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Post by elorens on Jun 27, 2023 19:28:46 GMT 12
I was talking to a colleague today and he has an idea for an event which would celebrate NZ music, and NZ-made AM radio receivers. On one floor of a large building, vintage receivers would be dotted around, all tuned to a single station provided by a low-power AM transmitter, which would transmit a programme of locally composed and played music. Coverage would be restricted as far as possible to within the building.
So, I’m looking for some feedback on the idea, especially with regard to the AM transmitter. The intention is to make this legal and above board, but what are the rules with regard to low power AM transmitters? Is there a power level below which licence rules don’t apply? Is it necessary/possible to get a licence for a one-off event?
I know that in the UK, as AM broadcast transmitters are turned off one by one, vintage radio enthusiasts are turning more to so-called pantry transmitters to fill the gap. The name suggests something secret or semi-legal, but I’d be interested to know what the position is here in New Zealand.
Thanks, Lawrence —
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Post by Richard on Jun 28, 2023 12:31:26 GMT 12
I'm not sure there is an actual "number" on the transmitting power as such, but if your home-built transmitter interferes with the reception of licensed broadcast stations, then spectrum management authorities might prosecute. I've built the low power AM transmitter from Silicon Chip magazine March 2018 and it performs very well, its supposed to be very low RF output power i.e a fraction of a watt.
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Post by elorens on Jun 28, 2023 18:09:51 GMT 12
I'm not sure there is an actual "number" on the transmitting power as such, but if your home-built transmitter interferes with the reception of licensed broadcast stations, then spectrum management authorities might prosecute. I've built the low power AM transmitter from Silicon Chip magazine March 2018 and it performs very well, its supposed to be very low RF output power i.e a fraction of a watt. Thanks, Richard. Are you able to post a copy of the article? I guess if we can ensure the signal avoids local broadcast station frequencies, and is in any case at extremely low levels outside the building, we are likely to be ok. But we may be able to avoid the issue by running an audio feed to the gram inputs of the various radios: the overall effect would be very similar.
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Post by Peter Walsham on Jun 28, 2023 18:22:44 GMT 12
Feeding an audio signal in to the 'external modulation' terminals of an RF signal generator, and connecting a long wire to the signal generators RF output terminal may provide enough signal strength to feed those radios that don't have a gram input.
Richard: I built up a valve based AM transmitter many years ago. Unfortunately, hum in the carrier wave plagued it to the point where the audio quality was severely compromised. I was never able to completely solve the problem, but plugging it in to a power point as close to the switchboard reduced the hum, but never silenced it. The hum was so bad that I completely gave up on using it. I built a second unit with the same hum problem. I have a copy of the Silicon Chip article that you mentioned, and had planned to make one of that design up. Your comments about it performing really well is very encouraging. Does it suffer from any carrier and/or modulation hum at all?
Peter
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roy51
Society Members
If it ain't broke, fix it anyway
Posts: 96
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Post by roy51 on Jun 28, 2023 21:50:22 GMT 12
I have also built the Silicon Chip micro transmitter, works well in the confines of my “man cave” Do a Google search, Silicon Chip, AM transmitter. I got the pc board and some parts direct from Silicon Chip in Aussie Cheers Roy
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Post by Richard on Jun 29, 2023 7:54:14 GMT 12
. Does it suffer from any carrier and/or modulation hum at all? Peter Hi Peter, No the unit doesn't suffer from modulation hum, its very quiet and uses a balanced modulator chip
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Post by Richard on Jun 29, 2023 8:04:34 GMT 12
Peter, here's my transmitter, built into an old modem case, I did build my own regulated power supply, so omitted the regulator and components around the one shown in the schematic above. I purchased the PC board directly from Silicon chip and had it posted to a relative in Melbourne, who posted it to me in NZ, as Silicon Chip mag wanted some outrageous amount for postage to NZ at the time.
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Post by Richard on Jun 29, 2023 9:13:25 GMT 12
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Post by Richard on Jun 29, 2023 9:14:15 GMT 12
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Post by Richard on Jun 29, 2023 9:15:12 GMT 12
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Post by elorens on Jun 29, 2023 21:30:19 GMT 12
Great, thanks Richard!
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Post by Peter Walsham on Jun 29, 2023 21:51:27 GMT 12
Very encouraging, thanks Richard. Will look in to building that up (and possibly 'illegally' experiment on increasing the RF output).
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Steve
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vintageradio.co.nz
Posts: 732
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Post by Steve on Jun 30, 2023 13:13:10 GMT 12
I bought one of these, which works well enough - albeit with only a few meters decent range... www.aliexpress.com/item/1005001279170402.htmlThere are plenty of alternative AM transmitter devices on AliExpress - it would be interesting to see how the different ones perform Cheers, Steve
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Post by Richard on Jul 2, 2023 11:17:56 GMT 12
I just remembered, that the PC board had an error, the 2.2 Meg resistor that biases the base of Q2 went to the wrong side of the 220pf capacitor! so check if you buy the PC board !
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Post by stanley on Jul 10, 2023 18:07:09 GMT 12
I saw this not long ago, it would be interesting www.radiomuseum.org/dsp_cohiradia.cfmI built an am transmitter using a 1mhz crystal oscillator, 5v regulator and 600ohm telephone transmitter. It worked well but it had to be almost in the cabinet or connected to the antenna wires
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