Talking House ATU On Roof of House

You need to ensure that your ground doesn't radiate.

One way is to install radials, perpendicular to the antenna (and 180 degrees from each other, to cancel out any radiation).  That will give you your RF ground.  If you require a safety ground, you can always attempt to install a filter, but there have been varying opinions here on the effectiveness of that.

Or you could just mount the antenna without any type of ground.  Some have reported success with a Talking House/ATU combination with no ground wire attached to the ATU.

Elevated mounts are always dicey, at least in this day and age, and the FCC inspector will have the final word as to its legality, no matter what opinions you get here or what you might think.

You're always safer to ground mount your antenna (or ATU), run a short ground wire to copper stakes and use radials.

In reality, it's not possible (3 Meter Length Rule).

Otherwise, best performance is achieved with a ground wire from the ATU down to a ground radial system. Without a ground wire, the ground is through the coax shield, then through the transmitter AC wiring connection, and finally to a the poor RF ground at the AC power service entrance. Both grounding methods will radiate causing the signal to be stronger than if the antenna were at ground level, thus the 3 meter length violation.

Lightning protection is much better with a dedicated ground wire. The ground wire will tend to divert lightning surges from flowing through the coax shield into your house.

Many are operating with a roof mount while still remaining under the FCC radar. AM band FCC rule enforcement is at a negligible level. It's your call.

Here's what appears on this subject in the user guide for the T.H. ATU.

The paragraph at the bottom of this clip should be of interest.

By certifying the use of ATUs, which the FCC has done, they contradict their own 15.219 rule by allowing transmission lines that exceed the 3-meter limit.

The ground question, since ATUs are certified for use above the ground, becomes even more mystifying, since the FCC has not restructured the printed rules to make room for the certified ATU type transmitters.

If I am incorrect in any portion of what I've said, please notify and I'll modify what I say in the future.

Carl: The ATU when used is part of the final stage of the transmitter. One has to switch the rear of the transmitter to use the ATU.  When the ATU position is selected the auto-tune unit in the transmitter is removed from the circuit. Therefore the"transmission line" is not really a transmission line but simply an inter-connect to the final stage.

Both systems compared below assume 15 ohm loss in the loading coil of the ATU, 30 ohm loss in the r-f ground path provided by conduction from the ATU chassis ground along the outside of a 50-foot coax cable shield through the transmitter and power supply to the power line ground rod at the service entrance, an operating frequency of 1650 kHz, 35 mW transmitter output power, and average earth conductivity (5 mS/m d.c.15).

• Horizontal distance to the 150 µV/m groundwave field for a T.H. system where the remote ATU is located 10 meters above the earth = 1740 meters (1.08 miles).

• Horizontal distance to the 150 µV/m groundwave field for a T.H. system where the remote ATU is located a few inches above the surface of the earth = 360 meters (0.22 miles).

The difference in range for these two configurations is due to the radiation from the coax cable to the elevated ATU -- which, unless some means is provided to prevent that radiation, effectively adds 10 meters of radiating length to the ~3-m whip at the output of the ATU.

The section of the coax cable that is lying horizontally on the earth has negligible useful groundwave radiation because the propagation loss for horizontal polarization is very high.

WDCX said what I like to hear: "The ATU when used is part of the final stage of the transmitter."

The ATU is a passive device, NOT an RF amplifier stage, and since it is accepted as being a final RF stage, I am inclined to believe that in any case the true final RF stage referred to in 15.219 is the filter, ATU, loading coil, or antenna on the OUTPUT of the RF final amplifier!

Many have argued that the language "exclusive of filament or heater power" proves somehow that the final RF amplifier is "the final RF stage." But the only thing it might prove is that the final RF stage needs to be a vacuum tube. That hasn't happened since the 60s.

The Talking House with the ATU is FCC Part 15 certified.  Therefore, if you mount the ATU on a roof with no ground, using the ground in the coax to the AC mains, you are legal.  At least in the U.S.

Yes, Rich, that's exactly what I am trying to resolve by finding that "IF THE ATU IS CERTIFIED THE FINAL RF STAGE IS THEREFORE THE ANTENNA SYSTEM."

That may not be exactly what you just said, but I think the audio cable is an indisputable part of any Part 15 transmitter, and if it happens to radiate, what can you do?

The more thoughtful FCC man would get it. If one of them doesn't get it, he should be taken to task.

Again, it depends upon the individual inspector.  At least one of our members using a roof mounted ATU was inspected and passed.  There was a separate ground wire connected.

My opinion regarding lightning protection is to use a COAXIAL in-line protector at the point of entry grounded to a suitable ground rod rather than a separate ground wire attached to the ATU.

We've been down this road before.  The FCC OET certification test data details using coax cable to connect the transmitter to the ATU and was accepted.  The discussion always centered on placement of the ATU with respect to earth.  

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.

"...if you mount the ATU on a roof with no ground, using the ground in the coax to the AC mains, you are legal."

Legal in the U.S. in that such hardware (but not the roof-mounted installation of the remote ATU component of that hardware) might have been granted certification under FCC §15.219, based on certification tests not conducted directly by the FCC.

Maybe not legal in the U.S. with respect to the physical reality permitted by FCC §15.219(b), which limits the radiating length of the antenna system to 3 meters.  This could be violated by radiation from the outside of the coax shield leading to an elevated ATU + 3-m whip, if such coax shield radiation was not suppressed by effective means.

"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."

While the manufacturer and major vendor reportedly agree with themselves on this issue, the FCC has yet to publicly support such a conclusion, AFAIK.

Please re-read my Post 10 above, the final edit of which was made during Posts 11 and 12.

"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."

While the manufacturer and major vendor reportedly agree with themselves on this issue, the FCC has yet to publicly support such a conclusion, AFAIK.

Certainly that conclusion is not supported by physics unless no radiation is produced by the r-f currents flowing on the outside of the coax shield leading to that elevated ATU -- which would require some means to suppress.

"Which would require some means to supress."

That's an interesting subject.

Say more about that.