David Ryeburn wrote:

I get the impression that this whole discussion about reception
difficulties for 90 MHz FM signals is hypothetical, rather than an
attempt to solve an existing real problem.

Are you sure about the Yagi being better than a dipole under the
circumstances described? He said to assume that he is on hilly land. The
predicted performance of a Yagi is far-field performance. If the
surrounding terrain is bumpy enough close enough, perhaps the
performance might be rather different. If the field near the director is
substantially different from that near the driven element, and the field
near the reflector is different from both of those, perhaps a Yagi might
do a poorer job of discriminating against unwanted signals than a
smaller, simple, dipole would do, since the field in the neighborhood of
the smaller dipole would be more uniform. I'm not asserting that this
would be have to be the case, but it seems reasonable to me that it
might be the case.

Entirely unreasonable for the following reasons:

The spacing between elements of a 90 MHz yagi is about 2 to 3 feet. It is
hard to imagine a 3 foot wide hill.

In other words, it would take some rather bizarre terrain to have a
different effect on the driven element than the director.

Further, the far field at 90 MHz is about 10 feet or so; one would
hope the antenna is at least 10 feet off the ground and the next
hill is more than 10 feet away.

With a dipole, you are depending on the null to minimize the undesired
signal and have little gain in the direction of the desired signal.

With a yagi, you still have the null (assuming a proper yagi) plus
additional gain in the desired direction. The net result is the ratio of
the signal strengths of the desired to undesired signals becomes greater.

Consider, for example, how FM broadcast signal strength and cleanliness
can vary markedly as one slowly moves one's car just a few inches, e.g.
in heavy traffic. This can happen both in downtown skyscraper jungles
and in the more natural mountainous terrain near where I live (Greater
Vancouver, BC, area), though in the mountains the heavy traffic is
blessedly absent. Clearly far-field assumptions are unwarranted under
such circumstances.

Clearly, far field assumptions ARE warrented under such circumstances.

What you are seeing is multiple reflections (in the far field) off the
various structures.

Yet another reason to get the antenna high; i.e. above the influence
of such structures.

David, ex-W8EZE


--
Jim Pennino

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