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Philips210 · 9th Jan 2018, 11:41 pm

Hi.

I have just completed building the MW pantry transmitter from mictesters design and can report success with it. I decided to design a PCB layout for the circuit rather than stripboard. It was made simply by applying the transfers to the copper clad board guided from my pencilled design on square grid matrix paper. I'll have to get back into doing the photo etching method again as it's a lot less time consuming. My old RS UV lightbox works well so I've no real excuses not to use this method of fabricating the PCBs in future.

The pantry transmitter is working generally well but I believe the aerial and matching circuit is important to get right in order to achieve a reasonable RF output power.

My unit was powered from either the bench power supply or a regulated wall wart. Supply voltage in both cases was exactly 12V and the current taken was about 15mA. Two audio sources were used for the tests, one being my old IBM T42 laptop computer, the other a Panasonic FM/DAB radio with line audio output.

When testing the unit, I got the impression the RF output power varied considerably over the MW band. This may have been due to mismatching of the aerial to the transmitter. I found that a wire aerial of 2m length worked quite well. With this type of transmitter, the output power level is important in that sufficient power will be needed to operate radios within the confines of your home but not too much power that's going to interfere with your neighbours' equipment. Personally, I prefer it to be at a minimum power level that's usable and this seems to be the case with my unit.
I found that bypassing the first three inductors gave the highest RF output but I also found I needed extra capacitance across the 120pF trimmer.

Experimenting with the DIP switches and setting them to MSB 01101100 LSB gives divide by 108. The carrier frequency is (108+1) x 9kHz = 981kHz or 305.6 metres. This seemed to fit into a quiet part of the band and reception was quite reasonable.
I also found the audio peak indicator circuit worked very well. Any slight overdriving of the modulator showed up with the LED flickering in sympathy with the modulating signal. Backing off the 47k pot to just extinguish the LED was the best setting.

I am very pleased so far and thanks to mictester for providing us with this interesting circuit. I am going to do more detailed tests on matching the aerial to the output stage so see what can be achieved. I wondered about using a coiled loop with low RF output for close range operation.
Attached is a couple of pics of my pantry transmitter.

Regards
Symon.

9th Jan 2018, 11:41 pm	#52
Philips210 Nonode Join Date: May 2007 Location: Redruth, Cornwall, UK. Posts: 2,578	Re: Another Pantry MW Thing Hi. I have just completed building the MW pantry transmitter from mictesters design and can report success with it. I decided to design a PCB layout for the circuit rather than stripboard. It was made simply by applying the transfers to the copper clad board guided from my pencilled design on square grid matrix paper. I'll have to get back into doing the photo etching method again as it's a lot less time consuming. My old RS UV lightbox works well so I've no real excuses not to use this method of fabricating the PCBs in future. The pantry transmitter is working generally well but I believe the aerial and matching circuit is important to get right in order to achieve a reasonable RF output power. My unit was powered from either the bench power supply or a regulated wall wart. Supply voltage in both cases was exactly 12V and the current taken was about 15mA. Two audio sources were used for the tests, one being my old IBM T42 laptop computer, the other a Panasonic FM/DAB radio with line audio output. When testing the unit, I got the impression the RF output power varied considerably over the MW band. This may have been due to mismatching of the aerial to the transmitter. I found that a wire aerial of 2m length worked quite well. With this type of transmitter, the output power level is important in that sufficient power will be needed to operate radios within the confines of your home but not too much power that's going to interfere with your neighbours' equipment. Personally, I prefer it to be at a minimum power level that's usable and this seems to be the case with my unit. I found that bypassing the first three inductors gave the highest RF output but I also found I needed extra capacitance across the 120pF trimmer. Experimenting with the DIP switches and setting them to MSB 01101100 LSB gives divide by 108. The carrier frequency is (108+1) x 9kHz = 981kHz or 305.6 metres. This seemed to fit into a quiet part of the band and reception was quite reasonable. I also found the audio peak indicator circuit worked very well. Any slight overdriving of the modulator showed up with the LED flickering in sympathy with the modulating signal. Backing off the 47k pot to just extinguish the LED was the best setting. I am very pleased so far and thanks to mictester for providing us with this interesting circuit. I am going to do more detailed tests on matching the aerial to the output stage so see what can be achieved. I wondered about using a coiled loop with low RF output for close range operation. Attached is a couple of pics of my pantry transmitter. Regards Symon. Attached Thumbnails Last edited by Philips210; 9th Jan 2018 at 11:52 pm.