The setting: city to the west, mountains to the east. For 2 meter
transmissions, should one feel bad that half the coverage area of ones
vertical antenna is wasted on unpopulated mountains, or might the
mountains reflect most of the signal anyway, especially if right
east of us is an idealized vertical mountain wall? Thus no hurry to
upgrade to a yagi?

On Tue, 15 Feb 2005 02:39:50 +0800, Dan Jacobson
wrote:

The setting: city to the west, mountains to the east. For 2 meter
transmissions, should one feel bad that half the coverage area of ones
vertical antenna is wasted on unpopulated mountains, or might the
mountains reflect most of the signal anyway, especially if right
east of us is an idealized vertical mountain wall? Thus no hurry to
upgrade to a yagi?

Hi Dan,

That's how it works here in Seattle for some. We have a lot of hills
that hide repeater antennas. We also have a lot of mountains that
reflect them (even from the opposite angle).

73's
Richard Clark, KB7QHC

The setting: city to the west, mountains to the east. For 2 meter
transmissions, should one feel bad that half the coverage area of ones
vertical antenna is wasted on unpopulated mountains, or might the
mountains reflect most of the signal anyway, especially if right
east of us is an idealized vertical mountain wall? Thus no hurry to
upgrade to a yagi?

My guess - the mountains reflect some of the signal, but in a rather
unpredictable manner and with much of the energy being reflected
upwards.

The mountain reflections might help fill in a few gaps in your
coverage pattern. However, the reflections are likely to _hurt_
coverage in some areas, due to multipath interference with the direct
(non-reflected) signal. You might find that your signal suffers from
a greater amount of picket-fencing due to multipath, than it would if
the mountains were not present or if the antenna's pattern didn't
include the mountains. The more vertical (and the more reflective)
the mountain wall is, the stronger the reflections, and perhaps the
more pronounced the picket-fencing.

It might well be worth your while to experiment with a simple
somewhat-directional antenna. A single reflector, located perhaps .1
to .2 wavelengths on the mountainside side (sorry :-) of your current
vertical, cut to perhaps 5% longer than a half-wavelength, could be
used to shape your antenna's pattern into something vaguely cardioid.
This would send your transmit energy (and your receive sensitivity)
where it will do you the most good. Play around with the antenna-to-
reflector spacing and see what it does to your pattern.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

D It might well be worth your while to experiment with a simple
D somewhat-directional antenna. A single reflector, located perhaps
D .1 to .2 wavelengths on the mountainside side (sorry :-) of your
D current vertical, cut to perhaps 5% longer than a half-wavelength,
D could be used to shape your antenna's pattern into something
D vaguely cardioid.

Yes! Say, can one just cut the reflector 5% longer than the antenna?
The antenna is some complex double 5/8 wavelength job.
If the reflector is shorter than the antenna, it becomes a director?
What if the reflector is just a pole jabbed into the ground, thus
grounded and longer downwards than the 5%? What if the pole is "very
much longer than the antenna in both directions"?
How about the thickness of the reflector? Only as thick as the antenna
itself (but wait, the antenna has two thicknesses, top thin, bottom
thick), or just grab any iron pipe (wait, metal type and shape
important?)

In article ,
Dan Jacobson wrote:
D It might well be worth your while to experiment with a simple
D somewhat-directional antenna. A single reflector, located perhaps
D .1 to .2 wavelengths on the mountainside side (sorry :-) of your
D current vertical, cut to perhaps 5% longer than a half-wavelength,
D could be used to shape your antenna's pattern into something
D vaguely cardioid.

Yes! Say, can one just cut the reflector 5% longer than the antenna?
The antenna is some complex double 5/8 wavelength job.
If the reflector is shorter than the antenna, it becomes a director?
What if the reflector is just a pole jabbed into the ground, thus
grounded and longer downwards than the 5%? What if the pole is "very
much longer than the antenna in both directions"?
How about the thickness of the reflector? Only as thick as the antenna
itself (but wait, the antenna has two thicknesses, top thin, bottom
thick), or just grab any iron pipe (wait, metal type and shape
important?)

Well, I think you'd probably need to do a bunch of modelling in NEC2
or similar to get a really solid analysis, for any given antenna and
reflector.

The "5% longer" rule of thumb applies to antennas which use halfwave
radiators - it works with Yagis, and Cebik has an analysis which
indicates that it works with Yagi-like beams based on vertical J-poles.

I suspect that the issue is rather more complicated for antennas which
use 5/8-wave or stacked/collinear radiators. The phase relationships
between driven and parasitic elements in such antennas are probably
different enough that this simple rule might not apply.

I believe that you could probably get a fairly effective
pattern-warping by using a non-tuned reflector which is significantly
longer than your collinear radiator. Try cutting a piece of EMT or
other conduit (or aluminum pole, or whatever) so that it's a few feet
longer than your antenna, and then install it 6" to a foot on the
undesired side of the radiator... have one end of it sticking up at
least a foot above the top of the radiator and the other extending
down below the bottom of the radiator.

Some commercial multi-bay stacked dipole antennas use the mast or
tower in just this fashion, to convert a circular omni pattern into a
cardioid or off-center near-circular pattern. Varying the antenna-to-
mast/tower spacing changes the shape of the pattern.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:
I believe that you could probably get a fairly effective
pattern-warping by using a non-tuned reflector which is significantly
longer than your collinear radiator.

Passive non-resonant elements have very little effect.
That's what allows a lot of multi-band beams to work.
--
73, Cecil http://www.qsl.net/w5dxp


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"Cecil Moore" wrote
Dave Platt wrote:
I believe that you could probably get a fairly effective
pattern-warping by using a non-tuned reflector which is significantly
longer than your collinear radiator.

Passive non-resonant elements have very little effect.
That's what allows a lot of multi-band beams to work.
_____________

As a point of reference, the radiation patterns of sidemounted FM broadcast
transmit arrays are affected by a non-resonant mounting structure (the
tower) -- especially for vertical polarization. Measured patterns from the
manufacturer's test ranges demonstrate this, as do the NEC-2 studies in
several of the papers on http://rfry.org .

RF

Richard Fry wrote:

"Cecil Moore" wrote
Passive non-resonant elements have very little effect.
That's what allows a lot of multi-band beams to work.

As a point of reference, the radiation patterns of sidemounted FM
broadcast transmit arrays are affected by a non-resonant mounting
structure (the tower) -- especially for vertical polarization.

True, and please note that I was talking about *antenna
elements* not support structures.
--
73, Cecil http://www.qsl.net/w5dxp


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I believe that you could probably get a fairly effective
pattern-warping by using a non-tuned reflector which is significantly
longer than your collinear radiator.

Passive non-resonant elements have very little effect.
That's what allows a lot of multi-band beams to work.

As a point of reference, the radiation patterns of sidemounted FM broadcast
transmit arrays are affected by a non-resonant mounting structure (the
tower) -- especially for vertical polarization. Measured patterns from the
manufacturer's test ranges demonstrate this, as do the NEC-2 studies in
several of the papers on http://rfry.org .

Check... that's just the sort of situation I was thinking of.

The effect on the pattern isn't all that strong, though... only a few
dB. As one example, the old ARRL VHF book gives the plans for a
four-bay stacked dipole antenna system. With the dipoles placed on
four sides of the mast, the antenna system has a gain of about 6 dB
(they don't say dBi or dBd but I assume it must be the latter). With
all four dipoles on the same side of the mast, the pattern is said to
be a cardioid of about 9 dB gain.

They don't say how deep the back-side null is. The cardioid pattern
might have a high enough F/B ratio to help with the original poster's
situation (mountain-side reflection), or he might need to use a tuned
reflector of the 5%-longer-than-resonant variety to get a more
directional pattern.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Cecil, W5DXP wrote:
"Passive non-resonant elements have very little effect."

True in a parasitic array such as a Yagi.

In some antenna arrays all the elements are fed by a transmission line.
Directional arrays are also made by using elements which are not
connected to a transmission line. These disconnected elements have
current induced into them by the driven element and are called parasitic
elements.

The effect of a parasitic element depends on the magnitude and phase of
the current flowing in it. To get substantial current, the element must
be near the driven element or other element carrying large current, and
it must be almost resonant. A non-resonant element has a large reactance
limiting its current flow and effectiveness.

A resonant length for a conductor in space is 1/2-wavelength.

A resonant length for a grounded conductor is 1/4-wavelength.

The 3-element Yagi is a common parasitic antenna. It has a driven
element, a reflector element, and a director element. The length of all
three elements is nearly 1/2-wavelength in most cases. To get large
currents, near-resonance is essential.

The trick is to get the radiation from all elements to reinforce in the
desired direction and cancel in other directions.

Phase of current in the reflector is often delayed by making it slightly
longer than a 1/2-wavelength. Phase of current in the director is often
slightly advanced by making it slightly shorter than a 1/2-wavelength.

Spacing between elements is close to get a large excitation of the
parasitic elements. This makes a compact antenna. Close spacing means
mutual impedance (coupling) between the elements, and this affects the
drivepoint impedance. Close spacing increases directive gain but lowers
the array`s radiation resistance. See Terman`s Fig 23-36 on page 904 of
his 1955 "Electronic and Radio Engineering".

Large non-resonant reflectors are also used. Backward radiation can be
eliminated with a plane conducting sheet. It`s impenetrable and must
reflect. The earth often acts as such a reflector. The parasitic
1/2-wave reflector is a "degenerated" (Kraus` word) case of the plane
reflector. I recall that either Kraus or Terman suggested that the best
way to handle non-resonant reflectors was on an image basis.

Best regards, Richard Harrison, KB5WZI