Now that we have covered transmitters and some basic signal types, we are ready to discuss actually getting those signals on the air. This article will cover feeders and antennas which will be common to the average home user rather than commercial, high power, transmitters. Many of the concepts will be transferable but the technology of high power transmitters will be different. We will also stick to signals under 10GHz as its around here that the technology begins to change to waveguides. Technically, it is possible to push signals of 10-40GHz through cables, but this is very specialised.
Feeders
A feeder is one or more electrical cables which take a signal from your equipment to an antenna. We will look at two types of feeder cable, one very common and one not so common.
According to Wikipedia, ‘Coaxial cable, or coax , is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric (insulating material)’. Coax is one of the most common cable types found on domestic systems and is pictured below.
Coax has the following properties:
- Inner conductor carries the signal
- Outer braided (screen) keeps the signal within the cable and prevents external signals from entering (can be leaky)
- “Unbalanced” – meaning there is only one signal carried
An older type of cable is the Twin Feed. Once common in older CRT aerials, it can be hard to find these days given how ubiquitous coax is. Twin Feed looks like this:
Twin Feed cables have the following properties:
- This is known as “balanced” – Each of the two wires carries the signal to the antenna, with the currents flowing in opposite directions, balancing each other for efficient feeding to a balanced antenna (such as a dipole)
- There is a constant separation between the two wires
- There is no screen. The properties of twin-lead feeder mean that interfering signals are cancelled out
- It is harder to work with, as it needs to be kept away from metal to work effectively
Connectors
Feeders are typically terminated with some form of connector and numerous standards exist depending on the frequency range. We will look at some of the more common types used in domestic settings.
The PL-259 connector is mainly found on HF equipment (3MHz – 30MHz), although technically it can support the frequency range 0-100MHz.
Next up is the N connector. The N connector can carry the frequency range 0-18GHz. Its commonly used in VHF and UHF communications.
The BNC connector is very common, particularly on handheld HF radios. Capable of supporting frequencies in the range 0-4GHz. There are two basic varieties, 50 Ohm and 75 Ohm. With average domestic radios the 50 Ohm version is used.
Finally, there is the very popular SMA connector. Often found on VHF and UHF handheld radios, as well as SDRs. The connector supports the frequency range 0-26.5GHz.
Antennas
An antenna is any object where charged particles can move freely in response to a driving signal. In domestic usage, this will be a length of conductive metal. There are other possibilities, such as Plasmas, but we will focus on common domestic antennas used with HF, VHF and UHF devices.
An optimal antenna will be the same size as the wavelength being transmitted/received. There are a few caveats here around energy levels but we’d need to discuss quantum mechanics and that is too advanced for now. Antennas of 1/2 or 1/4 the wavelength size will also function well.
The reason why these fractions are allowed is that the actual electric and magnetic fields can satisfy suitable boundary conditions that allow effective radiation and reception, arising from the mathematical structure of the fields. Shorter antennas, such as 1/3 or 1/8 wavelength, correspond to neither the nodes nor the maxima of a sinusoid, and therefore will not convert energy efficiently. We can still use them, but we’ll need to compensate for the lower energy absorption/radiation.
The first antenna we will look at is the half wave dipole. This antenna is split into two parts and is, in total, half the wavelength being received/transmitted.
This type of antenna is described as “balanced”. Each portion of the antenna feeds separately into a unit at the “feed point” called a balun. A feed point is where the feeder connects to the antenna. This can be seen in the below photo.
The balun effectively merges the two signals into one signal which is sent across the feeder. The Balun acts as an interface that separates the antenna system from the feeder cable, so that the feeder doesn’t “radiate” or become part of the antenna system. Not using a Balun can result in interference issues, as without one, RF energy will radiate down the screen of the co-ax. Let’s have a quick look inside the balun and see this in action.
In the picture above, the two lengths of antenna would attached via the butterfly screws on either side. We can then see, on the bottom, the feed point where our feeder attaches to the balun.
The next antenna type we will discuss is the 1/4 wave ground plane. This type of antenna is 1/4 the wavelength of the receiving/transmitting signal. So for 10m (28MHz), the antenna would be 2.5m in height. This antenna is omni-directional, so signals spread out in all directions.
In the above picture of a 1/4 ground plane antenna, the vertical rod is the antenna. The other four rods are the ground plane. A ground plane serves as a mirror reflecting signals onto the antenna.
A related antenna is the 5/8 ground plane antenna. This VHF/UHF antenna employs a little electrical wizardry known as a loading coil to improve performance. This loading coil is a distinctive feature.
Another common antenna is the End Fed Antenna. This type of antenna is just a random length of wire. Likely to cause a lot of electromagnetic interference (EMI/RFI), this needs special ‘matching’ to work correctly. We will discuss matching later in this article.
Finally we come to the Yagi. The Yagi is unique in this group of antennas we have discussed, as it is the only one that is directional. This directionality is known as ‘gain’ and we will discuss that later in this article. Most people will be familiar with a Yagi antenna, as this is often used with TV. Also, most should be familiar with its directional quality in the sense that it must be pointed at the local transmitter.
Polar Patterns
Polar patterns are diagrams which show how radio frequencies (RF) emits from an antenna. Let’s look at the pattern for half-wave dipole.
The axis in the above diagram represents the horizontal half-wave dipole and shows that the radiation is emitted at right angles. Now let’s look at the Yagi antenna.
In the above picture we can see that the majority of energy is released in one direction, we call this the main lobe. We can also observe that radiation is ‘leaking’ in other directions but to a smaller degree. These leakages are called sidelobes and backlobes. Side lobes and back lobes form as a result of not being able to confine radiation to one direction.
Antenna Gain and ERP
Antenna gain is the focusing of radiated power in one direction. This concentration of power is the same as increasing the output power of a transmitter which is using an omni-directional antenna. Gain is measured in Decibels (dB) and is used to calculate the Effective Radiated Power (ERP).
A directional antenna will have a gain listed in dB. The below table lists some common gain factors:
| Antenna Gain | Multiplies the power by a factor of |
| 3dB | x 2 |
| 6dB | x 4 |
| 9dB | x 8 |
| 10dB | x 10 |
To calculate the ERP we use the following formula:
ERP (watts) = power fed to the antenna x gain
Some examples using this formula:
| Antenna Gain | Gain (times) | Power into antenna | ERP |
| 3dB | x 2 | 10 watts | 20 watts |
| 6dB | x 4 | 10 watts | 40 watts |
| 9dB | x 8 | 10 watts | 80 watts |
| 10dB | x 10 | 10 watts | 100 watts |
Polarisation
Electromagnetic waves, such as radio waves, can be polarised. A radio wave is composed of one electric and one magnetic field that oscillates in a repeating pattern. Polarisation describes the way the electric field of the radio wave is oriented. The following video quickly demonstrates this.
Feed Point & Impedance
An antenna has an ‘impedance’ (measured in Ohms). The impedance of an antenna depends on the dimensions of the antenna, how it’s positioned and the wavelength of the applied signal. Antennas are designed to be used on specific frequencies, and the feed point impedance of the antenna should ideally match the impedance of the feeder and the transmitter.
As an example, transmitters tend to expect to see an impedance of 50 ohms. Coax feeder has an impedance of 50 ohms, so an antenna with a 50-ohm impedance at the feed point would be a perfect match.
As antenna impedance varies, matches are rarely perfect. But there is a way to handle this.
Antenna Matching & SWR
The impedance between the transmitter, feeder and antenna, all need to match. A mismatch will cause power to be reflected back into the transmitter, as standing waves, and can cause serious damage.
An SWR (Standing Wave Ratio) meter is used to test whether an antenna presents a correct ‘match’. The meter does this by measuring the flow of power received back from the antenna. These meters typically show the forward and reflected power.
A ratio of 1:1 is ideal – This means that all of the power from your transmitter is being radiated by the antenna, with none reflected back. As an example:
- An SWR of 2:1 would mean that over 10% of your power is reflected back to the transmitter.
- An SWR of 4:1 would mean that over 35% of your power is reflected back to the transmitter.
High SWR (over 2:1) can damage the transmitter, especially at high power.
If there is a mismatch (high SWR), then this is resolved using an AMU (Antenna Matching Unit). This is sometimes referred to as an ATU (Antenna Tuning Unit). Some antennas provide a mechanism to tune the antenna for a low SWR, but that does not help where the same antenna is being used multi-band. In this case, an AMU/ATU is required.
We have covered a lot of ground, once again. So, we will end it here for now. As homework, why not look at other connectors, antennas, their polar patterns and the frequencies they work on? Any questions on the series so far? Feel free to reach out in the comments below on each article in this series.
Further articles in this series:
Yet Again Another Hack 