How Eggbeater Antennas Work: Construction, Radiation Pattern, and Trade-Offs for Satellite work

Egg Beater antennas are a good choice for operating satellites for several reasons, but there are trade-offs to be made.  Let’s look into what an Egg Beater is, why they’re good, what their limitations are, and how to make one.

What Is an Eggbeater Antenna?

Egg Beater antennas are two full-wave loop antennas, mounted at right angles to each other.  They’re called Egg Beaters because they look like a whisk.

Need Image: Two full-wave loops mounted at right angles.

They are mounted at 90 degrees to each other, they also have to be fed 90 degrees to each other.  We accomplish this with a ¼ wavelength length of transmission line between the feed points of each antenna.

Need Image: electrical configuration, not physical: feed point, first aerial loop, ¼ wave phasing loop, second aerial loop

Radiation Pattern Characteristics

Radiation Pattern of an Egg Beater antenna

A single full-wave loop mounted vertically has a radiation pattern that looks like a donut on its edge: It transmits in-line with the loop vertically and in two directions horizontally.  But it has deep nulls (the hole of the donut) on the broadside of the loop.

Need Image: radiation pattern of a single full-wave loop.

By adding a second full-wave loop mounted perpendicularly to the first, the second antenna fills in the null of the first antenna and the first antenna fills the null of the second.

Need Image: two radiation patterns overlapped with each other

The result is an antenna with a roughly spherical radiation pattern: It can see in all directions at once.  

Need Image: the sum of the two radiation patterns

Well, it would if it were in free space with nothing around it.  In the real world, the earth under the antenna reflects the downward radiation back up and out, making the radiation pattern look more hemispherical: It can see the entire horizon, and directly upwards.

Need Image: radiation pattern of a practical Egg Beater antenna.

Satellites

This hemispherical radiation pattern makes the Egg Beater antenna perfectly suited for operating satellites, which can come up at any azimuth (point on the horizon), then spend a fair amount of time directly overhead, before dropping back down to a different azimuth.

The common vertical antenna can also see to the horizon in all directions, but has a deep null directly up.  They work for low satellite passes where they don’t get very high in the sky.  But as soon as they get above the radiation pattern of the vertical, they drop out.

Need Image: Radiation pattern of a vertical ground plane antenna.

Gain and Trade-Offs

The downside to that hemispherical radiation pattern is that the Egg Beater has zero gain.  Literally, 0dBi: it is as close to an isotropic radiator as you’ll ever find.

Common hand-held dual-band yagi antennas have about 10dBi of gain (give or take).  That’s 10x the amount of energy!  Except, that it’s only in the direction the antenna is pointed.

Think of it like this:  You have a fixed amount of energy.  If you want that energy to go everywhere in the sky, you have to spread it pretty thinly.  Instead you could put all that energy in one thick patch, but then you have none left for the rest of the sky.  You can spread out, or concentrate, that energy however you want.  That’s exactly what happens in antennas:

Gain doesn’t give you any more energy, it just concentrates it into a smaller area.

The trade-off here is: gain for simplicity.  The Egg Beater antenna has no moving parts, you can mount it to the side of a building or on a mast, and it can see the whole sky.  A Yagi has more gain, but it must be pointed at the satellite as it moves through the sky, requiring heavy, complex, expensive dual-axis rotators for permanent installs, or for the operator to hold and point the antenna manually.

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Polarization

Polarization of Satellites

Most ham satellites use linearly polarized antennas: dipoles or ¼ wave ground planes.  However, the orientation of the satellite relative to us on the ground isn’t predictable, and changes all the time as the satellite tumbles in space.  So while they are linear, we can’t know whether it’s horizontal, vertical, or (more likely) somewhere in between.

A linear polarization mismatch can cause as much as 30dB of path loss in the worst case, when the transmitting and receiving polarizations are at right angles to each other.

One way to compensate for this is to use circularly polarized antennas on the ground.  The path loss between a linear polarized transmitter in any orientation, and a circularly polarized receiver, is only 3dB.

Polarization of Egg Beater Antennas

The polarization of an Egg Beater depends on the elevation: at the horizon, the Egg Beater antenna is horizontally polarized.  As the elevation goes up, the polarization starts to twist and becomes circular directly up.

{Maybe: explain why/how?  Might be too much content.}

Right-Hand vs Left-Hand Circular Polarization

Whether an Egg Beater is left-hand or right-hand circularly polarized depends on how it is constructed.  It’s a decision made when building the antenna.

Just like how a linear polarization mismatch can cause path loss, so can a circular polarization mismatch.  If the transmitter is transmitting LHCP and the receiver is expecting RHCP, the loss is about 30dB.  This is why it’s important to select the correct handedness when building an Egg Beater antenna if you intend to operate a satellite with circular polarization.

In the case of amateur radio satellites, most of our sats transmit unpredictable linear polarization, not circular.  So this is less important for us.

Feed System Phasing and Impedance Matching

System Impedance

Amateur radio systems have standardized on 50ohm impedance for transmitters, receivers, and transmission lines.  So we want an Egg Beater to have a 50ohm impedance at its feed point.

The full-wave loop antenna that makes up the Egg Beater has a roughly 100ohm impedance at resonance.  By putting two full-wave loop antennas in parallel, that 100ohm impedance presents as a 50ohm impedance.  So we will feed the two full-wave loops in an Egg Beater in parallel.

Phasing loop considerations

However, we need a ¼ wavelength of transmission line between the two loops to provide the 90 degree phase shift:

• That transmission line is connecting two 100ohm antennas in parallel, so it also needs to be 100ohms.
• The full-wave loop antenna is a balanced antenna, so the transmission line should also to be balanced.
• There’s a lot happening at the feed point of an Egg Beater antenna, so the transmission line needs to be shielded.

Finding: 100ohm, balanced, shielded transmission line can be a challenge.  So we often compromise one or another of these requirements.

• 100ohm transmission line can be faked with two 50ohm transmission lines “in series.”  Using two equal lengths of 50ohm shielded transmission line, one between each pair of terminals on the two loops, with the shielded grounded at the feed point, can look like 100ohms.  The difficulty here is cutting and assembling both 50ohm transmission lines exactly identically.  Any difference here will affect the phase shift, which affects the radiation pattern of the antenna.
• There’s an unbalanced coax that’s close to 100ohms: RG-62 is 92ohm which is pretty darn close.  The un-balanced-ness is not ideal, but since the currents are balanced, they’ll stay on the inside of the coax, which satisfies the other two requirements.  The problem with RG-62 is that it’s not nearly as common as it was in the 80s and 90s when it was used heavily for ARC-NET local area networks.

When cutting the phasing loop, be sure to consider the velocity factor of the transmission line used.  Velocity Factor represents how much slower than the speed of light that RF travels through this cable.  For example, a VF of 0.5 means that RF will travel half the speed of light through this cable.  Wavelengths of RF signals are measured in free space where they travel at the speed of light.  So a signal with a wavelength of 2 meters will have a quarter wavelength of 0.5 meters in free space.  But in our VF=0.5 coax, a quarter wavelength of that same signal will only be 0.25 meter.

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Common Performance Challenges

Now we bridge to other pillars gently.

Pattern Distortion from Nearby Structures

• Mast interaction
• Feedline routing

The Role of Common Mode Current

• Feedline imbalance
• Importance of choking

Installation Height and Ground Interaction

(Notice: we reference common mode behaviour without re-explaining it.)

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Eggbeater Antennas vs Alternative Designs

Eggbeater vs Crossed Dipole

Eggbeater vs Turnstile

Eggbeater vs Directional Yagi for Satellite Work

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When an Eggbeater Antenna Is the Right Choice

Ideal Use Cases

• Portable setups
• Fixed satellite monitoring
• Simplicity-first installations

Situations Where a Directional Antenna Is Superior

• Weak signal satellites
• Long-distance satellite work

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Frequently Asked Questions About Eggbeater Antennas

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