In addition to the coax serving as a ground connection, there is a grounding connection on the ATU.
On the ATU I am using, the grounding connection is a binding post located next to the whip antenna.
The instructions recommend connecting it to an earth ground to improve the range.
I guess I really mean "2-questions."
1. Guessing that if the transmission line to the ATU is long, the transmitter might need to put out MORE than 100mW, to account for cable loss. The objective would be to have 100mW at the input to the ATU. That's a question without a question mark.
2. Does this Talking House system with ATU produce the same coverage as a conventional part 15 AM system, or is it superior?
Just a quick reminder that no certified AM transmitter would ever put out anything near 100 mW RF.
100 mW is the DC power input limit and the RF output is usually a fraction of that, say 10 - 30 mW for argument's sake.
It does seem that if the transmitter is certified with the ATU, which has a fixed length cable connected, then the manufacturer could add a circuit to that "internal/external" switch on the back of the unit so that when the ATU was connected a different current limiting resistor was in the DC power in circuit so that the RF out from the internal and external antennas was similar.
But I have no idea if this is the case.
The schematic I saw from the Patent info shows no attenuator switched in-line when connected to the external antenna. It appears to connect to some type of feedback circuit which may be to ensure the output power is limited to a legal level.
The antenna cable is not permanently attached to the ATU. They sell it with something like 25 or 50 feet of cable. The "InfoSpot" system sold by ISS suggests using it with up to 300 feet of cable.
I can offer that my experience is that the range is considerably greater using the remote antenna; 300 feet with the internal wire antenna and between 1/2 to 1 1/2 miles with the remote antenna depending on conditions.
First, to clear up a misconception. Laboratory certification of Part 15 devices (transmitters) is NOT done by the FCC. Certification is done by independent laboratories for manufacturers and the written reports or studies are submitted to the FCC for acceptance. In many instances, if the proper language is used, the units are certified for manufacture and marketing in the United States. If you can find the original certification of the TH w/ATU units, you will probably find that the FCC has never taken the units into the lab themselves for a second look. Hamilton's Rangemaster 1000, on the other hand has had further study by the FCC (refer to KENC NOUO). Just because a Part 15 transmitter has FCC certification does not mean they have their own lab rats take it through its paces and make sure it passes regardless of it's final application. That is why most Part 15 broadcast transmitter retailers print and publish disclaimers as to their use.
Now to the ATU. Looking at the schematic, the signal is coupled to the input by a 10 nf capacitor and through the primary winding of the UNUN (unbalanced to unbalanced) inductor. This is used to isolate the signal from the antenna/output circuit. Isolation is used to keep the re-radiation of the coaxial cable shield to a minimum and to allow the metering circuit to more accurately measure the output current in the antenna. The metering circuit contains all the components from the 10 pf capacitor all the way to the meter to ground potential. The ground lug next to the antenna is connected directly to the cable shield making this conductor and the shield VERY MUCH a part of the antenna (not just lightning protection). The result would naturally be greater range.
The new TH units with ATU come with 100 feet of 75 ohm RG-6 cable and the associated "F" connectors. The switch/resistor network used in the transmitter unit is for impedance transformation and expected change in load to the final output amplifier between the wire antenna and the ATU. NO attenuation is intended by the use of the networks.
O.K. boys and girls, now that we have that straight, tubes and transistors are different types of devices that do similar things differently. Let's be careful out there.
Isolation is used to keep the re-radiation of the coaxial cable shield to a minimum and to allow the metering circuit to more accurately measure the output current in the antenna.
A couple of picky points re this. The transformer does not isolate the coax ground from the ATU ground if shown correctly on the schematic since the primary and secondary windings are both tied together to ground. The purpose of this transformer is more likely to match the low impedance of the coil resonated antenna to the transmission line impedance.
The metering circuit responds to the voltage at the base of the radiator and not to the current. Since the antenna is linear I = V/Z so the current is indicated indirectly.
Neil