The original post starting this thread is simple and direct. "How do you properly mount and ground a Talking House ATU to the roof of a house while staying compliant with the rules? (3 Meter Length Rule)." The simple and direct answer is: you can't.
I don't believe it was ever in question in this thread that the TH and ATU were certified together. An 8 meter length of coax was included between the TH and ATU in the certification EUT configuration. It was apparently labeled as an "interconnect cable" , instead of a transmission line.
ANSI 63.4 specifies exactly how to handle interconnect cables: "All interconnecting cables that hang closer than 40 cm to the ground plane shall be folded back and forth forming a bundle 30 to 40 cm long, hanging approximately in the middle between ground plane and table".
The lab test report and test setup photos are not available on the FCC OET web site as they are for more recently certified transmitters like the Procaster, so we can only assume that they were both certified in substantially the same way.
Go to https://apps.fcc.gov/oetcf/eas/reports/GenericSearch.cfm and type in the two parts of the FCC ID in the first two boxes and click search.
FCC ID: DLB 5LTT98 for the Talking House
FCC ID: VCJ AMTX100 for the Procaster
Labeling the TH coax as a simple "interconnect cable" rather than a transmission line does not negate the fact that with the ATU elevated, the coax IS a transmission line attached to an elevated base-loaded monopole antenna.
Anyone expecting to get a free pass from the FCC by claiming the elevated TH/ATU is compliant with the 3 meter rule because the combination is certified should be prepared for failure. As usual, it all depends on the inspector. Some are laid back and don't really care, and some will nail you.
Well, I'm no radio engineer. Don't claim to be one. But I am pretty good at common sense.
The TH and ATU combination is certified. A given.
So if you install that combination at ground level, with no ground, it is OK. And if you install that combination with an elevated ATU with no ground, it's not OK? Doesn't make sense. The coax, if it does radiate, is radiating the same whether it is elevated or not. And there are certainly no indications in the documentation that you can't mount it elevated - and the documentation forms part of the certification. They DO say that installing a ground wire may void certification and that does make sense.
But then, perhaps common sense goes out the window with Part 15.
The definition of common sense at dictionary.ocm is: "sound practical judgment that is independent of specialized knowledge, training, or the like; normal native intelligence". Another definition at merriam-webster.com is: "the ability to think and behave in a reasonable way and to make good decisions", leaving out the part about specialized knowledge.
In the broad field of science and technology, most people who may be judged to have common sense don't have specialized knowledge, so the simpler, second definition is more appropriate.
The technical details concerning the behavior of monopole vertical antennas are widely published and well understood by engineers and scientists. Fundamental aspects of their operation have been presented in bits and pieces on this and other Part 15 forums. Those technical posts have probably been largely ignored by those that aren't interested or don't have the technical background to fully understand them.
Here are some very basic fundamentals.
1. A monopole antenna ABSOLUTELY requires an earth ground connection to complete the electrical circuit from antenna, through the air, to ground and then back to the transmitter. This is not true for dipole antennas or variants such as elevated counterpoise antennas.
2. RF current flows back to the transmitter through the ground wire.
3. Any wire that is conducting RF current will radiate an RF signal.
4. Any conductor connected between earth and a monopole antenna or transmitter/antenna will conduct the same amount of RF current that is conducted in the antenna,
5. The ground "conductor" below the antenna or transmitter/antenna includes the combination of all wires including, power, ground, audio and the outer surface of any coax shield.
6. Any portion of the wires contributing to the combined ground connection that are substantially vertical will radiate. Any portion of the combined wires that are horizontal and substantially at zero elevation from earth will not radiate significantly and, in fact, will act similarly to ground radial wires, which do not radiate.
' "Which would require some means to supress." Say more about that.'
In Post 16 of this thread I wrote in general terms about suppressing r-f current flowing on the outer surface of the coax shield leading to/from the ATU. Such current always produces radiation, as noted by PhilB in his recent post in this thread. The only way to ~eliminate that radiation is to ~eliminate that current flow.
One means of this would be to install ferrite chokes around the coax cable where it connects to the ATU. However if those chokes were effective then they would block the flow of r-f current to and from the power line r-f ground that is necessary for most efficient radiation of the 3-m whip attached to the ATU. Such chokes also might reduce the effectiveness of the connection of that cable shield to the building a-c ground needed for protecting the transmit system from nearby lightning surges.
However this or a similar approach apparently was used by one manufacturer of FCC-certified AM transmitters, and reportedly the results were not satisfactory to the FCC, or to the operator of that system.
Others have suggesed installing a "surge protector" in series with the coax shield where it connects at the ATU. Such a device has very high impedance during normal operation of the system, but low impedance when it conducts during nearby lightning transients. Once again, if r-f current cannot flow along the outside of the coax shield, then radiation from the 3-m whip at the output of the ATU will be very low.
This configuration also means that the 3-m whip and ATU have no way via that surge protector of draining off static charges that can build up on the 3-whip. That could be mitigated to some extent if the the surge protector had a parallel path across it supplied by several thousand ohms of (non-reactive) resistance.
Also it is unclear how such a surge protection device could be installed so that it would prevent current flow on the outside of the coax shield without doing the same for the current flow on the inside of the coax shield, during normal operation And if it did, then the coax could deliver no r-f power to the ATU.
This topic is more complex than it may appear.
Perhaps a few people need to step away from the trees (i.e. the technical details) and view the entire forest (i.e., apply some basic common sense).
I'm not arguing with any of these technical details (I understand them better than you may think). What I'm saying is that there is no restriction spelled out in the FCC Part 15 rules vis a vis elevated mounts. Nor, in any of the documentation with the TH and the ATU, is it stated that you cannot mount this combination in an elevated fashion.
My contention is that as long as you ensure that the antenna mounted on the ATU plus the ground wire (as well as the ground you're grounding to outside of soil) on the ATU does not exceed 3 meters, then you are legal. You are meeting precisely the Part 15 rules, with a certified transmitter.
I understand that this may fly in the wishes of some manufacturers of Part 15 compliant transmitters, who may feel that the TH/ATU combination is unfair competition. I myself don't know how they got these FCC certified, the then DOC - Department of Communications - in Canada would NOT certify a similar setup for a Talking Sign/ATU combination (Talking Sign then turned into Chez Radio). But the FCC did. So if I were in the U.S., I wouldn't have any compunctions about using it - again, as long as you meet the 3 meter rule.
"My contention is that as long as you ensure that the antenna mounted on the ATU plus the ground wire (as well as the ground you're grounding to outside of soil) on the ATU does not exceed 3 meters, then you are legal. You are meeting precisely the Part 15 rules, with a certified transmitter."
Even when the ATU and attached 3-m whip are mounted within a few inches of the suface of the earth, there will still be radiation from the outer surface of the shield of the coax cable connecting the transmitter to the ATU, unless some means is provided to prevent it. This is the result of the "ground" connection of this system being made via the a-c ground of the power supply, to a ground rod at the utility service entrance.
It would be a rare case where the conductor length to that a-c ground rod plus the length of the whip would not exceed 3 meters -- which would mean that the system does not effectively comply with FCC §15.219(b), in spite of any certification it might have.
If the ATU+whip was raised several meters above the surface of the earth while having the ground terminal of that ATU connected to a buried earth ground (e.g., a ground rod) directly below the ATU, that configuration would not effectively comply with FCC §15.219(b), either, due to radiation from the conductor leading from the ATU ground terminal to the ground rod -- in spite of any certification those hardware components may have.
My take on what has been said above is that certain FCC Certified transmitters will violate the rule for Part 15.219, and probably 15.209, and be subject to a NOUO.
Play it safe. Join the ALPB.
Rich is still effectively in the trees. Ah well.
In reality, a ruling on compliance is pretty much up to the FCC inspector in any case (regardless of the armchair physicists). I still maintain that if you are using an unmodified, certified TH + ATU, with the ATU installed elevated (using the supplied coax), and no ground wire connected to the ATU (you're using the ground to the transmitter's AC mains), you are legal.
It's not about physics. It's about the RULES.
Just earlier a member wanted this discussion closed because it covers old ground, but there are always new thoughts and new members and the whole reason for part15.us is to talk about low power radio, so further discussion is welcome.
PhilB said something worth recalling:
"If we assume the coax was considered an "interconnecting cable", folding it to a short length would suppress most of the RF that it might radiate. Additionally, if the ATU were grounded with a wire to the ground plane, much of the remaining possible radiation from the coax would be bypassed by the ground wire. There is a valid case for saying that these connection steps effectively remove the long coax from the 3 meter requirement. Same goes for the audio and power cables. They are also fan-folded down to a shorter effective length for RF."
Think about that. It would explain how such a system gained acceptance.
But it would not explain what the long coax is for if only to be folded into a short package.
In my opinion, no matter how you slice it, this is a curious set of circumstances.
I don't think we have a basis for reaching any sound conclusions about it and it remains a standing mystery.
Artisan,
You said: "What I'm saying is that there is no restriction spelled out in the FCC Part 15 rules vis a vis elevated mounts".
There most certainly is a rule pertaining to elevated mounts. It's called the "3 meter rule". I know you are aware of this rule because you have mentioned it. Exactly which part of the 3 meter rule don't you understand? Here it is quoted again in case you forgot:
Section 15.219 (b) The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.
You have made derogatory remarks about being in the trees and "armchair physics". I have a BS and an MS degree in electrical engineering, so I don't take your comments lightly.
I have attempted to state the "physics" in the simplest terms, and yet it seems the most basic fundamentals still ellude you.
Here is the bottom line in one sentence. When the ATU is elevated, the coax shield IS the ground wire.
Simply measure the length of the portion of the coax from earth level to the ATU and add that to the length of the antenna and see if the result is 3 meters or less. That's not so hard, is it? If you are confused by the words "coax" and "wire", you can do a Google search to get the definitions. Jeez!
I want to thank everyone for your thoughts and input on this subject. I never knew that this post would get so much attenetion. You guys have really helped me get a better understanding of the Talking House ATU and the grounding for it. I would be breaking the 3 meter rule if I run a grounding wire from the ATU into the Earth. I will try running the ATU without any grounding and see how well it operates. I may temporary run a grounding wire until I think of a better solution. Again, thanks everyone. 🙂
FACT-We've been down this road before.
FACT-The FCC OET certification test data details using coax cable to connect the transmitter to the ATU and was accepted.
FACT-The discussion always centers on placement of the ATU with respect to earth.
FACT-Based on patent language and FCC acceptance, both the manufacturer and major vendor of Talking House/I AM Radio systems maintain it is acceptable to elevate the ATU/antenna provided no separate gounding line is used.
FACT-At the end of the day it doesn't matter if you put it in the clouds or bury it in a hole, an interference claim will shut you down-legal or not.
OPINION-People, it's just not worth the time to discuss it.
The ALPB took care of it.
PhilB said "When the ATU is elevated, the coax shield IS the ground wire."
Yes, and the Talking House/ATU combination was FCC Part 15 certified in exactly that configuration. What exactly do YOU not understand about that? I don't believe that you're exactly unbiased in your pronouncements.
In any event, since this is apparently getting personal, I will take my leave of this thread. And I still stand behind my statement re the legality.