On 12/12/2014 7:53 AM, Lizeth Norman wrote:
Hi Lizeth,
This is my take "in plain language"and undoubtedly there are others
All passive antennas gain, gain, or it'd be better to say they achieve
gain by removing signal from all but the desired direction in various
amounts. Some of those amounts vary widely over the sphere.
The big, or huge difference between HF domain and the higher VHF and up
domains is the amount of interaction with the terrain. The Radiation
angles with HF in all directions including f/b are dependent on the
terrain and its makeup out to quite a distance. UHF and up, see terrain
more as a reflective object, but their height above ground is many
wavelengths so I'd expect them to behave more like an antenna in free
space with a mirror far below...BUT they see the atmosphere as an
absorber, with buildings being more like they are constructed of mirrors.
I'd think Cosmic radiation would make for a relatively steady signal
source (depending on where you are pointed) I don't know much about the
construction of UHF and up, antennas.
I believe UHF and up see the horizon and hills as refractive objects.
When I lived South of Breckenridge, MI, the Edmore repeater on 2-meters
was well over the horizon and beyond where I'd expect reliable
communications. Drawing out the elevation profile including the earth's
curvature showed a rather sharp ridge between us, a little closer to
them than me. Communications were always good with a mediocre antenna
at 20 to 30 feet. Not quite good enough for consistent mobile work. I
was surprised at the width of acceptable reception and they received VHF
stations well out to 50 miles.
When TV went digital, I had a pair of long UHF Yagi antennas with corner
reflectors (Radio Shack models. Although a bit fragile they worked as
good as the high priced spread.)
http://www.rogerhalstead.com/ham_files/Tower26.htm. The one pointed
NW is prominent. The one to the SSE is invisible in the photo. It's
highly visible in http://www.rogerhalstead.com/ham_files/tower41.htm,
but the one to the NW is above the frame. At 90 to 95 feet these
antennas reached the Detroit to Lansing/Jackson area to the S and
Traverse city to the NW with solid coverage. I did have antenna mounted
preamps due to the very long coax runs, but I had 20 to 22 UHF digital
stations that were solid into my location. Not much worth watching, but
I had 22. The top mounted 144 and 440 Yagi antennas gave comparable
performance. Unfortunately at 30 feet of mast between them ant the
thrust bearing, the flexing of the mast beat the snot out of them. I was
surprised that all it really did was tip the elevation angles on the
antennas. The left, or port 2-meter Yagi ended up pointing about 30
degrees above the horizon. OTOH it did destroy the TB-3 thrust bearing.
73
Roger (K8RI)
Hi Roger!
Good to hear you again. All you say is true with HF antennas. On the
other hand, what about VHF and higher?
Looking around I've found a bunch of stuff on optimizing and
qualifying antennas by using solar/cosmic noise. Every explanation
(makes a bunch of sense) that I've read says that the characteristics
(f/b, g/t)of the antenna are necessary to proceed with the
measurements.
As it's been said bunches of times, the figures provided by
manufacturers are suspect at best.
K3NG has an arduino rotator control project that has a sun tracking
feature, which almost automates the qualification process, if I
understand the process correctly.
What to do?
Constructing an antenna range is a possibility. There is a "how-to" in
a piece of software called HDL (horn, dish, lens).
Is this worth it? Time is fun to waste but I'd like something to show for it.
Norm n3ykf
On Thu, Dec 11, 2014 at 7:51 PM, Roger (K8RI) on TT
<K8RI-on-TowerTalk@tm.net> wrote:
As I see it:
There are several absolutes in antenna gain and many probably, might, maybe,
could be attributes.
The two absolutes are the 2.1 db dipole to Isotropic source (I've been a ham
since 61 and the books have always listed that figure) and gain measurements
are only true for the test range where they were made. Hence even though
the manufacturer may be honest and make an honest attempt at measuring gain
figures in all axises, comparing one manufacturers antennas against another
manufacturers antennas from advertized data only gives ball park figures and
tells me little about how they will perform in my installation.
Doubling the number of antennas theoretically doubles the gain which is an
additional 3 db for each doubling although it's unlikely due to losses in
feeding and matching that this will be achieved. Losses in the additional
coax can be substantial.
Even with computer modeling of the installation, the inputs must be accurate
and few if any know the ground characteristics near or far to any great
precision.
The radiation pattern at vertical angles is a crap shoot. You are playing
the percentages for atmospheric conditions that will match the maximum
signal radiated will be at the most desirable angle to match those
conditions. That's why the contesters have stacks that allow them to select
the antenna height that lets them put the maximum signal into the desired
range/distance.
As most of us have both budget and land constraints that limit us to the
number, size, and height of our antenna(s) It's unlikely we can depend on
equaling the performance in the advertized figures. Often for 40, 75, and
160, a simple wire antenna, or vertical will out perform all but the largest
and/or sophisticated antenna or array.
I've had extremely good luck with sloping, center fed, half wave dipoles.
compared to some stations running Yagi antennas at reasonable heights. Many
times the first word in the response to my call, is "Wow". Yes, I do run
QRO, but the best sounding signal out of my tetrode amp is at, or around the
legal limit so there is little incentive for pushing beyond the legal limit
for that extra 1 db and those tubes are expensive.
--
73
Roger (K8RI)
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73
Roger (K8RI)
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