Lately I have been back to work.. the nicer the weather, the longer the days.. I've just returned home about 30 minutes ago, so no Rich, my silence was not due to your post -- which I have just read twice and not quite understood.
Your reply did not have relation to the installations I described.. You referenced 2 ground mounted transmitters co-phased with each other as a combined system, whereas I described elevated ungrounded transmitters on the the same frequency each outside of each others range, working indepentently... There would be a dead zone between them, therefore no concern of sync or phase.
Carl, did you speak in jest? Give the FCC a call and ask for some prefered treatment you say?.. Hmmmm.. maybe if I could get an agent drunk enough, he'd agree.. Only problem would be when sobrietry returns..
Anyway, back to the idea at hand, Perhaps a visual will help..
Presently, I have only one of my Rangemasters in operation, ungrounded and mounted on the roof of the single story building next door (The stone-faced building), about an inch above the peak of its metal roof (I thought this might do better on top the metal roof instead of atop my own building (the little white building) without ground, but it made little difference).. With this installation and no ground I'm only getting a few blocks in each direction on a car radio..
Here's the same from a top view..
So I figure, ok fine at least it's legal, I'll just keep positioning additional roof mount ungrounded transmitters, far enough apart so their signal ranges don't touch, all along the main stretch untill it's covered fom end to end... No radiating grounds to fret about, no questionable installs.. Less than perfect, but legal, and effective enough..
By the way, and I've talked about it here before, but several years ago I had it installed on the roof peak of the pavilion (as you see in one of the pictures the middle of the octogon shaped roof with the peir extending off it) it was grounded to the pavilions ground and my signal was strong on the next island over about 7 miles away!.. I played around with it a few months but then ceased before trouble came knocking!..
I suffer from a naive belief that sometimes just talking things over person to person can do wonders, but attorneys advise "don't say anything," and they know what they are talking about. So please don't contact anyone (except us). WDCX is right about staying clear of the referee.
Your little white building is a dream location.
Access to the adjoining buildings is a nicely negotiated arrangement.
Tell you something about the adjoining uilding which I am quite proud of.. Until very recently it belonged to a man I used to work for about ten years, and am still closely associated with.He's now 92 years old. He built that build from the ground up about 50 years ago or so, and has sold and re-bought the property numerous times..
Well about 4 years ago he bought it back again and gutted the entire inside for a thourough remodel, and a prepare for resale.. That's where he called me in..
Almost every single litle aspect of the remodel was designed and directed entirely by me!.. about 95% me. Everything from the front rock face, the awning, the raised lettering lighted signs.. All the interior, where and how the deviding walls where structured, the diner, design of the kitchen, selection of kitchen equipment, the bar equipment, the booths, chairs, the lighting, including the indoor neon, the pictures, the menu contents, the cosmetic design of the menus, the walls, floor, stools.. everything was hand picked and/or designed by yours truly.. Then once completed, we opened it and I ran it for three years until some one bought it for $600 thousand about six months ago..
It's honestly is one of the most beutiful bars on the island. and it was all done by my eye, and past experience know-how.
Wish I was as good about it with part 15
Damn.. I typed that all to fast.. Please impliment a spell check for this forum
Sorry 'bout that..
"I don't know what you mean by "any leads to the sources going to mess you up.." ..?"
Any leads (wires) leavin' the Xmtr that are not RF isolated at the Xmtr will act as an extended ground. (Power, Audio in)
You referenced 2 ground mounted transmitters co-phased with each other as a combined system, whereas I described elevated ungrounded transmitters on the the same frequency each outside of each others range, working indepentently... There would be a dead zone between them, therefore no concern of sync or phase.
Below is a NEC calculation of the groundwave field on 1700 kHz radiated by a single, elevated, ungrounded, Part 15 AM transmit system for the other conditions shown.
If a field intensity of 500 uV/m from transmitter A operating alone produced a relatively noise-free signal in an AM receiver located along Butler Avenue, fields added by operation of transmitter B (same specs) would have to be at least 10 dB weaker than that 500 uV/m field in order to have ~tolerable interference to transmitter A in the receiver output.
The chart shows that a 500 uV/m field is produced about 370 meters away from transmitter A. Interference on the same frequency would have to be less than 158 uV/m for a protection ratio of 10 dB or more to transmitter A, at that distance.
The chart shows that transmitter B (same specs as A) produces a 158 uV/m field at a distance of about 1 km.
The bottom line here is that for this set of conditions, transmitters A&B would have be located 1.37 km apart along Butler Avenue, and each would have ~acceptable performance along Butler Avenue for a horizontal distance of 370 meters toward the other transmitter.
There would be a zone about 0.63 km wide centered midway between the two transmitters where mutual interference could be a problem.
This analysis does not consider the effects of non-synchronous carrier frequencies and modulation timing/polarity.
If another transmitter spacing would be better I can check that.
Hopefully this will provide information that can help.
Thanks Rich.. Ok, I'm mulling your response over (and converting meters and km to miles so I can better get an idea). I going to let this sink in some first, but there's a couple things that jumped out at me in regard to your chart..
Why is applied power at 75 instead of 100mw?..
Also, my ground conductivity is reported as 8 here..
Thanks for the questions.
FCC §15.219(a) limits the input power of the final r-f stage of the transmitter to 100 mW. No amplifier is 100% efficient, although Class E designs can be better than 90% efficient. A typical value for Class C is about 75%, so I used that. Also -- this is the power applied to the input of the loading coil, not the power that is radiated by the antenna system even when it is perfectly resonant.
Here are the coverage contours for an earth conductivity of 8 mS/m, d.c. 13 (other things the same):
The 500 µV/m contour moves from 0.230 miles to 0.239 miles, a 3.9% increase from my chart value.
The 158 µV/m contour moves from 0.621 miles to 0.684 miles, a 10% increase from my chart value.
To keep a 10 dB minimum protection ratio for the 500 µV/m contour of transmitter A, transmitter B (same specs) would need to located 1485 meters (0.923 miles) away from transmitter A, an 8.4% increase in separation from my earlier post.
The width of the zone where mutual interference could be a problem changes to 715 meters (0.44 miles).
I should have mentioned that the effective conductivity of the earth in an urban area such as along Butler Avenue on Tybee Island can be less than the value shown on the FCC M3 conductivity map you posted.
So the first analysis I posted could be close to valid, or even optimistic.
I should have mentioned that the effective conductivity of the earth in an urban area such as along Butler Avenue on Tybee Island can be less than the value shown on the FCC M3 conductivity map you posted.
I assume you say this in regard to the interferences caused by.. well - everything from neon signs to power transformers etc..
However, I'm curious to know how the already high conductivity of this area might be affected by the fact that this particular stretch of land is only about a half mile wide and surrounded on three sides by ocean..
I'm also not quite grasping the neccesity to move transmitter B almost a mile away to avoid interference when the audio signal of (ungrounded ) transmitter A can only be received for a few short blocks before it fades to hash.. If interference between each xmtr takes place within an area that the audio is unreceivable anyway, what differece does it make?
I would hope to be able to accomplish having shorter sections of dead space between the recievable audio of each transmitter.
You report that your ungrounded systems are received for a few short blocks now. If you are going to duplicate those systems up and down Butler Avenue then from that experience you could locate them closer together.
I was suggesting 3-m, center-fed dipoles, which would produce higher fields than an ungrounded monopole (other things equal). Those setups would have a greater potential for mutual interference at close spacings.
Probably the best solution you will find will come from your own experiments, with whatever you decide to install.
I was just trying to give some perspective to the issues.
I like the idea of the center loaded, center fed vertical dipole. I have toyed with that idea myself and wondered:
A. Should I link couple to the loading coil with a few wraps around it.
B. Being vertical, how will the proximity of the coax affect it.
Perhaps if it was setup like the "coaxial vertical" antennas where as the mast pipe runs up through the lower half of the antenna and the coax runs up through the mast pipe. By doing that maybe skin effect would help shield the coax from the field of the lower half.
The cleanest implementation would be to use equal length conductors a bit shorter than 1-1/2 meters each, one attached to the r-f output connector of the transmitter and one attached to the transmitter chassis, running in the opposite direction.
But as I mentioned earlier, the loading coil used in the AMT5000, Procaster, Rangemaster etc may not have enough range to resonate that antenna -- which according to NEC has a free-space input Z on 1700 kHz of about 0.6 -j 4000 ohms.
The coax vertical antenna would work technically, but then per FCC §15.219(b) some means would be needed to prevent the mast pipe from radiating.
And even if the coax shield didn't radiate, would the FCC still consider it to be a feedline? If so, it couldn't be very long if it had to be counted in the 3-m length of 15.219(b).
Sorry.. the "center-fed dipoles" was flying right by me... Just been tired from some long days lately and was not paying proper attention to your details you were describing.. I only now after googling "center-fed dipole" am beginning to realize what you were saying.
I need to get some rest, back up, and approace your post again..
Placing the transmitter directly at the center of the dipole antenna would avoid needing a transmission line.
By constructing the antenna elements from hollow copper pipe, the DC power and audio could be inside the bottom pipe, fed from below.
To minimize the number of total wires, the DC power could be phantomed on the balanced audio cable.