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art · September 25th 06, 01:29 AM posted to rec.radio.amateur.antenna

wrote:
Hi Art,

A few points below, but first just remember that the TOA of any
horizontally polarized antenna is primarily a function of ground
reflections which vary according to height above ground. Previously
you mentioned that your antenna was designed for a TOA of 10 degrees.
That cannot be true except for a specific height above ground. Whether
Yagi, Quad, Log, Rhombic or any non-vertically stacked antenna.
Something like a Sterba curtain is different because it has multiple
elements stacked vertically which CAN be steered by phasing.

That is entirely correct for known technology........... at this time !

art wrote:

yes it is twenty meters and I am
located in the couintryside that I understand is the highest between
Chicago and New Orleans and since this is the bread basket of the U.S
the ground loam is excellent.

1. Absolute height above sea level means nothing. What is important
to TOA is your relative height above the terrain within a mile or two
of your tower. I operated from Colorado for ~30 years and always got a
chuckle from the guys who said, "My antenna is over 1 mile high". In
fact what is important for determining TOA is not height above sea
level but height above surrounding terrain.

It is the highest point for the U.K. period

2. Ground conductivity has minimal effect on horizontally polarized
ground reflections. You may be thinking of vertically polarized
antennas like verticals where it has a huge effect. HFTA does have
conductivity as an input parameter but it has minimal effect, at least
in my case (average ground versus salt water).

Agreed, there was a Australian article that showd that very well

I use 7/8 andrews plus a long length
underground and the tower is hinged in two places
as well as another one for array tilting.

Phsical tilting has minimal effect on the ground reflections for the
angles of interest (4-10 degrees) because the vertical lobe is not very
narrow (typically a Yagi has ~50 degree 3 dB vertical beamwidth). In
other words tilting has a far secondary effect on TOA versus changing
the antenna height.

Lawson W2PV wrote about that while he was working in Schenectady G.E.
on the computor that replaced zillions of vacuum valves but with
respect to planar arrays
Mine is not a planar array

You can prove this to yourself by modeling with a
program like EZNEC (HFTA does not allow tilting because it is
physically impractical and has little effect).

As I said above mine is not a planar array. The computor program I use
is AO-PRO
by Beasley

Pretty simple question for
those who know the answer after all you start off with 3 db gain in two
different directions thereffore it would seem to me that a yagi was
only 50% efficient

I believe you're referring to a dipole which has 2+ dB gain over
isotropic in the two directions broadside to the element.
Yes, I rounded it off to 3

If we add ~6
dB from ground reflection gain, we get 8 dB gain over isotropic, but
this is ONLY for a specific TOA which is determined by the antenna's
height above ground.

Fully undertstood when referring to planar arrays

Of course a good Yagi will typically have 25-30
dB Front to Rear, so its backward lobe has very little of the total
energy (far less than 50%). F/B for planar arrays is for a single angle , front to rear is more appealing but at the same time it ignores pattern lobes in all directions including upwards. I have found that TOA on its own is pretty wothless as one can get the gain of a larger antenna with a wider but narrower lobe that follows the bottom half of a higher gain lobe. Lets face it for planar arrays gain is obtained at the axpense of half power area.

Bottom line to all of this is that your antenna's height above ground
has the primary influence on TOA. The only other way to "steer" the
vertical lobe is to mount your antenna on a motorized tower (unless you
go to vertically stacked elements and phasing).

Obviously height is important and totally constrictive of all planar
arrays where the TOA is captive. As soon as you add another vector such
that the array is three dimensional as with a stack things change

Put your single antenna at 120' and the vertical pattern will be
centered on about 9 degrees (assuming flat terrain).

I assume you mean vertical elevation and yes mine cannot be much over
100 feet
and I will have various lobes available at various TOA's and most
important variable beam depth to accommodate propagation changes as
arrival angles change
Thanks for the contact The most interesting thing that I am looking
forward to is operation at the bottom of the cycle to see if there is
still propagation but only for low angles. As you gat older you look
for any excuse to stay indoors
Art

73 & GL!

Bill W4ZV

P.S. Here are some results using HFTA for my 10 meter 3-stack:

Yup there was an extensive article on that for the NE area where it
shows the impact
of a ground projection on a collective angle.

http://users.vnet.net/btippett/terrain_&_toas.htm