antenna-theory.com

[DonPMitchell]

I'm curious how the Soviets designed the antennas on Sputnik-1. The 58 cm sphere had four antennas, two were 2.4 meters long and two were 2.9 meters long. They were angled at 35 degrees from the longitudinal axis, and two transmitters alternately emitted 20 or 40 MHz.

Nobody knows how the 20 and 40 MHz transmitters were connected to the antennas, but I'm assuming that each transmitter was connected the 2.4 or the 2.9 meter "V". There were Russian articles (not by the Sputnik designers) published in 1957 that discussed the idea that satellites should emit circularly polarized waves. So it is even possible the antennas were a V-shaped turnstile.

The Soviets claim this was meant to create a maximally uniform spherical coverage. Why 35 degrees? Why these odd lengths, which seem to not be a simple fraction of the wavelength, or is that a well known consequence of "V" antenna designs?

How difficult would it be to generate a 3D plot of the radiation patterns?

Something I wrote up a few years ago on Sputnik: http://www.mentallandscape.com/S_Sputnik.htm

[bigSteve]

Hi Don,

That's an interesting question. I looked at some pictures and here is my guess.

There were actually two antennas on the sputnik, where the 2 2.4meter rods made up the structure of one antenna and the other two 2.9 meter rods made up the 20MHz antenna.

Basically, you can view a dipole antenna as having two arms and fed in the center:
http://www.antenna-theory.com/antennas/dipole.php

Now you can take those arms and change the angle - the result is a loss in bandwidth, a decrease in resonant frequency (i.e. you need less than lambda/2 to achieve resonance) and a loss in efficiency.

As far as the 35 degrees goes, that's not a magic number in antenna theory. I'd guess it was a complex optimization between the antenna, structures and aerodynamic people.

In terms of the size of the antennas, you can use impedance matching to lower the resonant frequency (at further decreased bandwidth for the antenna). Since they weren't mass producing these things, you can tune an antenna pretty accurately when you're just making a couple, and get away with smaller bandwidth than you would in, say the consumer electronics industry.

On circular polarization, I highly doubt these antennas were circularly polarized. And a turnstile would not work - to achieve circular polarization the turnstile would have to spin at close to the speed of light! Circular polarization is advantageous for satellite signals - for instance, gps is Right Hand Circularly Polarized. This is because at that frequency, one of the E-field components (Horizontal or vertical) is absorbed or reflected in the atmosphere. It could be the same story at 20MHz. However, you don't need circularly polarized - just two orthogonal E-field components - i.e. antenna sets that are spaced by some angle between each other.

I don't know about their claim about maximally uniform spherical coverage. No antenna could claim that. In general, the smaller an antenna is the more uniform it is in radiation pattern. As an antenna gets larger (in terms of wavelengths) it becomes more directive (less uniform). Possibly they were claiming spherical coverage over the "sphere of earth" as viewed from the satellite at orbit height. Not sure.

The 3D pattern....well it would be fairly simple to generate it with some software. I imagine it resembles a dipole radiation pattern though.

Anyway, those are my musings on your post. Thanks

[DonPMitchell]

I know on Luna-1 you had a "V" shaped turnstile for generating circularly polarized waves. You don't have to spin the antenna, turnstiles are just two dipoles, driven 90 degrees out of phase. The USSR used this idea on planetary probes until the mid 1960s, and then switched to using helical antennas.

But I suspect Sputnik was very simple. So I'm guessing the two sets of antennas were just connected like you say, one pair to the 20 MHz transmitter and the other pair to the 40 MHz transmitter.

A dipole generates a well known pattern in free space, a donut shape that has bad dead zones in the longitudinal direction.

I'm going to try to ask some of my Russian friends. The designer of the Sputnik satellite (Oleg Ivanovsky) is still alive, but very old. I'm trying to relay this question to him via some friends.

[rachana]

Hi guys,

I'm in my final year of my physics degree and for my first semester project I had to make a dipole radio antenna, and it's proven to be quite tricky (at least in the theory side of things). I would seriously appreciate some help with a few things if anyone has anything to offer

Firstly, I don't understand the beamwidth thing at all. How is it calculated? The best I've managed to find is images of the ideals but with no information as to how they're found, or an explanation. Here's the images I found of the patterns for the height I'm using:

Single Dipole ~7m (lambda 1/2) above ground

x ... ground plane - z ... zenit