5 ms·

I was super into the ham world many many moons ago (DE K2KD), and my curiosity is definitely piqued with the arrival of SDRs. Dumb question(s): Is it possible

by nlh 5y ago

I was super into the ham world many many moons ago (DE K2KD), and my curiosity is definitely piqued with the arrival of SDRs. Dumb question(s): Is it possible to do "traditional" HF/VHF ham stuff with these mini SDRs? Is there a good software package that emulates the old-school rigs (but with fancy new features)? Are these radios powerful enough to drive an amp to get to something like 100W (or 1500W) on HF?

superkuh 5y ago
Not without paying a bunch for bandpass filters and power amps to get it up high enough to actually drive your 100w amp. Most SDRs are just the transceiver without all the front-end parts you really need for a "usable" radio.
- jacquesm 5y ago
  Yes, true, you'll need a bunch of intermediary stage amps as well, you won't drive 100W without something in the middle. Typically a set of usable stages would be an SDR, a very sharp bandpass filter, a 1W or so buffer, then something on the order of 5 to 10W and then into real power stages 50-100 or so and if you want to go substantially higher than that you'll need yet another stage. And each of those will need to be perfectly regulated to avoid dumping a bunch of harmonics into the air.
- trothamel 5y ago
  That's correct, though getting those is possible, and something of a fun project. I've found the stuff 60dbmco ( https://www.ebay.com/sch/60dbmco/m.html?_nkw=&_armrs=1&_ipg=&_from= https://www.ebay.com/sch/60dbmco/m.html?_nkw=&_armrs=1&_ipg=... ) makes to be useful for this . You're probably not saving any money this way, but you do learn a lot.
jacquesm 5y ago
Before you start pumping out that kind of power I'd invest in a spectrum analyzer (which are surprisingly affordable today) to ensure that the signal that goes out is clean. Those mini SDRs when used for transmission will output a ton of harmonics because they are almost by definition not going to have any filtering on their output because that would limit their usability. When used for reception that is not a problem, if you start using them for transmission it's a completely different matter. Keep in mind that a power stage that responds well to a particular signal absent filtering is going to respond quite well to the main harmonics too!
- drmpeg 5y ago
  That's for sure. Here's what an Ettus B210 (now $1472, not cheap) looks like with a fundamental at 145 MHz. 3rd harmonic is only 10 dB down. Power was 0 dBm (with a 20 dB pad on the analyzer input). https://www.w6rz.net/harmonic.png https://www.w6rz.net/harmonic.png
  - jacquesm 5y ago
    That's even worse than I thought it would be. A cheap one will likely not do better. At least they picked the HAM bands smart so harmonics tend to end up in the next HAM allocated frequency range up. But SDRs being what they are there are no passive components on board to ensure the signal stays 'in its lane'.
  - rfdave 5y ago
    It's inherently a broadband device, with no output filtering. No surprise that the output is full of harmonics. I built several application specific RF front ends for Ettus SDR's at a previous job, it's mandatory for any kind of professional use.
gmiller123456 5y ago
As far as transmitting, it's not really practical. Receiving, yes. What I normally do is look for signals using my SDR, and when I want to make a contact, use my FT991A. Even though the 991 has a SDR in it, the dedicated ones have a much wider bandwidth, and the better display and UI of a computer make things a lot easier.
cschneid 5y ago
Check out the Hermes Lite, it's an SDR with a transmit ability, and some other additional features. I think it's 10w by default, but a lot of people put that right into an amp to get up to 100+ watts.
- gorkish 5y ago
  I immediately hit reply to suggest the Hermes Lite also; despite the crazy supply chain situation, there is a current manufacturing run going right now. With N2ADR's filter board, you have a complete 10W HF direct sampling transceiver. Add a hardrock PA and you can take that easily to 50W or 500W at an insanely low price point. I hand build a few of the early prototypes and have a couple of the production versions too. I cannot emphasize how underrepresented I think this radio is compared to a lot of the other SDR hardware out there. It compares favorably in performance and vastly exceeds the features of most amateur radio HF transceivers in the <= $2500 range. Because it speaks the OpenHPSDR protocol it can also run a digital predistortion algorithm called PureSignal that samples the PA output and distorts the input signal to correct for amplifier phase and power nonlinearity. Digital predistortion is relatively common in modern radios like cellular baseband, but to my knowledge PureSignal is the only open source implementation of such and the only implementation publicly available for an HF radio.
jmrm 5y ago
AFAIK that radios aren't usually that powerful (at least, legally to non-commercial use), and usually amplifies the modulated signal from the machine, they doesn't have an input of a direct signal to amplify and send. Anyway, I think you can send signals in baseband from the computer or the SDR and let the radio modulate it.

by nlh 5y ago

superkuh 5y ago

Not without paying a bunch for bandpass filters and power amps to get it up high enough to actually drive your 100w amp. Most SDRs are just the transceiver without all the front-end parts you really need for a "usable" radio.

jacquesm 5y ago
Yes, true, you'll need a bunch of intermediary stage amps as well, you won't drive 100W without something in the middle. Typically a set of usable stages would be an SDR, a very sharp bandpass filter, a 1W or so buffer, then something on the order of 5 to 10W and then into real power stages 50-100 or so and if you want to go substantially higher than that you'll need yet another stage. And each of those will need to be perfectly regulated to avoid dumping a bunch of harmonics into the air.
trothamel 5y ago
That's correct, though getting those is possible, and something of a fun project. I've found the stuff 60dbmco ( https://www.ebay.com/sch/60dbmco/m.html?_nkw=&_armrs=1&_ipg=&_from= https://www.ebay.com/sch/60dbmco/m.html?_nkw=&_armrs=1&_ipg=... ) makes to be useful for this . You're probably not saving any money this way, but you do learn a lot.

jacquesm 5y ago

Before you start pumping out that kind of power I'd invest in a spectrum analyzer (which are surprisingly affordable today) to ensure that the signal that goes out is clean. Those mini SDRs when used for transmission will output a ton of harmonics because they are almost by definition not going to have any filtering on their output because that would limit their usability. When used for reception that is not a problem, if you start using them for transmission it's a completely different matter. Keep in mind that a power stage that responds well to a particular signal absent filtering is going to respond quite well to the main harmonics too!

drmpeg 5y ago
That's for sure. Here's what an Ettus B210 (now $1472, not cheap) looks like with a fundamental at 145 MHz. 3rd harmonic is only 10 dB down. Power was 0 dBm (with a 20 dB pad on the analyzer input). https://www.w6rz.net/harmonic.png https://www.w6rz.net/harmonic.png
- jacquesm 5y ago
  That's even worse than I thought it would be. A cheap one will likely not do better. At least they picked the HAM bands smart so harmonics tend to end up in the next HAM allocated frequency range up. But SDRs being what they are there are no passive components on board to ensure the signal stays 'in its lane'.
- rfdave 5y ago
  It's inherently a broadband device, with no output filtering. No surprise that the output is full of harmonics. I built several application specific RF front ends for Ettus SDR's at a previous job, it's mandatory for any kind of professional use.

gmiller123456 5y ago

As far as transmitting, it's not really practical. Receiving, yes. What I normally do is look for signals using my SDR, and when I want to make a contact, use my FT991A. Even though the 991 has a SDR in it, the dedicated ones have a much wider bandwidth, and the better display and UI of a computer make things a lot easier.

cschneid 5y ago

Check out the Hermes Lite, it's an SDR with a transmit ability, and some other additional features. I think it's 10w by default, but a lot of people put that right into an amp to get up to 100+ watts.

gorkish 5y ago
I immediately hit reply to suggest the Hermes Lite also; despite the crazy supply chain situation, there is a current manufacturing run going right now. With N2ADR's filter board, you have a complete 10W HF direct sampling transceiver. Add a hardrock PA and you can take that easily to 50W or 500W at an insanely low price point. I hand build a few of the early prototypes and have a couple of the production versions too. I cannot emphasize how underrepresented I think this radio is compared to a lot of the other SDR hardware out there. It compares favorably in performance and vastly exceeds the features of most amateur radio HF transceivers in the <= $2500 range. Because it speaks the OpenHPSDR protocol it can also run a digital predistortion algorithm called PureSignal that samples the PA output and distorts the input signal to correct for amplifier phase and power nonlinearity. Digital predistortion is relatively common in modern radios like cellular baseband, but to my knowledge PureSignal is the only open source implementation of such and the only implementation publicly available for an HF radio.

jmrm 5y ago

AFAIK that radios aren't usually that powerful (at least, legally to non-commercial use), and usually amplifies the modulated signal from the machine, they doesn't have an input of a direct signal to amplify and send. Anyway, I think you can send signals in baseband from the computer or the SDR and let the radio modulate it.