At 06:55 AM 7/30/2006, john@kk9a.com wrote:
>I'm not sure why my message posted three times when I only sent it once.
>Yes, I meant using an elevation or azimuth rotator won't change the angle
>of radiation. This is assuming level ground, if your antenna is on the side
>of a mountain it will vary depending on direction.
>
>John KK9A
Sure, changing the pointing direction *will* will change the elevation
angle for peak of the pattern. However, the ground reflection effects will
mean that you don't get a "degree for degree" change. This is especially
so for a relatively low gain antenna (e.g. 10dBi has a beamwidth of about
60 degrees, in both horizontal and vertical planes). The effective
"vertical beamwidth" from the ground reflection (particularly for a low
antenna) is much narrower and will dominate.
Start getting a reasonably high gain antenna (say, 20dBi, not that this is
practical for HF), which has about a 20 degree beamwidth, and elevation
rotation (either mechanical or electrical, through phasing) starts to be
more interesting.
However, I think that the real value is in "null placement" more than just
"raw gain". It's well known that atmospheric noise is highly directional,
so reducing the gain in the direction of the noise will improve the
received SNR of desired signal. The same applies for reducing interfering
signals. Say you want to work someone in Maine (and your QTH is Los
Angeles) and there's better propagation, say, to Washington DC. A
practical HF antenna (i.e. you don't have W6AM's array of rhombics) isn't
going to have a narrow enough beam to allow suppressing DC while pointing
directly at Maine. However, pointing a bit north of Maine will start to
put DC down on the lower part of the lobe, into the null, while only
slightly reducing the gain in the direction of Maine.
Same thing works for elevation angle.
The challenge, as Mark and Howard pointed out, is that we (hams in general)
don't have much experience with how to use "elevation control", or, for
that matter, "null placement control" in azimuth Heck, the vast majority
of hams have a single antenna, in whatever position it's in, determined by
where the trees, houses, etc. happened to be, and that works just fine for
most of the time.
So, discussions of the value, or possible performance, of a clever phasing
scheme using the adjustability of a SteppIR, are relevant to a small
fraction of HF using hams, who, by nature, are sort of at the bleeding edge.
For myself, I see this sort of thing (clever phased arrays) as highly
interesting, because it might (and *might* is the operative word) provide a
way for hams in restricted situations to do something beyond the "single
flagpole vertical" or "dipole on the roof", and *that* is something of
great future value. We've been using multielement passively coupled phased
arrays (Yagis) on tall towers for the better part of a century, and for
those that have the place to do it, it works great. But, we're not really
doing much to "advance the state of the radio art" by doing so, and the
availability of "places to do it" is getting less.
The real future is in something like "What can you do in a box that is
50x100x30 feet with a house in the middle of it", and great questions about
"how will you know whether it actually works better" are an important part
of it. We've all gotten beyond the "I worked 300 countries using a wet
noodle and a bedspring" sort of thing.. That sort of achievement is more a
testament to patience, operating skill, and ionospheric luck. We deserve
better, and tools like SteppIRs and computer controlled phasing networks
are something that will get us there.
Jim, W6RMK
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|