Friday 16 July, 2021, 13:17 - Amateur Radio, Broadcasting, Licensed, Pirate/Clandestine, Electronics, Radio Randomness
Posted by Administrator
Any radio transmitter needs a final output device whose job is to deliver the required amount of radio frequency (RF) energy. For the purposes of the Wireless Waffle lockdown project, an output power of 1 Watt was determined to be sufficient for most purposes. Most purposes including:Posted by Administrator
- having fun by designing transmitters during lockdown;
- generating enough RF to leak signals around the house whilst connected to a dummy load;
- warming up a 1 Watt resistor to 'just about untouchable' heat levels;
- driving a higher power amplifier to several hundred watts for whatever purpose that might be used for.
Most modern RF power devices are based on various versions of a field effect transistor (FET). One of the complexities of using these devices (such as the RD01MUS1) is that they require a specific bias voltage on their gate in order to perform correctly. That would be all fine and dandy except for the fact that the aforementioned 'specific bias voltage' is different for each transistor. That's right, not for each type of transistor, but for each and every transistor. This is due to the manufacturing process of FETs and although this might be to tight tolerances, it still leaves the end device with sufficient variability that individual adjustment is required. The only way to get this right in a circuit is to manually set the bias voltage. This is usually done by monitoring the amount of current that the device is drawing without any RF passing through it and then adjusting the bias voltage until the current consumption is correct.
Although setting the correct bias voltage is a 'do once and walk away' kind of activity which doesn't need to be re-done once set, the need for any kind of variable component on a design is a nuisance for several reasons:
- Variable components (such as resistors or capacitors) are prone to age badly. Variable resistors, for example, often go open circuit due to the ingress of dust, or through corrosion.
- If the circuit is not working properly, there is a tendency to start fiddling around with anything that is variable, which could end up with a completely incorrect setting of the variable component.
- There is also a tendency to fiddle with anything variable to see if 'more power' can be generated. Whilst some settings may result in a higher output power they are also likely to reduce the life of the output device, or worse, blow it up due to excess current consumption.
The simplest alternative to the FET bias problem is to not use a FET. Bipolar junction transistors (BJT) are biassed very differently and do not succomb to the same problems. Instead they can be set up with no bias at all and what's more, this arrangement is perfect for amplifying constant signals (such as generated by an FM transmitter) and even has it's own name 'Class C'. The thing is that BJTs are decreasingly common for RF given many other (as yet undiscussed) factors which favour the use of FETs.
In the 1980s and 1990s, the 'go to' BJT for 1 Watt VHF transmitters was the 2N4427. This transistor would happily provide 1 Watt of output on most frequencies up to around 175 MHz and was relatively readily available and cheap. Manufacture of the 2N4427 ceased some years ago (some Chinese companies make a 'clone' which can provide 1 Watt at a push but is not the same device at all and is certainly not a drop-in replacement), and their availability and price are both becoming stretched. Whilst there are a number of equally aged transistors which can do a similar (or indeed a better) job, these too are no longer manufactured. A selection of these are listed below.
Device | Output Power | Maximum Operating Frequency | Package | Gain |
---|---|---|---|---|
2N6255 | 3 Watts | 175 MHz | TO-39 | 7.8 dB |
2SC730 | 1.5 Watts | 150 MHz | TO-39 | 10 dB |
2SC1947 | 4 Watts | 175 MHz | TO-39 | 10.5 dB |
2SC2329 | 2.5 Watts | 175 MHz | TO-39 | 13 dB |
2SC2131 | 1.6 Watts | 500 MHz | TO-39 | >7 dB |
2SC2851 | 0.9 Watts | 175 MHz | TO-92 | 13.5 dB |
2SC3017 | 2 Watts | 175 MHz | TO-39 | 11 dB |
2SC4767 | 0.9 Watts | 175 MHz | TO-92 | 13.5 dB |
MRF227 | 3 Watts | 225 MHz | TO-39 | 13.5 dB |
MRF237 | 4 Watts | 175 MHz | TO-39 | 12 dB |
MRF555 | 1.5 Watts | 470 MHz | Power Macro | 11 dB |
MRF607 | 1.8 Watts | 175 MHz | TO-39 | 11.5 dB |
NTE472 | 1.8 Watts | 175 MHz | TO-39 | 11.5 dB |
SD1127 | 4 Watts | 175 MHz | TO-39 | 12 dB |
TP2314 | 3 Watts | 175 MHz | TO-39 | 15 dB |
Just about the only device being manufactured today which could theoretically replace the 2N4427 is the MRF4427 which, given it's name, is supposedly a surface-mount replacement for the original device, but which, like the Chinese clones, is not a drop-in replacement. It has a very different gain profile and works up to much higher frequencies. The secondary issue with the MRF4427 is that being a surface mount device, it is much more difficult to get rid of the heat which is generated in a transmitter. RF output devices are often no more than around 60-70% efficient, meaning that a 1 Watt output transistor will also be generating around half a Watt of heat. Whilst this is easily gotten rid of on a traditional transistor by sliding a heatsink onto it, it is much more difficult to get rid of with a surface mount device (SMD). There are heatsinks available for surface mount devices but these are typically 'glued' onto the device which, though it may work, is not a perfect way. Better, perhaps, might be to try and get rid of the heat by distributing it around the printed circuit board (PCB).
2N4427 (TO-39) | 2SC2851 (TO-92) | MRF555 (Power Macro) |
For standard, double sided, FR4 PCB material, the thermal characteristics of the PCB are such that it will rise around 500°C per Watt per square centimetre of board. Copper via's between the top and bottom of the board can help reduce this. Dissipating a half Watt of heat in a centimetre square of PCB will therefore raise the temperature of the PCB by up to 250°C which is far too hot a temperature for any transistor to operate. However, if the transistor is mounted on a bigger piece of PCB, the heat will be distributed over a wider area and it becomes possible to get rid of half a Watt of heat without being left with a device whose main purpose would be boiling tin.
Given all the above, for the Wireless Waffle lockdown project, after scouring the planet for sources of 30 year old transistors, the decision was eventually made to use an MRF555 device. This has nice, healthy sized leads which can be soldered onto pads on a PCB and the heat will be distributed across the PCB from all four pins of the device. It's still readily available (though no longer manufactured), relatively cheap, and being a device designed for UHF use, it loafs along at VHF frequencies, making it more efficient and therefore producing less heat. When mounted on a PCB, producing a Watt of RF, it heats up to around 65°C which is hot to the touch, but won't damage the device (or your skin too badly if you touch it).
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Saturday 5 June, 2021, 12:20 - Amateur Radio, Broadcasting, Licensed, Pirate/Clandestine, Electronics, Radio Randomness
Posted by Administrator
Hasn't lockdown been a bore? There is only so much Netflix or Amazon Prime that it's possible to watch, let alone enjoy. Mind you, having said that, and if you haven't already seen it, Halt And Catch Fire is a must-watch for any kind of nerd or geek who remembers the dodgy, unreliable and overheating computer designs of the early 1980s.Posted by Administrator
In order to maintain some kind of sanity, in the Wireless Waffle workshop work has been proceeding on the crudely conceived idea of an instrument that will not only supply emissions for use in radio transmissions, but one which would also be capable of receiving them. Or to put it simply, a wideband VHF transmitter and a matching receiver which could be used as an audio link. Having designed and built many of these devices back in the 1990s, it ought to have been relatively straightforward to revisit those designs and modernise them. Alas, RF design is a fickle mistress and in addition to resulting in some truly awful designs, some very nice ones have also emerged. Along the way, many topics have had to be re-learnt all of which makes for some (very techy) content for a series of blog posts which may, in some small way, help those who might seek to replicate this pointless pass-time.
As Julio Iglesias once said, let's Begin (at) The Beg(u)in(e)(ning). There are dozens of designs for simple VHF variable frequency oscillators (VCO) on the internet, so getting something to generate the intial signals ought to be relatively straightforward. However, there is also lots of evidence to suggest that in cases where signals from oscillators are amplified to high power levels, and sent through antennas which are relatively close to the transmitter, radio frequency (RF) feedback can occur causing buzz and hum to the transmitted signal. The way around this is to have the oscillator work on a different frequency to the one being transmitted.
This might seem like an odd thing to do, however it is not uncommon. Two primary methods are used to achieve this:
- A variable oscillator is mixed with a fixed oscillator and the sum (or difference) of the two is then filtered and amplified. For example, a variable oscillator covering 100 to 125 MHz could be mixed with a fixed oscillator at 75 MHz, resulting in outputs either at 175 to 200 MHz (if the two signals are added) or 25 to 50 MHz (if the two are subtracted). The difficulty with this approach is that mixers are, almost by necessity, non-linear devices. In the first case, with the oscillator on 100 MHz, and the wanted output on 175 MHz, the second harmonic of the oscillator would fall at 200 MHz which, being within the 'wanted' output range, could not easily be filtered out. Careful selection of the variable and fixed frequencies can help overcome this, however this limits the possible range of output frequencies and also requires lots of filtering which is fine if the wanted frequency range is relatively narrow, but more difficult if the frequency range is wider.
- The varible oscillator operates at a frequency which is a sub-harmonic of the wanted frequency, and a multiplier is then used to double, triple or multiply the frequency by even higher orders. If we therefore wanted an output from 25 to 50 MHz, we could, for example, use an oscillator running from 8.33 to 16.66 MHz and triple it. Once again, multipliers are also non-linear devices and the 16.66 MHz signal would also be doubled to 33.33 MHz, which being within the wanted output frequency range would also be difficult to filter out. This method therefore is not without its problems and also requires filtering with all the issues associated with that.
Digging around the internet for 'Kalitron' circuits in which the double frequency (or '2f') output is available, yielded very few results. An article entitled, High Frequency VCO Design and Schematics by Iulian Rosu did discuss a Differential Cross-Coupled VCO and though Iulian's design is meant for very high frequencies, his article does provide some useful guidance. One of these is that the transistors should ideally be biased at the point between their saturation and linear regions (i.e. almost fully turned on). It was therefore decided to try and adopt this circuit.
Since getting involved in designing radio equipment in the 90s, the use of PNP transistors for oscillators has always been the preferred Wireless Waffle approach. The primary benefit of this is that the inductor used to set the frequency of the oscillator is grounded, making applying some kind of variable capacitance across it far easier. And so experimentation began. It would have been easy to copy the Veronica approach, however this requires 6 individual coils to be wound, and finding a simpler way to achieve the same results was sought. Getting such an oscillator to work was not difficult, but finding the correct balance of transistor current and bias, and then tapping off the doubled frequency component with sufficient 'oomph' to do something useful with, whilst not disturbing the oscillator's stability proved a complex balancing act. In addition, to keep the costs of the associated (and yet to be designed) phase locked loop (PLL) down by using off-the-shelf high speed CMOS chips also required a method to extract the un-doubled output to be found.
The final design works. Actually, that was meant to be the start of a sentence, but having got as far as 'the final design works', seemed sufficient. An inductor wound on a toroidal core was used for the oscillator, with a secondary winding (not shown in the picture) used to tap off the un-doubled output. This is somewhat fiddly to wind, but with practice becomes much easier and is the alternative to winding the 6 coils used in the Veronica design.
Once oscillating a new issue was identified: the varicap diodes being used to tune the circuit were rectifying the RF generated by the circuit causing mountains of unwanted non-linear signals to be generated. This was partially fixed by not connecting the varicaps directly together but by short-circuiting them at RF with a capacitor whilst driving the DC level through separate resistors. Keeping the drive voltage above around 4 Volts keeps the diodes in their non-rectifying, more linear region though slightly reducing the potential tuning range. Reducing the amplitude of the oscillations would solve this a little but also reduce the potential output power. RF design is nothing if not a set of complicated compromises.
The doubled output from the oscillator is around 10 dBm (10 milliWatts) which is not quite enough to immediately drive a power amplifier to a reasonable level, so a buffer will be needed. The addition of a buffer will add (at a later date) the option to implement an 'out of lock power down' function in which the output of the transmitter is switched off until it has settled on the required frequency, without which a multitude of problems can occur.
The final circuit (or 'schematic' in American English) of the voltage controlled frequency-doubling oscillator (or 'uppy-downy-frequency-makey-matey' in Australian English) is shown below. Layout should be kept as symmetric as possible to minimise the amount of 'f' which is present on the '2f' output. The transistors used were originally type BF451 which are ideal for the task but other PNP RF transistors such as the BF509, BF939 or MPSH81 would work equally well.
Future articles in this series may explore other unbelievably exciting topics such as:
- how not to blow up RF power transistors
- how well soldering irons burn things like skin and phone covers
- why 40 year old transistors trump modern ones
- glaringly obvious mistakes to make when sending PCBs for production
- forgetting that receivers are sensitive devices
- badly matching one stage to another
- getting PLL loop filters to oscillate wildly
- and much, much less...
Car journeys can be long and laborious, and, in many cases, just plain boring. However, for the avid radio DXer, they can also be an excellent opportunity to try and receive radio signals from far afield. This is especially the case for journeys that either run close to the sea (where tropospheric propagation may enhance distance reception), or over hills and mountains (where the elevated position of the receiver may increase line-of-sight). Further, many car radios are both sensitive and selective, making them great receivers to eke out weak signals.
On a recent car journey across the UK, for example, Wireless Waffle was able to add Omroep Zeeland to the FM DX Logbook, despite radio reception conditions being what can only be described as 'as flat as the mountains of the Netherlands'. We thought, though, that it might be interesting to share our experiences of how a car journey can be turned into a DX-pedition (as they are known), safely and easily, as well as some very intriguing and unusual results.
Playing around with your car radio whilst driving is probably a bad idea, and not that safe either, so the best thing to do is to tune to just a single frequency and see what crops up. How do you choose which frequency to tune to? In the example of Omroep Zeeland above, it was 87.9 MHz. This frequency is not used by any FM transmitters in the UK (check the list at frequencyfinder.org.uk) and as such, any transmissions you do hear are either going to be pirate radio stations, or iTrip type devices (more of which later). Many of the frequencies at the bottom end of the FM band including 87.8 and 87.6 are also good candidates but there may be others, depending on which area you live in (105.0 and 105.1 are also relatively ununsed in much of the UK).
On these 'empty' frequencies what you will hear most of the time is 'ssssssshhh' (a.k.a. white noise). If there are others travelling with you on the journey, this can be extremely annoying, so this is a sport best played alone. However, from time to time, this noise will be punctuated by snippets of speech or music (which for a non radio funster, can be even more annoying than the noise).
If you get enough speech or music to be able to identify the station, or (on occasion) get a signal for long enough, and strong enough, for RDS to appear, bingo, you've just landed yourself a DX catch. Even catching the language of the station and the style of programming may, in some cases, be sufficient to lead to an identification, especially if there are only a few stations in that language on a given frequency.
This is how Omroep Zeeland was identified. The language heard was clearly dutch, and it was a speech based program with occasional oldies music. According to fmscan.org, the following FM stations are within tropospheric range on a 'flat' band (i.e. a few 100 km at most), operate on 87.9 MHz, and are in the dutch language:
One thing that did crop up whilst driving around listening to 87.9 and 87.5, was the number of strong signals that popped up with a strange variety of content from stories being read, to asian music and hip-hop. Whilst it would be easy to dismiss these as pirate radio stations, the fact that they appeared and disappeared relatively quickly tends to suggest that they were, in fact, 'iTrip' type transmitters connected to people's mobile phones, to allow music to be played on their car radios. These two frequencies are obvious choices for such devices given that they are not used for (legal) broadcasting in the UK.
An additional fun game that you can therefore give a go, is trying to work out which car driver is listening to which material. Who is the hip-hop fan, and who is listening to the x-rated audio book? Give 87.5 a try for this as it's repleat with lots of these iTrip type devices. I know what you are up to EK59 ESX!
On a recent car journey across the UK, for example, Wireless Waffle was able to add Omroep Zeeland to the FM DX Logbook, despite radio reception conditions being what can only be described as 'as flat as the mountains of the Netherlands'. We thought, though, that it might be interesting to share our experiences of how a car journey can be turned into a DX-pedition (as they are known), safely and easily, as well as some very intriguing and unusual results.
Playing around with your car radio whilst driving is probably a bad idea, and not that safe either, so the best thing to do is to tune to just a single frequency and see what crops up. How do you choose which frequency to tune to? In the example of Omroep Zeeland above, it was 87.9 MHz. This frequency is not used by any FM transmitters in the UK (check the list at frequencyfinder.org.uk) and as such, any transmissions you do hear are either going to be pirate radio stations, or iTrip type devices (more of which later). Many of the frequencies at the bottom end of the FM band including 87.8 and 87.6 are also good candidates but there may be others, depending on which area you live in (105.0 and 105.1 are also relatively ununsed in much of the UK).
On these 'empty' frequencies what you will hear most of the time is 'ssssssshhh' (a.k.a. white noise). If there are others travelling with you on the journey, this can be extremely annoying, so this is a sport best played alone. However, from time to time, this noise will be punctuated by snippets of speech or music (which for a non radio funster, can be even more annoying than the noise).
If you get enough speech or music to be able to identify the station, or (on occasion) get a signal for long enough, and strong enough, for RDS to appear, bingo, you've just landed yourself a DX catch. Even catching the language of the station and the style of programming may, in some cases, be sufficient to lead to an identification, especially if there are only a few stations in that language on a given frequency.
This is how Omroep Zeeland was identified. The language heard was clearly dutch, and it was a speech based program with occasional oldies music. According to fmscan.org, the following FM stations are within tropospheric range on a 'flat' band (i.e. a few 100 km at most), operate on 87.9 MHz, and are in the dutch language:
- Omrope Zeeland, transmitter site: Goes, transmitter power: 15kW
- Radio 10, transmitter site: Vught, transmitter power: 7.9 kW
One thing that did crop up whilst driving around listening to 87.9 and 87.5, was the number of strong signals that popped up with a strange variety of content from stories being read, to asian music and hip-hop. Whilst it would be easy to dismiss these as pirate radio stations, the fact that they appeared and disappeared relatively quickly tends to suggest that they were, in fact, 'iTrip' type transmitters connected to people's mobile phones, to allow music to be played on their car radios. These two frequencies are obvious choices for such devices given that they are not used for (legal) broadcasting in the UK.
An additional fun game that you can therefore give a go, is trying to work out which car driver is listening to which material. Who is the hip-hop fan, and who is listening to the x-rated audio book? Give 87.5 a try for this as it's repleat with lots of these iTrip type devices. I know what you are up to EK59 ESX!
Back in 2008, Wireless Waffle discussed the fact that the licensing of new community radio stations by Ofcom had forced many pirate radio stations to change frequency and in some cases to move to using two (or more) frequencies simultaneously in order to provide wide area coverage without treading on the toes of officially licensed stations.
As the number of community stations continues to grow, the problem of finding a 'clear' frequency for the pirates to operate on becomes increasingly difficult. Indeed, one might even go so far as to suggest that part of the reason for Ofcom's relatively newly found interest in licensing community radio stations is to purposefully force pirates off the airwaves, which would, of course, be in perfect alignment with their objectives as the regulator of the radio spectrum.
For many years Ofcom claimed that there were no FM frequencies available for new radio stations as the band was full, but over the past 10 years or so, they have licensed dozens of community stations all over the country. Being lower power and covering a smaller area they have managed to squeeze a number of stations into a band which had been previously claimed to have no space in it. What is interesting is how similar the frequencies chosen by Ofcom for the community stations are to those that were previously employed by the pirates themselves, suggesting that the frequencies that the pirates were using were well chosen so as to try and avoid causing interference to legitimate stations.
Take, for example, the handful of radio pirates operating in Brighton on the south coast of the UK. The table below shows some of the frequencies used by pirates in the area, and the frequencies used by the four community stations now broadcasting to the city. You will note that there is a large degree of commonality.
One station, CodeSouth, has changed frequency four times since it first went on air in 2012. Initialy on 108.0, it moved to 88.8 in 2013, then to 98.5 (the frequency previously used by another Brighton pirate 'FIP') from 2014 to 2015, then to 105.6 until late 2018 and is currently on 88.2 MHz. Incidentally, the choice of 88.2 MHz may not be the best, as it is the frequency used by a low-power relay of BBC Radio 2 in nearby Bexhill. Though the relay does not put a strong signal into Brighton and thus the frequency may appear empty, the same may not be true of CodeSouth's big signal heading in the opposite direction (88.0 may have been a better choice). Of course there are a decreasing number of frequencies available and any choice is likely to cause potential interference to someone.
The other pirate stations have not been heard of for some time. It seems at least partially feasible that some of the presenters have found their way onto one of the new community stations and as such there is no longer a need for them to fly the jolly radio roger. In this respect, it could be argued that Ofcom's community radio policy has had a double whammy effect and both taken away the frequencies from the pirates, and taken away the needs of the DJ's to use pirate stations as an outlet for their music.
Despite all the above, the simplicity of Internet streaming (which many of the pirates also do) and the opportunity of space on local DAB multiplexes, the UK's licensing laws must still be failing some part of the musical community, otherwise there would no longer be a need for the pirates. The whole situation sounds hugely reminiscent of the very early days of the pirates, when the BBC launched Radio 1 to try and take away the need for the offshore stations playing 'pop' music, yet the pirates persevered. What will it take to finally find a way to end unlicensed broadcasting, and give everyone who wishes to have a radio show the audience they seek?
As the number of community stations continues to grow, the problem of finding a 'clear' frequency for the pirates to operate on becomes increasingly difficult. Indeed, one might even go so far as to suggest that part of the reason for Ofcom's relatively newly found interest in licensing community radio stations is to purposefully force pirates off the airwaves, which would, of course, be in perfect alignment with their objectives as the regulator of the radio spectrum.
For many years Ofcom claimed that there were no FM frequencies available for new radio stations as the band was full, but over the past 10 years or so, they have licensed dozens of community stations all over the country. Being lower power and covering a smaller area they have managed to squeeze a number of stations into a band which had been previously claimed to have no space in it. What is interesting is how similar the frequencies chosen by Ofcom for the community stations are to those that were previously employed by the pirates themselves, suggesting that the frequencies that the pirates were using were well chosen so as to try and avoid causing interference to legitimate stations.
Take, for example, the handful of radio pirates operating in Brighton on the south coast of the UK. The table below shows some of the frequencies used by pirates in the area, and the frequencies used by the four community stations now broadcasting to the city. You will note that there is a large degree of commonality.
Pirate Station | Frequency | Community Station | Frequency |
---|---|---|---|
InFront FM Haven976 | 97.9 97.6 | Gaydio | 97.8 |
Radio 4A | 101.4 | 1BTN | 101.4 |
CodeSouth FM | 105.6 | Platform B | 105.5 |
- | - | Radio Reverb | 97.2 |
One station, CodeSouth, has changed frequency four times since it first went on air in 2012. Initialy on 108.0, it moved to 88.8 in 2013, then to 98.5 (the frequency previously used by another Brighton pirate 'FIP') from 2014 to 2015, then to 105.6 until late 2018 and is currently on 88.2 MHz. Incidentally, the choice of 88.2 MHz may not be the best, as it is the frequency used by a low-power relay of BBC Radio 2 in nearby Bexhill. Though the relay does not put a strong signal into Brighton and thus the frequency may appear empty, the same may not be true of CodeSouth's big signal heading in the opposite direction (88.0 may have been a better choice). Of course there are a decreasing number of frequencies available and any choice is likely to cause potential interference to someone.
The other pirate stations have not been heard of for some time. It seems at least partially feasible that some of the presenters have found their way onto one of the new community stations and as such there is no longer a need for them to fly the jolly radio roger. In this respect, it could be argued that Ofcom's community radio policy has had a double whammy effect and both taken away the frequencies from the pirates, and taken away the needs of the DJ's to use pirate stations as an outlet for their music.
Despite all the above, the simplicity of Internet streaming (which many of the pirates also do) and the opportunity of space on local DAB multiplexes, the UK's licensing laws must still be failing some part of the musical community, otherwise there would no longer be a need for the pirates. The whole situation sounds hugely reminiscent of the very early days of the pirates, when the BBC launched Radio 1 to try and take away the need for the offshore stations playing 'pop' music, yet the pirates persevered. What will it take to finally find a way to end unlicensed broadcasting, and give everyone who wishes to have a radio show the audience they seek?