OK, so has there been any concrete resolution to what exactly constitutes a ground wire? I read on the Procaster yesterday that they suggest you can mount it on a pole as long as the pole is grounded in earth or to a flat metal roof, etc. and as long as the actual grounding wire was shorter than the specified length. We have a steel tower bolted to the metal superstructure of the building, the superstructure is bonded to grounding rods everywhere they go into the earth. All electrical power is also grounded to these rods. The tower is about 60 to 75 feet at the top and that is of course where I would want to install the transmitter. We are also located on the top of a hill with clear line of site to the general populace, especially from on top of that tower.
Legal, illegal, still an unknown and depends if the FCC field engineer is having a bad day?
This also assumes that I can get any of the faculty interested enough to even consider AM, last time it was quickly shot down as useless. Yes I do know that I can run as much power as I desire to cover the campus as long as the field density at 200 feet from the perimeter is under XYZ level (basically gone by 200 feet off property).
Hell Greg_E
The length of ground lead in a campus installation may be any length so long as your signal at campus boundaries measures properly.
Even with that, your description of the building, pole, and substantial earth grounding sounds like it would qualify as "true ground" even in an ordinary Part 15 situation.
Here's some technical information so the reader can decide for themselves what counts as ground lead length.
When a transmitter sends a signal by pushing electrons up the antenna it is necessary that it pulls electrons into its ground terminal (those antenna electrons have to come from somewhere). In other words, the antenna current going out the antenna terminal is equal to and in the same direction as the current going into the transmitter ground terminal.
By definition a RF ground does not radiate. For a "ground" to not radiate, at least two of the ground currents summing together at the transmitter ground terminal need to be equal and traveling in opposite directions. This will be the case if the RF ground is two horizontal equal length wires strung 180 degrees apart and connected together at their abutting ends as a ground terminal. The current from each wire will be equal and each wire will radiate but since the direction of the two currents is opposite the RF fields from each wire will be 180 degrees out of phase and they will cancel yielding no net radiation.
The same idea applies to a ground stake in the earth. The current coming in from the north direction through the soil or radial is matched by the current coming in from the south direction and the fields will cancel. Pick any direction and there is an equal and opposite current coming from the direction 180 degrees opposite.
The key to not radiating is that there is cancellation of the fields because of the equal and opposite currents. Now consider a vertical (or even a horizontal) conductor connecting the RF ground to the transmitter ground. The current will flow through this conductor and produce a field but there is no equal and opposite current available to produce a canceling field and a RF signal will be radiated.
So, a wire, a metal tower, a metal building, or any conductor used as a ground will radiate a signal unless there is an equal and opposite current in a nearby parallel conductor.
Knowing this, consider why the FCC would write a rule which limits both the antenna and ground lead length to a combined total of 3 meters. They know the ground lead is part of the radiating system.
As to the legality, many have suggested that this is up to the discretion of the FCC inspector and this may or may not be the case, but now you know the physics.
Neil
So then I take it we still do not have a real answer to exactly what is allowed. Last time this went around it was pretty much decided that anything that was not ground mounted would technically fail, since I read on yet another certified device that tower mounting should be OK I thought I would check and see if things have changed. Ground mounting is not an option for us.
Maybe I'll write a letter and send it to the local field engineer and ask if he would get back to me or visit next time he is in town.
Greg,
Keep in mind that if you are operating on a campus this restriction may not apply and therefore may not be an issue.
The difficulty with compliance with the rules for a campus is making a defensible documentation of the field strength at the boundary.
Here's a thought. As you recede from the antenna the field strength drops off rapidly at first and then more slowly farther away. Depending on the real estate, to achieve the legal field strength at the boundary may mean that you will have to have a rather weak field strength on campus as well. My experience with campus radio was with carrier current where I could maintain a high field strength inside the buildings and meet the boundary limits. Carrier current systems are complicated but can deliver a good signal in the buildings despite the shielding effects of the buildings.
Neil
I was aware of the regulations regarding on campus use, realistically I want it to go as far as possible while still being legal. If I had the room in the middle of campus I would just put in a ground mounted system and it would probably get most of campus and the perimeter road, but I can not put one anywhere in the middle, so up on the tower it must go. And it seems that up on the tower would still be illegal do the the almost obvious radiation that will occur down the mast part of the tower before it gets to the first set of bolts. I have a feeling that the entire building will radiate until it finally gets into the ground, and that's a really tall antenna at the probably 70 feet to the top of the tower.
What would really be nice is if there was a concrete interpretation for all of us, even if that interpretation is that only ground mounted systems were allowed due to the total radiator length limit. It's kind of dishonest to suggest that these devices can be mounted on a pole that is 25 feet tall as long as the ground wire used to connect to that pole is only 18 inches.
It is almost to the point where I would have to out of pocket the cost of one of these transmitters just to prove to the rest of the department what we could have. I even talked about contacting the FCC to see about a lower power waiver now that there is some precedent set, we could maybe support a 400 watt rig on top of the tower if it was allowed, I know we could deal with a 100 watt rig, but I site the 400 watts since they were issued a couple years ago to one of the guys on here. They wanted no part of AM at all, even if we invest in some digital equipment to have a nice clean stereo output.
A nice dipole running up the tower should throw really far up in the 1600 range on top of this hill, bet it would go 15 to 20 miles out over the lower lying city, might even jump the small hill to the south and go that far that direction too.
Once upon a time before I worked here, they had an FM rig up with about 1 watt output... It went an easy 10 miles up here. After I started I removed the transmitter and stuck it on a shelf never again to see power. It's a nice 1 to 10 watt sucker and we even have the stereo generator and processor to go with it. Wish I could sell it since we will almost never get a chance to get an FM channel here. It would take several LPFM stations to go under, and then we would need to beat the AM commercial guys to the punch or we would have another translator (making about 20 in the market) including ones down on the educational/PBS area.
It's really very depressing when everything I try to do to make the station better gets stomped on.
It seems, Greg_E, that you are at the mercy of decision makers who may not have the necessary vision to appreciate what you envision. But there is additional technology that you will find described elsewhere on the party15.us website which could be added into the equation as follows:
I'll be talking AM in this scenario.
Let's imagine you mount the best AM transmitter you can achieve on your available mast. It is also legal to send a cable AM signal to key points where your reception needs a boost and you can drive a circuit which accepts a 100 mW input (or more on a campus) and load another 10-foot (or on a campus larger) antenna.
What I'm describing is a master transmitter feeding several remote "final" amplifiers.
Perhaps someone will come behind me and link you to the appropriate information.
I think the FCC (other fed agencies do the same thing) keep their rules vague, it gives them latitude they can later use. This makes it frustrating for those who want exact detailed guidelines. Really I don’t see anywhere the rules define what is ground. It does not define what the ground lead is connected to. Here is something that was sent out to agents on the matter last year though.
The 3-meter combined length specified in Section 15.219(b) refers to the length of all radiating elements. Attaching the ground lead to an unshielded radiating object, or the addition of a ground screen, will cause the effective length of radiating elements to exceed 3 meters, in violation of Section 15.219(b).
My take is they are more concerned with a ground designed to radiate, the issue here as I see it is you have conflicting regulations, NEC codes requiring grounding for safety purposes. So consumers are caught in the middle. There are over a million inside table top AM transmitter in operation for example that use the electrical outlet ground for a ground. Would it be recommended that the consumer cut off the ground prong to comply with a FCC rule? Even the idea of telling the consumer to cut their ground is horridly bogus, because of the many other safety and protection laws about grounding in force, to protect the consumer.
So it is just one of those things that really has no resolution, conflicting laws exist, agents just deal with it the best they can. I talk to agents about it, they don’t like it either.