What benefit does real SDR transmitter give you for HF?

Discussion in 'Software Defined Radio (SDR)' started by N4VDI, Apr 4, 2021.

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  1. N4VDI

    N4VDI Ham Member QRZ Page

    I know SDRs have real and tangible benefits right now as receivers (on HF, and elsewhere), but what specifically is there to gain by using a SDR as a transmitter for HF (specifically, the 20, 30, and 40 meter bands) if you're an American ham subject to FCC rules?

    I love the idea of SDR. I think it's unquestionably the way radios ought to be, and in another 20 years or so, almost certainly will. But I'm struggling badly to find of any concrete advantage to using one as a HF transmitter today.

    For VHF, UHF, and microwave bands... sure, there are plenty of good and obvious reasons to want a SDR transmitter... like cool, exotic, wideband transmission modes that would be impossible or impractical with today's normal rigs. But on HF, as far as I can tell, pretty much anything a SDR transmitter could uniquely do that a conventional rig can't accomplish with a soundcard + SSB is, as far as I know, prohibited by the FCC.

    Am I wrong? Is there some cool, bleeding-edge experimental digital transmission mode that's legal to use on HF, has concrete advantages over modes like MT63, THOR, and Olivia, and actually needs a SDR transmitter?

    As an alternative, how viable is it to use a SDR as a receiver, together with a conventional HF radio as the transmitter (just ignoring the transceiver's audio output, and using some kind of PTT-triggered automatic coax switch to connect the antenna to the transceiver a fraction of a second before transmission begins, and switch it back to the SDR a fraction of a second after transmission ends)? Is there some huge drawback I haven't thought of, or would this actually be a sane & sensible compromise that doesn't really have any significant downside besides the loss of bragging rights & desk space?
  2. DL2JML

    DL2JML Ham Member QRZ Page

    As I don't live in the US, I am not fully aware of what the FCC allows and prohibits on HF. Do you mean that the FCC limits the bandwidth of any HF transmission to 2.7 KHz? That indeed would insure that any transmitted signal could be accomplished with a sound card + SSB.

    (I know that AM and FM can be used on some bands and need more than 2.7 KHz, but there may be a specific provision for these modes. I also know that D-Star can be used on 10m on some rigs, but I don't know whether the FCC allows that.)
  3. SM0AOM

    SM0AOM Ham Member QRZ Page

    The wide-band transmission possibilities available with SDR transmitters are quite pointless on HF due to the coherence bandwidth limitations of the ionosphere.

    Some wider-bandwidth spread-spectrum waveforms, say in the 20 to 100 kHz ranges, may be usable exploiting their frequency-diversity and coding gain properties, but will be more difficult to co-exist with conventional narrow-band emissions, and to accommodate into small antenna systems.

    3 kHz per channel is about the limit bandwidth-wise which is actually usable, especially on short sky-wave propagation paths.

    However, the main "point" of SDR technology is flexibility.
    It becomes very much easier to synthesise quite arbitrary waveforms having adaptive properties, and to optimise the characteristics of the whole system. SDR transmitter technologies excel in improving keying characteristics and adjacent channel emissions when properly applied.

    An increasing number of commercial and military HF systems use SDR technologies, primarily adaptive predistortion, for improving their distortion and adjacent channel properties to levels previously unattainable using conventional means.

    KA0HCP, K0UO, K2XT and 5 others like this.
  4. WA4SIX

    WA4SIX Ham Member QRZ Page

    ANAN & Hermes Lite 2 can use active predistortion when running an amplifier to reduce harmonics/IMD. It's "Brand name" is Pure Signal.

  5. W0XA

    W0XA Premium Subscriber QRZ Page

    A true SDR transceiver will get firmware and software updates as needed.
    That is one of the best reasons for running an SDR.
    The Japanese look at updates as a failure to their radio design.
    Anyway.... I am glad I made the switch to SDR and along with the outstanding performance it's just plain fun to operate.
    NY7Z and KA0HCP like this.
  6. N2DTS

    N2DTS Ham Member QRZ Page

    Not only can Pure Signal give you a clean transmitter, it can clean up a poor amplifier into one of the cleanest signals on the band.
    You also have control over a lot of things other radios will not allow, like bandwidth control.
    You CAN open it up and sound hifi if you want and band conditions allow, or narrow it down to whatever you want.
    Lots of space on the band, what is the problem opening the audio up to 4000 Hz?

    Some software includes pro audio tools and other features, and since its software, any and all things could be added in the future.
  7. N4VDI

    N4VDI Ham Member QRZ Page

    @WA4SIX and @N2DTS, something like PureSignal is exactly what I had in mind when I wrote my post 2 days ago exploring the possibility of a SDR that implements a built-in class D 100-watt amp using vacuum tubes... the idea that if an amp is slightly nonlinear in consistent & predictable ways, a SDR could just pre-compensate for it at the waveform-generation level so it goes in, gets predictably distorted, and comes out the other end as the desired waveform (kind of like cell phone cameras with intentionally-distorted lenses that are telephoto towards the center, and become fisheye as you move towards the periphery... calibrate them at the factory, then apply a deconvolution transformation to produce photos with lots of center detail and distortion-free straight lines from edge to edge).

    The thing is, right now there seems to be a pretty hefty cost premium or power-penalty for going the all-SDR route for HF. At the moment, I'm looking at 3 basic scenarios:

    • Brand new non-SDR transceiver in the neighborhood of $600-750. Say, Icom IC-781 or Yaesu FT-891.
    • Old (but still respectable) non-SDR transceiver from eBay (say, Yaesu FT-757GX or Icom IC-735) to use as my 100w transmitter, paired with something like HackRF+HamItUp as my SDR receiver
    • The Unicorn... an all-SDR transceiver that gives me everything I'd get with one of the other two scenarios, at a cost that isn't substantially worse than a brand-new non-SDR transceiver.
    I had hopes for the Xiegu G90, but then I saw how much an XPA125B costs, and spent a few days in nihilistic despair... if even the radio and amp combo that everyone hails as being "affordable" is expensive, there's probably not a lot of near-term hope for anything else.

    I've also read a few threads here that gave the distinct impression that QRP rigs work poorly when matched up with a "big gun" amplifier built for 1500W output... basically, that they end up drawing almost as much power when amplifying 5-20w to 50-100w as they need for amplifying 100w to 1500w, and throw off most of it into the room as pure waste heat while doing it. I looked into THAT scenario because my dad has a Dentron 2500 amp (from sometime in the late 1970s) that I could borrow.

    In any case, I've gotten the overwhelming impression that if 80-100w output is a make-or-break requirement, it's (almost) never cost-effective to simultaneously buy a QRP rig and suitable amp, and rarely a good choice even if you can get a big amp "for free".

    I'm pretty adamant about 100w, because I had a few weeks at my dad's house in SW Florida (Naples) to discover the realistic limits of 100w with an antenna that's about as good as the best antenna I can ever realistically hope to have at my own house (his antenna was constrained by my desire to ensure he could take it down by himself without tools or a ladder within 3 minutes... mine is constrained by the fact I live in a townhouse with a HOA).

    I could probably go as low as 80 watts on 40m... but it would have to be 80 solid RMS watts that I could confidently sustain for 3-5 minutes at a time, not 80 "SSB phone" peak marketing-watts that can barely pull off a series of 15-second FT8 transmission cycles without the radio's thermal-protection kicking in.
    Last edited: Apr 4, 2021
  8. N6YWU

    N6YWU Ham Member QRZ Page

    An SDR radio doesn't need a "sound card" for any non-voice modes, but can do all the digital processing in the digital domain, using "sound" only for any actual speaker output or mic input. Sound is not an optimal channel unless your data is speech or music.

    Using digital generation of the Tx signal, exotic equalization methods, such as adaptive pre-distortion to improve amplifier output, or even adaptive pre-distortion to match HF propogation channel conditions might be possible. Using digital control of the Tx function, one can compensate for component variation, aging, and temperature, etc. using more accurate differential equation models than simple analog circuits might allow. Etc.
    Last edited: Apr 4, 2021
    AA5BK and KA0HCP like this.
  9. WA4SIX

    WA4SIX Ham Member QRZ Page

    THe Hermes Lite-2 is a 5W out rig. There are a few amplifier configurations with automatic bandswitching. 50W-100W.
    Now, if you want big guns, just build a BLF188 amplifier & drive it direct with the HL-2 for about 800W or a double BLF188 for 1600W. Bansdwitching can still be automatic.
    Or, you can grab a junker HF rig & pull the 100W amp & LPFs out of it.
    So many choices with a bit of imagination...

  10. N4VDI

    N4VDI Ham Member QRZ Page

    Well, I'd be perfectly content with 100 (real) watts. My only reason for mentioning the big-gun amp was because I could get one from my dad to use for free. But from what I've read, even for free, that's not necessarily a good idea.

    Now that you mention Hermes... I could SWEAR that ~2 months ago, I stumbled over something about a shockingly affordable module for it that combined a 100w amp with automatic antenna tuner (I think it was something like $250). I think it was somehow related to Apollo. Does that ring a bell? From what I recall, it was mentioned in the wiki and/or mailing list, with links to Apollo that are all dead now.

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