cb 27/81 limits

I remember the days when CB radio for the UK came about, the powers to be chose Narrow Band FM “NBFM” for the UK introduction of CB radio.

There was much talk whether if “NBFM” would work at all, well the American CB radios had much to offer, multi-mode radios there where.

However, when things were launched onto the consumer, a 1 metre whip antenna was the limit, using a mag mount. However, as things turned out, many other antennas’ then appeared upon the market place, having found that the 1m whip antenna had its limits of distance.

Below the CAD model results of using a 50ohm whip antenna both ends, say between two vehicles, or even between two dwellings.

I remember a local shop retuning the radio’s RF PA stage with more powerful final stage transistors, business as it turned out was to be brisk.

Then RF power amplifiers appeared, termed as burners, used at night-time when the local radio interference regulator GPO man, was asleep in their beds.

Then one summer, some CB radio operators in Scotland decided to use 4 element 10m beam antennas. Now living at the near Swansea, on the Gower at the time, picking up the Scottish CB operators was a novel thing, but the powers to be put it down to the weather patterns for the year.

However, from the CAD propagation model, below, new version to post, good distance could be achieved with 4 watts using a 4 element beam both ends.

An antenna that become well used, was the quarter wave ground plane, which was effectively and half wave dipole, in essence.

This itself was useful antenna design, compact with the loading coils on both the vertical and ground plane elements.

The distance achieved was more respectable than just the whip antenna, able with more chance to chat north towards the boarder, or even wider a field.

Then the subject of a signal lift came into the CB radio language, termed skip or a sprog, or put another way, “ ‘E’ layer propagation “. Distance ranges improved thus so, even with just a 3dB signal lift.

It was never really announced why the UK CB radio tech was as so, but the licencing today is more liberal so, just as well, but even though, CB radio still deserves a 100 Watt radio transmitter to work correctly so.

While the EU CB radio operators seemingly use larger than just the 4 watts, things need to be levelled up for UK CB radio operators, hence the 100 watts RF output.

The below image shows the point, the CB radio using a full wave wire through a 9:1 balam.

It is perhaps worth noting, that most CB radio's do not have a tuned front-end circuit. This would mean that the front-end circuit does not have a band limiting bandwidth, which in essence means that the front-end circuit is wide open to all that the antenna would collect, cb radio signal and all everything else.

The wide open front-end would also not limit the static noise that the CB radio would pick up, which in effect the static noise bandwidth needs to be limited to the band plan in use.

Referring to the Uniden Export service manual, copyright date 1984, the Uniden President Grant CB radio ( export model ), component labelled as "L7", can be re-arranged to perhaps improve the cb radio receiver performance. As shown within the circuit diagram, the twin wound coil connected to the emitter of component "TR17", is the primary of the RF transformer. The antenna is connected to effectively the secondary of the RF transformer.

A trick here, is to swap around the RF transform connections.

Re-arrange to circuit so that : (1), the RF transformer primary coil is fed with the signal from the antenna signal at one end and the other end of the RF primary winding connected to the transistor emitter terminal together with a 100ohm resistor to the junction of "R75" and "C48". (2), the RF secondary circuit, is tuned as a resonant circuit, to the CB band plan centre frequency and in doing so left as an un-loaded but grounded secondary winding circuit at one end.

The arrangement that then have, is a tuned secondary circuit, which will peek the signal fed through the primary winding at the resonant frequency of the secondary winding. With the secondary winding only grounded at one end of the secondary winding, now a tuned resonant circuit, from a discovery made with an RF spectrum analyser, the signal fed through primary winding gets around a 2 or 3dB lift in signal amplitude, but only within the bandwidth of the tuned secondary winding resonance "Q" quality.

Two things have now occurred.

Firstly, the CB radio receiver now has a bandwidth limitation, thus to limit the static noise pickup, say maybe 3MHz, while secondly, giving the signal a pick up boost in signal strength.

The improvement of the CB radio is quiet different. On this image, a full wave wire is 475 ohms, improved program code.

Using a two wavelength wire antenna, an 20metre wire feed through a 9:1 followed by a 2:1 impedance transformer, matching to 50ohms radio, still with just 4 watts of cb radio transmitter power, here the example using FM signal.

Notice in both cases, the 3MHz front end bandwidth, and the RF bench test sensitivity has reduced from 0∙5uV to 0∙35uV. Notice also the range distances of contact possibility, just on 4Watts of TX power.

All in all, an interesting thought.