# Practicality of SDR-based TX/RX

When it's not possible to be at the same location as a TX/RX appliance, some form of remote control is needed.

There's basically two ways to TX/RX:

• classical radio appliance, with or without digital functions
• SDR appliance solely manipulated in a digital fashion

The advantage of option #1 is that there's usually an amplifier built-in which provides a decent amount of power output. Most #2 solutions would require an additional device to achieve a similar state.

Another key difference is flexibility. Indeed, a digital system with a single SDR at its core could hop between multiple bands (and so antennas) with the help of a switching mechanism, whereas a "normal radio" can only TX/RX over bands it's been made and has antennas for.

I might skip on some pros and cons, but being a starter in the field of amateur radio (though with a strong IT background) I'm wondering which of the two options is best for remote operation.

Is there any definite answer to this, taking into account the latest capabilities given to us by new IT & RF hardware, or is it more of a personal thing ?

• Hello and welcome to ham.stackexchange.com! – rclocher3 Jun 10 '20 at 14:10
• Thanks @rclocher3 ! – Dreadlockyx Jun 10 '20 at 15:09

The advantage of option #1 is that there's usually an amplifier built-in which provides a decent amount of power output. Most #2 solutions would require an additional device to achieve a similar state.

I'll challenge that.

As a counterexample, the FTDX101D is an SDR that provides 200W of output power without an external amplifier.

The QRPver-1 v.3 is a superheterodyne transceiver with only 3 W of output power.

Whether a transmitter modulates a signal in software or with analog electronics, the problem of subsequently amplifying that signal to 100 W or more is the same problem. Consequently, being an SDR or not has no fundamental bearing on the attainable output power.

The bigger determinant in output power is cost. RF power transistors aren't cheap, so you won't find more than a few watts coming from a \$30 radio.

Another key difference is flexibility. Indeed, a digital system with a single SDR at its core could hop between multiple bands (and so antennas) with the help of a switching mechanism, whereas a "normal radio" can only TX/RX over bands it's been made and has antennas for.

I'll challenge this too.

The Softrock Lite II is an SDR and can not change bands -- in fact it can't tune at all. This is because it utilizes a simple crystal oscillator, and has only a single filter.

Meanwhile, the FT-897D is not an SDR, yet covers all HF bands plus 6m, 2m, and 70cm bands. It can do this because it has a variable local oscillator, and it has a bank of filters. You can hear the relays switching when changing bands.

An SDR would need precisely the same hardware to achieve similar coverage. And in either case, a multi-band antenna, or a way to switch between antennas is still required.

It seems like the attributes you ascribe to SDRs are not really a property of SDRs generally, but perhaps just the SDRs you have seen. What distinguishes an SDR is the use of software and digital processing to perform modulation and demodulation. But these other attributes, like transmit power and band coverage, are not part of the (de)modulator. They are part of the other bits of the radio, which will be the same whether that radio is an SDR or not.

To answer your question, an SDR transceiver is entirely practical. In fact if you were to go out today and buy a "nice" commercial radio, there's a pretty good chance it's an SDR. It comes down to economics: the price of digital electronics has decreased so much that an SDR architecture usually yields better performance per dollar.

• I think my misconception of an SDR being necessarily a USB-based device tricked me into thinking a normal tabletop radio wouldn't fit the bill. Now as you put it, the filtering & oscillator parts are what really matter in the end. – Dreadlockyx Jun 25 '20 at 17:48

I think you're over-estimating the flexibility of SDR for transmitting on ham bands. SDR may simplify modulation from a hardware point of view, but transmitters for most bands and modes require amplification of the signal, and filtering to suppress harmonics, phase noise, etc. Filtering circuits are definitely band-specific. Also, hams are limited to transmitting on designated bands. What I'm trying to say is that I've never heard of a legal SDR-based multi-band ham transmitter that is just SDR > amplifier > universal antenna. The ones I know of are more like SDR > amplifier > band-specific filter > band-specific impedance matching circuit (as required) > band-specific antenna.

To sum up so far, it is possible to have an SDR-based multi-band receiver and antenna that is inexpensive and has simple hardware. A multi-band ham transmitter and antenna(s) that puts out a useful level of power, SDR-based or not, are more complicated and expensive. In fact commercially-made ham SDR-based multi-band multi-mode transceivers are comparable in price to the more traditional kind. (I doubt there are many new designs that don't use SDR for modulation.)

Regarding remote control of a ham station, for commercially-made equipment it makes little difference whether the "appliance" is SDR-based or not; most premium multi-band HF radios have been remotely-controllable for 10 or 20 years now. Some of the SDR-based radios make getting remote audio in and out a bit easier, but that's just one of the challenges of setting up a remote station.

• I think you're underestimating the applicability and maturity of SDR: Just because your filtering happens in software doesn't mean it's any worse! – Marcus Müller Jun 10 '20 at 17:30
• @MarcusMüller quite possibly! But don't typical 100 W ham transmitters need bandpass filters to handle harmonics and phase noise generated or made worse by the final amplifier stage? – rclocher3 Jun 10 '20 at 17:35
• Hm, depends on the amplifier. In fact, if you have a somewhat sensible system that is at least "band"-preselective (e.g. an antenna that simply doesn't work over multiple octaves), then OOB radiation can pretty effectively be countered with digital predistortion. – Marcus Müller Jun 10 '20 at 22:18
• Many HF ham antennas are multi-band antennas that work over multiple octaves. – rclocher3 Jun 15 '20 at 14:21
• absolutely, rclocher3, but that's why I wrote "many". But you really don't need a great amount of selectable filters to suppress the remaining harmonics. So, when wondering whether to build something with fantastic analog circuitry, or something that involves a good digital side, mediocre analog circuitry, and a low-effort selectable bandpass filter for the same quality: you still get better quality-per-buck when going digital :) – Marcus Müller Jun 15 '20 at 14:27

There are at least a couple differences that aren't just based on the specs (wattage, etc.) of current commercial offerings known to the OP.

One advantage of SDRs is that some SDR setups (Hermes Lite 2, et.al.) allow not only monitoring the final RF output, but also allow the use of computer/DSP pre-distortion to help clean up the signal all the way from or through any added linear/final amplifier(s). This would be much harder to do using a legacy analog transceiver with just a remote front panel.

Another advantage of many SDR setups is that they can stream a remotely observable panadapter or waterfall display to see if the band is active enough to bother, or where the activity is located within the band. An legacy analog setup would require extra equipment (boxes, cables) to do this. Some SDRs even allow (locally or remotely) monitoring multiple HF bands simultaneously (4 or more band "slices" in the case of some OpenHPSDR systems), if you have a receive antenna system that allows this.