Billboard Part 15 Compliance

CGC #720 carried the following story questioning the legality
of a new "talking billboard" in San Diego:

"TALKING BILLBOARD" ON 1610 KHZ IN SAN DIEGO IS POWERFUL

A new "talking billboard" on 1610 kHz is reportedly located
on the west side of I-805, just before the southbound Sorrento
Valley Road on-ramp. It's said to be a tall billboard with an
eight foot whip on top. It has been playing a 60 second BMW
spot for the past few weeks. The problem is that the signal
is reportedly audible in El Cajon some 14 miles away.

(This was a Submitted Report)

Now, Richard Fry comments on the story as follows:
_________________________

Talking billboards typically operate under Part 15, but the coverage reported
above could be related at least in part to a transmit antenna system length
not compliant with Part 15.  Paper 3 at the link below develops this situation
in some detail.   Other papers on this website may be of interest, also:

  http://rfry.org/Software%20&%20Misc%20Papers.htm

Below are some calculated values for field strengths at several distances
from the Part 15 compliant r-f system described there.  These calculations
are based first on a NEC-2 analysis to determine the FCC efficiency for the
antenna, and then using that value with the FCC's propagation curves for
the frequency and ground conductivity, to determine the distances to the
contours shown.  This is a proven process which [of course] applies even
at these low powers.

INSTALLATION DATA:

Item	Description
Frequency	1700 kHz
Applied Power	80 milliwatts (~output power of Part 15 AM tx)
Radiator	3-meter total length including the conducting path from the tx chassis to the ground plane (antenna is ground-mounted)
Antenna System RF Resistance, Loading Coil plus Ground	10 ohms
Ground Conductivity	8.0 mS/m (typical value)
Radiation System FCC Efficiency	21.12 mV/m at 1 mile for 1 kW applied

This information should assist anyone wanting to know the approximate
coverage capabilities of a legal Part 15 AM r-f system, as described.
Usable signal strengths at much greater distances than shown may indicate
that Part 15 Rules are not being observed.

Richard Fry, January 28, 2006
(email to: rfry at adams.net)

Field Strength	Distance
2 mV/m	0.09 miles (good signal)
1 mV/m	0.18 miles (fair)
0.5 mV/m	0.34 miles (noisy)
0.05 mV/m	2.5 miles (very noisy)