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RE: The crossed yagis on the Dayton AO-40 demo station



Jack Gelfand wrote:

> Is there a simple way to explain which yagi goes in front
> of the other to get RHCP or LHCP?

Wayne replies:

> Howard G6LVB emailed me a little diagram to show the correct
> relationship between the two dipoles to obtain RHCP.

Here's that note about ways to achieve RHCP... figures are in metric and
calculated for a 435MHz antenna.

Look from the back towards the front (reflectors closest to you) in the
direction of the main lobe's maximum radiation.

        |
        |+
        |
        |
--------+---------
        |      +
        |
        |
        |

The + represents the Gamma match [or folded side of the DE for the WA5VJB
antenna] end of the driven elements.

Three alternatives.

(a) First let's assume that the two driven elements are physically
positioned at the same location along the boom. For RHCP, the vertical
element needs feeding 90 degrees before the horizontal element, so you'd
need to feed the horizontal element with a feeder with an additional delay
of 90 degrees (or, for example, 11.5cm [4.5"] of RG58 67% VF longer).

(b) Or, instead of using a 90 degree delay line, you could situate the
horizontal elements 90 degrees (or 17.2cm [6.75"]) behind (i.e., closer to
you when looking from behind the antenna).

(c) Or, a mixture of both. For example, if the horizontal elements are 2cm
[0.75"] behind the vertical elements that's 10.4 degrees behind. So you need
an extra delay of (90 - 10.4) = 79.6 degrees, or 15.2cm [6.0"] free space,
or 10.2cm [4.0"] of 67% VF RG58 delay line in the rear feeder.

73 Howard G6LVB


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