A Ham Radio Series 31 - Very Directional, Very High Gain Antennas

There's a niche area of directional antennas that I want to touch on briefly.  The highest gain antennas are going to tend to be the ones that are extremely directional.  Remember the analogy of antenna gain coming from "squeezing the balloon" of the fields coming from the antenna?  The more the fields get squeezed, the narrower the radiated beam gets and the higher the gain goes.  

The king of antenna gain is the parabolic dish antenna.  These are the ones that are used for all of the biggest radio telescope observatories, the Deep Space Network, and even for home satellite TV dishes.  

The National Radio Astronomy Observatories Very Large Array in New Mexico - also used by the Search for Extraterrestrial Intelligence.  That's the NRAO VLA also used by SETI for the acronym-aware.  NRAO photo from the SETI Institute.

Regardless of size, they're all governed by the same physics.  The gain of a parabolic dish antenna in dB with respect to an isotropic antenna is given by this equation: 

where k is a constant for the efficiency, typically 0.5 to 0.6, D is the diameter in meters and the Greek letter Lambda is the wavelength in meters.  It's important to note that the units cancel out algebraically, so while the original said meters, there's really no particular reason to use meters.  Just make sure if you use feet in the diameter, you use feet in the wavelength.  Since the gain is the ratio of dish diameter to the wavelength, the bigger the dish, the higher the gain.  The higher the gain, the narrower the beam.  And just like the gain, there's a simple expression for the beamwidth

(yes, that's 70 - seventy).  So let me create an example based on an aviation weather radar antenna I used to work on.  It's diameter was 0.6 meter, with a frequency of 10.00 GHz.  Wavelength is C/f or 3*10^8/10.00*10^9, or 0.03 m (3cm).  The gain in dBi is

Since the beamwidth depends on the ratio of lambda to D, or 0.03/0.6, BeamW is 70*0.03/0.6 or 3.5 degrees. 

That requires quite a bit more precision in pointing and feeding back the direction than your typical HF or VHF antenna, and one reason dishes tend to only be used by microwave experimenters in the ham bands.  All of those dishes in the Very Large Array are computer controlled to all point at their required target - and don't forget they need to track that object across the sky just like an optical telescope.  In the case of the weather radar these numbers approximate, it was precisely driven by stepper motors so that the radar sent out a pulse and waited 8-1/3 milliseconds for the return echoes before stepping over for another pulse.

Have you seen dish antennas that aren't circular?  Many of the small antennas people get on their houses for various services are more elliptical than circular.  Those antennas produce radiation patterns that are also elliptical - they're receive better and the beamwidth is narrower in the direction that the dish is bigger.  For Satellite TV, that's OK.  The loss of gain happens in directions away from the orbit that the signal is coming from.  That part of the antenna wasn't needed.  Also note, they don't need the tracking motors because the satellites aren't moving in the sky.  They always appear to be in the same spot in the sky. 

So what's up with all this?  Why do I go down this road?  Because there are ways to do these things without parabolic dishes that I want to talk about.