21-04-2011, 08:14 AM
The question often arises ‘how will we be able to use our old radios when DAB transmissions take over, and FM/AM stations close down?’ or ‘how can I play an i-pod/MP3 player through my old radios?’ Often, the answer that people give is build or buy a ‘micro-transmitter’ (sometimes colloquially called a ‘pantry transmitter’). However, such devices, whether commercially produced such as the SSTRAN or ‘Spitfire’ are not licensable anywhere in Europe, and to use one is a criminal offence, in the admittedly unlikely event that Ofcom detected one and decided to prosecute the user. I doubt this is high on Ofcom’s enforcement agenda, and as an ardent homebrewer, I wouldn’t wish to discourager anyone from experimenting in any way that they wish – ‘ride at your own risk’ is the motto. But I’m aware that they have taken enforcement action against one UK supplier, and another supplier says on its website that ‘any orders placed by UK customers for its micro transmitter kit will routinely be cancelled’.
FWIW, the Radio Equipment and Telecommunications Terminal Equipment (R&TTE) Regulations 2000 regulate the placing on the market and putting into service of this type of apparatus in the UK. The use of apparatus is regulated by the Wireless Telegraphy Act 2006. For such apparatus to be lawfully sold in the UK it would have to meet the R&TTE regs.
For now, one supplier has tried to get around the regulations by stating:
Clip:
EU R&TTE directive and EMC Directive 2004/108/EC - in order to purchase this item, you must agree that:-
1. You intend to re-export the item to an address outside the European Union. Under no circumstances will this item be resold or put into service within the European Union.
or:
2. The item will only be used within a completely RF screened and shielded metal enclosure or environment for testing, evaluation, experimental or laboratory purposes and any other use that does not radiate electro-magnetic waves into free space. I am not an unqualified end user and I have RF engineering and electromagnetic compatibility skills at my disposal.
Clip.
To comply with such terms would of course render the device completely useless, and I doubt it lets the supplier off the hook, but that’s an issue between those parties and of no concern to us. Ofcom are far more concerned at curbing illegal pirate broadcasting stations which plague most large cities.
Maybe in time, if and when DAB takes over (a long time from now, given that at least 17 million cars have FM/AM only radios) maybe the regulations will be amended to allow such devices to be produced, as with so many other low power short range devices, but for now, the only legal way to do it is by using a Medium Wave modulator.
Such a device appeared in a 2004 EPE Magazine, and a short while ago, I finally got round to building it. This enables input from audio sources such as an FM or DAB radio, or ipod etc, to be fed into the modulator and played through a valve radio at a point on the medium waveband, so that you can play your favourite programmes through your favourite old time radio.
It's a simple project, consisting of a Varable frequency oscillator (VFO) with a centre frequency of 650kHz (c 460 Metres) which tunes from about 610 - 690 kHZ, (490 - 435 Metres). This is then modulated by the audio input, and the resultant signal fed into the aerial input of the radio on which you want to play your programme.
It should not be confused with a micro-transmitter as it does not connect wirelssly to the radio, and does not radiate RF. There are three sections to the device – the power supply, the VFO, and the modulator. However simple a device is, long experience of homebrew over more than five decades has taught me to follow the practice of ‘build a bit, test a bit’. I therefore built and tested the power supply first – no problems there. When it came to the VFO, the only snag is that the original design uses a Toko coil, which is now unobtainable. Only one tapped winding of the coil is used, so it isn't used as a transformer – simply a coil, so I wound my own on a scrap transistor radio IFT core.
Initially I wound on 270 turns, then tapped it and wound on another 30 turns to see how it panned out. The original design specified a 50 pF tuning cap, but I only had a 75 pF to hand, so I used that. To get the VFO on frequency, I had to add 47pF in parallel. The VFO is very stable and didn't drift when left running for several hours.
Once I'd got the VFO working, I built the rest of the modulator - the modulator board and PSU board, and built in into the case. It has three controls - the tuning cap to vary the tuning to a suitable spot on the dial of the radio, the modulation control, to adjust so that it isn't driven into distortion, and an attenuator control, so that the radio isn't overloaded. I adjusted the device to tune from 430 – 490 Meters on the MW band. In use, I simply find a vacant spot on the dial, connect whatever input to the modulator that I wish – such as my i-pod, then adjust the depth of modulation for clear undistorted output. I wouldn't say it's "Hi-Fi" but I've tried it with an FM Radio, a DAB, and an i-pod through my Ekco A22, and it sounds fine.
It’s the sort of project where a frequency counter and scope come into their own for testing and adjustment purposes. To check and adjust the VFO, all that is needed is a small loop of wire placed close to the VFO coil to read out the tuned frequency. With the tuning cap fully closed, the lower end of the tuning range can be adjusted by tweaking the core of the VFO coil. The tuning cap can then be fully unmeshed, and the frequency checked in the same manner at the upper end of the VFO tuning band.
To check that the VFO is working, its unmodulated output can be coupled to the scope, whereupon a clean sine wave should be observed. If that’s fine, the modulator can then be checked by injecting a tone from a waveform generator to the input of the device, whereupon the modulated signal should be seen on the scope. I've attached some pics, which might be of interest, showing the inside and outside of the finished project.
For anyone wishing to convert frequency to wavelength or vice versa, without doing the maths, this is a handy online calculator. Put in any designator – kHz, MHz, frequency, (or any wavelength) and it instantly does the sums and gives the answer:
http://www.onlineconversion.com/frequenc...length.htm
In another post under 'Test Gear' I'll show some scope traces and how a frequency counter can be used to check the frequency of an oscillator - in this case, the VFO.
Hope it's of interest.
David
FWIW, the Radio Equipment and Telecommunications Terminal Equipment (R&TTE) Regulations 2000 regulate the placing on the market and putting into service of this type of apparatus in the UK. The use of apparatus is regulated by the Wireless Telegraphy Act 2006. For such apparatus to be lawfully sold in the UK it would have to meet the R&TTE regs.
For now, one supplier has tried to get around the regulations by stating:
Clip:
EU R&TTE directive and EMC Directive 2004/108/EC - in order to purchase this item, you must agree that:-
1. You intend to re-export the item to an address outside the European Union. Under no circumstances will this item be resold or put into service within the European Union.
or:
2. The item will only be used within a completely RF screened and shielded metal enclosure or environment for testing, evaluation, experimental or laboratory purposes and any other use that does not radiate electro-magnetic waves into free space. I am not an unqualified end user and I have RF engineering and electromagnetic compatibility skills at my disposal.
Clip.
To comply with such terms would of course render the device completely useless, and I doubt it lets the supplier off the hook, but that’s an issue between those parties and of no concern to us. Ofcom are far more concerned at curbing illegal pirate broadcasting stations which plague most large cities.
Maybe in time, if and when DAB takes over (a long time from now, given that at least 17 million cars have FM/AM only radios) maybe the regulations will be amended to allow such devices to be produced, as with so many other low power short range devices, but for now, the only legal way to do it is by using a Medium Wave modulator.
Such a device appeared in a 2004 EPE Magazine, and a short while ago, I finally got round to building it. This enables input from audio sources such as an FM or DAB radio, or ipod etc, to be fed into the modulator and played through a valve radio at a point on the medium waveband, so that you can play your favourite programmes through your favourite old time radio.
It's a simple project, consisting of a Varable frequency oscillator (VFO) with a centre frequency of 650kHz (c 460 Metres) which tunes from about 610 - 690 kHZ, (490 - 435 Metres). This is then modulated by the audio input, and the resultant signal fed into the aerial input of the radio on which you want to play your programme.
It should not be confused with a micro-transmitter as it does not connect wirelssly to the radio, and does not radiate RF. There are three sections to the device – the power supply, the VFO, and the modulator. However simple a device is, long experience of homebrew over more than five decades has taught me to follow the practice of ‘build a bit, test a bit’. I therefore built and tested the power supply first – no problems there. When it came to the VFO, the only snag is that the original design uses a Toko coil, which is now unobtainable. Only one tapped winding of the coil is used, so it isn't used as a transformer – simply a coil, so I wound my own on a scrap transistor radio IFT core.
Initially I wound on 270 turns, then tapped it and wound on another 30 turns to see how it panned out. The original design specified a 50 pF tuning cap, but I only had a 75 pF to hand, so I used that. To get the VFO on frequency, I had to add 47pF in parallel. The VFO is very stable and didn't drift when left running for several hours.
Once I'd got the VFO working, I built the rest of the modulator - the modulator board and PSU board, and built in into the case. It has three controls - the tuning cap to vary the tuning to a suitable spot on the dial of the radio, the modulation control, to adjust so that it isn't driven into distortion, and an attenuator control, so that the radio isn't overloaded. I adjusted the device to tune from 430 – 490 Meters on the MW band. In use, I simply find a vacant spot on the dial, connect whatever input to the modulator that I wish – such as my i-pod, then adjust the depth of modulation for clear undistorted output. I wouldn't say it's "Hi-Fi" but I've tried it with an FM Radio, a DAB, and an i-pod through my Ekco A22, and it sounds fine.
It’s the sort of project where a frequency counter and scope come into their own for testing and adjustment purposes. To check and adjust the VFO, all that is needed is a small loop of wire placed close to the VFO coil to read out the tuned frequency. With the tuning cap fully closed, the lower end of the tuning range can be adjusted by tweaking the core of the VFO coil. The tuning cap can then be fully unmeshed, and the frequency checked in the same manner at the upper end of the VFO tuning band.
To check that the VFO is working, its unmodulated output can be coupled to the scope, whereupon a clean sine wave should be observed. If that’s fine, the modulator can then be checked by injecting a tone from a waveform generator to the input of the device, whereupon the modulated signal should be seen on the scope. I've attached some pics, which might be of interest, showing the inside and outside of the finished project.
For anyone wishing to convert frequency to wavelength or vice versa, without doing the maths, this is a handy online calculator. Put in any designator – kHz, MHz, frequency, (or any wavelength) and it instantly does the sums and gives the answer:
http://www.onlineconversion.com/frequenc...length.htm
In another post under 'Test Gear' I'll show some scope traces and how a frequency counter can be used to check the frequency of an oscillator - in this case, the VFO.
Hope it's of interest.
David
