In another thread, http://www.part15.us/forum/part15-forums/transmitter-talk/part-15-fm-fcc-legal-considerations a problem a person operating on AM had with the FCC was mentioned.
I looked for NOUO’s in Stayton OR for AM operations, and found 2. (Did I miss any relevant ones?)
http://transition.fcc.gov/eb/FieldNotices/2003/DOC-297435A1.html
http://transition.fcc.gov/eb/FieldNotices/2003/DOC-294572A1.html
They both seem pretty clear. The transmitter was installed elevated and the structure elevating the transmitter was radiating, so a NOUO was issued.
So, in the other thread, was there more to the story than elevated installs and the inspector saying it was non-compliant?
From the other thread, I got the feeling someone may have been saying “the manufacturer of the certified transmitter said it was okay to do an elevated install, so the end user should have been okay to do an elevated install.”
An article means little to figuring out what happened. http://www.oregonlive.com/news/index.ssf/2009/11/fcc_shuts_down_folksy_low-powe.html The NOUO’s tell the story that we care about to keep from getting an FCC visit.
Elevated installs of AM transmitters are asking for trouble. Most of the AM NOUO’s relate to breaking the 3m rule.
Things could have been worked out. But he got mouthy with the FCC agent and things went down hill from there.
Below is an illustration of the effect of using long, radiating paths between the ground terminal of a Part 15 AM transmitter and an earth r-f ground.
Some prominent hobbyists have contended (and maybe still do) that the ground lead described in FCC §15.219(b) applies only to a short conductor connecting the transmitter to the top of a "massive wire," metal flagpole, tower, steel framework, "lightning ground" wire etc where the bottom of those conductors were connected to the earth.
However the performance of such systems using mutiple conductors wired in series is the same as if that path to the earth used a single wire of the same length.
The added radiating length from such long "ground" conductors can have a large affect on radiated field intensity. as shown below.
This has all been posted/discussed many times before, but some newcomers to Part 15 AM may not have seen it yet.
Can you define or have ever seen an "unshielded tower."
I think you may have taken that out of context. In the NOUO’s it says “unshielded radiating metal water reservoir tower” and “unshielded radiating metal tower”. The concern is the tower is radiating, and nothing is containing the energy radiated by the towers. The way the NOUO is written I think unshielded and radiating go together to emphasize the structure the transmitter is mounted on is part of the ground path so the transmitter is noncompliant.
I know: Humor here. Just wondered how one would shield a water tower?
But what if an elevated ground wire were shielded.
Some have claimed that will not prevent radiation.
But if shielding is useless to stop radiation, why would anyone mention shielding in any elevated ground scenario, be it wire or tower?
Your graph shows a fs at 1600 meters to be about 70 uV/m. This distance (1 mile) is one for which many (myself included) report a usable signal on a car radio.
In order to compare this fs for the elevated systems would it be reasonable to extend the graph to greater distances so the distance for an elevated system can be illustrated for the car radio experience?
Neil
That is my point exactly.
Extending that graph to show the range to the 70 µV/m field for the elevated systems would have a lot of error, because those curves do not account for groundwave propagation across the lossy earth. The longer the path, the greater the error.
Here are some numbers that do account for those propagation losses, using NEC4.2 ...
System Description:
- Frequency = 1670 kHz
- Earth Conductivity = 5 mS/m, d.c. 13 (average earth)
- RF Ground Configuration: 12 x 3-m Radial Wires, buried 0.1-m
- Tx Power Output at Loading Coil Input = 85 mW, unmodulated
- Loading Coil Resistance at 1670 kHz = 18.5 ohms
- 3-m Vertical Whip Attached to the Transmitter
- Vertical Conducting Path Connecting the Transmitter Chassis to RF Ground
- Loading Coil Adjusted for Best System Performance
This system with its transmitter located 0.2 meters above the earth produced a groundwave field of 57 µV/m at a horizontal distance of 1 km.*
If the transmitter is elevated 5 meters then the distance to the 57 µV/m field is 2.9 km.
For an elevation of 10 meters the distance to the 57 µV/m field is 4.2 km.
IMPORTANT The increased range is not due to the height of the transmitter+whip, but to the greater radiating length of the antenna system conductor leading from the transmitter chassis to r-f ground, when the transmitter+whip are elevated (see illustration below).
* Note that this field at 1 km is almost 4X that permitted by FCC §15.209 at a distance of only 30 meters.
A long shielded conductor (coax) with the center conductor attached to the transmitter and the shield attached to the earth only should work because the shield and the earth connection are at the same potential.
...the shield attached to the earth only should work because the shield and the earth connection are at the same potential.
The top of a vertical shield attached to the earth only at the bottom does not have the same r-f characteristics at the top of that shield that exist at the bottom of it.
Consider that a grounded, shunt-fed vertical monopole is sometimes used as a transmit antenna. That wouldn't be possible if that conductor had the same potential as the earth, along its entire length/height.
I was refering to the DC potential. Accounting for resistance of course.
However the suggestion of using shielding was to prevent the radiation of r-f, not DC.
Such a shield will not prevent the radiation/reception of radio frequencies.
for the additional information regarding elevated tx field strength. I didn't think it would be simple.
The numbers pretty clearly show why the FCC has the 3 meter limit in 15.219.
Neil