SDR Transmitting Gets The Power

Most hobby-grade software defined radio setups don’t transmit. Of the few that do, most of them put out anemic levels around one milliwatt or so. If you want to do something outside of the lab, you’ll need an amplifier and that’s what [Tech Minds] shows how to do in a recent video. (Embedded below.)

The video covers LimeSDR, HackRF, and the Pluto SDR, although the amplifiers should work with any transmitter. The SPF5189Z module is quite cheap and covers 50 MHz to 4 GHz, amplifying everything you throw at it. The downside is that it will amplify everything you throw at it, even parts of the signal you don’t want, such as spurs and harmonics.

There are other modules, depending on your needs. The CN0417 covers a very narrow range from 2.4 GHz to 2.5 GHz. (If you can call 100 MHz bandwidth “narrow”.) The RF2126 will cover from 400 Mhz to 2.7 GHz.

None of these are powerhouses. The maximum 20 dB gain will only give you a watt or so out with the minimal drive from most SDR transmitters. But for very many applications, that’s plenty, especially with some filtering.

Unfortunately the gain isn’t stable, and we wonder about the linearity which would affect some modulation modes. There are datasheets for these devices around, such as this one for the SPF5189Z.

If you are really into SDR, the SDR Academy went virtual this year. You might also enjoy this book.

27 thoughts on “SDR Transmitting Gets The Power

  1. Please, use at last low pass filters… It’s really easy to find yourself on many others, unplanned and restricted bands using this stuff and unwillingly affect somebody’s (eg. emergency response teams) communication.

  2. “anemic levels around one milliwatt”, the reason for that is to fall under FCC Part 15. They are exempt from most FCC requirements, because they are sold as test equipment, most of which output a maximum power of about 10 dBm (10 mW).

    1. Probably also because people really do mostly use them as test and development equipmet. SDRs aren’t exactly ready to be walkie talkies and WiFi adapters, and probably will never be till we have cheap integrated transceiver and FPGA chips or something, which I’d imagine will happen eventually.

      1. There is SDR WiFi but they use FPGA chips that alone cost from $150 to $4,000 for the 80k to 600k Logic Cells/Logic Elements required to function.

        SDR walkie talkies and WiFi adaptors could be a thing today, it is just that the price point makes them a very niche market. You would need FCC testing and compliance, for useful power levels, and I do not believe that is even possible for hardware and open source software that can broadcast without restriction at any frequency.

        1. I would be far more interested in moderate power levels, IF the system had quality filtering, both high pass and low pass, plus band pass filtering.
          Personally, all amps I have designed for UHF and above, had proper filtering in the R.F chain, as well as D.C.
          I despise garbage, and if you hear my signal across the band, I’d smash it…Junk is unacceptable.
          No better than C B running 120%mid index, and FMing. Splatter boxes need to go.
          Clean signals show a professional mindset.

      2. i believe wifi dongles use sdr tech, it’s just not user accessible. all the radios in mobile phones are SDR too iirc.

        regarding walkie-talkies it is probably not going to become very popular, as most communications in VHF/UHF is FM or DMR and there are specific chips for that already.

        it’s not that technology isn’t ready (military uses sdr handhelds for a while) but there is no use for consumer handheld to be SDR

        1. Aren’t the Baofeng’s and cheaper handheld’s SDR? I was under the impression when doing RDF that I had to consider dealing with not being able to disable their ACG issues when using.

          Amazing seeing the Harris military handheld clones and even surplus. Almost want to buy one.

      3. You’re thinking in terms of an infinitely programmable device.

        I’d guess all cellphones are SDR. I remember years ago getting Maxim mailouts that had all kinds of devices that were a start at sdr. Look inside a current cellohone, there’s nothing much.

        I’d guess that cheap FRS walkie talkies use SDR, especially since I’m sure I saw a datasheet for such things.

        Cheap consumer shortwave portables are now SDR, it now cheaper than analog.

        Given all this, I assume cheap Bao-Feng type walkie talkies are SDR too.

        But most of these aren’t infinitely programmable, they have a sort of firmware. They are intended for specific applications, and specific frequency range.

        Note that none if these amplifiers cover shortwave, and many are general purpose SDR doesn’t bother going lower.

  3. “The downside is that it will amplify everything you throw at it, even parts of the signal you don’t want, such as spurs and harmonics.”

    It is worse than that. Even without modulation, harmonic distortion can splatter power over other user’s frequencies, potentially making them unusable. With a little imagination you can think how that might endanger life and limb.

    Expect Government Agencies with men in black suits to appear at the door, and rightly so.

    1. This is a myth. The FCC barely has the manpower to take action against the most egregious offenders. Even with these amplifiers, I suspect you’ll barely be able to make a dent in the RF noise out there being generated by millions of sh*tty LED lights and other poorly made electronics.

      Hams love to talk up big FCC fines (and “men in black” appearing at your door), but the reality is something entirely different. Just take a look at their enforcement actions for the past 5 years.

        1. It’s all public record. Find me an enforcement action in the last 10 years for spurious emissions from an end user. Heck, show me any action involving the ham bands for anything less than the most egregious and blatantly harmful behavior.

          Case in point, the entire community of Baofeng users which, by the way, far exceeds the spurious emissions seen on these tiny amps.

    2. Men in black will not show up.
      Such wide band garbage will be list in the noise floor.
      I hear a lot of garbage, and once the signals drop in dBuV levels, they lose their interest.
      It would require a hefty signal, not a Bao Feng level signal that is intermittent at best.
      Spurs, harmonics, images, multiplication, a require the fundamental signal to be quite strong, not some 5 Watt H.T The offending junk will be far too weak off the major carrier.
      The fear is unfounded.

      1. Why not just design a feedback loop that auto bandpass filter tunes for the transmit frequency? If the response with the amplifiers isn’t linear seems can do some sort of auto attenuation also… though that might be a little more tricky.

  4. It will be nice to see more microwave amp options for amateur radio SDR at least. There are so many QO-100 users that use a SDR and a 2.4ghz wifi/ISM amp to work the first functional geostationary transponder. Even in the S and L bands you are already seeing the microwave squirrelyness of unintentional inductance and capacitance in the board design which makes DIY trickier than the more DC like what you see/design is what you get at HF and below bands.

    I would like to try more microwave DIY but am sort of stuck in that coffin-corner where I am too disabled to do much DIY anymore but in my case disabled=poor so no money and in this case few offerings to buy off of the shelf either.

    1. The limeSDR can handle just about anything , but it is pricey if you mess up modding the PCB for ham use. The $35 50ohm RTL-SDR with TCXO is great for testing things out ( ).

      Most SDR dongles have these general limitations:
      1. they are wide-band noise-sensitive devices, and the LNA built into most front-ends is limited (requires different inputs for the limeSDR). Use a bandpass filter for your channels, and a 88-105MHz bandstop filter if you don’t want to saturate the receiver. Note too, many devices have zero input protection.

      2. they are energy hungry with higher bandwidth models. I use a silicone sheet on the aluminum case for my limesdr pcb backing, and also heat-sinks on the chips (use a metal caged fan for cooling). The laptop you use should also be fairly good as well… the limesdr fpga code needs more filter features in my opinion, as even USB3 will limit your max raw data rates (closer to a 50MHz wide scan limit on my setup).

      3. Most SDR transmitters need a bandpass filter on the RF output, and are limited in how fast you can update the internal mixers VCO. Turn off the transmitter between VCO updates if possible, as some models handle these changes less gracefully. In my country it is a $5k fine and or a year in prison if you or your transmitter is not following the rules. KerberosSDR makes tracking down illegal transmitters fairly trivial. They are goofy to setup the antenna array, but work well for most ham bands.

      4. Most high frequency work requires a decent reference 10MHz oscillator, and this is especially true if you build something like a VNA. If you can find a recent BG7TBL 10MHz GPSDO model that isn’t counterfeit, than they stabilize things enough for GHz band work.

      5. Practically speaking the software installers can be unforgiving to students. I would recommend a standalone Ubuntu 18.04 LTS on a cheap SSD if you are working with limesdr, gnu radio, and gqrx. The FOSS Windows/MacOS software ports out there is just not going to be fun… the rtlsdr driver setup can also be a pain for win users.


  5. Daniel Dunn,
    Most newer military handheld/backpack stuff is 100% SDR inside, even the $20 Baofengs are partly SDR mixed into a FM chip to be tuned and then amped.
    People forget that the high bandwidth wifi especially is a near-miracle tech like modern CPUs and GPUs and they use custom silicon to grab and process the RF as fast as they do, you cant really SDR or FPGA around that science with the current state of the art. But even the HackRF portapack gives a walkie-talkie mode and is only missing a snap-on batt pack and some RF amplification.

    1. Part of this is an unhealthy obsession people have with incredibly wideband systems. It may be convenient to have a 1MHz-4GHz SDR for testing, but if you want real radio you want as narrow bandwidth you can get away with, starting with a narrowband antenna and highly selective filter/matching circuit between that and your LNA/PA.

      There just will never people a 1Mhz-4GHz transmitter that works without unhealthy spurs/harmonics, analog just doesn’t work like that. But that can’t stop people wishing for it.

      1. Antenna bandwidth will not affect overall bandwidth, just reception/transmission of those signals.
        A great example of a wide band antenna, is the Log Periodic, favored by broadcasting for this ability
        Curtain antennas, and wire.
        Specialized, such as a Yagi, Quad, or other multi-element antenna will not be useable outside its design specs. Same for parabolic antennas. You can cover a wide frequency spread, but all are tuned within a narrow portion of that band, for maximum signal, not TX and RX. Side lobe rejection also has to be taken into account in dense locations.
        As the story goes…Crap in, crap out.
        Filter your signals, both directions…no excuse.

  6. There is a popular Brand of Chinese Radios, HTs and Mobiles which are based on SDR ‘Radio Systems on a chip’ which have just about taken over the Amateur use Brandmeister DMR. Just saying – properly filtered SDR chips are already here. The HT’s (D868 & D878) output between 1 and (allegedly) 8 watts TX on 2 meters and have FCC Part 90 Approval you can look up for more details. The Mobiles output up to 50 Watts with FCC Part 90 approval for commercial uses. Thess radios have earned the respect of most Amateur Radio Operators who are particular about FFC compliance & legal interoperability with Public Safety freqs when so authorized. Cheers.

  7. SignalsEverywhere,
    My first thought is where would an amateur radio DIY get the tools, but really a resistor and an RTL-SDR is actually pretty good for hunting down splatter and harmonics within it’s range.

    1. By being a major Chinese Manufacturer of Public Safety Communications equipment used in China & presumably other Asian markets, … and yes you are correct about missing the fun to be had making homebrew mods & improvements to existing low cost devices.

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