Noob question: Why is Band 1 so popular for linking

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Persona Non Grata
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Noob question: Why is Band 1 so popular for linking

Post by Persona Non Grata » Thu Sep 16, 2021 8:55 pm

I would have thought Band 3 would have been a better option for several reasons:
  • Less prone to sporodic E interference.
  • Directional aerials meaning less multipath distortion.
  • Aerials less cumbersome (and conspicuous)
Is know DAB multiplexes need to be avoided in parts of band 3 but is there much PMR usage these days ?

I seem to remember Veronica used to do band 3 kits but I don't see many around nowadays.

Albert H
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Re: Noob question: Why is Band 1 so popular for linking

Post by Albert H » Fri Sep 17, 2021 2:20 am

Band III is absolutely full! There are a few gaps, but Band I has the "wide open spaces". The downside of Band I is that the aerials are huge!

TBH, I have always preferred Band IV and V - it's easy and cheap to get high gain aerials (albeit matched to 75Ω), but this permits the use of cheap TV coax, really high gain (and cheap) Yagis, and there's plenty of room in the TV bands if you look around. Also (and most important), a TV aerial doesn't ever look out of place on a rooftop!

The earliest Band IV links used hacked Pye Pocketfone PF1 pairs. We used to disable the transmit phase modulator, add a simple ~90MHz PLL as the FM source, and peak them up for a couple of hundred milliwatts on the output frequency. The receivers required a bit more work - the crystal oscillator and the multiplier chain was still used, along with the front end, but the IF, squelch and audio parts were chopped off the board (with a hacksaw!). A couple of ceramic filters. a transistor IF amplifier stage, and a CA3089 IF / discriminator IC, and you had the receiver. The transmitter was matched to 75Ω, as was the receiver input, and it was possible to get fully noise-quietening links over tens of miles! We used to buy the PF1s from "Garex" for about £8/pair!

Later on, I developed a really nifty PLL TX for Band IV and V that would deliver either 1 or 5 Watts into 75Ω. There was a matching RX that used a TV tunerhead and a nice PCB for the 35MHz to 10.7 MHz IF down-mixing and a fairly conventional 10.7 MHz IF and demodulator (though the last run of these included a PLL demodulator to improve the receiver's useful sensitivity by more than 10dB). These link set-ups were used by many stations in the 80s, and only one of them was ever tracked by the DTI! The distance record for one of the Band IV links was a little over 45 miles!

The very final Band IV / V link system used a receiver that mixed the incoming signal down to the required output frequency, so it never actually received the modulation, it just took ~500MHz in, and gave ~100MHz out, using a PLL controlled local oscillator. This made it possible to (slightly) move the output frequency of the main rig from the studio end! One set-up like this was used in the Hollywood Hills in the 80s, with great success. The output rig was powered by a couple of big "gel-cells", and these were charged by a mixture of solar cells and a little wind turbine. We were putting out around 140 Watts, and with the directional transmit aerial, we used to have an ERP in the region of 500 Watts - enough to get a good, solid signal into much of the greater Los Angeles area.
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Re: Noob question: Why is Band 1 so popular for linking

Post by Persona Non Grata » Sun Sep 19, 2021 11:54 am

Thanks Albert.

I've always wondered why the UK needs to allocate so much broadcast spectrum to PMR ? In the rest of the world the broadcast bands are used for....Broadcasting (radio or TV) but in the UK the development of FM broadcasting was held back for decades by the fact that most of the band was taken up by emergency services, British rail and the gas board. Then it all got moved up to Band 3 which was supposed to be a TV band ? Suppose it all boils down to the incompetence of OFCOM and its predecessors.

Is there still much usage of the PMR frequencies ? Most taxi's and delivery companies these days seem to be using cellular (mobile phone) or digital systems.

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Re: Noob question: Why is Band 1 so popular for linking

Post by Albert H » Mon Sep 20, 2021 12:26 am

There's a huge amount of PMR and other comms stuff in Bands I & III. The old (rather inefficient) use of those bands was for monochrome, 405 line TV. Each analogue TV channel was 6 MHz wide !!! In Bands I & III there were 13 TV channels - a bandwidth of 78 MHz. When the old 405 line TV was shut down (in the 80s) most of the PMR services were moved there. For mobile use, Band III was preferred because of the smaller aerials. When DAB started, it was put into one (ultimately three) of the 6 MHz wide analogue TV channels.

The real Band II scandal is the stupid mismanagement of frequencies for the National services - there is no need for BBC Radio 2 to spread from 87.9 to almost 92 MHz for example. With proper synchronisation, it's trivially possible to use just two (or at most three) adjacent FM frequencies for a National service. With proper planning, all four BBC Nationals could be below 92 MHz. As in many other countries, this would obviate the need for RDS - you just need a receiver with "AFC", so even PLL tuning is unnecessary. Car radios could automagically re-tune as you drove around the country, locking to the best National signal....

Back in the 1980s an engineer (and friend of mine) called Fred Wise was engaged by the Government to suggest a broadcast band plan for Band II. He recommended three (or possibly four) "classes" of radio station:

Type A: National, high powered ones from 87.6 to 93.5 MHz, using pairs of adjacent channels,

Type B: Regional medium powered ones from 93.5 MHz to 99.5 MHz - commercial stations for the bigger cities, and larger area coverage elsewhere with extensive frequency re-use,

Type C: City or Town-wide stations at lower power, providing "local" radio from 99.5 MHz up to 103.5 MHz, again with plenty of frequency re-use,

Type D: "Parish Pump" or "Community of Interest" stations at lowest power, covering neighbourhoods from 103.5 MHz to 107.9 MHz.

He pointed out that with the improvements to domestic receivers, the channel spacing (with intelligent planning) could be as little as 200 kHz, to allow as many as 60 stations in a big city! Sensible frequency planning and Capture Effect would ensure that listeners would get reliable, interference-free reception.

If the Wise Report recommendations had been adopted, DAB would have been completely unnecessary! Unfortunately, the UK has always had technically illiterate politicians, so the report fell on deaf (and very stupid) ears, and we continued to have the BBC wasting almost 70% of the whole band!
"Why is my rig humming?"
"Because it doesn't know the words!"
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Re: Noob question: Why is Band 1 so popular for linking

Post by reverend » Wed Oct 13, 2021 1:28 pm

There are a few gaps in Band-III but these are generally reserved for PMSE (i.e. 191.7, 199.7, 199.9, and 200.1) so are not ideal for 'unlicensed' broadcasting. Also, the DAB signals in/around the band, being very strong, produce a lot of noise in the band which makes it difficult to use. One other thing with Band-III is that if you want to use it for linking, you need to be careful with your FM transmit frequency, as the second harmonic of transmissions in the FM band will be in Band-III and you need to avoid this being too near the link frequency and, in many cases, the image too (generally +/- 21.4 MHz from the Band-III receive frequency).

Using Band-I, there are many more 'empty' frequencies and there's no problem with harmonics or images (unless you're towards the top of Band-I with images +21.4 MHz which could cause problems). For example, a Band-I receiver on 68 MHz might struggle to filter out an FM signal on 89.4 MHz if the local oscillator in the Band-I receiver is on the 'high' side. This can be solved by using an LO on the low side of the wanted receive frequency.

I agree with Albert that Bands IV/V are good due to the smaller antennas, however these frequencies are getting congested too and are full of noisy, high-power DTT transmissions which are just as annoying as the DAB transmissions. Unlike the old analogue TV transmissions, which had useful spaces between them, the gaps between the DTT transmissions are relatively small (and very noisy).

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