This is the first of a two part article covering what is meant by “Feedlines” and why they are used.

Feedlines come in many shapes and sizes, often dependant on the frequency of the signal to be carried.

For carrying microwave signals (frequencies above 1000MHz or 1GHz are generally considered to be microwave signals), it is common to find that rectangular shaped box section, known as “waveguide” is used. The dimensions of the waveguide determine which frequencies it is designed for, and deviation from this frequency will often result in degradation or loss of the signal in the waveguide.

For signals in the lower end of the microwave spectrum, it is common to find “flexible waveguide” used. This is round in shape and can be best described as copper/brass pipe with a PVC weather shield surrounding it. Termination of this at each end is by special adapters who function is not dissimilar to antennas in free space.

Around the 1GHz area, and for lower power applications, possibly up to about 1400MHz, it is more common to find Coaxial Cable used to convey signals from transmitter/receiver to antenna. Coaxial cable generally consists of a copper centre conductor (of varying diameter dependant on frequency and power levels used) encased in a PVC/foam insulator with a copper braid and or foil surround shielding it. Normally, the outside of the coaxial cable is of a UV resistant PVC to keep the weather out.

When using VHF/UHF (30-300MHz/300-1000MHz), coaxial cable is the norm. Coaxial cable comes in many varieties. There are generally 2 impedances of coaxial cable (“coax”) available, 50ohm and 75ohm. The type to use in a situation is defined by the impedance that the output of the transmitter is designed for, and a suitably designed antenna. There are many different types of coax connectors available which are used for terminating the cable to the transmitter or antenna.

Some of the more common ones are:

PL259/SO239 – Generally used up to 150MHz (Losses increase rapidly with increasing frequency).

“N” Type – similar size to the PL259, generally used up to 1500MHz, more complicated to fit than PL series.

BNC – Bayonet fitting version of the “N” type, similar characteristics but difficult to find in a low-loss coax size. Very popular for audio and instrumentation connecting leads also.

“Belling-Lee” – is the standard “tv coax plug” in the UK. 75ohms impedance, and used for Band II (88-108MHz) radio receivers as well as Band IV/V television reception systems. Generally lossy above 100MHz, but historically easy to use.

For RF in the HF (3-30MHz), there is often a mix of coaxial cable and “openwire” balanced feeders. For lower power installations, coaxial is most popular, however, there are a lot of higher power transmitters on the HF section of the spectrum. Most of these are broadcast stations with multi Kilowatt output transmitters, and the large antenna arrays these stations posses are almost always fed with open wire feeder.

In Part 2 I will go deeper into Open Wire feeder and why its used in Low Frequency (LF) applications.

Till then….

Alan

As seen previously, there are many different ways of transmitting information via voice on a radio transmission. This article is concerned with the more complex component of an Amplitude Modulated transmitter, called SSB or single sideband. This mode is most commonly used for HF communications and is very popular with radio amateurs wishing to talk long distances (DX)

SSB (Single Sideband)

Single Sideband was developed in the 1920’s, following a patent registered in 1915, but didn’t really make an impression on the amateur communications arena until the 1950’s. This was mainly due to the level of technology catching up with the requirements to implement the mode.

Single Sideband is 1/3 of a modulated AM signal envelope. When audio is applied to a radio transmission, there are 3 components to the resultant signal. These are: the carrier, which contains no information but is the reference for the signal, and the upper and lower sidebands, which contain the same information as each other.

So, if we were to remove the carrier (which contains no information) and one sideband (seeing as the sidebands are identical), we could have a transmitter that uses considerably less power to produce the same output power, or for the same input power, could have up to 3 times the final transmitted output power.

This is a very simplistic view on SSB. In reality, it is relatively complex to create a single sideband transmission. There are two reasonably common methods of creating an SSB signal, with this first method being the most common. The simplest method is to feed the audio signal and the oscillator into a “balanced modulator” (enhanced mixer) which is configured to remove the carrier part of the signal. The resultant “double sideband” signal is then fed through a crystal filter with a very narrow (approx 2.4Khz @ 6db) bandwidth and very steep skirt response to remove the unwanted sideband.

Generally (although not exclusively) there is only 1 filter, and the frequency of the generated signal is shifted up or down to allow the filter to remove the unwanted sideband. Certain high grade transmitter manufacturers have included 2 filters, but these transmitters often have the facility to switch between variations of the SSB mode, such as: SSB with no carrier (the most common), DSB (double side band) with no carrier (quite popular on the HF Amateur Radio bands, SSB with reduced carrier (used by some HF stations to allow easier reception by having a percentage of carrier in the signal as reference) and DSB with reduced carrier.

The second, and possibly less common method of SSB generation is to use 90degree phase separation modulators. These use a network of amplifiers and capacitive/resistive filters to alter the phases of the audio information component and the generated carrier until the carrier and the unwanted sideband have cancelled each other out and the only component left is the wanted sideband. In years gone by, this method wasn’t popular due to the high component count, and the inability to get cheap close tolerance capacitors to create the filtering networks. However, with the advancement of digital signal processing (DSP) electronics, this method is finding more popularity due to the reduced cost of implementation. Common forms of this type of modulator are the Hartley and Weaver modulators.

Reception and recovery is also more difficult than for an AM signal. Because there is no carrier for the receiver to reference against to recover the audio, a carrier insertion method has to be employed. This is usually called a BFO or Beat Frequency Oscillator, and inserts the carrier normally at the second IF (intermediate frequency) in a super-heterodyne receiver, which is often at 455khz. On most communication receivers, there is a control known as “clarifier” or “fine tune”. This moves the received signal around the 455khz BFO and allows the detector to “de-modulate” the audio by making it more intelligible. After the carrier reinsertion, the detection process follows the same path as the AM detection. This method is known a synchronous detection and is highly reliant on oscillator stability.

There are other methods to de-modulate an SSB signal. One way used by high grade communications receivers is the envelope detection method. As above, this method is now starting to become more popular following the advancement of DSP