On 12/12/14, 4:53 AM, Lizeth Norman wrote:
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.
Different kinds of antennas for different uses in different
environments. The usual HF antenna is <1 wavelength in size and has
<10dBi gain and the main lobe has a half power (3 dB) beamwidth is
probably 100 degrees wide. The HF environment is one where you're
looking at skywave propagation for long distance, typically at low
angles relative to the horizon, where there is atmospheric noise from
all directions, as well as interfering signals from other hams and
broadcasters.
The usual VHF/UHF antenna has fairly high gain (20dBi isn't unusual) is
more than a wavelength in dimensions and has a narrow beamwidth (20dB =
20 degree HPBW). The environment is "quieter" since there's very little
"over the horizon" propagation from thunderstorms and other interfering
sources.
The stuff about using sun noise is one way to "measure" the gain and
beamwidth. The sun is about 1/2 degree wide and there's no other
sources near it, so you can sweep your antenna past it, measure the
noise power, and see if your antenna has the beamwidth you expect.
The other thing about UHF and higher and when you're pointing up into
the sky (e.g. amateur DSN or moonbounce) is that the noise from the
earth is significant (compared to the sky, which is very cold), so you
want low back and side lobes to not pick up thermal noise from the
surroundings.
In terrestrial VHF/UHF, low sidelobes are important to reduce the effect
of multipath reflections.
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