radio antennas

see also:

• radio basics
• remote communication for campers and hikers
• radar and radar motion detector sensors for campers
• radio signal intelligence (SigInt)
• software defined radio (SDR) receivers
• Ham amateur radio
• satellites and their radio signals
• digital TV for portable devices
• vector network analyzers (VNAs)
• oscilloscopes

• radio antennas are generally capable of both receiving and transmitting radio waves - the transmission is quite wavelength dependent whereas reception is more tolerant of a wider range of wavelengths
• radio signals are generally line of sight just like light hence most are limited by the curvature of the earth to some 50km depending upon height above ground, although long wavelengths can bounce off the ionosphere and in this way can be received 1000's of km away in some cases
• a transmission radio antenna is generally tuned to resonate and thus transmit optimally at a single frequency and its design will determine the shape of the field that is being transmitted

• Gain: Measures how efficiently the antenna directs or receives energy in a particular direction.
• Directivity: The focus or aiming of transmitted/received power, important for range and minimizing interference.
  • the direction of the antenna is often important as the waves have an electrical component which is perpendicular to the magnetic component and the receiving antenna generally needs to be in the same orientation as the transmitting antenna for best reception hence most should be vertical (although in practice the waves do bounce around near field objects a bit)
• Bandwidth: The frequency range over which the antenna operates effectively.
• Polarization: The orientation of the electromagnetic field (vertical, horizontal, left or right circular).
• Radiation Pattern: The spatial distribution of the radiated energy (e.g., omnidirectional vs. directional)
  • omnidirectional is ideal for applications requiring broad area coverage, such as Wi-Fi routers, mobile base stations, and broadcasting systems
  • directional provide longer range and targeted communication with less interference and is used in point-to-point links, satellite dishes, radar, and long-distance communications
  • the effective radiation pattern can be distorted by surroundings (buildings, terrain)
  • higher frequencies tend to produce higher directivity as the wavelength becomes shorter relative to the antenna size, which generally results in a narrower beamwidth and more directional radiation patterns
  • higher frequencies allow antennas to achieve higher gain with the same physical size, thus concentrating radiation more effectively
  • the current distribution on an antenna surface controls the amplitude and phase of the radiated electromagnetic fields, which in turn affects the shape and directionality of the radiation pattern

often measured in dBm (eg. a HackRF One is usually 5-15dBm maximum)

Power in watts = 10^dBm/10/1000 thus 15dBm = 32mW

dBd gain is expressed in dB relative to a half-wave dipole which has a gain of about 2.15 dBi hence dBd gain = dBi gain - 2.15dB

dBi gain expressed in dB relative to an isotropic radiator, which is a theoretical antenna that radiates equally in all directions and has 0dBi by definition.

resonant frequency in Hz = 1 / 2πLC, where, L = inductance and C = capacitance

• inductance is the antenna's ability to store energy in a magnetic field and represents the antenna's reactance component that opposes changes in current due to magnetic fields created by the antenna's coil or loop geometry
• inductance is measured in Henries (H) and is frequency-independent as a property, but its effect (reactance) depends on frequency
• inductance contributes only the imaginary (reactive) part of the total antenna impedance
• the resonant frequency of an antenna is dependent upon the inductance and the capacitance as follows.

inductance L(μH) for a single loop = 31.416 x r² / (3r+10d) where r = radius in inches, d = wire diameter in inches

finally for multiple loops of wire, multiple this inductance by N² where N = number of wire loops

required Resonance Tuning Capacitor in Farads = 1 / (2πf)²L, where f = resonant frequency in Hz

• antenna impedance is measured in ohms
  • impedance is a comprehensive parameter representing how the antenna resists and reacts to an AC electrical current flowing through it at a given frequency
  • it is a complex quantity expressed as impedance = R + jX, where:
    • R = resistance (real part), representing power actually radiated as electromagnetic waves or lost as heat
    • j = the imaginary unit in electrical engineering, equivalent to the square root of −1
    • jX indicates that reactance contributes a phase shift of 90 degrees to the current relative to voltage. For inductive reactance, this means current lags voltage; for capacitive reactance, current leads voltage
    • X = reactance (imaginary part), combining inductive and capacitive effects, including antenna inductance
      • X is mostly inductive, it's related directly to the antenna's inductance via: X in ohms = 2πfL where f = frequency in Hz and L = inductance in Henries
  • impedance dictates how much voltage and current are produced for a given applied signal and includes both power radiation and stored energy
  • for an antenna system to function efficiently, the antenna's impedance must match the impedance of the connected transmission line (commonly 50 Ω) and the transmitter or receiver. When impedances are matched, nearly all available power is transferred from the source to the antenna for transmission, or from the antenna to the receiver for reception. If the impedances are not matched, significant power is reflected back toward the source, causing signal loss and reduced system performance.
  • most radio systems use a characteristic impedance of 50 Ω for both antennas and cables, so antennas are typically designed to have an impedance as close to this as possible
  • impedance changes as the antenna’s electrical length approaches resonance or harmonic frequencies
  • height at which an antenna is installed above the ground (or above a conductive surface) can alter impedance due to changes in ground reflection, especially for low-frequency or HF antennas
  • for antennas like dipoles, moving the feed point toward the ends increases impedance, while center-feeding yields the lowest impedance (about 73 Ω in free space for a half-wave dipole
  • the kind of conductor, as well as PCB substrate properties (permittivity, thickness), impact impedance, especially in microstrip or embedded antennas
  • nearby conductive or dielectric materials (buildings, trees, human presence) influence impedance by altering electromagnetic fields and the “ground” effect
  • factors such as humidity, rain, nearby foliage, or ice can absorb, reflect, or detune the antenna, leading to significant changes in impedance
  • the resonant frequency of an antenna can be measured using a NanoVNA
    • these typically have a measuring range of 10KHz -1.5GHz and come with a calibration kit

• mono headphone plugs and jacks
  • these are commonly used in portable radios to allow attaching an external antenna
• SMA
  • these are the common cable adapters for antennas, dongles, filters, etc for portable equipment
  • there are male and female versions
  • generally should only be tightened via the nut (turning the cable or device will eventually damage the inner part) and tightened just beyond hand tightening using a torque wrench or a 8mm wrench (too tight and it will break it)
• N-type
  • much larger than SMA
  • 50 Ohm N-type are physically different to 75 Ohm N-type, as the 50Ohm has a larger centre pin and attempting to connect a 50Ohm male to a 75ohm female will destroy then 75Ohm connector
  • you can get 75Ohm-50Ohm converters for using 75 Ohm on a 50 Ohm VNA for instance but to reduce losses a matching L pads is best (although you lose 7.5dB of dynamic range per pad) and is placed near the DUT at the end of a 50 Ohm cable (see https://www.youtube.com/watch?v=CGh40GTld7A)
    • NB. most 75 ohm systems and cables were designed to operate to only 3GHz
• F-type
  • 75 Ohm screw on style
  • often used for satellite and cable TV
  • are not metrologically sound as a N-type as the centre pin is not as stable
• BNC
• PAL/Belling-Lee connector
  • push-in type which is the usual 75 Ohm TV antenna cable connector in Australia
• MCX
  • developed in the 1980s and are about 30% smaller than SMB connectors, with an outer diameter of approximately 3.6 mm
  • impedance of 50 Ω (with some versions at 75 Ω), and are designed for frequencies from DC up to 6 GHz
  • gold-plated contact surfaces for reliable signal transfer and offer a long mechanical life, rated for at least 500 mating cycles
• etc

• antenna cables for radio are usually 50 Ohm (TV antenna coax cables are usually 75 Ohm with a IEC DIN connector including most DVB-T USB digital TV dongles)
• if long distances, then a high quality cable is recommended to reduce signal losses and also to reduce interference
• can use less expensive coax cables for frequencies < 30MHz as there is less loss at lower frequencies
• if using above 100MHz, then you need to consider the type of coax - check the loss in dB/m at the frequencies you are wanting
• there are many coax cable types:
  • RG58 - not adequate for 1GHz if using a long 25m cable
  • Mini8x
  • RG213
  • LMR400
  • RG223
  • RG58CU
  • M17/75 - RG214
  • TMC-LMR-240
  • TMC-LMR-200
  • TMC-LMR-195
  • TMC-LMR-400
  • TMC-LMR-600
  • LMR-SW540
  • M17/113 - RG316
  • RG316
  • RG316DS
  • HF047
  • HF086
  • HF141
  • HSMultiplex86

• High Z antennas
  • high impedance usually above 90 Ohms and can range into the several hundreds or even kilohms
  • often used with antennas like random wires, certain loops, or short whips for lower frequencies (HF, MF)
  • common for portable receivers or specialized low-noise receive arrays
  • often favored for receive-only low-band work or where long wire layouts are practical
  • used with High Z inputs on radio but require special impedance-matching networks (like transformers or matching amps) to interface with 50 Ohm radios or receivers, especially to minimize signal loss and optimize transfer
  • NB. High Z FETs are high impedance field effect transistors
• 50 Ohm
  • the standard “low Z” impedance used in most RF systems, matched to coaxial cables and radio equipment inputs/outputs
  • used with center-fed resonant antennas (like dipoles and many verticals), as well as tuned loops and commercial VHF/UHF antennas
  • designed to minimize signal reflections and optimize power delivery with 50 Ohm feedline
  • the normal choice for general-purpose amateur radio operations using standard coax and equipment

• wire antennae
  • tend to pick up the electrical component mainly and are primarily used for longer wavelengths
  • eg. if using SW, consider a 3m long wire antenna outdoors
• loop magnetic antennae
  • such as a ferrite rod with wire strung around it multiple times, tend to pick up the magnetic component mainly - eg. for AM radio
  • eg. BB Loop Antenna
    • most effective as receiving antennas, particularly below about 15 MHz, where atmospheric and man-made noise dominate and antenna inefficiency is less critical
    • are directional with sharp nulls, enabling the user to rotate the loop to maximize desired signals and nullify interference
    • often preferred for weak AM signal reception below 15MHz primarily because of their high signal-to-noise ratio (SNR) and their ability to reject interference
    • small loops respond to the magnetic component of radio waves, which often makes them less susceptible to local electric noise sources prevalent in urban or electronic environments - they have a much improved SNR, often 10 to 20 dB better than conventional dipoles or vertical antennas in noisy urban settings
    • act as high-Q resonant circuits, providing narrowband filtering naturally and helps avoid front-end overload on receivers like RTL-SDRs
    • often available with built-in amplifiers or preamps, which further improves reception in challenging conditions
    • generally will not outperform larger, outdoor wire antennas in signal sensitivity but offer superior convenience, directionality, and noise immunity
    • compact hiking versions with Internal Thread Inner Needle and a low impedance converter generally provide 9.9 kHz to 181 MHz coverage
• dipole antenna
  • two equal lengths of metal connected to the centre pin and the shield respectively - each length should be quarter a wavelength long
  • has an omnidirectional pattern in the horizontal plane
• monopole ground plane antenna
  • in addition to a single metal vertical antenna, these have a ground component (often as a magnetic base) which may be the metal of a car, or your body via your hand for hand held devices
  • functions as a half of a dipole and is omnidirectional horizontally
  • eg. mobile phones, car radios, handheld radios, and vehicle-mounted communication systems
• large loop antenna
  • these are NULL perpendicular to the plane of the loop and thus will not detect anything exactly perpendicular
    • this feature can be used for locating a transmitter outside - when the signal drops out - it is in the direction perpendicular to the plane of the loop
  • they detect transmissions in the plane of the loop
• Yagi-Uda antenna
  • these are a series of parallel metal rods which create a more directional antenna
  • the 2nd rod is usually the actual dipole antenna and the other rods act as directors and reflectors
  • used in television and radio reception, long-distance communications, and amateur (ham) radio
• log periodic antenna
  • another directional antenna
  • array of elements of varying lengths, offering wideband frequency coverage and directional gain
  • used where frequency agility is needed, such as television, communications, and laboratory measurements
• Parabolic (Grid Parabolic) Antenna
  • uses a parabolic reflector (dish shape) to focus energy into a narrow beam, achieving very high gain
  • used for point-to-point communication, satellite dishes, radar, and radio telescopes
  • examples:
    • R80P is a single-pack 80cm dish for KU band satellite reception suitable for the reception of VAST, Pay TV, and other available KU band satellite services between 10.7 GHz and 12.75 GHz
• Helical Antenna
  • a wire wound in the shape of a helix. Can be omnidirectional or directional, with circular polarization
  • used in satellite communications, space communication, and some portable radio applications
  • some can be used to point at a satellite dish
    • see https://www.youtube.com/watch?v=aLsOZbsGJ1Q L band helix antenna
  • NB. there are left and right handed circular polarised versions to match the polarity of the transmissions
• Collinear Antenna
  • stack of dipoles arranged in line for increased gain and an elongated omnidirectional radiation pattern.
  • used in base stations for mobile networks and other communication systems requiring broad coverage
• Patch Antenna
  • flat, rectangular antenna mounted on a surface; usually directional with a low profile
  • used for Wi-Fi devices, mobile phones, and GPS receivers due to its compactness and ease of integration
• Discone antenna
  • often designed to be relatively compact, wide spectrum, indoor antennas often covering 25MHz to 2GHz, mostly with an SWR < 6
  • https://www.youtube.com/watch?v=XCDpOj6Mof0
• Electronic phased array antenna
  • eg. Starlink Mini - 2.9kg 30x30x3.9cm square flat transceiver antenna with WiFi which uses 25-40W DC/USB-C PD
  • eg. SatKing Pro Max for satellite TV such as Foxtel in caravans
    • motorised GPS enabled; can receive both DVBS and DVBS2 transponders (SD & HD MPEG4);
    • dual LNB outputs to suit twin tuner STB’s like Foxtel IQ/Austar MyStar and VAST twin tuner receivers

• the point where the antenna meets the 50 ohm coax cable is the feed point
• the feed point impedance of an antenna can change depending on its height above ground, ground condition, location, etc
• you can use a VNA to measure the feed point impedance of an antenna but in practice this is not what is usually done as the SWR is usually just measured at the radio end of the coax and the correct unun/balun/tuner is used for the known system, or one uses a resonant antenna
• the antenna at this feed point can have a far higher impedance than the 50 Ohm coax
• a matched antenna system will give a low Standing Wave Ratio (SWR) near 1 and this will allow transmissions with minimal power losses (the higher the SWR, the more power loss in that more of the power used to transmit is just dissipated as heat)
• if SWR is higher than 2 you not only lose transmit power significantly but can cause harm to your radio transmitter
• example antennas:²⁾
  • a quarter wave vertical wire antenna (length in m = 71.5/freq MHz) generally has a perfect feed point impedance of 50 Ohms
    • you don't really need a unun for this but you could use a 1:1 balun to act mainly as a choke (see below)
  • a half wave vertical wire antenna (length in m = 143/freq MHz) generally has a feed point impedance of 2450 Ohms
    • you can use a 49:1 unun (2450/50) to join it to the coax and get matching impedance
  • a quarter wave dipole antenna (each arm being quarter wave length) and this generally has a perfect feed point impedance of 50 Ohms
    • you don't really need a balun for this but you could use a 1:1 balun to act mainly as a choke (see below)
  • off-centre fed dipole antenna has asymmetric arms to provide multi-band resonance use
    • eg. 2/3 wave arm and a 1/3 wave arm gives a feedpoint impedance of 200 ohms and so you need a 4:1 balun (200/50)
  • vertical random wire antenna
    • 5.33m or 7.6m have a feedpoint impedance of 200-350 Ohms hence need a 4:1 unun with ground plane plus an antenna tuner
    • 8.84m or 10.82m or taller have a feedpoint impedance of 450-500 Ohms hence need a 9:1 unun (or an antenna coupler) with ground plane plus an antenna tuner
  • full wavelength delta loop antenna (triangular)
    • gives a feedpoint impedance of 200 ohms and so you need a 4:1 balun (200/50)

• power loss in Watts = power in Watts x (SWR-1)² / (SWR+1)²

• ununs are transformers paced at the feed point to improve antenna impedance matching for assymetric unbalanced antennas which rely upon a connection to the ground
  • eg. a random wire driven element vertical antenna may have 300-600 Ohms and require a 9:1 unun at the feed point
• baluns are used at the feed point to match impedance for balanced, electrically symmetric antennas which do not rely on a connection to the ground
  • eg. a 1:1 balun is often used for a dipole antenna with each of its arms being 50 ohms
  • eg. a 4:1 baun is often used for a delta loop (triangular) antenna which may have an impedance of 200 Ohms
• other examples of antenna impedance matching devices include:
  • antenna tuners - often used at radio end of coax to more finely tune the impedance matching
  • matching subs
  • antenna couplers

• for optimum reception / transmission, your antenna needs to be “tuned” to the frequency you wish to use and the antenna impedance needs to match the coax impedance of 50 Ohms
• this is particularly important for transmissions
  • if the SWR is > 2 then there is a risk of the reflected energy causing damage back into the radio transmitter
  • if antenna is too short, you will reduce efficiency and more of your power will be dissipated as heat instead of RF waves
  • if antenna is too long, you will lose the omnidirectional pattern and instead develop jagged regions of nulls around the pole where no RF is transmitted
  • just adding a tuner box will not remedy this but it will possibly make your transmitter happier by providing a 50 Ohm box
• while receive-only systems generally tolerate a higher VSWR than transmitters (since power reflection is less critical), a well-matched antenna will still yield a lower noise floor, improved signal-to-noise ratio, and more sensitive reception, which is crucial for weak-signal radio work

• you can buy antennas that are already designed for a specific use such as:
  • 868 or 915MHz LoRA - 50cm
  • 1090Mhz ADB-S

• this will only give an approximate tuning as environmental factors will also come into play
• eg. length of total dipole in m = 143 / frequency in MHz

• see vector network analyzers (VNAs)

• see https://www.youtube.com/watch?v=eYN7dhdt1Dw

• software can automatically control antenna rotators to keep it optimised for a satellite
• examples:
  • AntRunner Satellite Tracking System
    • 12V
    • https://www.aliexpress.com/p/tesla-landing/index.html ~$AU560

• a RF choke can be used to block unwanted radio frequency currents from traveling along a coaxial cable, which can interfere with the intended signal or cause other issues.
• it acts as a high-impedance barrier at radio frequencies, preventing RF energy from flowing back into the radio or other devices connected to the cable.
• this helps to improve performance, reduce interference, improve the standing wave ratio (SWR) of an antenna system, and protect equipment from damage
• they are primarily used to suppress common mode currents that can flow on the outer shield of a coaxial cable
• these currents can be induced by nearby noise sources or by the antenna itself, effectively turning the coax shield into an unwanted part of the antenna system
• RF chokes are typically placed at the antenna end of the coax, and sometimes near the radio as well
• types of RF chokes:
  • ferrite beads: these are often placed around the coax cable to create a high impedance at RF frequencies
  • toroids: coax can be wound around a toroid core, creating an inductor that acts as a choke
  • air chokes: In some cases, coax can be wound into a coil without a core, creating an air choke

• when a receiver antenna tuned to the frequency of a nearby transmitter receives the radio waves, it converts it into electricity
• the efficiency of doing so is generally 70-90%
• for RF energy harvesting systems using rectenna (antennas designed to convert received RF signals into DC power for powering low-energy devices), RF-to-DC conversion efficiencies up to about 40-65% have been experimentally demonstrated at certain frequencies and input power densities, the actual usable electrical energy generated by a receiving antenna depends heavily on the receiving electronics following the antenna, such as impedance matching networks and rectifiers that convert RF to DC
• at long distances the power is further reduced by free-space path losses
• at 1km from a 7kW AM radio transmitter antenna broadcasting on 630kHz and using a tuned 30cm loop antenna with 5 wire loops whilst you could achieve 15V of DC harvesting for charging a 12V battery, the power after all the efficiency issues are taken into account is likely to be only 1mW so NOT really an option whilst camping off-grid
  • one would need a much more efficient directional antenna to improve this, and things get worse the further you go from the antenna

the power received by the antenna = transmitter power x transmitter antenna gain x receiving antenna effective area / 4πd²

receiving antenna effective area = λ² x receiver antenna gain / 4π where λ = wavelength in m

antenna gain for these equations is linear gain = 10^{(dBi gain /10)}