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I have been bugged with the idea of building a radio in a suitcase, but I have not many clues about where to start. My idea is to make a portable FM station so podcasts can be broadcasted in a small area and later uploaded to the Internet.

The functions the "case" should accomplish are:

  • Power the entire unit
  • Take analog MIC/Line signals in a mixer
  • (some long term magic here, such as digital processing)
  • Modulate the analog signal for FM broadcasting
  • Broadcast the signal trough a small antenna

My sub-questions here are:

How much could it cost? How great can I expect a "small antenna" to reach? How much power do I need in relation to the whole equipment? Which parts would I need?

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  • $\begingroup$ Have you tried searching for schematics? $\endgroup$ – Phil Frost - W8II Jan 28 '14 at 12:44
  • $\begingroup$ Also, are we talking about operation on the commercial FM broadcast band? As in, the one used by car radios and such? $\endgroup$ – Phil Frost - W8II Jan 28 '14 at 13:03
  • $\begingroup$ Thank you for the hint, Phil. I am a total amateur. Indeed, I was talking about the FM band. $\endgroup$ – user648 Jan 29 '14 at 12:28
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    $\begingroup$ You should know then that what you are contemplating is probably illegal in most jurisdictions, which require a license to operate in that band. It's certainly illegal in the USA. It is legal to operate very low power FM transmitters in the USA, such as for connecting a music player to a car radio, however these have a range of just a couple meters and require a certification to FCC standards. You could just buy one of those transmitters. Using it for broadcast is still probably illegal. $\endgroup$ – Phil Frost - W8II Jan 29 '14 at 12:51
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If the output power is low and you know programming, then you can make such a device for under $30 in parts, if we don't count the suitcase.

I made a device which is nearly the same as you need using a TI Tiva C launchpad for 13 USD and a AD9850 board for 10 USD. There are numerous similar boards on eBay and related sites, so you could get one for even less, if you shop around. You can replace the Launchpad for a similar board, such as for example STM32VLDISCOVERY or something like that.

Basically, the Launchpad has an analog to digital converter that I use to sample the audio. In my particular case, the ADC's internal reference is set to 3 V, so I used a 3.0 V voltage reference and a resistor divider plus a capacitor to move my audio input so that it's DC value is 1.5 V.

In the launchpad, I have a loop that samples the ADC, waits for the sampling to complete and then based on the amplitude of the signal calculates the deviation.

The ADC has 12 bits of resolution, so we can expect values from 0 to 4095. Using a simple proportion, you can scale that to the range of -deviation to +deviation for your band. What I did next was to add the deviation to a fixed frequency which represents input when the ADC provides zero.

Next step is to output the data to the AD9850 and set correct output frequency. A problem here appears if you want to use frequencies above around 40 MHz, due to the limitations of the chip. There is a way to work around that, by using harmonics of the output and a bandpass filter to just let the frequency range you want out. The procedure is explained in Analog Devices application note AN-543: High Quality, All-Digital RF Frequency Modulation Generation with the ADSP-2181 and the AD9850 DDS.

Using this procedure, I was able to output high-quality mono audio signal to an FM radio with very little effort.

The main advantage of this approach, compared to using an analog transmitter is that on the hardware side, except for the output filter, you don't really care about the frequency band you're using (as long as AD9850 can produce required frequency) and you don't care about the deviation, since you can set it in software.

As for the power supply, m solution requires +5 V and uses less than 100 mA and is normally powered from a USB port.

For the antenna, very little is needed. For example, just plugging the AD9850 module I got into a bread-board was enough to cover big part of my room.

In the end, be sure to monitor the output of your radio transmitter to be sure that it only outputs in the band in which you have a license to operate! Additional filtering will probably be required for that.

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  • $\begingroup$ Do you have any picture or scheme of this? :) $\endgroup$ – user648 Jan 29 '14 at 12:31
  • $\begingroup$ @user648 I don't. Unfortunately, I have an exam in two hours, so I don't have enough time to make a reasonable picture right now. I will post a picture and a block diagram of connections later today, when I get back from the exam. $\endgroup$ – AndrejaKo Jan 29 '14 at 12:35

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