Gents:
Hello, new here and am trying to get a community Part 15 off the ground. Been playing around with the TH5 for about a year now and just acquired the external atenna box from Radio Systems. Fairly impressed with the audio quality, and it should meet my needs from what I'm aiming to do.
My (probably first of many) question is this: Does elevation really make much of a difference with these antennas? I have a few options in the installation - a 8' elevation in the center of a metal roof, a 15' elevation on the side of a metal roof, both using grounds to the roofs themselves (it does work) or a 22' elevation (which I have not tested yet) which is installed and grounded to a vent flashing. The 22' would put me at equal treeline and adjoining building level, but just barely. It'll be quite an undertaking to get the thing up there, but it's certainly do-able.
I've been concocting and thinking up various ways of getting audio from my office to the transmitter, but that's another topic I'll save for later.
Have began to play around with it some, but am looking for pointers on how to max that range - it's a fairly congested area I'm in with few open spaces, lots of trees, but not many tall structures. Water less than two miles to the west and south.
Looking forward to hearing some ideas. Let me know if you need more details. Thanks!
MM
Elevation does make a (postive) difference, but if you elevate that ATU 8 feet or 15 feet or 22 feet above the roof and ground it to that metal roof, you may be deemed to be in violation of Part 15 rules - the antenna (whatever the length of your whip is) + feedline (no feedline with the Talking House ATU, per the FCC) + ground wire cannot exceed a length of 3 meters or roughly 10 feet. At least, that's the way the FCC has been evaluating long ground wires for the past while.
If that is an issue for you (it certainly would be for me), you can mount your ATU just above (i.e., inches) the centre line of your metal roof. You should not need to run a ground wire, as there will be a capacitive coupling effect between the roof and the ATU. That way, your ground wire will be of 0 length, and you shouldn't run into any problems, should the FCC come calling. If you do run a several inch in length ground wire to the roof, your installation may still be deemed to be exceeding the 3 meter rule, as the metal roof will probably radiate.
The safest installation, in terms of passing an FCC inspection, is to ground mount your ATU, and use radials (they will increase the efficiency of your antenna without radiating, much as mounting your ATU several inches above the metal roof).
But of course, the final decision is yours.
My personal experience with the ATU finds that elevation does improve coverage. Some will argue that a counterpoise is needed and when ground mounted will offset the difference between elevated and ground mounting.
I tried the ATU at ground level with 12-20 foot radials. I also tried it mounted about 5 feet above the roof of a wood frame 1.5 story house. The roof top installation far out performed the ground mount with radials.
The roof top ATU install was not grounded whereas the ground mount ATU was grounded to the radial system.
Common theory would dictate the results should be reversed but its hard to argue with real world results.
MRAM's experience that the elevated ATU out performed the ground mounted and grounded installation is not too surprising even if the roof top installation was not grounded considering that the connecting coax shield could function as the ground lead back to the transmitter and produce radiation.
I'll leave the interpretation of this up to our legal scholars.
Neil
The elevation numbers I was referring to was the total height above ground level. Meaning the base box of the antenna would be placed on one of three different roof levels at (approximately) 8', 16', or '22, not above it... (Indeed, that would have been WAY out of FCC regs!) The ground lead from the ATU to the roof would be... 12 inches or less, maybe?
Also, would grounding to the roof cause enough radiation on the roof itself (which as I said is metal) to not pass the rules, or would it even matter? Would the material of the physical mount of the antenna box make a difference (pvc, aluminum, copper/metal, wood) so as to not count in the 3-meters?
Obstructions in and around the neighborhood are another matter... Lots of trees, power lines, and buildings of various height and construction. Not much space for a radial system, I did consider that. Height may be my best bet, but I also have an option of a location next to a large bayou which feeds into a larger bay that surrounds the town... Anyone know how these things are with signal skip?
It is likely that if you physically connect your ground wire to the metal roof, it will radiate. It's possible to add a choke to the ground wire (it's been discussed here before) to reduce that radiation. But whether or not a non choked or choked ground wire connected to that metal roof would pass an FCC inspection is up to the particular FCC inspector - they have the final say.
Like I said before, you could try mounting it just above the metal roof without the ground wire. When I had my FM station on Bowen Island, it was first mounted on a tripod about 6 feet above a metal roof. I had to relocate it as I changed studio locations and decided to mount it inches above that metal roof - the signal strength (and range) increased substantially! And that was with no physical connection to the roof. Since unlicensed FM is governed by field strength only, I decided to move the transmitter and antenna (it was a Decade MS-100 in a weatherproof box) back to the original height. But it just goes to show that a metal roof can act as a ground, in the real world, with no actual ground wire.
The potential installation site of near a bayou sounds particularly promising (I'm assuming that it would be ground mounted). The soil there should have excellent grounding capabilities (earth grounds don't radiate), and coupled with radials would make an interesting comparison to an elevated mount.
I don't think there are any absolute answers. There are a great many factors that go into the range seen by a Part 15 AM transmitter. Sometimes you just have to experiment to find what will work best for YOU in your particular location and with your particular setup.
Assuming your transmitter is on the ground floor, run a short wire from the chassis out the a ground rod with your radial system attched. A review of the ATU schematic indicates that the ground lug of the ATU is attached to the shield of the coax. So no long ground lead from the ATU is required.