delara news Delaware Amateur Radio Association, Delaware OH   VOL 36 NUMBER 3
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© 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.

DELARA News April, 2012

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

polarization.

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.

OBSTACLE AVOIDANCE

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.

FUTURE DEVELOPMENTS

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

available soon.

Thanks to Craig W8CR for suggesting that I write this article.