Parabolic antennas are common high-gain antennas in the ultra-high frequency and microwave bands, often used for satellite links, radio telescopes, and some wide area network (WAN) links. In amateur radio, they primarily serve as satellite link antennas. I am providing a brief overview of my limited knowledge to help those interested in entering satellite communication.
First, let me reiterate the regulations: When using such equipment, do not intentionally receive signals outside of amateur business use, to avoid exceeding the scope of amateur radio licenses.
Individuals are prohibited from installing and using satellite ground receiving facilities. In special circumstances, if an individual genuinely requires the installation and use of a satellite ground receiving facility and meets the licensing requirements stipulated by the State Council's Department of Radio, Film and Television Administration, they must apply to their respective units. After obtaining approval from the local county or municipal People's Government's Department of Radio, Film and Television Administration, the application must be submitted to the People's Government of the province, autonomous region, or directly-administered municipality for final approval. (Article 8 of the "Regulations on the Management of Satellite Television Broadcasting Ground Receiving Facilities")
The electrical components of a common parabolic antenna (or one type thereof, commonly referred to as a satellite dish) can be divided into three parts:Parabolic reflector, transmitter (also known as illuminator), and down-converter。
If you are an astronomy enthusiast, you will quickly think ofReflectionReflecting telescopes (particularly those based on the Cassegrain design) also have a similar structure.
Interestingly, common parabolic antennas also have a Cassegrain structure. In fact, they all utilize a parabola to reflect approximately parallel incoming electromagnetic waves to the focal point, or conversely, to emit signals from the focal point in a parallel manner.
Besides the Cassegrain structure, there is another called the Gregorian structure, which is also widely used in optical telescopes/parabolic antennas. It has two reflective structures: the additional second reflector can more effectively focus electromagnetic waves upon reception, thereby achieving a larger gain than the Cassegrain structure. However, it requires precise positioning and alignment during installation and debugging, resulting in higher costs.
Parabolic reflector (reflector)
Starting from first principles, a parabolic reflector only needs to perform one task: it must be able to reflect radio waves to a specific point. In other words, Conductive . If there's one more thing to consider, it needs to be strong enough.
Therefore, the most cost-effective approach is to fabricate a curved galvanized steel sheet and then coat it with a layer of polyurethane powder for rust protection. Some portable parabolic reflectors are made into structures resembling folding fans (or foldable steamer racks).
There's also a more creative method: attaching aluminum foil tape to the inside of a large umbrella.
(The image shows a DIY antenna made by a Bilibili user using an umbrella to receive signals from civilian weather satellites.)
A proper parabolic antenna, those of a slightly higher grade are made from aluminum and magnesium alloy, which protects itself through its own oxidation process. The surface protective layer has also been upgraded from plastic powder to fluorocarbon paint, which is more expensive. There are also those made with glass fiber + aluminum foil heat-bonded, which have a smoother and flatter appearance due to their one-time molded form.
In terms of engineering, honeycomb aluminum is used to achieve a balance between structural strength, weight, and thermal expansion/contraction properties. The surface can be either an aluminum composite panel (I'm not sure about this, need to verify), or a zinc steel frame + aluminum mesh + aluminum skin construction, which is then riveted together. Some specific applications may also include silver plating.
Some mobile broadcast vehicles use a combination of carbon fiber and aluminum foil, but the aluminum foil is tested inside the shell, so it doesn't show a very strong metallic sheen. An outer layer of polyurethane coating is applied to protect against rust while also reducing weight.
The image shows an ON4MU 2.4m diameter foldable parabolic antenna (100W power) used for EME communication in the 1.2 GHz band. It also functions like an umbrella.
The size of a parabolic antenna is clearly best when it's as large as possible. When the diameter is less than approximately one wavelength, the gain drops sharply, and below 0.5 wavelengths, there is virtually no directivity. The gain G of a parabolic antenna is given by: G = 4π * Ae / λ² (where λ is the wavelength, Ae is the effective area of the antenna, Ae = πD²η/4, η is the transmission efficiency, and D is the diameter of the parabolic dish. Interested HAMs can easily derive this themselves.)
Larger antennas can capture weaker signals, such as those from the Moon's reflection.（EME）For example, a 2-meter antenna typically requires hundreds of watts of power, while a 26-meter parabolic dish only needs 3 mW (milliwatts) to receive the echo! (Related information: "World Lunar Reflection Day" on June 29, 2009, celebrating the 40th anniversary of Apollo 11's landing; launched by the University of Tasmania in Australia, received by the De Wit telescope in the Netherlands).
However, due to space constraints, even consumer-grade antennas would not be very large. Thanks to the development of TV satellite phased array technology, using even small pots or dishes with a diameter of 60cm or 45cm can stably receive television program signals transmitted from geostationary orbit (e.g., "户户通"). Another reason is the widespread adoption of satellite television in China.Household connectionThe wavelength used by the Zhongxing-9 satellite, at 11 GHz, is only a few centimeters. Compared to this wavelength, parabolic antennas are large enough, which is another advantage of the Ku band in terms of size.
What are larger antennas used for? According to some publicly available information, operators use Ku-band antennas around 1 meter for VSAT (satellite internet) small stations. Antennas with a diameter of approximately 1.8 meters are typically used to receive C-band satellite television signals. Some satellite earth observation stations/space-to-earth gateways require apertures ranging from 3 to 9 meters, and the largest ones are likely intended for radio astronomy purposes.
The image above shows a satellite ground control station in Shaanxi province.
By the way, in addition to the abbreviations for frequency bands used by satellites, such as UHF (300 MHz - 3 GHz) and SHF (3 - 30 GHz), there is also a notation system that uses letters to represent specific frequency ranges. For example, C represents 4-8 GHz, Ku represents 12-18 GHz, and Ka represents 27-40 GHz.
In the broadcast and television service sector, the C-band spectrum is almost fully allocated. Although it experiences less rain fade compared to other bands, it requires larger antenna sizes (1.8m+). The K-band suffers from significant rain fade and is primarily used for high-speed WeChat communication, HD TV, real-time news gathering, and direct-to-home (DTH) services. However, it still feels like a relatively new and emerging area.
High-frequency operation implies smaller antenna sizes and larger bandwidth capacity, but it also suffers from rain attenuation and multipath effects. This requires high precision in the design and manufacturing of modulation and amplifier circuits.
In addition to rain (which causes signal absorption and irregular scattering), satellite communication can also be affected by solar eclipses. This means that when the sun is behind the satellite, it acts as a large electromagnetic radiation source, causing the satellite's signal to be partially submerged in noise, thereby reducing the signal-to-noise ratio. The "Solar Eclipse" warning in the Look4sat app is related to this phenomenon.
Parabolic antennas do not necessarily need to be continuous; they can also be mesh-like with holes. As long as the size of the holes is less than 1/10th of the wavelength, 2.4 GHz Wi-Fi grid antennas (often used as high-power outdoor access points) are also variations of parabolic antennas.
When the antenna is pointed in the correct direction, software such as Look4sat, satdump, and Gpredict can provide elevation (El) and azimuth (Az) data to assist. Alternatively, a signal strength meter (especially those with beacons that some satellites transmit) can be used to determine the tracking angle based on the beacon's strength. For asynchronous satellites, a servo motor can be used for continuous tracking (there are affordable open-source solutions available).
The parabolic reflector of a parabolic antenna does not inherently involve electrical signal resonance, so it can operate over a very wide bandwidth. (Similar to different colored optical fibers, the image formed on the human retina is independent of their position.) The actual electrical processing occurs in the feed section. The feed section consists of the source and LNB.
Source (Emitter)
Then, he explained the feedback.Source(feedhorn): The word itself is quite descriptive: "feed" (to provide) + "horn" (animal horn, or a loudspeaker). It's located at the focus of the parabolic reflector. The feedhorn can direct electromagnetic waves from the reflector to the receiving device at the back end (such as an LNB). When receiving, it acts as an external signal source, responsible for feeding the focused signal into the LNB. When transmitting, it acts as an electromagnetic wave transmitter (also known as an "illuminator"), emitting radio frequencies onto the parabolic surface, and using the reflection of the parabolic surface to emit the radio frequency in a directionally controlled manner. Although it is called a "feed"Source"However, it is an unstructured structure."
A structure with no external source represents a structure that does not require an externalElectricityIt can operate as long as there's a supply. The LNB mentioned later is an active one.
The three commonly used feed structures are Poty, Septum Feed, and Choke Ring Feed. This section involves in-depth knowledge of waveguides…
In practice, the feedpoint shape can vary. At lower frequencies, Yagi-type or DP ( Doherty) antennas can also be used as feedpoints.
The polarization method is an important factor in antenna installation. "Polarization" refers to the orientation of electromagnetic waves, and only when the polarization of the transmitting part (typically a short copper wire connected to the feed) matches that of the receiving part, can a relatively strong signal be received.
To improve installation reliability, or to receive signals from two different directions simultaneously, a dual-polarized antenna can be used. A dual-polarized antenna consists of two orthogonally polarized antennas: vertical and horizontal.（V/H）The two polarization calculations are orthogonal; one for left-handed and one for right-handed.（LHCP/RHCP）Both of these polarizations can also be considered orthogonal. There's a metric called XPD (cross polarization discrimination) that measures the ability of this dual-polarized antenna to separate the two polarizations.
There's a problem here: we'll quickly find that having a feed and support structure directly in front of the parabolic dish will obviously block the beam, thereby affecting overall efficiency (this is technically called aperture blockage). This is especially problematic with small, home-use antennas where the feed and dish are roughly the same size. So, how do we get the focus away from the front of the parabolic dish? One solution would be to design the parabolic dish as part of an ellipse, with a short, single arm extending from the edge of the dish. This wouldn't block the main beam. Another option is to make the feed rotatable and movable, allowing for flexible adjustment of the polarization direction. (A more modern approach is phased array technology, such as Starlink's flat antennas, which use phase shifting to control the beam direction, eliminating the need for mechanical movement.)
| Forward feed | Unbiased feed |
|---|
| Face covering | (Pole + Power Source) → Efficiency 55-65% | Almost no obstruction → Efficiency 70-75% |
| Sideband / Noise | The antenna diameter is large, and the noise temperature is high. | The weather is mild and slightly rainy. |
| F/D (Zhibi) | 0.3-0.4 (deep) | 0.6-0.7 (light) |
| Polarization adjustment | Full-size rotating pot or feed system | Only rotate the feed source disc for convenience. |
| Multi-frequency sharing | Bands in the C-band, dual polarization, dual-band are easy. | The Ku band is the primary focus, while the C band is more challenging. |
Two parameters are particularly noteworthy here: the aperture efficiency eA (or Ae), which represents the actual gain of the antenna divided by the ideal, uniformly illuminated aperture's gain. This is expressed as a percentage and describes how effectively the energy is utilized. So, where does the power go that isn't transmitted along the intended path? It could be that the radiated energy doesn't hit the reflector surface, is obscured by the support pole, or that the center of the feed is offset, leading to an imperfect parabolic shape due to installation factors, etc.
Another is the focal ratio, F/D. F/D = Focal length F ÷ Aperture diameter D. This is easy to understand: a smaller F/D indicates a "deep dish" shape, while a larger F/D indicates a flatter dish (shallow focus). A deep dish shape means that a wide beam feed source is required (e.g., a horn-shaped opening).
LNB
The LNB (Low Noise Block downconverter) is likely unfamiliar to many people. It's not as well-known as the LNA (low noise amplifier). However, the LNB has a common nickname called:High-frequency head. The scientific name for an LNB is Low Noise Block Downconverter (LNB). Block (downconverter), where "Block" refers to "block" frequency conversion, meaning converting the entire band at once.
Before the LNB performs down-conversion, it first amplifies the high-frequency portion (which is handled by a low-noise microwave amplifier), then performs mixing and oscillation, and finally amplifies the resulting intermediate frequency. Some tutorials for the down-conversion section also distinguish it separately as an LNC (converter).
When downconverting, the commonly used local oscillator (LO) frequencies are typically 9.75 GHz and 10.6 GHz. After mixing the LO frequency f0 and the signal source frequency f1, the higher output frequencies (f0 + f1) are filtered out, leaving only f0 - f1, which corresponds to an output frequency of only a few hundred MHz. At this point, in the downstream signal processing (such as SDR), you should observe this output frequency.
The intensity of the observed output frequency can also serve as a reference for determining the optimal polarization matching angle for the rotating feed source.
The image shows an LNB (the circular component on the left) connected to the feed source.
In practice, in common satellite parabolic antennas, the LNB (Low Noise Block) is often integrated with the feed horn and housed within a rectangular or cylindrical white plastic box (the white color helps to reduce solar heat absorption, and the enclosure provides waterproofing). Its function is to convert high-frequency signals into low-frequency bands. For example, it can convert signals around 10 GHz in the Ku band to 900 MHz in the UHF band, making subsequent signal processing easier. This frequency conversion circuit is quite mature, consisting of a crystal oscillator and a mixer, followed by filtering, which keeps costs down (a single unit typically costs around ten dollars).
The LNB has a feature called22 kHz switching audio: When the receiver transmits a 0.5 Vpp, 22 kHz square wave on the coaxial cable, and the LNB's internal filter detects it, the local oscillator is switched from 9.75 GHz to 10.6 GHz (or vice versa). This allows the same downlink to cover the low band of 10.7–11.7 GHz and the high band of 11.7–12.75 GHz without manually disassembling and replacing the high-frequency head. However, I personally don't find this feature very useful for amateur applications.
Due to the high frequency stability requirements of amateur satellites, HAMs frequently modify commercially available LNBs. The purpose is to suppress the frequency drift of the internal 25 MHz (or 27 MHz) crystal oscillators in the LNB, which are caused by temperature and time variations. In this case, the crystal oscillator serves as the local oscillator. The local oscillator is then used for hundreds of times multiplication to reach a frequency of 10 GHz using a phase-locked loop.
There are two specific methods to implement this: one involves using a GPS-based oscillator as the clock reference for the PLL (Phase-Locked Loop), and the other involves using a temperature-compensated crystal oscillator.（TCXO）As a replacement for the original crystal oscillator. The former achieves stability of ±50 Hz, while the latter has slightly less performance (approximately still exhibiting slow drift of several hundred Hz).
TCXO – Temperature Compensated Crystal Oscillator: "This oscillator is designed to operate under conditions of significant environmental temperature variations. The TCXO incorporates a thermistor, which continuously monitors and compensates for changes in the surrounding temperature. Its accuracy is higher than that of standard crystal oscillators, but lower than that of GPS-disciplined crystal oscillators."
As mentioned earlier, the LNB is an active device. However, we typically don't provide a separate power supply to the LNB. In reality, there are three ways to power an LNB:
1. The most common method for consumer devices is to provide power to the LNB through coaxial cable. This cable will have a voltage of around 18V.
2. An integrated circuit (IC) such as D4202 inside the LNB converts this voltage into the LNB's operating voltage.
3. Some LNBs also use the voltage as a switching signal for polarization.
Some HAMs who have experimented with RTL-SDR to receive satellite signals should remember that the LNA requires a bias-T (biasing tee) to provide power. The bias-T is also used in some LNBs that require an additional power connection, and its function is to ensure that the current flows towards one side of the LNB, rather than flowing back into the SDR/receiver.
Duplexer, amplifier, and driver chain
Many beginners mistakenly believe that "placing the dish + LNB will transmit signals," but in reality, ordinary LNBS can only receive. For optimal transmission, it's best to use a separate feedline + amplifier + duplexer.
In simple terms, the LNB circuit used for consumer applications is designed to receive low-power signals. The high power output it generates cannot be handled by a single amplifier. Therefore, we also need to install a duplexer or circulator to achieve simultaneous transmission and reception (feed-sharing).
Duplexer The principle is frequency splitting, which involves using two bandpass filters (or high-pass/low-pass, bandpass/bandstop) to separate the transmit and receive frequency bands. Specifically, this is achieved by connecting a power amplifier → duplexer common port → feedline, or a feedline → duplexer common port → LNA (or SDR front-end). This means that the connection from the duplexer to the feedline only requires one coaxial cable or waveguide, enabling single-feed dual-band operation.
Circulator— Or, it can be called a unidirectional RF tri-connector. It contains a piece of ferrite plus a magnet. When the RF signal passes through, the ferrite is "twisted" by the magnetic field, and the signal can only flow in one direction (clockwise). Alternatively, think of it as a traffic roundabout with three entrances. The three entrances are sequentially: antenna, SDR, and amplifier. The signal can flow unidirectionally within this roundabout, from SDR to amplifier, or from amplifier to antenna, but not backwards.
Conclusion
Thanks to the development of modern software-defined radio (SDR) technology, such as the widespread adoption of hardware components like FPGAs and direct conversion chips, and the evolution of open-source tools/frameworks like GNU Radio, SDRangel, and gqrx, it has become easier for amateur enthusiasts to modulate and demodulate signals at high frequencies like C and Ku bands. As a result, HAM radio operators are gradually moving into the microwave frequency band, with parabolic antennas becoming essential tools for experimentation.
A parabolic dish antenna achieves three key missions – directional, efficient, and bidirectional communication – using a seemingly simple "dish" shape. It can be a small feed antenna on a rural rooftop (45 cm) or a ground station spanning several meters on a mountaintop; it can capture signals as weak as 0.1 microwatts from deep space, and also transmit carrier waves over 35,000 kilometers to synchronous orbits.
Due to limited bandwidth, human technology and imagination are limitless. May this "channel" continue to hold humanity's curiosity about the sky, transforming every small radio wave into a resounding echo that connects the world. (These two paragraphs were written by AI; additionally, operation of a carrier frequency requires a license, and any communication experiments exceeding power limits require prior approval.)
I hope everyone can benefit from this, pursue their hobbies in moderation, find a good balance, and have fun.
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