When installing an antenna, don't forget that the satellite signal is very weak and comes from above. For example, trying to protect the antenna from rain with a tarpaulin leads to almost complete loss of reception.
Sometimes it can be even worse. Once, we were called for diagnostics because a "satellite compass was not working." It turned out that the crew couldn't find a better place for the antennas than under a metal platform. Of course, as soon as the antennas were given access to the sky, everything started working.
Now let's talk about climate. Few people consider antennas as consumables. Most would like an antenna to last at least 10 years. That's why antennas are divided into classes, depending on the operating conditions.
Marine antennas.
This is very tough. For example, during a storm, a couple of cubic meters of salty water can be poured onto an antenna mounted on the roof of the wheelhouse. In high latitudes, not only water but also a mixture of snow and ice. Coaxial cables for marine antennas are made as thick as a thumb, but I've been told of situations when even such cables were broken by a snow-ice charge. Interestingly, the antennas themselves survived. No wonder - marine antennas are designed for 20 years of service in very harsh conditions.
A separate topic is marine connectors. They are expensive, reliable, and waterproof. The degree of protection for a marine antenna is IP68, which means the antenna is actually submerged in a bathtub of water, and it is checked that no bubbles enter, and the antenna works after 24 hours underwater. The marine antenna is not 100% hermetic; there is a hole for balancing pressure outside and inside the antenna, but the hole is covered with a filter so that water does not pass through it.
These antennas are usually only L1, meaning they are not suitable for most applications. Mounting is typically on a threaded inch-sized peg, sometimes on a "geodetic" 5/8 inch peg. The main thing to remember here is that if you are sold an antenna in a "marine housing," the housing will be excellent. But the connector may be ordinary, not very waterproof, and afraid of water. So check the protection class of the entire antenna, not just the housing.
Aircraft antennas.
These are antennas according to the ARINC-734A standard. They are very reliable, but their high price is due more to the lengthy certification process according to aviation standards rather than reliability. For use outside of an aircraft, they are unsuitable, as their mounting and connectors are very specific. Such an antenna looks like a patch on an airplane. In the photo - a typical aviation antenna for a large passenger aircraft. In terms of characteristics, there's nothing special about them, only L1, GPS, and sometimes GLONASS.
Geodetic antennas.
This is another monster of antenna construction, but in a completely different sense. There is a concept called the phase center of antennas, roughly speaking, the very point where the signal is received. So, in the documentation for a severe geodetic antenna, it is written something like, "the phase center for the L1 frequency is located at 68 mm ±0.2 mm from the bottom edge of the antenna, horizontal error ±0.1 mm. The phase center size is 1.3 mm ±0.1 mm vertically and 1.1 mm ±0.2 mm horizontally." And similarly for L2. In the most advanced cases, the measurement results for a specific antenna sample are provided, with graphs showing the dependence of the phase center location on the satellite's elevation above the horizon.
Why is this necessary? For heavy geodetic tasks such as monitoring mountain movements, continental drift, or measuring a first-class geodetic network on a country scale. There, every millimeter of error counts. Moreover, they calculate both the maximum and average error. Significant part of geodetic antennas are "smart" antennas, meaning that there is at least a receiver inside the antenna housing, and sometimes even a battery and radio modem. Understandably, such antennas are quite expensive. They usually have a 5/8-inch geodetic peg mount.
Timing antennas.
Another interesting class. To receive very accurate time signals and even more precise synchronization of two events occurring in different places, it is necessary to receive signals exclusively from satellites, eliminating all possible reflections from below and from the sides. And even more so - interference.
Even more importantly, for determining position, we need relatively low (close to the horizon) satellites. But for determining time, it is better to use only high satellites (from 30 degrees above the horizon), as they are less delayed by the troposphere, resulting in fewer errors in the solution. Therefore, timing antennas are antennas that poorly receive both low satellites and interference and reflections coming from below and the sides.
Another trick of timing antennas is polarization. Satellite signals are RHCP, meaning they have right-hand circular polarization. With each reflection, the polarization changes to the opposite, i.e., from right to left, from RHCP to LHCP, and vice versa. Timing antennas are designed to receive RHCP polarization much better than LHCP. This feature is called anti-jamming (our anti-jamming solutions).
Photo: anti_Jam.jpg In the photo, you can see how the RHCP satellite signals (black) are received much better than the reflected LHCP (red) signals. Additionally, it is noticeable how the reception level decreases as the angle from the vertical increases.
Another feature of timing antennas is the precise time signal (called 1PPS) that needs to be transmitted to the equipment. A signal with a one-nanosecond leading edge is a signal with a spectrum ranging from a gigahertz to hundreds of gigahertz, depending on the required edge steepness, and it is difficult to transmit it through a regular cable; a very good coaxial cable is needed. Plus, it's challenging to ensure proper impedance matching so that the leading edge doesn't reflect off the cable's end and create a secondary peak.
Therefore, timing receivers are placed closer to devices that use precise time, and they are connected to the antenna with a long (up to hundreds of meters) cable. This results in another feature of timing antennas – a high gain (up to 40 dB, which is 10,000 times) of the built-in low-noise amplifier.
The fact that timing antennas are usually mounted on metal masts leads to three more interesting features.
Firstly, the antenna is designed in the shape of a rocket to prevent birds from perching on it. The red color is a bird-repellent paint. For a regular antenna, an interruption of a few seconds caused by a bird landing on it may be somewhat acceptable, but for continuous process synchronization, it can be disastrous.
- Secondly, a tall pole is an attractive target for lightning. Therefore, timing antennas are the only ones with some lightning protection. Some protection is provided because it is usually designed to withstand 5-6 strikes. It should be the last line of defense against lightning, not the first.The third feature is clearly visible in the photo – it's the side mounting to the pipe from which the mast is made. However, it's essential to understand that the pipe should not be higher than the border between the upper (white) and lower (red) parts. In the photo on the left, you can see improperly installed antennas – the red pipes are too high and cause significant reflection in the antenna. Additionally, the antenna system on the right is lower than these pipes and also receives a reflected signal.
- Since we're discussing installation errors, let me tell you about the "piano in the bushes"As you can see, the antenna is placed at the edge of a grove, significantly lower than the surrounding trees. The creators of this wonder complained that they could only receive signals from 2-3 satellites from each system. I'll repeat once again – antennas need a clear sky. Trees, especially wet ones, can significantly attenuate GNSS signals and cause reflections if antennas are placed at the edge of a forest.
Stationary antennas.
These are a lightweight version of geodetic antennas. For surveyors, they are like fake Christmas tree decorations – they cost ten times less but lack precise figures describing the phase center position. For everyone else who doesn't mind losing a few millimeters of accuracy, they are an excellent working tool.
These antennas are typically designed in the shape of a mushroom with a ground plane inside, and they screw onto a geodetic stake. The most crucial role of this stake is to raise the antenna above the water or snow level, which may accumulate on a roof during heavy rain, snowfall, or snowmelt. The connectors of these antennas are standard and can rust underwater. Therefore, I recommend finding out the maximum daily precipitation level before installing the antenna on a "bordered" roof and making the stake slightly higher. In northern regions, learn the maximum snow level that falls during the winter – not in millimeters of water, but in actual snow level. Alternatively, you can clear the antenna buried in the snow a couple of times during the winter, or learn from experience that antennas perform poorly under snow.
Another function of the stake is the ability to replace the antenna. This may be necessary to obtain an official document with the antenna coordinates from surveyors.Another purpose of stationary antennas is mounting them on the roofs of truck cabins and cars. The reason is the same – to avoid cleaning the antenna from snow, it's easier to raise it above the roof. This means having a separate ground plane in the antenna and a mushroom shape.
Magnetic antennas.
These are antennas without their own ground plane, but attached to someone else's using a magnet. For example, to a car roof, a steel sheet, and so on. They are ideal for testing. Since magnetic attachment does not last for years, some of these antennas have tabs for screwing. It is a very convenient option if you want to remove the antenna at night. Sometimes such antennas are used instead of groundless ones and are placed on a ground plane inside a housing.
As stationary antennas, they are not very suitable. IP67 allows for immersion in water for up to one minute, but not lying in a puddle for hours. During the spring snowmelt, several antennas on our roof died (reducing the signal strength). It turned out that in this batch, the housing had a hair-thin crack. The amplifier rusted while lying in the puddle. By the way, the manufacturer fully recognized the problem. So it's better to use antennas attached to a stake, which are always above the puddle. Or an explicit IP68, which will not be affected by puddles.
Drone antennas.
The peculiarity of a drone is that it can tilt significantly during maneuvers, so helical antennas with a radiation pattern that includes the lower hemisphere are used for drones, and a ground plane is not used. The second feature is the very lightweight antennas, weighing 30 grams.
To reduce weight, the attachment reliability was sacrificed, and drone antennas are simply screwed onto the CMA connector. This is logical because if the drone crashes and breaks, but the antenna survives, it will hardly console anyone. Therefore, the reliability of the antenna attachment in collisions is approximately equal to the reliability of the other parts of the drone.
Groundless antennas.
These are semi-finished products for creating stationary antennas and custom devices (smart antennas). They are usually mounted on a multilayer printed circuit board, the ground layer of which represents the ground plane, and the receiver and processor of the device are mounted on the bottom.
© Eltehs SIA 2023
