To see how spacing affects take-off angle, install and run the YT (Yagi/Terrain)
software that comes on the CD-ROM with your ARRL Antenna Book. Basically,
stacking couples two or more yagis such that energy normally released at higher
angles to the ground is radiated as part of the antenna system's primary
(lowest-angle) lobe. Stacking doesn't lower the take-off angle of the primary
lobe, but it does re-route the higher-angle energy that is radiated by even a
very high single yagi. This is not necessarily an advantage on 6M, as I will
discuss.
For sporadic-E work on 6M -- the workhorse propagation mode on this band --
understanding and being able to manipulate take-off angle is, IMO, more
important than raw low-angle gain, for a number of reasons. One is that
sporadic-E clouds are very thin -- just a few kM, according to most
ionosonde-based research -- and in many cases relatively small. That means that
the effective range of the angle of incidence between the radio wave and the
cloud is pretty small, and you have to hit it just right. Large, deep nulls in
the elevation pattern of your yagi system may make this impossible. The
sporadic-E MUF also plays a big role in this computation. If the MUF is well
above the 6M band (meaning the sporadic-E clouds are very highly ionized), a
higher-take-off angle can take advantage of the larger critical angle produced
by the higher MUF, and allow communication to take place over comparatively
shorter paths, effectively reducing the extent of the usually dead "skip" zone.
A yagi with little energy in these higher-angle lobes may make a poor showing
over such a path, while a low-mounted yagi (sometimes as low as twenty feet off
the ground!) will often excel.
Just the other night, for example, I was working stations in the midwest over a
very typical sporadic-E path of about 1000 miles. Most of the signals from this
area were S9 or better. I finally and came back to a call from a comparatively
weak station in the same area and gave him a 5x5 report, only to find that he
was running a kilowatt to stacked 9-element yagis at 150 feet! If 6M had been
open to Australia via F2 propagation, he would have cleaned everybody's clock
--
but not over this typical sporadic-E path to New Mexico.
On the other hand, low-angle propagation modes also exist on 6M, from ground
wave and tropo-scatter to some arguably mysterious propagation modes that nobody
clearly understands yet, such as supposed "cloud-to-cloud" sporadic-E
propagation and "E-ducting".
So on 6M it pays to try to cover as much of the useful elevation range (from 0
to about 16 degrees) as possible. YT will show you that, for a flat terrain
model in the far field, probably the best overall 6M yagi configuration is one
which stacks two or more beams about 30 feet apart (with the lowest being at
around 30 feet above ground level), AND which allows each antenna to be used
separately as well as driven in phase. A 60-30 stack of two yagis with a
StackMatch produces a pretty ideal coverage of all the useful elevation angles
within around 3dB. Adding a third at 90 feet lowers the lowest possible take-off
angle a little. This would help for F2 propagation (next solar maximum), as well
as for tropo-scatter and ground-wave modes, but doesn't help much for most
sporadic-E paths.
Hope this info helps!
Bill / W5WVO
Keith Dutson wrote:
> May be, but I have always heard 1/2. Anybody out there with
> experience?
> How does spacing affect take-off angle?
>
> Keith
>
> -----Original Message-----
> From: Pete Smith [mailto:n4zr@contesting.com]
> Sent: Monday, July 12, 2004 7:39 AM
> To: keith@dutson.net; towertalk@contesting.com
> Subject: RE: [TowerTalk] Stacking Distance for M2'd 6m7JHV?
>
> At 08:38 AM 7/12/2004, Keith Dutson wrote:
>
>> Half wavelength is standard for optimum coupling, or about 10 feet
>> for 6 meters ((6*39.4)/(12/2)).
>>
>
> Isn't maximum gain somewhere in the 5/8 - 3/4 wavelength range?
>
>
> 73, Pete N4ZR
> The World HF Contest Station Database
> was updated on June 5, 2004
> 2728 contest stations at
> www.pvrc.org/WCSD/WCSDsearch.htm
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers",
> "Wireless Weather Stations", and lot's more. Call Toll Free,
> 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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