This is the second part of the FeedLines article, and deals with lower freqencies and the use of open wire feeder.

The impedance for openwire feeder varies with construction, but is generally in the order of 450ohms. Openwire can be manufactured in the hobby environment quite easily by stretching out 2 lengths of wire (hard drawn single strand copper is the best, but any will do) parallel to each other, then placing spreaders between them at regular intervals.

The distance between the wires is dependant on the output power of the transmitter used, the higher the power, the wider the spacing. This is to prevent “flash over” on high voltage peaks on the line.

For lower output transmitters, 300ohm ribbon cable (often sold with Band II FM tuners as the antenna) is also usable. If using this, it should be noted that the output power should not be allowed to exceed 150watts, or flashover in the clear dielectric between the conductors could occur. This feeder is not recommended without some form of treatment for outdoor use, as water will collect on the dielectric when it rains increasing the risk of flashover.

Another variation on this theme is the “slotted ribbon line”. This is a “half way house” between 300ohm ribbon cable and full open wire feeder. Its impedance can be anywhere between 300 – 450 ohms, and due to its construction, is easier to deal with than full openwire feeder. The slots in the dielectric mean that it is not as susceptible to the effects of dampas ribbon feeder, however, the dielectric between conductors can harden an snap allowing the conductors to close up. This causes an impedance hump in the line and in extreme circumstances, flashover on transmitter voltage peaks. If this feeder is to be used, then it must be inspected regularly to ensure that it is in good mechanical order.

For any of the above open wire type of feeders to be used, it is imperative that matching units are used to match the impedance of the transmitter to the feeder. Most modern low power transmitters are in the order of 50ohms impedance, so direct connection to a frequency resonant dipole via openwire feeder will immediately result in an SWR of 6:1 or greater, which some valve PA’s will tolerate for a modest period of time, but can be considered to be suicide for a transistor PA. The design and use of “baluns” (balanced to unbalanced) and ATU’s (Antenna Tuning Units) is beyond the scope of this article, but both are required for the above.

Finally, for signals in the VLF (9 – 30Khz), LF(30 -300Khz) and MF (300KHz -3Mhz), any of the above systems used for HF signals can be used. The constraints about the construction of openwire feeder are even more important, due to the inability to erect efficient antenna systems (a half wave dipole or doublet antenna at 198Khz (BBC Radio 4 Long Wave frequency) is 758m long or nearly 1/2mile!!!) means that the transmitters are usually far higher output power. Also, the matching units for such antennas are substantially larger, again to reduce the possibilities of flash over during transmitter voltage peaks.

This concludes my brief look at feedlines. There is loads of information available both in books and on the internet regarding this subject. Although not the most exciting aspect of radio work, get this part wrong and you will lose that important output power, and in a transciever envirionment, this  loss is also there on recieve. So, that really large 17element 2metre beam you have just erected doesn’t work as well as joe’s 4 element quad down the road?? Did you use the right feeder and install it properly…..? Think on…!!

See you soon,

Alan.

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

Standing Wave Ratio (or to be more correct, VSWR – Voltage Standing Wave Ratio), is the measured ratio of transmitted power travelling up the feed line from a transmitter to the antenna, as opposed to the quantity of reflected power returned down the feed line because the antenna is not at resonance.

SWR is measured with an SWR meter. An SWR meter is essentially a Wheatstone Bridge device with the meter in one side and the detection line in the opposite side. This allows power readings to be taken for power in both directions on the detection line.

An SWR meter is used with coaxial feed line and often consists of 2 antenna sockets on the rear (usually marked up as “TX” and “ANT”) for connection to the transmitter and the antenna to be checked. On the front it is usual to find a sensitivity adjustment in the form of a variable resistor, a switch to switch between “Forward” (transmitter to antenna) and “Reflected” (antenna to transmitter) power, and last but not least a moving coil meter to display the readings.

To use the meter, the antenna is plugged into the rear socket marked ANT, and the transmitter is plugged into the socket marked “TX”. The SWR measurements should always be initially taken using the minimum output power of the transmitter that will allow the unit to show full scale on the moving coil meter.

Firstly, switch the meter to “FWD” position on the front and key the transmitter. With the sensitivity control, ensure that the moving coil meter reads full scale. Then un-key the transmitter, switch the unit to “REV” and key the transmitter again.

In an ideal world, the meter should read no reflected power, but it is more common to find that there is some level of reflected power displayed. If the reading is less than 1.5, then it is reasonably safe to use the transmitter for longer periods on that frequency with out the likelihood of damage occurring to the output stages. If the reading is around 2, then approximately 10% of the transmitted power is being returned to the transmitter and causing heating in the output stages. This heating will eventually lead to permanent damage, and possibly an expensive repair bill, so it is wise to look further into the reasons why this is happening.

If the reading is 3 or above, stop transmitting immediately!! A reading of 3 or more means that 25% (or more) of the transmitter power is being returned and WILL cause damage to the output stages. A reading this high is often indicative of either a short-circuit or an open circuit on the feed line.

Go back and check the route of the installed coax, the connection to the bottom of the antenna and the installation of the coax plug where it goes into the SWR Meter. Once the culprit of the high reading has been established and cured, the above procedure can be repeated.