Below is my experience with two different antenna designs using only
ham equipment for data collection and reference over a period of
about six months of day and night comparisons (June-November 1993).
The test period took me through the late summer and fall weather
conditions for north central Texas. No snow conditions were
experienced only one violent hail storm.
I compared a 34' tall 40m vertical monopole antenna located about
75' from a 31' top loaded folded vertical (4 skirt wires) antenna
about 8 years ago. I compared the two antennas A-B fashion on 40m
for noise as well as signal strength. The monopole was matched with
a loading coil to the 50 ohm feed line. The top loaded folded
vertical was capacitor coupled to the skirt system for a 50 ohm
match. Both antennas had the same number of radials (60) 34' long
(thanks to a neighbor to my east).
The folded top loaded vertical was generally quieter (noise wise)
than the monopole in most cases by about 2 S-units as compared
between my two transceivers on 40m with no distinguishable signals
present. One radio was a FT1000D and the other a Icom 775DSP. Both
antennas and transceivers could be switched in and out instanteously
for the A-B comparison. The DSP on the 775 was disabled for the A-B
comparisons. S-Meter center scale positions were within 5% or each
other using a MFJ noise bridge with a 47k resistor in series with
the antenna input to the receivers.
The test results were usually always the same no matter what time of
day the measurements were taken. Receive signal levels were just
about 1 S-unit better with the top loaded antenna. This may have
been because the top loading added some gain to the antenna. Top
loading was via a 17' diameter (8 spokes) top hat assembly with the
outer perimeter of the spokes tied together with #14 wire. The
natural resonance point for the top loaded vertical was about 2.4
Mhz.
I lifted both antennas from ground at the bottom to detune them to
see if there was any interaction between the two at their feed
points. I did not see any change in impedance using either a MFJ or
AEA bridge while measuring the test antenna, and comparing it
against the open antenna.
The top loaded antenna always did hear better (signal quality-my
ears) than the monopole. These tests were conducted from a 185' by
200' residential lot in North Dallas with overhead power lines
running behind the lot in the alley way. Power line height is 35'
with a transformer located on a pole adjacent to my driveway. I
believe this line is a 24kv line which is located about 45' away
from the top loaded vertical.
I will add that I have manufactured the folded top loaded vertical
commercially in the past. There are about 1,400 of them in service
worldwide with most of them in commercial service. The above tests
were documented in my engineering log and dated. The folded test
antenna was not a production antenna, only a prototype. However, it
is electrically and mechanically the same. The comparison tests
convinced me to begin manufacture of the antenna which I can say has
been a very rugged and effective antenna for its size.
Again, this test was conducted using only amateur gear from my home.
The only calibration standard used was a precision 10W 50 ohm
standard for the calibrating the impedance bridges.
Your comments and questions are welcomed ... Ed - N5NUG
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