The Mosley PRO-67B at K9MA
August 9 ,1993
Background:
The Mosley PRO-67B, S/N 562062 was delivered June 10, 1993. It was intended to
replace the Hygain TH-7, primarily to provide improved performance on 40
meters using single tower.
Installation:
A 70 foot guyed Rohn 25 tower is located near the center of a level 60 by 120
foot residential lot. The guy wires are attached to the tower at the 33 and 65
foot levels and are anchored 35 feet from the tower base. The upper guy wires
slope downward from the tower at an elevation angle of about 60 degrees. The
guy wires are broken with insulators, with a maximum unbroken length of 19
feet. However, for the purpose of the PRO-67B tests, the upper guy wires were
replaced with non-conductive material. During the tests, there were no
conductive guy wires or other antennas within 37 feet of the PRO-67B, and no
conductors within 70 feet that were resonant on any of the bands tested.
Antenna assembly and installation:
The PRO-67B was assembled according to the manufacturer's instructions for the
Code I (CW) setting. All dimensions, element locations, trap locations, and
trap orientation were verified several times. Additional verification when the
antenna was disassembled confirmed that it was properly assembled.
The feedpoint connection was made according to the manufacturer's instructions,
using a coaxial cable balun.
Test Results:
Tests were performed with the PRO-67B connected directly to a Kenwood TS-930S
transceiver through 130 feet of Belden 9913 coaxial cable. The TS-930 SWR
meter was used for all SWR measurements. For front-to-back ratio and pattern
measurements, Hewlett-Packard step attenuators with 1 dB steps were used to
maintain a constant S-meter indication. The signal source was a horizontally
polarized yagi about 2 km away, on the other side of a broad valley providing a
nearly line-of-sight path.
Initial performance was extremely poor, with high SWR on most bands and very
little directivity. It was determined that all the traps were defective, with
high losses caused by the plastic material used for the coil forms.
Apparently, these traps were not tested by Mosley before shipment. Replacement
traps had much lower losses, and Mosley gave assurance that they had been
thoroughly tested. Mosley also verified that the resonant frequencies
measured by K9MA were correct. Replacement of the traps required removal and
re-installation of the antenna.
With the new traps, the SWR of the antenna matched the curves in the Mosley
manual very closely, except on 40 meters. On 40 meters, SWR ranged from 1.5:1
at 7.00 MHz to 2:1 at 7.100 MHz and 3.0:1 at 7.200 MHz, indicating that
resonance was somewhere below 7 MHz.
Front-to-back ratios were 2, 5, 17, 10, and 8 dB on 40, 20, 17, 15, and 10
Meters, respectively. Mosley's specifications are 12 dB on 40 meters, and 20
dB on the other bands tested. (Front-to-back ratio was not measured on 12
meters.) The low front-to-back ratios were confirmed by observation of
long-distance signals. These measurements were made with non-metallic upper
guy lines on the tower. When the steel upper guy wires were installed, the
front-to-back ratio changed by 2 dB or less. The front-to-back ratio was
generally higher without the steel guy wires, but on 40 meters it was 1 dB
higher with the steel guy wires installed. These guy wires attach to the
tower 6 feet below the antenna, and slope away at a 60 degree elevation angle.
Previous measurements, using the same measurement techniques, showed that the
TH-7 had front-to-back ratios in excess of 20 dB at most frequencies, and
values which showed excellent agreement with the manufacturer's data. This
comparison shows that the test method was accurate.
Conclusions:
The PRO-67B, properly assembled and installed showed front-to-back ratios far
below the values in Mosley's published specifications. Although the antenna is
known to be very sensitive to detuning by nearby conductors, the small effect
of the upper guy wires shows that detuning by the lower set, at least 37 feet
below the antenna, is extremely unlikely to be responsible for front-to-back
ratios 10 to 15 dB below their specified values. It appears that either the
antenna is defective through manufacture or design, or that the specifications
are inaccurate.
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