AMSystemComparison.gif

//

 

[LibertyNT]

Well My station is about a year old now. ... A better example would be KLSR 1650 in Rowlett, TX. he broadcasts from a 30 foot pole. and im assuming his ground wire is that long. he has been operating for a few years now and he has never been cited. His range is about 15 miles or so. But hes by a lake.

For perspective on this...

The link below leads to plots of field intensity vs. distance from the transmit antenna for three "Part 15" AM systems. These fields were calculated for a perfect r-f ground, which is much better than can be provided even by salt water conditions (let alone an inland lake).

The blue curve there shows the field intensity from a typical Part 15 system elevated about 33 feet above the surface of the earth, using a "ground" conductor of about that length.

Note that the field intensity for the blue curve at 2 km (about 1.24 miles) is less than 400 microvolts/meter. Even at 5 miles for these conditions, it would be 24.8% of that, or about 100 microvolts/meter -- which could not provide very useful reception by a typical indoor AM radio.

Two considerations apply:

- A "Part 15" AM system with a long ground conductor installed at this elevation is non-compliant with 15.219(b), and

- Propagation loss from such a system due to earth conductivity over a 10- to 15-mile terrain path would reduce the groundwave field at the receiver to a value well below that which would be listenable -- and maybe even detectable by that receiver.

http://i62.photobucket.com/albums/h85/rfry-100/Part15AMSystemComparison.gif

//

No Doubt that KLSR is not 100% compliant but certain FCC offices will let this slide. in which case The Dallas Office let him slide. Did I mention it was by lake Ray Hubbard? Most of that 15 mile or so is Around that lake. Gotta love water!
check out his coverage map at

http://www.klsr1650.com

Ive had many chances to pick up that station and his map is pretty accurate. Ive picked it up inside across the lake with a pretty decent signal. The usable signal is probably only 10 miles or so. So Fry your pretty correct about that.

Also gotta tell ya that graph doesn't take into consideration of local Ground Conductivity. Which North Texas is blessed with great ground conductivity.

 

[rfry]

Also gotta tell ya that graph doesn't take into consideration of local Ground Conductivity. Which North Texas is blessed with great ground conductivity.

As I mentioned in my post, the plots in the graph were made for perfect ground conductivity, ie, zero loss.

If they showed the true ground conductivity for the useful coverage area around the transmit site then the field intensities in the plots would be less than shown in the graph.

So the graph gives an upper limit for field intensity vs. distance. Real-world performance would be less than that.

//

 

[radioman148]

>>Well My station is about a year old now. a nice antenna on the ground attached to my outside wall has done just fine for me out here. A good ground probably is what makes my signal so good. i just shoved an old rabbit ear dipole into the ground and hooked up a wire to that. It's served me well.>>

Am I understanding you correctly, You're using the old rabbit ear as a ground? What kind of antenna are you using?

 

[LibertyNT]

>>Well My station is about a year old now. a nice antenna on the ground attached to my outside wall has done just fine for me out here. A good ground probably is what makes my signal so good. i just shoved an old rabbit ear dipole into the ground and hooked up a wire to that. It's served me well.>>

Am I understanding you correctly, You're using the old rabbit ear as a ground? What kind of antenna are you using?

A wire antenna...
The Rabbit Ear is at full length shoved in the ground. which is about 3 feet or so. I didn't say MY signal was great. Just covers my Block and some surrounding blocks with a good to decent signal.

Also gotta tell ya that graph doesn't take into consideration of local Ground Conductivity. Which North Texas is blessed with great ground conductivity.

As I mentioned in my post, the plots in the graph were made for perfect ground conductivity, ie, zero loss.

If they showed the true ground conductivity for the useful coverage area around the transmit site then the field intensities in the plots would be less than shown in the graph.

So the graph gives an upper limit for field intensity vs. distance. Real-world performance would be less than that.

//

Ah i see.. So that graph would be a perfect example for that lake. I have noticed on land the signals not as good but its listenable. 5 of those 15 miles it fluttering of the signal as you pass buildings and whatnot. but overall his signal is very impressive you have to admit.

 

[radioman148]

>>A wire antenna...
The Rabbit Ear is at full length shoved in the ground. which is about 3 feet or so. I didn't say MY signal was great. Just covers my Block and some surrounding blocks with a good to decent signal.>>

OK, thanks.

 

[rfry]

So that graph would be a perfect example for that lake.

Sorry, but that belief is incorrect.

The typical conductivity of a freshwater lake for MW frequencies is about 8 mS/m. This is true even for the U.S. Great Lakes (Superior, Michigan, etc).

This 8 mS/m conductivity is far, FAR less than the value used in the chart I posted. The groundwave propagation loss over a path with 8 mS/m conductivity is much greater than shown in my chart.

So the plots in my chart do NOT provide a "perfect example" for the coverage areas ascribed to KLSR, IF they are operating in compliance with Part 15 AM.

//

 

[LibertyNT]

So that graph would be a perfect example for that lake.

Sorry, but that belief is incorrect.

The typical conductivity of a freshwater lake for MW frequencies is about 8 mS/m. This is true even for the U.S. Great Lakes (Superior, Michigan, etc).

This 8 mS/m conductivity is far, FAR less than the value used in the chart I posted. The groundwave propagation loss over a path with 8 mS/m conductivity is much greater than shown in my chart.

So the plots in my chart do NOT provide a "perfect example" for the coverage areas ascribed to KLSR, IF they are operating in compliance with Part 15 AM.

//

Well They use a Rangemaster Transmitter. No Mods or anything. So im guessing the only infraction they would have is the ground wire.

Didnt you have a chart at one time that showed how better Part 15 systems run from second floor of a 2 story house?