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Delaware Amateur Radio Association, Delaware OH VOL 36 NUMBER 3
© DELARA News, the official monthly newsletter of the Delaware Amateur Radio Association, Delaware, OH
Circle Path Propagation
A new mode of working stations you never could before.
By Bob Dixon, W8ERD
An unfortunate antenna mishap has led to the discovery of a new mode of propagation that is dis-
tinct from the more familiar short path, long path and
skew path modes.
How it was discovered.
Last April, my yagi antenna was bent in the center in an unfortunate accident. The front half now
points somewhat to the left
of the rear half (Figure 1).
This has caused a strange effect to occur, which has not been recognized before. Thru much experi-
mentation, I have found that the bent
antenna now launches a signal which curves to the left, and travels along a big circle. I have named
this Circle Path Propagation, and it is more easily understood by thinking of how a pitched baseball
curves because it is launched with a spin. (Figure 2) Note that this has nothing to do with circular
How it is used.
The diameter of the circle depends on propagation conditions and the bending angle of the an-
tenna. If the signal absorption is low, it travels all the way around the circle and returns to me.
Then I can measure the round trip time and calculate the diameter of the propagation circle, and
hence how far away the band is open to. In very good conditions, the
signal makes multiple trips around the circle, so a distinctive echo is heard on my signal, making it
easy for other stations to identify me. If the delay time is just right to make the return signal be in
phase with my originally transmitted signal, then my signal is amplified and a new phenomenon
called Circle Path Gain is achieved. If the signal makes multiple turns around the circle, the additive
gain can be huge.
For maximum range, both stations have to point their antennas along the circle. So I have to point
my antenna 90 degrees clockwise from the other station, and they have to point theirs 90 degrees
counterclockwise from me. And if I am using Counterclockwise Circle Path, then they need to use
Clockwise Circle Path (see antenna discussion below for how to achieve this).
Other noteworthy characteristics
This propagation mode is very useful in eliminating QRM from stations at other distances who are
not on the circle path, or those who do not know
which direction to point their antenna,
This new propagation mode could also help explain the phenomenon of long-delayed echoes,
where the timing of the echo does not correspond
to the round-the-earth trip time. In reality, Circle Path Propagation is really the most general case of
propagation, with all others being special cases of it. Short and long path just have zero curvature,
and skew path is the case where the circling signal does not make it all the way back.
If you happen to live behind a large obstacle such as a mountain or a big building, then circle path
propagation is the solution for you. Just point your antenna to the side of the obstacle, and your
signal will circle around it and enable you to work stations you never could before. This could also
have military applications, where it is desirable to send a signal around the enemy territory to
friendly forces behind them, without the enemy being able to
intercept the signal.
It would be desirable to enable both clockwise and counterclockwise circle path propagation, so
you could choose whichever is best for a given
situation. It would also be useful to vary the bending angle of your antenna and refine the effect
that has on the circle propagation radius.
So a small motor could be mounted at the mast, to point the front half of your antenna at various
angles, relative to the back half. (figure 3). The longer the yagi is, the more effectively it can impart
curvature to the signal.
A further refinement could be obtained by having the antenna boom curve continuously along its
entire length, instead of having an abrupt bend at just one point. This could be done by having a
long cross boom sticking out from the mast perpendicular to the antenna boom, with pulleys at the
far ends. Then attach
ropes to the ends of the boom, run them out to both ends of the arm, thru the pulleys, and back to
the mast at the center of the antenna. At that point
you install a motor-driven capstan drum with a vertical axis, and wind all the ropes separately
around the drum. When the drum turns, it pulls on the antenna boom
in one direction and releases it in the other direction, bending the boom in a smooth curve. (figure
4). Thus you can match the curve propagation radius
very closely. Of course you have to be careful not go too far and break the boom. Perhaps this will
lead to a new yagi design with a more limp boom,
to accommodate this need.
There are rumors that some major antenna
manufacturers are looking into this mode, and we may see commercial versions of bendable yagis
Thanks to Craig W8CR for suggesting that I write this article.
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Circle Path Propagating
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