Antennas and radiating systems
The antenna is the transducer between the cable and free space. It is also the component that pays off the most when it is well chosen and well installed: in a link, doubling the antenna gain is worth more than doubling the power, and it is much cheaper.
01The parameters that define an antenna
| Parameter | What it means |
|---|---|
| Gain | How much it concentrates energy in one direction, compared with a reference. In dBi relative to the ideal isotropic radiator and in dBd relative to a dipole: dBi = dBd + 2.15. |
| Radiation pattern | The “map” of where it radiates, in the horizontal and vertical planes. An antenna does not amplify: it distributes. |
| Beamwidth | The angle within which the power drops by less than 3 dB. More gain always means a narrower beam and more critical aiming. |
| Polarization | The orientation of the electric field: vertical, horizontal or circular. Two crossed antennas lose 20 to 30 dB. |
| Impedance | What the antenna “shows” to the line. The standard is 50 Ω (75 Ω in TV and video). |
| SWR | How well matched it is. 1:1 is perfect; above 2:1 it is worth checking. |
| Bandwidth | The range over which it keeps its SWR and its pattern. A simple dipole is narrow; a log-periodic covers decades. |
| Front-to-back ratio | How much less it radiates backward. It matters for rejecting interference. |
An antenna behaves exactly the same transmitting as receiving: the same gain, the same pattern, the same polarization. That is why everything studied for transmission holds equally for reception, and why an antenna that transmits poorly also receives poorly.
02Types and their patterns
| Antenna | Gain | Impedance | When it is used |
|---|---|---|---|
| Isotropic | 0 dBi | — | It does not exist: it is only the theoretical reference. |
| λ/2 dipole | 2.15 dBi | ≈ 73 Ω | The basic antenna. It radiates all around the circle perpendicular to its axis. |
| λ/4 monopole with ground plane | 2 to 5 dBi | ≈ 36 Ω | Mobile units, handheld radios, car roofs. Half of the dipole, with the ground plane acting as a mirror. |
| Yagi | 7 to 15 dBi | 50 Ω with a matching device | The classic directional antenna: one driven element, one reflector and several directors. TV, point-to-point links. |
| Log-periodic | 6 to 10 dBi | 50 Ω | When a very wide frequency range has to be covered with a single antenna. |
| Panel | 12 to 20 dBi | 50 Ω | Cellular and WiFi: covers one sector; they are installed in threes for 360°. |
| Parabolic dish | 20 to 50 dBi | 50 Ω | Microwave and satellite links. Enormous gain, a beam of a few degrees. |
- Dipole for 100 MHz (FM band): 143/100 = 1.43 m, that is, 71.5 cm per arm.
- Dipole for 2400 MHz (WiFi): 143/2400 = 5.96 cm. That is why a router's antennas are so small.
- λ/4 monopole for 433 MHz: 71.5/433 = 16.5 cm. It is the little piece of wire on radio modules, and cutting it “to fit in the box” ruins the range.
- 1.2 m dish at 10 GHz (λ = 3 cm): G = 10·log(0.55 × (π × 1.2/0.03)²) = 10·log(0.55 × 15,791) = 39.4 dBi. With a beam of barely 1.8°: you have to aim seriously.
03The transmission line
The cable between the equipment and the antenna is not an ordinary cable: it is a transmission line with its characteristic impedance, and everything that is not matched comes back.
| SWR | Reflected power | Interpretation |
|---|---|---|
| 1.0 : 1 | 0 % | Perfect match. Theoretical ideal. |
| 1.5 : 1 | 4 % | Very good. This is what you look for in a real installation. |
| 2.0 : 1 | 11 % | Acceptable. The usual limit for calling an antenna good. |
| 3.0 : 1 | 25 % | Needs checking: connector, cable, antenna or wrong frequency. |
| ∞ | 100 % | Open circuit or short circuit: all the power returns to the transmitter. |
With the output open or shorted, all the power returns to the final stage and destroys it. Modern equipment has protection that reduces power at a high SWR, but not all of it does, and it does not always act in time. The rule is simple: antenna or dummy load, always, before transmitting.
| Cable | Z | Loss at 100 MHz | Loss at 2.4 GHz | Use |
|---|---|---|---|---|
| RG-58 | 50 Ω | ≈ 0.17 dB/m | ≈ 1.0 dB/m | Short runs, VHF. At 2.4 GHz, 10 m eat up 10 dB: unacceptable. |
| RG-213 | 50 Ω | ≈ 0.07 dB/m | ≈ 0.5 dB/m | Standard for HF and VHF. |
| LMR-400 | 50 Ω | ≈ 0.04 dB/m | ≈ 0.22 dB/m | UHF and microwave links. |
| RG-6 | 75 Ω | ≈ 0.07 dB/m | — | TV and video: a different impedance, not mixed with the 50 Ω ones. |
At 2.4 GHz, 20 meters of RG-58 mean 20 dB of loss: of 100 mW, 1 mW is left. And since the antenna is reciprocal, that loss is paid twice, in transmission and in reception. That is why at microwave frequencies the coax is kept as short as possible, low-loss cable is used, and often the equipment is placed at the top of the mast, next to the antenna, carrying data over Ethernet.
- A balun joins a coaxial cable —unbalanced— to a balanced antenna such as the dipole. Without it, the coax shield radiates and the pattern is distorted.
- The velocity factor tells how much slower the signal travels inside the cable than in vacuum: 0.66 in RG-58 with a solid dielectric, 0.85 in foam ones. It matters whenever a length of calculated size is cut, as in a quarter-wave matching section.
04Mounting and installation
- Height: it pays off the most, especially in VHF and UHF (see the radio horizon in analog communications).
- Clearance: nothing metallic or vegetation nearby, and a clear line of sight.
- Matching polarization with that of the other end.
- Aiming: with a 2° dish, being off by one degree already costs several dB.
- Never near power lines: the safety distance is calculated assuming that the mast falls.
- Grounding of the mast and a surge arrester on the line, before it enters the building.
- Sealed connectors with self-amalgamating tape: water inside the coax ruins it in weeks.
- Guy wires properly tensioned and anchored; work at height with a harness and with another person present.
- Never go in front of a dish or a panel with the transmitter on.
05In the lab
Calculate and build a half-wave dipole for the FM band or for 433 MHz, with a homemade balun. Measure the SWR with a meter or an antenna analyzer and adjust the length to the minimum. Note how the resonant frequency shifts as each arm is shortened: it is the way to understand the relationship between length and frequency.
With a fixed low-power transmitter and a receiver with a signal indicator, rotate the antenna under test in 15° steps and note the received level. Plot it in polar coordinates. Compare a dipole with a Yagi: you can see the difference between distributing and concentrating.
With two identical antennas, measure the received level with both vertical, both horizontal and one of each. The cross-polarization loss is 20 dB or more: the signal practically disappears.
Measure the received power with a short length of coax and with 20 m of the same cable. Calculate the loss in dB per meter and compare it with the manufacturer's specification. Repeat with two cables of different quality.
06Common mistakes
| Symptom | Usual cause |
|---|---|
| High SWR on a new antenna | Incorrect length, badly assembled connector, damaged cable or frequency outside the antenna's band. |
| The SWR improves with a long cable | Bad sign: the cable loss masks the reflection. A very bad cable “improves” the SWR and worsens the link. |
| The link works much worse than calculated | Crossed polarizations, or underestimated coax loss. |
| The antenna works on the ground and not on the mast | The environment changed: coupling with the mast, the guy wires or nearby metal structures. |
| The equipment burns out when transmitting | No antenna or no dummy load: all the power comes back. |
| The installation degrades after months | Water inside the coax from badly sealed connectors. |
| The equipment was damaged after a storm | No grounding and no surge arrester on the line. |
| The pigtail of a 433 MHz module was cut “to make it fit” | It no longer measures λ/4 and the antenna is mismatched: the range drops from hundreds of meters to a few. |
07Self-assessment
How long is a half-wave dipole for 145 MHz?
L = 143/145 = 0.99 m, almost a meter, with two arms of about 49 cm.
An antenna has 12 dBd of gain. How many dBi is that?
dBi = dBd + 2.15 = 14.15 dBi. You always have to pay attention to the reference: mixing them up overstates the gain by more than 2 dB.
What does it mean for an antenna to have gain if it does not amplify?
That it concentrates the energy in one direction at the expense of taking it from others. The total radiated power is the same; what changes is how it is distributed.
What is the relationship between gain and beamwidth?
They are inverse: more gain implies a narrower beam, and therefore more critical aiming and less tolerance to mast movements.
What happens if two antennas with crossed polarizations are connected?
20 to 30 dB are lost: the signal falls to one hundredth or less. It is one of the most expensive installation mistakes and one of the easiest to avoid.
With an SWR of 2:1, how much power comes back?
About 11 %. It is the usual limit for accepting an installation; above 3:1 you have to look for the problem.
Gain of a 60 cm dish at 12 GHz.
λ = 300/12,000 = 0.025 m. G = 10·log(0.55 × (π × 0.6/0.025)²) = 10·log(0.55 × 5685) = 35 dBi.
Why is the equipment put at the top of the mast at 2.4 GHz?
Because coax loses a great deal at that frequency —on the order of 1 dB per meter in ordinary cable— and that loss is paid in transmission and in reception. By raising the equipment, the coax is reduced to a short pigtail.
What is a balun for?
To join an unbalanced line —the coax— to a balanced antenna, such as the dipole. Without it, current flows on the outside of the shield, which radiates and distorts the pattern.
What basic precaution must be taken before transmitting?
Have the antenna or a dummy load connected. Transmitting with the output open or shorted sends all the power back to the final stage and destroys it.