Turnstile and Eggbeater Antennas for Satellite Work

A low-orbit amateur satellite crosses the sky in about ten minutes. It rises in one corner of the horizon, climbs, passes overhead and sinks into another corner, and during this time its antenna turns and tumbles and the polarisation of its signal changes as it does. A station that follows it with a pair of long yagis, one rotator for the azimuth and another for the elevation, can work it well, but many operators begin with something much simpler: a small fixed antenna that looks at the whole sky at once. The turnstile antenna and its relative, the eggbeater, are the classic answer. They are built from two crossed elements fed in quadrature, they produce circular polarisation and a nearly hemispherical pattern, and they need no rotator at all. This guide explains how they work, the differences between the two, how to build and feed one, and what to expect from it.

The Problem They Solve

A satellite signal passes through a long path and several changes of orientation. The satellite spins or is stabilised in an unpredictable way, and the signal reaches you with a polarisation that may be vertical, horizontal or anything between. With ordinary linear antennas on the ground, the match between your polarisation and the signal varies from one moment to the next and can fall to nothing for a short time, which operators know as polarisation fading. A circularly polarised antenna accepts any linear signal with a fixed loss of 3 decibels, whatever its orientation, which removes the deep nulls. The article on circular polarisation describes the physics and the two senses, right-hand and left-hand.

The second problem is the pattern. A satellite travels from horizon to horizon, and an antenna that must be aimed has to follow it. An antenna that radiates upward in a wide pattern, with a gain of a few decibels above the horizon, hears the satellite all the time without any tracking. The price is a modest gain, but for the strong signals of the FM satellites on 145 and 435 MHz that is enough, as the operating guide on getting started with amateur radio satellites explains.

How the Turnstile Works

The basic turnstile antenna consists of two half-wave dipoles placed at right angles to each other in the same plane, like the arms of a turnstile gate, and fed with currents of equal strength but with a phase difference of 90 degrees. When two perpendicular fields of equal amplitude are 90 degrees apart in phase, their sum is a field vector that rotates once for every cycle, which is circular polarisation. Looking at the antenna from above, the field rotates in one direction, and from below in the other, so the sense is right-hand circular on the one side and left-hand on the other, with linear polarisation at the horizon.

The pattern of a turnstile in free space is a figure-of-eight rotated around the axis of the crossing, with a null in the plane of the dipoles’ axis. To make a useful antenna for the sky, a reflector, a metal plate or a set of wires, is placed about a quarter wavelength below the dipoles. It folds the downward radiation upward and gives a pattern with its maximum straight up and a gain of about 5 dBi, falling at low angles. Nearer the horizon, the polarisation becomes linear and the gain drops, which is the main weakness of the turnstile for low passes.

The Eggbeater

The eggbeater antenna replaces each half-wave dipole with a full-wave loop, bent into a shape that resembles the beaters of a kitchen whisk. Two loops are mounted at right angles, with a common centre, over a ground plane or a set of radials. Each loop has a circumference of one wavelength, with a feed point that gives a good match, and the two are fed in quadrature, usually through a quarter-wave delay in the cable to one of them. The result is a pattern much like a hemisphere, with a good gain at low elevations and circular polarisation over most of the sky, which is why the eggbeater has become a favourite for the 70 centimetre and 2 metre satellites.

The reason for the better coverage at low angles is the loops’ own radiation pattern, which is broader than a dipole’s, and the fact that a loop over a ground plane has a different distribution of current. A single eggbeater for 70 centimetres is easy to build with copper tube or heavy wire and fits in a small space, so it is also popular for portable and balcony use. A larger structure is needed for 2 metres but it is still practical.

Phasing and Feeding

The 90 degree phase shift is the secret of both antennas. It is usually made with a length of coaxial cable. A cable a quarter wavelength long, measured in the cable and so shortened by the velocity factor, delays the signal by 90 degrees, so one element is fed straight from the splitter and the other through the extra quarter-wave length. The two elements are connected to a common feed with a power divider or simply in parallel, so that the radio sees a single 50 ohm connection. The effect of the velocity factor on the cable length is explained in the article on velocity factor, and the exact length matters only to a degree, since a few degrees of error cause only a small loss of circular purity.

Matching needs a few lines of arithmetic. A half-wave dipole over a reflector has an impedance somewhat below 73 ohms, often 50 to 60. Two such elements in parallel would give about 25 to 30 ohms, which needs a matching section to bring them to 50 ohms. A common method is to use 75 ohm cable for the feeds and a quarter-wave transformer, a technique explained in the article on matching networks, or to choose the element dimensions so that each has the right impedance. Many published designs give tested dimensions for the cables, and following one of them is the practical route.

Dimensions for Common Bands

For the 2 metre satellite segment near 145.9 MHz, each dipole is about 97 centimetres from tip to tip, and the reflector sits roughly 50 centimetres below it. For the 70 centimetre satellite segment near 436 MHz each dipole is about 32 centimetres long, with a reflector 17 centimetres below. These are approximations to begin with: end effects, the diameter of the elements and the shape of the reflector shift the figures by a few percent, and the final adjustment is done by trial with a meter or a NanoVNA, as explained in the guide to the NanoVNA. A combined antenna for both bands can be made by stacking a 70 centimetre turnstile on the axis above a 2 metre one, with separate feed lines and a diplexer at the radio, if the radio has only one connector.

Sense of Rotation and Polarisation Loss

Most amateur satellites transmit with linear polarisation or, in the cases of some, with right-hand circular, and a circular ground antenna of the same sense receives the circular signals fully. A circular antenna receiving a linear signal loses 3 dB, and one of the opposite sense to a circular signal loses a lot, often 20 dB or more. For most FM satellites the loss of 3 dB is the price of avoiding fading, and it is acceptable. The sense of your antenna is determined by the connection of the phase line: swapping which element receives the delay reverses the sense. When you transmit to a satellite that has a circular receiving antenna, the sense matters too, and the satellite’s documentation gives it. The overview in the article on Gpredict shows how to look up the satellites and their frequencies.

Building One in an Afternoon

A turnstile for 70 centimetres can be made from a few things. Take a metal plate or a disc of about 40 centimetres for a reflector, or use four or more radial wires of the same length as a lighter alternative. Mount a short mast or a plastic tube in the centre, and fix the two dipoles to the top, at right angles, using stiff wire or brass rod, insulated from each other at the crossing. Each dipole is cut in the middle and fed with its own piece of coaxial cable, the shield to one half and the centre conductor to the other. Run the cables down inside the mast to keep them out of the field.

At the bottom, join the two cables with a tee, taking the phase delay into account: one cable is a quarter wavelength longer than the other, in the cable. Cut the lengths with the velocity factor in mind, usually 0.66 for solid polyethylene coax, and leave a few centimetres extra for trimming. Add a choke of a few turns of coax or a ferrite sleeve on the main cable near the feed point, as in the guide to common mode chokes, because current on the outside of the feed line will distort the pattern. Seal the open ends of the cables with self-amalgamating tape, and give the joints a coat of paint or varnish if the antenna is for permanent use.

Tune the antenna with an analyser by trimming the length of the dipoles a few millimetres at a time until the resonance is in the middle of the satellite segment, and the SWR is below 1.5. The antenna is not very sensitive to a small error, as the bandwidth of the turnstile is wide, about 5 percent. Check the SWR with the antenna in its final position, because a roof, a railing or a tree changes it, and repeat after every change in the surroundings.

Testing With a Real Pass

The best test is a satellite. Choose a pass with a high maximum elevation, use a tracking program to find the time and the frequency, and listen on the downlink with the antenna in its final place. A good antenna produces a steady, full-quieting FM signal from low to high elevation, with only a little fading at the start and the end of the pass. If the signal drops at certain times, the cause may be a nearby obstruction, a cable fault or a damaged connector. If you hear the satellite well but the signal fades repeatedly in a slow rhythm, the polarisation sense may be opposite to the satellite’s, and swapping the phasing cable cures it. Once the downlink is reliable, try a transmission on the uplink with low power and listen for your own signal repeated by the satellite.

Performance and Limits

Expect a turnstile or an eggbeater to work FM satellites well above 20 degrees of elevation with an ordinary handheld transceiver of 5 watts, and to be useful with a small amplifier for the lower passes. At the horizon the gain falls, and the signal is also attenuated by buildings and trees, so a clear view of the sky matters more than anything. For SSB and CW satellites with weaker signals, particularly the linear transponders where Doppler shift must be followed, a directional antenna with a rotator gives a better margin, and a preamplifier at the antenna helps on receive.

Because the antenna is small, it is also the right shape for the roof or a balcony, for a camping trip, or for a field day. The weather poses a mild challenge: copper tubing and PVC supports stand outdoors for years, and the cable connections need to be sealed against water. Many operators build the antenna in a day for a few tens of euros, and use it for several years.

The Bottom Line

A turnstile antenna is a pair of crossed half-wave dipoles fed 90 degrees out of phase, placed about a quarter wavelength above a reflector, giving circular polarisation and a broad overhead pattern of 4 to 5 dBic. The eggbeater uses crossed full-wave loops over a ground plane and has better coverage at low angles. Both need no rotator and suit the FM satellites on 2 metres and 70 centimetres. Phase the elements with a quarter-wave cable allowing for the velocity factor, match the combined feed to 50 ohms, check with an analyser, and mind the sense of rotation. Use a directional antenna for weak SSB and CW passes.

Frequently Asked Questions

What is a turnstile antenna?

Two half-wave dipoles crossed at right angles and fed with a 90 degree phase difference, which together radiate circularly polarised waves.

How does an eggbeater differ from a turnstile?

It uses two crossed full-wave loops instead of dipoles, with a broader, nearly hemispherical pattern and better gain at low elevation angles.

Why use circular polarisation for satellites?

The signal orientation changes as the satellite moves and turns. A circular antenna receives any linear polarisation with a fixed 3 dB loss, so there are no deep polarisation nulls.

Do I need a rotator for a turnstile antenna?

No. The pattern covers most of the sky, so the antenna can be fixed. The price is a modest gain of about 4 to 5 dBic.

How do I make the 90 degree phase shift?

Add a quarter-wavelength of coaxial cable, shortened by the velocity factor, to the feed of one element.

Can one antenna cover 2 metres and 70 centimetres?

Yes, by stacking two turnstiles on a common axis with separate feeds, or by using a diplexer so that one cable runs to the radio.