CFA Tests Set to Begin
by Ted Nahil
CFA Test Site Nearly Complete for June Tests
While EH Developer Awaits Test Authority
With the weather finally moderating in Shropshire
in the United Kingdom, long-awaited tests on the Crossed-Field
Antenna should soon be underway.
Delayed by rain, snow, ice, illness and injury
– not to mention access restrictions thanks to foot-and-mouth
disease – the construction of a test CFA in the U.K. neared
completion at the end of May, with managers hoping to begin tests
this month (June).
In the meantime, developers of another new AM antenna
design, the EH antenna in Eatonton, Ga., await word from the FCC,
hoping they’ll gain permission to construct and test an antenna
in the United States designed for the AM band.
Observers say that real-world tests are critical
to determining both antennas’ viability as functional and
efficient AM radiators.
Dr. Fahti Kabbary, a co-inventor of the CFA, arrived
in early May to finish construction of the CFA antenna, according
to Robert Richer, president of Crossed Field Antennas Ltd., Farmington,
Conn.
Foot-and-mouth disease has played its part in the
CFA delays. The bovine illness produced a quarantine of the test
area and its perimeter, halting construction and accompanying
tests of the antenna.
"As the weather improves, foot-and-mouth diminishes,"
Richer said. "Everyone is scared to death, but we have access
to the site, which is a great step forward."
The British government has authorized facilities
of 2 kW at a frequency of 972 kHz, Richer stated. The government
also has allowed an occupied bandwidth of 20 kHz for the tests.
The frequency-agile transmitter is being installed
by ntl Group Ltd., a communications facilities provider headquartered
in Hook, Hampshire, U.K. The company is providing a transmitter
specialist and a medium-wave antenna expert to assist with impedance
matching at the site.
Ben Dawson of Hatfield & Dawson plans to take
extensive field measurements of the CFA, arriving on site this
month. George Hagn, now retired from the Washington office of
SRI International, is accompanying Dawson. SRI is a global organization
that develops applications for businesses, governments and other
organizations.
Hagn has developed an open-wire line kit to measure
ground conductivity, a measurement that will be critical at Shropshire
to help determine the antenna efficiency. The efficiency is a
measurement of how well an antenna produces an RF signal given
a known input power and impedance.
Richer hopes the field tests will be complete by
the end of June. These tests include the following procedures:
Horizontal Plane Radiation Efficiency
– This conventional, six-radial proof of performance will
measure the field strength on radials spaced 60 degrees apart.
Fifteen or 20 evenly spaced points will be used on each radial
at distances ranging from 0.1 to 3.0 kilometers from the antenna,
and 10 points are measured at distances from 3.0 to 10.0 kilometers
from the antenna.
Ground Conductivity Tests – These
tests will determine the ability of the ground to propagate an
RF signal and will be used as a second independent set of measurements
of field strength.
Vertical Radiation Pattern –
Measurements will be made using a helicopter provided by the BBC
and will be taken at known elevations and distances from the antenna,
with a 5-degree vertical resolution. These tests will be critical
in determining the shape of the antenna’s pattern as the
signal moves at an angle through the sky, ultimately defining
how the antenna propagates without causing undue skywave interference.
Near-Field Radiation – These
tests will determine if the CFA meets or exceeds the radio frequency
radiation standards of the FCC’s Office of Engineering and
Technology. CFA proponents claim exceptional performance in this
area.
Impedance Bandwidth – These tests
are planned to determine the antenna’s ability to produce
an audio output of suitable fidelity in a receiver. Critics of
the CFA maintain that short radiators, such as the CFA, generally
exhibit poor impedance bandwidth characteristics.
Pattern Bandwidth – Measurements
will be taken at carrier and sideband frequencies to determine
what effect the CFA has on audio bandwidth and to determine if
the antenna might create adjacent-channel interference under nighttime
(skywave) conditions.
Stability and Sensitivity – These
tests will determine how well stations will be able to maintain
their licensed radiation patterns with an operating CFA. Far-field
measurements will be used to determine the effects on the field
strength and pattern variations under different, but controlled,
variations in the system input voltage and current.
Frequency Scalability and Ground Effects
– Prior to any FCC approval, it will be necessary to be able
to predict the performance of a CFA at frequencies other than
the ones used for the tests, specifically across the AM band.
If the CFA’s performance is less dependent on wavelength
than conventional AM towers, it may be possible to define the
characteristics of the CFA in a frequency-independent way.
Richer stated that the measurements will be made
solely by Dawson and Hagn to eliminate any possible bias introduced
into the results by members of the CFA team, including Richer
and Kabbary, who will be on site during some of the tests.
Some preliminary test data may be available as
early as the end of June.
EH word
Meanwhile, developers of the EH antenna await word
from the FCC about a test permit.
In March, the Mass Media Bureau dismissed an application
filed in August 2000 by EH Antenna Systems, Eatonton, Ga., with
the FCC to test an EH antenna.
The request for authority to test an antenna on
1590 kHz at 100 watts was denied for a variety of technical reasons,
one of which concerned potential interference to co- and adjacent
channels.
In addition, in a letter to Hart and copied to
his consulting engineer Stu Graham of Graham Brockman Inc., the
bureau stated, "given the high level of existing interference
at the site," the proposed test facility would be subject
to interference, resulting in inaccurate measurements of the EH
antenna’s performance.
Graham has been working since that dismissal with
OET to obtain permission to test the antenna.
Ted Hart, CEO of EH Antenna Systems, stated that
OET has forwarded a revised application to the MMB for frequency
coordinatio
