Medium Wave Transmitter - UK Vintage Radio Repair and Restoration Discussion Forum

plaka78 · 24th Aug 2007, 7:23 pm

I have been busy designing and building this transmitter today. I have made separate modules for the various stages. The sound quality is superb. Next job is to build some kind of chassis

paulsherwin · 24th Aug 2007, 7:45 pm

Do you have a circuit diagram?

Paul

plaka78 · 24th Aug 2007, 7:52 pm

The Transmitter is at my place of work. I will draw the cct diagram next week and post it
on this site as soon as possible.

Paul Stenning · 24th Aug 2007, 9:16 pm

I assume this is a low power device with a range of no more than a few tens of metres and an output of no more than 100mW e.i.r.p.? Anything more than that cannot be discussed here.

plaka78 · 24th Aug 2007, 9:23 pm

hello Paul

best thing is to close the thread. I have not got anyway of measuring the output power

paulsherwin · 24th Aug 2007, 9:45 pm

Quote:

Originally Posted by plaka78

hello Paul

best thing is to close the thread. I have not got anyway of measuring the output power

Judging from your pictures there shouldn't be a problem with the output power. If it uses less than, say, 20mA from its power supply it's likely to be OK under the forum rules.

We just don't allow people to discuss transmitters which are clearly illegal and likely to cause a nuisance to others.

Paul

Ray Cooper · 24th Aug 2007, 10:04 pm

I see from the piccie that you are using a ferrite rod: is this being used just as a convenient inductor, or do you have a separate aerial wire connected too? (We've had quite a discussion about ferrite rods as tx aerials in these pages quite recently, and I think the consensus of opinion is that they radiate quite inefficiently).

I also see that your breadboard is resting on a work-surface that seems to include quite a lot of offcuts of wire, etc. Be advised that this is quite a good way of producing puzzling effects...

Paul Stenning · 24th Aug 2007, 10:08 pm

If it is intended for rebroadcasting in your own home (minimising interference to neighbours etc) it's fine. If it's intended for broadcasting to the neighbourhood or over any real distance then it isn't.

plaka78 · 24th Aug 2007, 10:12 pm

Basically the circuit consist of an oscillator driving a BFY51 transistor, The Output tuned circuit is a series tuned and is shunt fed by a 330 ohm resistor in the collector of BFY51, which gives out a very clean sine wave. The Modulator runs at 30v using an emitter follower stage therefore with no modulation the TX output transistor is running at 15v.

The circuit did start of as a cobweb circuit and then I rebuilt it bit by bit on vero
board. It worked quite well just radiating of the ferrite rod.

Paul Stenning · 24th Aug 2007, 11:00 pm

As Ray highlighted above and as has been discussed previously, ferrite rods are very inefficient radiators. With a 30V supply (capable of an amp or so by the look of it) and an output transistor with a heatsink, it looks like you are pushing significant power into a very inefficient antenna. So although the range will probably be within the confines of your home (which is fine), I wonder whether there is a more efficient way of achieving the results.

Bear in mind that some of the 100mW rebroadcasters run from a PP3 battery or a small plug-in adaptor, use a TO92 plastic packaged output transistor with a length of wire as an antenna, and can achieve a range of tens of metres or more.

You would probably find that if you attached a wire antenna to yours, the range would increase substantially. Then you could reduce the supply voltage and output stage drive to get it back down to the range required.

I am not a low power transmitter expert though. I have tested and reviewed a few here http://www.vintage-radio.com/reviews/transmitters.html so have seen what goes into them. I have also seen some of the circuits for 100mW transmitters on the MWA archive site here http://www.geocities.com/raiu_harrison/mwa/. They all seem to use lower voltage supplies and smaller transistors to achieve the same sort of results as your design.

This is not intended to be criticism; just questioning the approach you have used in your design.

paulsherwin · 25th Aug 2007, 1:26 am

I agree that reducing the power supply to, say, 9V and improving the aerial design would be the way to go. Such an approach would also be less likely to fall foul of the forum rules

Paul

plaka78 · 25th Aug 2007, 10:38 am

I will experiment with a frame antenna next week. When I got time I will design a circuit that will run on a PP3 battery. I have experimented in the past with modulating various oscillators but found that they don’t modulate in a linear way and also slightly deviate in frequency .

By the way medium wave here is totally useless there is so much interference from digi and sky boxes etc.

24th Aug 2007, 7:23 pm	#1
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Medium Wave Transmitter I have been busy designing and building this transmitter today. I have made separate modules for the various stages. The sound quality is superb. Next job is to build some kind of chassis Attached Thumbnails

24th Aug 2007, 7:45 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Medium Wave Transmitter Do you have a circuit diagram? Paul

24th Aug 2007, 7:52 pm	#3
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Medium Wave Transmitter The Transmitter is at my place of work. I will draw the cct diagram next week and post it on this site as soon as possible.

24th Aug 2007, 9:16 pm	#4
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,071	Re: Medium Wave Transmitter I assume this is a low power device with a range of no more than a few tens of metres and an output of no more than 100mW e.i.r.p.? Anything more than that cannot be discussed here. __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

24th Aug 2007, 9:23 pm	#5
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Medium Wave Transmitter hello Paul best thing is to close the thread. I have not got anyway of measuring the output power

24th Aug 2007, 10:04 pm	#7
Ray Cooper Retired Dormant Member Join Date: Jan 2007 Location: Porthmadog, Gwynedd, UK. Posts: 199	Re: Medium Wave Transmitter I see from the piccie that you are using a ferrite rod: is this being used just as a convenient inductor, or do you have a separate aerial wire connected too? (We've had quite a discussion about ferrite rods as tx aerials in these pages quite recently, and I think the consensus of opinion is that they radiate quite inefficiently). I also see that your breadboard is resting on a work-surface that seems to include quite a lot of offcuts of wire, etc. Be advised that this is quite a good way of producing puzzling effects...

24th Aug 2007, 10:08 pm	#8
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,071	Re: Medium Wave Transmitter If it is intended for rebroadcasting in your own home (minimising interference to neighbours etc) it's fine. If it's intended for broadcasting to the neighbourhood or over any real distance then it isn't. __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

24th Aug 2007, 10:12 pm	#9
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Medium Wave Transmitter Basically the circuit consist of an oscillator driving a BFY51 transistor, The Output tuned circuit is a series tuned and is shunt fed by a 330 ohm resistor in the collector of BFY51, which gives out a very clean sine wave. The Modulator runs at 30v using an emitter follower stage therefore with no modulation the TX output transistor is running at 15v. The circuit did start of as a cobweb circuit and then I rebuilt it bit by bit on vero board. It worked quite well just radiating of the ferrite rod.

24th Aug 2007, 11:00 pm	#10
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,071	Re: Medium Wave Transmitter As Ray highlighted above and as has been discussed previously, ferrite rods are very inefficient radiators. With a 30V supply (capable of an amp or so by the look of it) and an output transistor with a heatsink, it looks like you are pushing significant power into a very inefficient antenna. So although the range will probably be within the confines of your home (which is fine), I wonder whether there is a more efficient way of achieving the results. Bear in mind that some of the 100mW rebroadcasters run from a PP3 battery or a small plug-in adaptor, use a TO92 plastic packaged output transistor with a length of wire as an antenna, and can achieve a range of tens of metres or more. You would probably find that if you attached a wire antenna to yours, the range would increase substantially. Then you could reduce the supply voltage and output stage drive to get it back down to the range required. I am not a low power transmitter expert though. I have tested and reviewed a few here http://www.vintage-radio.com/reviews/transmitters.html so have seen what goes into them. I have also seen some of the circuits for 100mW transmitters on the MWA archive site here http://www.geocities.com/raiu_harrison/mwa/. They all seem to use lower voltage supplies and smaller transistors to achieve the same sort of results as your design. This is not intended to be criticism; just questioning the approach you have used in your design. __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

25th Aug 2007, 1:26 am	#11
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Medium Wave Transmitter I agree that reducing the power supply to, say, 9V and improving the aerial design would be the way to go. Such an approach would also be less likely to fall foul of the forum rules Paul

25th Aug 2007, 10:38 am	#12
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Medium Wave Transmitter I will experiment with a frame antenna next week. When I got time I will design a circuit that will run on a PP3 battery. I have experimented in the past with modulating various oscillators but found that they don’t modulate in a linear way and also slightly deviate in frequency . By the way medium wave here is totally useless there is so much interference from digi and sky boxes etc.