Towertalk
[Top] [All Lists]

Re: [TowerTalk] Antenna Gain and Reality

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Antenna Gain and Reality
From: Jim Lux <jimlux@earthlink.net>
Date: Sat, 13 Dec 2014 07:07:18 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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.


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>