Anatomy of an Antenna

Anatomy of an Antenna

This page was assembled to show images of some of the components I talked about in reply to a classmate's question on radio telescopes vs. satellite dishes. There are many images so please be patient while it loads.

Your description of how your satellite dish works is basically how a radio telescope works. The dish collects the incoming signal. You can get a pretty good idea of what kind of frequencies an antenna is optimized for by looking at the surface of the dish: the smoother the surface, the higher the frequency/ shorter wavelength. So however big the holes are in the wire mesh of your satellite dish, the radio waves it is detecting are at least 10 times bigger. Shorter wavelengths are scattered rather than focused, or pass right through the holes.
	Image at right is a piece of one of the original wire mesh panels of the NRAO 300-foot telescope constructed in 1962. These panels were replaced because they became deformed over several years. Obviously they weren't observing at very high frequencies. The 300-foot telescope collapsed under its own weight in 1985 due to the deterioration of a gusset plate in the support structure.

Radio telescopes observing at centimeter and millimeter wavelengths usually have a smooth metal surface rather than wire mesh (like those of the VLA and VLBA), and sub-millimeter telescopes have surfaces as smooth as glass (like the James Clerk Maxwell Telescope on Mauna Kea). Another important feature of the dish is that it not deviate from a perfect parabola by more than a fraction of the wavelength being observed. As you know, a parabola focuses the incoming radiation to one point; deviations from this shape scatter the radiation rather than focus it.
VLA & VLBA dishes are 25 meters in diameter (this is a VLA antenna).	Here I am inside a VLBA antenna. They're bigger than they look from a distance when you're actually in one!	Looking into a VLA antenna.

So the incoming radiation is collected by the dish (the bigger the better), reflects off of its parabolic surface (the smoother the better) and (most of it we hope) comes to a common focus at the "knobby widget". The knobby widget, located at the prime focus of the antenna, can be either another reflector or a receiver. I don't know about satellite dishes, but in the case of the VLA and VLBA antennas the knobby widget primarily serves as a secondary reflector (although there is a low-frequency dipole receiver mounted there as well). The common, though far less technical ;-), term for the reflecting type of knobby widget is "subreflector". The subreflector does just what its name suggests -- it reflects the radiation back down to the center of the dish where the feed horns are located. In the case of the VLA and VLBA antennas, the hyperbolic subreflector is asymmetrical and rotates into different positions. In each different position the reflected radiation is directed to a particular feed horn, the feeds being arranged in a ring in the center of the dish (the Cassegrain focus). The subreflector moves up and down axially as well, in order to bring the incoming radiation of differing wavelengths to a focus at the desired feed.

Subreflector on a VLA antenna.

Looking up from underneath the subreflector on a VLBA antenna..

Feeds in a VLBA dish.

We enter the dish from the vertex room through the housing for the feeds.

After being directed into the desired feed horn (which is basically a conical horn with a corrugated inner surface made from aluminum whose diameter and length is determined by the wavelength being detected) the radiation travels through a "wave guide" (a metal tube circular or rectangular in shape) down to the front-end in the vertex room, a room directly below the dish. The front-end is the first part of the receiver system, responsible for separating the incoming radiation into its right and left circularly polarized components, and then amplifying it. The front-end components are enclosed in a dewar which is cryogenically cooled (to about 15 K) in an effort to minimize thermal "noise", the result of heat produced by the electronics. The radio emission that is being detected is relatively weak, which explains why it is so important to both keep noise from being introduced into the signal and to amplify the signal.

Here you can see two different sized feed horns. The horns are different sizes for the different frequencies of radio emission being detected.

A feed horn leading into a front end unit. These components are located in the vertex room, immediately beneath the dish.

Another view. By the way, these are all in a VLBA antenna.

This feed horn is so large that it protrudes into the next level down in the vertex room.

Next, the split (RCP & LCP), amplified signals from the front-end are fed into a converter module. Basically, this module down-converts the frequency of the incoming radiation, the RF (radio frequency), to an intermediate frequency (IF) in the range of 500-1000 MHz while retaining the information in the original signal. Converting the RF to an IF within this small range makes it easier to process.

Up to this point we've been in the vertex room. From this point on I will describe the signal path of a VLBA antenna. The signal is now fed through cables (3/8" semi-rigid coaxial type) that run from the vertex room down through the antenna's support structure, down into the ground below the antenna, then (still underground) over to the electronics room in the control building. Here there are a few more modules that further process the signal before it gets recorded to tape.

The cable room underneath the VLBA antenna.

Cables in the cable room. One of these is the coax cable that runs underground to the control building.

VLBA antenna (in Pie Town, New Mexico)
and control building.

In the electronics room the cables carrying the signal terminate at an amplifier, then the signal is sent on to the IF distributor module. The IF distributors attenuate (adjust the level of) the signal as well as increase the gain of the signal if need be.

Finally, the signal goes to a baseband converter. This module amplifies the incoming IF signal yet again, produces two output bands with selectable bandwidths from the signal, then digitizes it. At this point it is sent on to the formatters, which arrange the digitized signal for recording onto tape.

There are some differences in the signal path depending on whether we're talking about a single dish, an array such as the VLA which correlates (combines) the data from the telescopes in real time, or an array such as the VLBA which records the data on tape and correlates it later. But I believe the basics are the same.

To see the full-sized diagram of the signal path for a VLBA antenna which has been described above, click on the thumbnail below.

For more information on radio astronomy (search for SiO masers research project) and operating the VLA, go to the following page and look at the two sites listed under "Research":

www.astrophys-assist.com/educate/index.htm

Also, if you want to see lots of images taken at the VLA, go to the url below and look at the archive of images page. I posted a new picture taken at the VLA (almost) daily for most of the time I worked at the VLA :

www.astrophys-assist.com/vvla/index.htm

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