LEO/Amateur Satellite Quadrifilar Helix Antenna (QFHA):
Frequently Asked Questions
Below are frequently asked questions of our Amateur Satellite antennas received from visitors via email. We hope these posts and their responses will help other visitors having similar interests and inquiries. Should you need additional information, our have any questions not addressed below, please feel free to email us, or call. We will be happy to provide a fair and objective response.
• Is a Quadrifilar Helix better than other omnidirectional satellite antennas?
A number of omnidirectional antenna structures exist suitable for satellite use. They include Monopoles (Verticals) [LP], Dipoles [LP], Turnstiles [CP], Loops [LP], Egg-Beaters [CP], Spirals [CP], (axial mode) Helices [CP] and Circular Arrays, including Quadrifilar Helices [CP], Lindenblads [CP] and mathematically optimized structures [LP or CP]. Excluding linearly polarized [LP] radiators, which may require regular adjustment (dynamic optimization) during a satellite pass, circularly polarized [CP] antennas are most convenient to enjoy a QSO without having to mind the technical details of the communications path at every instant.
Of the above [CP] list, the most popular structures are Turnstiles, Egg-beaters, Lindenblads and Quadrifilar Helices (QFH). The first two have natural gains of 0 dBic in their most basic form. Their popular renditions rely on a set of radials, a ground plane, or a phased replica for the exclusion of radiation in the lower hemisphere, and to realize gain, which is usually between +3 dBic and +5 dBic when well implemented. A Quadrifilar Helix is a phased array, its exclusion of the lower hemisphere is natural to its design. The same yields its typical +3 dBic to +5 dBic gain. This means that you get similar performance to the earlier described [CP] antennas in a smaller volume. . . A QFH's axial ratio is usually better conserved off zenith (overhead) angles. This is due to its symmetry and smaller dimensions. Outside these differences, all three: Turnstile, Egg-beater and Quadrifilar Helix can be made to have approximately the same gain. Thus, all 3 are good choices for satellite communications. Our -531 and -328 series QFHs were designed not to look as an obvious antenna on purpose, to enable Amateur Satellite operation for individuals residing in communities with restrictions.
A Lindenblad array, in free space, would be an ideal Amateur Satellite antenna, as its peak performance (gain and axial ratio) normally resides near the horizon, where the satellite is most distant, and where the path needs the most help. Overhead (zenith), the gain drops significantly; fortunately, this is where the satellite is closest to the ham station, where the gain is least needed. . . Sadly however, we do not live in free space. Instead, we live on a beautiful large planet with an atmosphere, where earth reflections and tropospheric instabilities generally demolish or otherwise significantly diminish the benefits of such an excellent antenna structure at low elevation angles (near the horizon). When including the effects of the ground, a Lindenblad loses out to the above three [CP] structures, when one averages performance over an overhead or a typically good satellite pass.
• I see your QFHAs are rated at 50 Watts, is that the maximum applied power?
We rated the power at 50 Watts, which is a substantial energy level for most LEO (Low Earth Orbit) satellite platforms. Having said this, the design of the -531 series antennas is quite tolerant. The VHF model should tolerate intermittent operation at levels as high as 100 Watts. UHF models are less tolerant, but brief operation up to 75 Watts should be OK.
• Is RHCP (Right Hand Circular Polarization) best for most Amateur satellites and the ISS?
Traditionally, popular OSCAR operation (Mode J, now called VU) was VHF LHCP up and UHF RHCP down (what we call our "default" pair). Currently, there are a number of satellites that follow other operation standards. Some present satellites are linearly polarized, but spin as they traverse their orbit (the spin keeps the satellite stable in its orbit). Some satellites approach at one polarization and flip polarization as they pass overhead, receding with the opposite polarization. Information on the ARISS (Amateur Radio on International Space Station) website suggests that the ARISS VHF antennas are vertically polarized, which means they are linearly polarized. . . Based on the mix of satellites and their designs/implementations, I still believe the "default pair" is a good choice. Some hardcore satellite operators use a pair of oppositely circular polarized antennas for each band, with a relay switch, to accommodate/optimize for all pass conditions and satellites. . .
As you can see, the "optimum" answer to the question is complex due to the experimental nature of Amateur Radio projects (there is no hard standard every satellite must comply with). This leads to confusion in trying to find the "best" solution for all orbiting platforms. However, a simple strategy that will work adequately most of the time is to select a Circularly Polarized antenna, whether RHCP (Right Hand) or LHCP (Left Hand). This strategy is compatible with linear and with a majority of antenna polarizations used for at least half the satellite pass. This also serves as a good strategy to minimize the variations due to atmospheric effects. . . It is for this reason, and in keeping with original popular Mode J OSCAR strategy that we usually suggest our default option (VHF LHCP and UHF RHCP).
• Can your QFHAs be mounted high above any ground?
Yes, but we usually recommend users to be practical: increasing the height of a ground satellite antenna increases its field of view, which has the benefit of extending the length (duration) of the satellite's "pass". However, increasing the height of any antenna subjects the same to greater wind and gust levels. Therefore, finding the best height should incorporate practical and safety considerations (wind, snow, trees, heights of nearby dwellings/objects, etc.) A height that is good enough is the lowest height that provides a generally clear view of the horizon and sky from the base of the antenna.
It is also best to place the UHF antenna at least 2 ft away from the VHF antenna, with the line between the two antennas arranged mostly perpendicular to a typical or likely overhead satellite path. The reason for this strategy is to minimize occlusion (blockage) by the VHF antenna. This formula is applicable to any satellite crossband antenna combination.
• What is the difference between the UC-AMSAT-KITP and UC-AMSAT-KITP-531 kits?
The difference between the 2 kits is small: the only difference is in the UHF antenna. The -KITP-531 is the newest of the two, it's the one we recommend, as eventually, the original -KITP will be obsoleted by the newer kit. Outside the historical difference, the new UHF element (UC-4364-531) has +1dB better performance than its predecessor, the UC-4364-328. The latter was designed to offer a very compact antenna, yielding over +3 dBic gain across the Amateur UHF band. Since many users also use the VHF model, there was little point in the kit having a compact UHF antenna, so we designed the full QFH version of the UHF antenna to match the performance of the VHF UC-1464-531 model. Thus the kit of both "-531" version antennas was named the UC-AMSAT-KITP-531.
Eventually, we will obsolete both the original kit UC-AMSAT-KITP (pair of passive QFHs) and the UC-AMSAT-KIT (-531 VHF +Mini -UHF QFH with built-in LNA). This will happen once we release the upcoming TX/RX LNAs for VHF and UHF this year (Q4).