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

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Filed under: "How To" Radio

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