My experience with the horizontal loop has led me to conclude that it
is a good antenna on its one wavelength band and 2 and maybe even 4
w/l, provided the loop geometry is achieved, i.e. 3 or 4 or more equal
sides and its height is 1/4 wave or more on its lowest frequency of
use. So for example, if you can have a 520 foot long loop with 3 or
4 equally long sides it will play okay if you want high angle, on 80
meters if you have it 60 feet high. It will be too low on 160 for
decent performance. On 40 and up you start getting into some wild
unpredictable patterns from the 3-D modeling I have seen.
I learned some of this from my own experience. I had a loop that was
just a bit less than one w/l on 75 meters. I fed it with ladder line
and a balanced tuner (which probably covered some of my construction
sins). It was 25 to 30 feet high. But the sides were not equal in
length; two were fairly long but two were so short that it barely
behaved like a loop on 75, however I believed it out performed a
center fed dipole at the same height on 75 and it seemed to. Where I
really goofed was that I bought into the NVIS information about cloud
burner antennas being perfectly okay at heights as low as 1/10
wavelength on the lowest frequency. I figured I was pretty low but
not low enough to experience much if any ground loss. I patted myself
on the back for a few years. I was very much mistaken. After a few
years I began to wish I had the loop higher up, but eventually
concluded that I could not afford the cost of elevating it an
additional 20 or 25 feet, but I could afford to get a 130 foot long
dipole that high.
By coincidence a couple of things happened during this evaluative
process earlier last spring: I happened across a nugget of wisdom in
one of Cebik's antenna musings and a ham in New England did some
modeling for me that showed a higher dipole 130 feet long would
outperform the loop on 75 meters. Cebik's nugget which I'll
paraphrase here because I can't recall it word for word, was that
height is everything for low band horizontal antennas and trumps every
other consideration. In other words, if you have a choice between a
small half-wave antenna that's high up and one that's big and low, the
high antenna wins every time. I had somehow missed this in the past.
I had figured the loop, due to its length, was superior. I took it
down and put up a 130 foot center fed dipole at 50 feet with the ends
hanging down. It is markedly superior to the loop at 25 -30 feet on
transmit and receive, and even though its takeoff angle is still
straight up, the ground loss must be significantly less because it is
out performing the loop by 10 to 15 dB on regional contacts. My
conclusion with horizontal wire antennas on 80 meters is that ground
loss starts to cause performance to deteriorate at 40 or 50 feet,
perhaps even higher, and the NVIS wisdom that such an antenna can
perform adequately as low as 0.1 wavelength is extremely incorrect.
Do not make the mistake of thinking that if the t/o angle doesn't
change there will be no performance difference between say, 30 and 50
feet on 80 meters (and this scales for higher and lower bands). There
is a critical difference on 3.5 - 4 MHz over just a few feet, 15 to 20
feet in height, going from 30 feet to 50 feet on that band, that
substantially reduces ground loss for a horizontal antenna, and
increases the amount of propagated RF, even though it is still
straight up. I imagine that if you could plot ground loss on a Y
axis with height on the X axis the curve would drop down quickly from
maximum and start to level off around 50 or 60 feet on the X axis when
moving from left to right. In other words, the Old Timers with their
high pre-World War 2 dipoles stretched between two windmill towers and
fed at the center with ladder line had an antenna that is hard to beat
even today.
I therefore no longer recommend any horizontal wire antenna on 160
meters unless it is possible to achieve an average height of at least
100 feet. For most hams this means a transmitting antenna that is
working against ground on that band and a separate rx antenna.
Certainly a lower horizontal antenna will work in the sense that the
user will make contacts, but its performance will be sub-par compared
to what could be experienced at a height of low ground loss.
73
Rob
K5UJ
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