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Old 26th Jan 2017, 8:38 pm   #1
SiriusHardware
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Default All about CB radio

To avoid hijacking Damo666's 'CB Wanted' thread any further, I thought I'd open this thread for general discussion about CB radio, with the emphasis mainly on vintage CB. For UK purposes this mostly means the late 1970s and 1980s - probably the time when most people with an interest in CB here were most active, although a few of us continue to use CB either sporadically (like me) or even regularly.

One thing I would be interested to know is - whose brainchild was the unique CB/27/81 set of UK 27Mhz frequencies?

I can guess why they chose the strange one-and-a-bit KHz end-of-frequency offset (all frequencies ending in ...0125, as in 27.60125MHz) - probably so that the harmonics did not end up on 'round number' frequencies higher up in the VHF low, Airband and VHF high bands.

The UK and Europe-wide PMR446 channels also have a similar channel frequency offset, probably for the same reason.

But which UK agency or individual actually chose or defined the original CB/27/81 frequencies? Does anyone know?

It wasn't only the end-of frequency offset which caused initial headaches for would-be manufacturers of radios for the UK market - all previous CB channel allocations had copied the original USA channel allocation scheme which (a) was interleaved with existing 27Mhz radio control channels, leading the CB channel scheme to have occasional 20Khz steps to jump over the RC frequencies, and (b) originally only had 23 channels and then was expanded to 40, with the new channels 24 and 25 added in between the existing channels 22 and 23.

This frequency cha-cha was hard-coded into many of the existing binary coded rotary channel switches which meant they couldn't be directly used to to generate the UK's new neatly ascending, consecutive 10Khz spaced channels. Later on, appropriately programmed EPROMS started to be used to unscramble the exotic order of channels produced by FCC pattern channel switches into a more orderly ascending sequence of 40 channel codes.

I have a diagram here for the 'Commtron Nato CXX', an early radio with the UK CB/27/81 channels on it: It uses the LC7120 PLL synthesiser which is normally dedicated to generating the FCC CB channels, but combines that with a bank of no less than eight loop mixer crystals which are switched in and out at need in order to produce all the correct frequencies for the radio's channels. As far as I know this was a uniquely complicated scheme, never repeated by any other manufacturer. The main chassis was a Korean-made Maxon chassis similar to the one used in all the blue-cased Midland 'Precision Series' radios, the Midland 100 / 150 and the original Cobra 19X.

Not long after that came the UK successor to / version of the LC7120, the LC7136/LC7137 which was a dedicated PLL for generating the UK's unique 40 channels. After that, UK CB PLL circuits became a lot simpler, albeit far less interesting.
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Old 26th Jan 2017, 9:15 pm   #2
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Default Re: All about CB radio

Having been around in the days when CB27/81 first went live, with a "Lowe TX40" transceiver (which used the LC7137 chip) it was generally believed that the odd UK channelisation was adopted to prevent interworking with the various illegal-import radios that were popular at the time.

The later Icom ICB1050 used a Motorola chip synth that could easily be pulled to work on the upper end of the 10M amateur band by adding a single resistor (a couple of them if you wanted repeater-shift).
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Old 26th Jan 2017, 9:39 pm   #3
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Default Re: All about CB radio

It was all a bit of a sop to the illegal 27MHz AM users giving then something about the same. In my opinion it should have been on low band VHF, easy to make, smaller better aerials and using a bit of (by then) redundant VHF TV service. It is all a bit academic now given the proliferation of mobile 'phones.
 
Old 26th Jan 2017, 9:53 pm   #4
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Default Re: All about CB radio

There was talk at one time (pre UK CB27/81) of an allocation around the dead-end 0f the TV 41-68MHz band for UK CB. The Italians had an allocation there: https://en.wikipedia.org/wiki/Person..._service#Italy and I remember in the very-early-80s seeing ads in places like "Exchange & Mart" for such radios offered very much on a "cash in hand no questions asked" basis.

I gather the UK Military objected to CB in this area - though they were happy for us army-cadets to use "88-sets" in the TV band!

Somewhere in the attic I've probabloy still got a copy of the original late-1970s UK "Green Paper" relating to options for what was then being called 'Open Channel' by the Government of the day.
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Old 26th Jan 2017, 10:37 pm   #5
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Default Re: All about CB radio

And don't forget the 934MHz band. I actually came across some 934MHz gear a few years back in a load of stuff someone asked me to identify. First I had ever seen. There wasn't a lot of it round back when CB was made legal.
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Old 26th Jan 2017, 10:53 pm   #6
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Default Re: All about CB radio

Ha, yes, amusingly, having finally grudgingly accepted that CB was basically unstoppable, they still tried to rename it so they wouldn't be seen to have given in and given us CB.

The original proposed 'Open Channel' UK CB frequency allocation was on what I would call very high UHF, and eventually became the 934Mhz CB band for which a limited amount of equipment was introduced and used by CB 'power users' - well heeled people who could afford to pay the several hundred pounds for something which could operate at such exotic frequencies, and used it to escape from the rest of us.

934Mhz CB coexisted alongside the 80 27Mhz channels for quite a few years until it was finally withdrawn and the frequencies reallocated to... analogue mobile phones? It's a shame we don't still have that high UHF CB allocation now - you'd have very short, unobtrusive aerials at that frequency and large multi-element beams would be perfectly practical. Even on 934Mhz, beam antennae were allowed. Thanks to mobile phones, RF PA modules which work at that frequency are widespread and cheap now, whereas before they were practically witchcraft, which made the equipment very expensive.

VHF would have been the best compromise band, and indeed it could be observed that the amateur 2m band has become the VHF CB system we never got. Certainly it represents a good model of what VHF CB might have been like. I think a four-metre-like low VHF allocation would have been even better, specifically with mobile operation in mind.

The eventual choice of an alternative 27Mhz allocation was good in one respect - everyone could re-use their existing auxiliary equipment like aerials, SWR meters and so on. The frequency shift (upwards) was in the right direction so that existing aerials only needed to be shortened slightly by lopping a bit off, whereas if the shift had been down to 26Mhz, adjustment of aerials to make them longer might not have been practical in many cases.

And actually, with hindsight I think we have the best of both worlds with the CEPT and CB/27/81 bands available to us- CEPT is good if you actively enjoy working skip during the summer months, especially now that we can legally buy and use SSB-equipped radios for that band, and the UK channels, being unique to the UK, are much less troubled by unwanted interference from overseas when the conditions are up - we get the occasional overseas SSB user coming up that high but they mostly concentrate in the gap between the CEPT channels and the UK channels.
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Old 26th Jan 2017, 11:09 pm   #7
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Default Re: All about CB radio

With regard to VHF Low Band, I own and use 4 Metre and 2 Metre equipment (usually just on local nets with friends) and we have compared propagation on occasion. 2 Metres seems to perform the best. Of course this could simply be down to antenna efficiency. I remember cars with massive CB aerials waving about on the roof. Just the thing to attract unwanted attention from the "authorities" when CB was illegal. I never heard of anyone being prosecuted though, considering the amount of illegal users around.
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Old 26th Jan 2017, 11:35 pm   #8
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I never heard of anyone being prosecuted though, considering the amount of illegal users around.
I had (still have, actually), a scrapbook full of cuttings of 'Exchange and Mart' type magazine adverts for CB equipment, letters which people wrote to newspapers about CB and newspaper articles - usually about someone having been prosecuted for using (then) illegal CB. One such describes how an individual who owned a local electronics business was intercepted, stopped and his equipment seized as he crossed the Tyne bridge. So it did happen.

I've also still got a whole series of consecutive issues of 'CB Radio' magazine, 'Citizen's Band' magazine and a one-off special from the publishers of Hobby Electronics entitled 'Citizens Banned', which was all about the UK CB scene probably from around 1979. It wasn't remotely unbiased or objective - it was completely pro-CB, despite it being illegal at the time.

As regards 4m vs. 2m, if you take away the fact that you have repeaters on 2m and consider only mobile simplex operation then I find that mobile 'flutter' is much less noticeable on 4m than on 2m (and considerably worse on 70cms). I suppose that's one reason why VHF low has always been a popular taxi band. It is true, though, that you can get a decent high gain 2m mobile aerial the same length as a basic 4m quarter wave whip, so there is that trade-off.

I knew a couple of people who drove around with unloaded full length (~9ft) quarter wave CB aerials on their cars. Car body panels were -so- much thicker in those days...
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Old 26th Jan 2017, 11:36 pm   #9
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Default Re: All about CB radio

The boom in CB radio coincided with the most favourable point in the sunspot cycle for propagation over long distances. I well remember CBers sitting in their cars at local high spots and there were several 'twigs' (some more like trees) in my street. The other attraction in the illegal days was its subversive nature. It was a bit like the excitement of offshore pirate radio. Getting long distances on 27MHz was far more fun than talking talking to someone two streets away on 934MHz using a licensed set.

My next door neighbour of the time, who knew I was a licensed amateur, proudly boasted that he'd got a rig. The situation had some advantages as he was less inclined to complain about my aerials. I did try to rattle him by saying that I had the capability to run full legal limit on the adjacent amateur band so he'd better watch out when I was on as his front end might get fried! Actually, though I was active on 15 and 20m I never went on 10m.
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Old 27th Jan 2017, 12:01 am   #10
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Default Re: All about CB radio

Yes I remember CB as Childrens Band

Myself and a colleague were trying to set up some equipment using legal FM CB sets around 1986 in Torbay but we were using formal radio protocol.

It really upset the local CB'ers who started to harass us so we set up a sequence of channels once we were discovered to keep them on the hop

It annoyed the hell out of them but we were always one step ahead.

It went on for Months but they were never able to block more than a dozen channels at a time.

It didn't cause us any inconvenience at all

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Old 27th Jan 2017, 12:14 am   #11
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Default Re: All about CB radio

Quote:
Yes I remember CB as Childrens Band
I can remember when it went legal and by that Christmas/New Year there were loads of kids on the band. I think you had to be 14 but many were much younger.

There would be several doing the same routine at the same time where they would ask 14 for a copy, then move channels and then it would be the same suite of questions:
What's your handle? What's your 20, How many candles are you burning? What's your rig? what's your twig? How many pounds am I pushing you? Then back to channel 14 for the same routine to be repeated. It could get quite annoying.

However, one reason I'm rarely active on the amateur HF bands (don't think I've used them for many years) is that radio hams are often MUCH worse than the 12 year old kids on CB. If you compare the above contact with a typical radio ham contact it contains a lot more information and probably lasted a lot longer that a typical ham contact on HF. Often a radio ham can get through several contacts in less than 1 minute and this isn't including contest contacts. Why do they do it?
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Old 27th Jan 2017, 1:43 am   #12
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And don't forget the 934MHz band.
When I briefly lived in the Midlands there was a CB shop in Hinckley Leics that used to stock Reftec 934MHz radios in the early to mid 1980s.

They often had at least one of them running on display in the shop but the only thing I ever heard when I visited the shop was the odd squelch pop. I can't remember the name of the shop but I think it began with an E and it was on the main road into Hinckley and they used to advertise as a 934MHz specialist dealer.
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Old 27th Jan 2017, 2:04 am   #13
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I do regret not having at least tried out 934Mhz.

At the height of my time using CB a little clique of us all used to hang out together on-air and socially - as it happened all of us had day jobs as electronics technicians with different employers and we repaired quite a few radios for locals between us. I can remember one of the other lads saying to me, "you know, if you bought (a 934Mhz set) I'd have to get one as well".

But I declined.

One of the nice things about 27Mhz was that we had built up a decent collection of test gear (frequency meters, scopes, synthesised RF signal generator, spectrum analyser and so on) all of which which was good enough for working on HF gear but couldn't get anywhere close to 900Mhz. I didn't want to own anything that I couldn't work on or service myself. That's the main reason I never bought 934Mhz gear.
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Old 27th Jan 2017, 2:29 am   #14
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Gosh, I didn't have a spectrum analyser back then. I had access to them in an RF lab from about 1985 onwards but I can remember a G8 radio ham who had one in the early 1980s and it was the equivalent of having a classic Ferrari in your garage back then.

From about 1982 I had a Tek 585 scope, a Marconi TF2015 sig gen and a Marconi 801B sig gen and a Marconi TF2430 counter. I had a (very good) homemade two tone generator based on a design in the ARRL handbook plus various meters etc.

I can remember there was another CB shop in Hinckley around 1981/2 and I think it was on Trinity Lane. It looks like it is a 'soapsuds' launderette nowadays. I think this is the right place, there is even an old antenna bracket on the side of the building. There was another one in Bedworth on Marston Lane/Orchard Street which is now a plumbing centre. I was still a spotty teenager when CB became legal and I can remember going to these CB shops quite a few times.
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Old 27th Jan 2017, 7:09 am   #15
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Default Re: All about CB radio

Sadly the odd frequency offset of 27/81 did cause problems in London. The old Police
Car radio repeater system (Car-receive 99MHz, transmit 81MHz, base station reverse)
was capable of being blocked on at least 2 of the CB channels. The base station
transmitters were carrier accessed.
Some anti social types even mistuned the CB power amp to maximise the 3rd harmonic.
This would never have happened if the FCC channels had been adopted.
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Old 27th Jan 2017, 6:18 pm   #16
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Actually, I don't think the 3rd harmonic thing was normally intentional. The best equipment most people had for tuning up a transmitter at the time was a power meter and (hopefully) a dummy load. What some of them didn't realise was that a power meter just adds up all of the output coming out of a transmitter no matter which frequency it is on, so when they were tuning for maximum indicated output they were, in many cases, increasing the amount of harmonic energy rather than increasing output on the wanted frequency. Sometimes, one of those tunable circuits in the transmitter would be a notch filter meant to be used for -minimising- the harmonics.

Jeremy, the spectrum analyser technically wasn't ours and belonged to one of our employers, but I think we used it a lot more than they did. Unfortunately, they did eventually reclaim it.

When I got into amateur radio I bought an add-on box by Thandar which effectively made any X-Y capable scope into a crude spectrum analyser. It basically just gave indication only and relative strength, but it went up to about 1GHz, so it was good for tuning up ex-PMR radio transmitters on 2m, while at the same time keeping an eye on the harmonic output. I've still got it, actually.
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Old 27th Jan 2017, 6:38 pm   #17
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Originally Posted by SiriusHardware View Post
Actually, I don't think the 3rd harmonic thing was normally intentional. The best equipment most people had for tuning up a transmitter at the time was a power meter and (hopefully) a dummy load. What some of them didn't realise was that a power meter just adds up all of the output coming out of a transmitter no matter which frequency it is on, so when they were tuning for maximum indicated output they were, in many cases, increasing the amount of harmonic energy rather than increasing output on the wanted frequency. Sometimes, one of those tunable circuits in the transmitter would be a notch filter meant to be used for -minimising- the harmonics.
True... likewise many amateurs who in the 1970s bought "Liner-2" 2-metre SSB transceivers and then went inside to tweak the PA trimmers for "maximum smoke" found that they'd got it horribly wrong.

A lot of simple "Power/SWR" meters have sensitivity which rises rapidly with increasing frequency!
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Old 27th Jan 2017, 6:43 pm   #18
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Happy days... My first experience of CB radio was when I was a teenager sat in the back of an old Triumph car in the late 70s. I wasn't old enough to drive but one of my mates had an older brother who had a car and an AM CB radio. We would often drive around and it was quite exciting to be driving around (with no grownups) and the CB radio was a lot of fun.

I bought my first 40 channel AM CB a few years later and it was a classic Sharp CB2460 'little window' model. My first 'new' CB radio was a UKFM Fidelity 2001 and these were on clearance sale for just £15 not long after the initial surge of CB sales in early 1982.

However, my first experience of UKFM CB was in late 1981 and I think it was just before it was legal. A mate managed to buy one of the little Cybernet 1000 radios and he wasn't allowed to use it in the house until it was legal and he had a licence. But his mum worked late shifts so in the evenings we would hook a wire antenna up through the loft hatch and all sit around on the stairs taking turns to use the radio.
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Old 27th Jan 2017, 6:44 pm   #19
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CB in the eighties was the equivalent to having a mobile phone nowadays I suppose. If you wanted to keep in touch with your mates, particularly on the move, it was very useful. The third harmonic interference thing used to be prevalent when I worked on VHF low band comms around the time. People with large power amps fitted in cars used to go and sit under the masts (always on top of nice big hills) and transmit a lovely big third harmonic of 27MHz straight into our main receivers. Even worse if it was a talk-through system and it was re broadcast all over the county. When everything moved up to high band things improved but by then the CB thing was over. It's true the sunspot maximum in the early eighties fueled interest in illegal AM CB. I still remember the band being absolutely alive with DX, even with a relatively small antenna.
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Old 27th Jan 2017, 7:20 pm   #20
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Default Re: All about CB radio

What a great thread.

Back in the late 80s/early 90s, I vividly remember that Car alarms & house alarms were regularly set off when using high power burners (amplifiers) - also, audio could often be heard over garage forecourt PA speakers if the mic' was modulated.

My favourites CB's back in the day were the Harvard 420 & York 863 which shared the same cybernet board, and I also had a soft spot for the Amstrad 901 and Harvard 402 which also shared the same circuitry.

I'm looking forward to giving the CB another go, if only for nostalgic reasons.

Happy days!
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