Circular Polarization in Ham Radio: RHCP, LHCP and Satellites

Most amateur radio antennas are simple about polarization: a dipole hung horizontally gives horizontal polarization, and a vertical gives vertical. Satellite operators speak a different language. Their antennas are described as RHCP or LHCP, their feed lines go through phasing harnesses, and their preamplifiers switch between two senses. This guide explains what circular polarization in ham radio actually is, why the two directions of rotation are called right-hand and left-hand, why the definition itself is a source of confusion, what happens when linear and circular antennas meet, and which antennas produce a circular wave. The physics follows the standard reference article on circular polarization, and the practical notes tie it to amateur satellite work.

What Circular Polarization Means

Polarization describes the orientation of the electric field of a radio wave. In a linearly polarized wave, such as the one from a horizontal dipole, the electric field oscillates back and forth along one line. In a circularly polarized wave, the reference explains, the electric field has a constant magnitude at each point and rotates at a constant rate in a plane perpendicular to the direction of the wave. If you could follow the tip of the electric field vector as the wave travels, it would trace a helix along the direction of propagation, like a corkscrew.

The reference gives the condition for circular polarization: the two orthogonal components of the electric field must be of equal magnitude and out of phase by exactly 90 degrees, which is one quarter of a wavelength. It is this quadrature relationship that creates the helix. If the components are not equal, or the phase difference is not 90 degrees, the result is elliptical polarization, of which circular and linear polarization are the two limiting cases. Real antennas never produce a perfect circle, and the closeness of the ellipse to a circle is called the axial ratio. The nearer to 1, or 0 dB, the better the circular polarization.

RHCP and LHCP

A circularly polarized wave can rotate in one of two senses. The reference defines right-handed circular polarization, abbreviated RHCP, as the case where the electric field vector rotates in a right-hand sense with respect to the direction of propagation, and left-handed circular polarization, LHCP, as the case where it rotates in a left-hand sense. Which way is which depends on where you stand, and that is where the confusion comes in.

Two Conventions

The reference points out that two opposing historical conventions exist. In the first, polarization is defined from the point of view of the source: you point your right or left thumb away from the source, in the direction of propagation, and the curl of your fingers shows the rotation of the field with time. This convention conforms to the IEEE standard and is generally used by the engineering community, as well as by quantum physicists and radio astronomers. In the second convention, polarization is defined from the point of view of the receiver, and it is used in many optics and chemistry textbooks. The same wave is called right-handed in one convention and left-handed in the other.

For amateur radio, the practical rule is to follow the engineering definition. When a satellite or an antenna manufacturer says RHCP, it normally means the IEEE sense, as seen looking in the direction of propagation. The reference recommends specifying the point of view when there is any doubt, and that is good advice if you build or buy antennas from different sources. It also gives a helpful fact about helical antennas: a right-handed helix radiates a right-handed circularly polarized wave under the IEEE definition, and a left-handed helix responds to left-circular polarization.

What Happens at a Mismatch

Polarization matters because a receiving antenna picks up only the matching component of the wave. The combinations are easy to remember with the table below. The loss values for linear against circular are standard results; real antennas deviate from them, particularly when the axial ratio is poor.

The last two rows explain the appeal of circular polarization. A circular receiving antenna does not care about the orientation of a linear transmitting antenna, and the price is a loss of about 3 dB compared with a perfect match. A linear-to-linear link can be perfect or terrible depending on rotation. That is why circular polarization is used where the orientation of one end is unknown, changing or both. Our article on vertical and horizontal polarization describes the linear case, where cross-polarized links lose a lot of signal and where ground reflections can complicate the picture.

Why Satellites Use It

Amateur satellites are the main place where hams meet circular polarization. A satellite in low Earth orbit tumbles or spins, moves across the sky and changes its position relative to you in a few minutes. A linearly polarized antenna on the satellite would present a constantly changing orientation to the ground station, and the received signal would swing from strong to weak as the angle changes. Circular polarization removes most of the sensitivity to orientation, so the signal is steadier. The reference article on AMSAT-OSCAR 7, notes that it carried a circularly polarized, canted turnstile VHF and UHF antenna system, an example of the technique at work in an early amateur satellite.

The ionosphere gives another reason. A linearly polarized wave passing through the ionosphere can have its plane of polarization rotated, a phenomenon known as Faraday rotation, and the effect is larger at lower frequencies. A circularly polarized wave is not affected in the same way, because rotation of the reference plane does not change a circle. That is one more reason why circular antennas are favoured for links that pass through the ionosphere.

Geostationary satellites do not tumble, but they still use circular polarization in some cases. In our guide to the QO-100 satellite, the uplink is described as right-hand circular, while the downlink is linear. Do not assume that every satellite link uses the same scheme; check the specification for the satellite and the band you plan to use. If you are getting started with satellite operating, read our introduction to amateur radio satellites first.

Reflections Reverse the Sense

The reference explains an effect that matters for anyone who bounces signals off something. The handedness of polarized light is reversed when it is reflected off a surface at normal incidence. The rotation of the plane of polarization of the reflected light is identical to that of the incident field, but because the propagation direction is now opposite, the same rotation that was called right-handed for the incident beam is called left-handed for the reflected one. Right circular polarization reflected from a smooth conductor at normal incidence comes back as left circular polarization. This principle holds strictly at normal incidence only; at other angles the reflected wave generally becomes elliptical.

For hams, this has several consequences. Reflections from buildings, hills or the ground can arrive with the opposite sense, and a circularly polarized antenna of the original sense will reject them to a degree, which can reduce multipath interference. It is also the reason why a signal bounced off a surface, such as a reflector dish, needs to be understood in terms of the reversal when you choose the sense of a feed. For example, a dish with a circular feed may need the opposite sense from that of the direct wave. Always check the documentation of the feed and the dish if you work with one.

Antennas That Make Circular Polarization

The reference lists the standard ways to produce a circular wave, quoting an antenna textbook. They fall into three families, and each is used by amateurs.

Crossed Dipoles and Crossed Yagis

Two crossed dipoles provide the two orthogonal field components. If the two dipoles are identical and fed with a 90 degree phase difference, the polarization along the axis is circular. One way to get the phase difference is to feed one of the two dipoles with a transmission line that is a quarter wavelength longer or shorter than that of the other. This is the principle behind the crossed Yagi, which satellite operators use at VHF and UHF: two sets of elements are mounted at right angles on one boom, and a phasing line gives the quadrature feed. The sense of rotation depends on which dipole is fed with the longer line, so changing the phasing harness changes between RHCP and LHCP. Many satellite stations include a switch so that they can select the sense that matches the satellite, or the one that reduces fading.

The Helical Antenna

The helix is the simplest mechanical circular antenna. For axial-mode operation, the textbook quoted in the reference gives an optimum circumference of about one wavelength and a spacing of about a quarter of a wavelength between turns. The helix radiates along its axis, and its handedness is fixed by the direction in which it is wound. It is popular for satellite work at UHF and microwave frequencies because it is broadband and has a single feed, with no phasing network. Our article on the Gpredict satellite tracking guide shows how to point such antennas with a rotator and track the Doppler shift.

Patch Antennas

For microwave bands, a patch antenna can produce circular polarization when two orthogonal modes are excited with a 90 degree time-phase difference. For a square patch, the easiest way is to feed it at two adjacent edges through a 90 degree power divider. Patch antennas are found in small, flat designs for GPS and for some satellite receive systems.

Practical Tips for Satellite Operators

• Check the satellite specification. Know whether the downlink and uplink are linear or circular, and which sense. The conventions differ between sources, so follow the IEEE definition unless told otherwise.
• Use a switchable sense if you can. A relay or a phasing switch lets you select RHCP or LHCP. If the signal fades, try the other sense, because reflections and the satellite’s orientation can change what works best.
• Do not expect miracles from a handheld antenna. A linear antenna such as a handheld whip or a small Yagi works, but the signal will fade as the satellite moves. Holding the antenna at an angle or rotating it slowly can compensate, and some operators tilt the antenna to find the best angle during a pass.
• Account for the 3 dB. A circular antenna on a linear signal gives up about 3 dB, which is acceptable for a strong signal and visible for a weak one.
• Keep the feed line short and low-loss. At UHF and above, feed line loss matters, and a preamplifier at the antenna may be required.
• Test the antenna. Use a receiver and a known signal to compare the two senses before you rely on the antenna for a pass.

The same ideas apply to other circular systems that amateurs use, including EME at higher frequencies and some ATV and microwave links. If you operate from the International Space Station using a handheld, see our guide to ISS contacts with ARISS for the typical antenna choices.

The Bottom Line

Circular polarization is a wave whose electric field rotates at a constant rate, produced when two perpendicular components of equal strength are 90 degrees apart. RHCP and LHCP describe the direction of rotation, and the engineering convention looks along the direction of travel from the source. A circular antenna picks up any linear signal with about 3 dB of loss, which is why satellite links use it. Reflections reverse the sense, and crossed Yagis, helices and patches are the common ways to build the antennas. Know the sense your satellite uses, bring a way to switch it and keep the 3 dB in mind when you work the weak signals.

Frequently Asked Questions

What is circular polarization?

It is a polarization state in which the electric field of the wave has constant strength and rotates at a constant rate in a plane perpendicular to the direction of travel, tracing a helix along the path.

What is the difference between RHCP and LHCP?

They are the two senses of rotation. In the IEEE engineering convention, the sense is defined from the point of view of the source, looking in the direction of propagation. Optics textbooks often use the opposite convention.

How much do I lose if I receive a linear signal with a circular antenna?

About 3 dB, regardless of the orientation of the linear signal. That is the trade-off for not having to match the orientation.

Why do satellites use circular polarization?

Their orientation relative to the ground station changes constantly, and a circular antenna avoids the deep fades that a fixed linear polarization would cause. It also avoids the effect of Faraday rotation in the ionosphere.

What happens when a circular wave is reflected?

At normal incidence the handedness reverses, so a right-hand wave comes back as a left-hand wave. At other angles the reflected wave tends to become elliptical.

Which antennas make circular polarization?

Crossed dipoles or crossed Yagis fed with a quarter-wave phase difference, helical antennas in axial mode and patch antennas fed at two points with a 90 degree divider.