The most common use of my J-Pole and Slim Jim antennas is for FM operation on the VHF and UHF amateur radio bands. Typically FM simplex and repeaters use antennas that are vertically polarized. J-Poles, being vertically oriented are also vertically polarized, which makes them well suited for this purpose. But there are many different communication modes available on the VHF bands and one frequent question I receive is that if these antennas will work with single sideband radios. The simple answer is yes, but with caveats.
To dig into the issue a little deeper, we need to define what antenna polarization is and how it affects reception of signals. Radio waves can travel in two different patterns, either in a linear form (think ocean waves) or as a circular pattern (like water going down a drain). The orientation of the antenna can affect how these waves are transmitted or received. This is called polarization. [pq]For best performance, it’s desirable to use antennas that have the same polarization.[/pq]
Antenna polarization types
There are three types of antenna polarization: vertical, horizontal, and circular. If the waves are traveling perpendicular to the earth, they are vertically polarized. Waves traveling parallel to the surface of the earth are horizontally polarized. The third type, circular polarization, creates waves that travel in a swirling fashion. Certain types of communication modes are better suited, or have historically used type type of polarization over the other.
For example: FM communications be it voice or data, has historically used antennas that are vertically polarized. Mobile and handheld radios all have vertically mounted antennas. It would be cumbersome to have an antenna on a car or HT that was oriented in a horizontal fashion, so vertical polarization stuck for FM work.
Sideband communications typically is used for long distance and weak signal work. Horizontally polarized antennas have a couple of distinct advantages over vertically polarized ones. First since most trees, structures, and buildings exist in the vertical plane, horizontal signals are less likely to be attenuated by them. Second, man made and natural radio noise typically emanates in a vertical fashion, so horizontal antennas can offer a lower noise floor for weak sideband signals.
Circular polarized antennas, ones that transmit signals on both the horizontal and vertical plane, are commonly used for communication with spacecraft and satellites. As a satellite orbits the earth, it is also rotating, or tumbling towards earth, causing the signals to mix on both the vertical and horizontal plane. A circularly polarized antenna increases the receptability of these types of signals.
Cross Polarization between antennas
With this simplified antenna theory behind us, you can start to get the idea of what happens when you try to use a vertically polarized antenna with a horizontally polarized signal. Because of the cross polarization your signal will be attenuated. This attenuation could be as little as 3db or as great as 20db. If you are communicating Line of Sight (LOS), that is both antennas are visible within the curvature of the earth, the attenuation will be greater. As distance between antennas increase, the cross polarization becomes less and the attenuation is lower.
The J-Pole and Slim Jim Antennas are vertically polarized. Due to their polarization, these antennas are best suited for FM communications. But the antennas are tuned and have enough bandwidth to cover the entire 144 – 148 MHz two meter amateur radio band. Sideband operation happens near the bottom of the band and these antennas are tuned to work that portion. Most sideband operators use horizontally polarized beam and yagi antennas, so using your J-Pole to talk to them will create a cross polarization loss. If the two stations are close enough to be within Line of Sight, the attenuation will be stronger, but as the station’s distance increases, the attenuation will decrease.
One option to work sideband would be to mount the antenna horizontally. Changing the orientation of the antenna does change the polarization of the antenna. You would then match the polarization of the other sideband station. The two meter J-Pole is small and light enough to be effectively horizontally mounted.
Summary: a vertical antenna can be used for sideband operation
Radio signals travel in a linear fashion and their polarization depends on the orientation of the antenna transmitting the signal. Sideband operators typically use horizontally mounted antennas and FM operators vertically mounted antennas. You can use an antenna designed for FM with the sideband mode, but you’ll have to contend with the cross polarization loss of up to 20db. Yet as distance between the two antennas increases beyond the line of sight (20+ miles, depending on antenna height), this attenuation becomes less of a factor. The J-Pole will work on sideband, you may not get the results that a horizontally polarized yagi could deliver, but it will certainly work.
Frequency Modulation vs. Single Sideband on the VHF Bands – What’s the Difference?
Cross-polarization for long paths on HF isn’t as much of a problem as you might think, since the ionosphere jumbles polarization quite a lot. Many people use verticals on HF looking for good DX at low elevation angles. A good ground/radial system is needed.
Thanks for the comments. You are correct about cross polarization not being so much of an issue, especially with HF. The further two stations move away from each other, the less of an impact that polarization makes. That’s why I make the distinction with VHF/UHF signals that when you move beyond the Line of Sight, the amount of attenuation due to polarization mismatch decreases.
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