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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"Cecil Moore" wrote in message
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.
___________________

But geing closer to the driven element(s), probably even non-resonant
antenna elements may have more affect on the net antenna pattern than that
produced by a supporting tower further away.

RF

Richard Fry wrote:
But geing closer to the driven element(s), probably even non-resonant
antenna elements may have more affect on the net antenna pattern than
that produced by a supporting tower further away.

Using EZNEC, one can observe that the current in a
closer non-resonant passive antenna element can be
pretty low while the current in a grounded tower
further away can be pretty high.
--
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