LimeSDR USB Version Transmit on HF - Here's the verdict (and it's GOOD!)


LIMESDR TRANSMIT UPDATE: …I let my Lime transmit on 40m WSPR (7.0386 MHz Upper Sideband) using SDRAngel overnight since the power level was small (I thought +10dBm) and didn’t collect any contacts - - without an amplifier, straight to my Hustler 6BTV vertical, but on receive I was reporting A LOT of stations with no drifting (well, one station - you can check out the stats on the WSPR database and look for the reporting station ‘KN0CK’ today 2/24)…For those that REALLY want to know what their LimeSDR is capable of on transmit I finally took matters into my own hands and took pics of the transmit power level right out of the Low Band port on the LimeSDR (USB version - I’ll take pics of the Mini later today)…Anyway, on AM at 90% modulation you can expect an output power of -6.7dBm at 75m, -2.3dBm at 40m, and 0.026dBm at 30m…There are a couple of pictures of what the Upper Sideband transmit level looked like and it was +2.34 dBm at 40m, +3.786dBm at 30m, so note that the power level is slightly higher using SSB than AM (as is true with all radios)…I will measure and report ALL bands from 160m to 70cm later today for the USB version of the LimeSDR and the Mini if I have time…This should go without saying, but if you plan to drive your LimeSDR with an amplifier, you need a low pass filter with a decent rolloff for the band of choice to eliminate the harmonics that are very prevalent in the spectrum…Obviously, the further out you go in frequency the spurs diminish, but 2nd and 3rd harmonics are strong enough to get transmitted - so use a LPF for your band of choice if you plan to ‘pump up the volume’…Overall, I was really impressed with the output of the Lime and SDRAngel…Really happy to see that the spectrum is about what you’d expect and very clean - no close-in spurs in the modulation…More to follow as I have it - Stay tuned…Pictures follow…


Thank you for sharing this. I got my ham license not a long time ago (R2AUK) and would like to try my LimeSDR USB as a transceiver some day. It’s unfortunate that a filter is required, but I believe I can live with it. I already have an EMI filter anyway (though I didn’t test it yet). Amplifying the signal is not a problem as well.

I had some issues receiving amateur radio signals in the city, most likely because of the high noise level (typically it’s 9) and wide bandwidth of LimeSDR. It literally doesn’t work, although RTL-SDRv3 + NooElec Ham It Up work in the same conditions just fine. This is most unfortunate since I spend 5 days a week in the city. Hopefully LimeSDR will work better in the suburbs.

As I understand to use one antenna for both transmitting and receiving LimeSDR (or maybe some MCU-based dev board) should be programmed to switch an external relay using GPIOs. By any chance didn’t you try to do this? I also wonder whether SDRAngel has any support of such functionality.


@afiskon - Alexsander,

Thanks for the very kind words and background, much appreciated. It’s my next intent to get the GPIO board running with my LimeSDR - I believe that SDRAngel made provisions for this since Edouard was provided with a GPIO board awhile back, so T/R switching is most likely already there in SDRAngel, but I will confirm this on the weekend.

73 de Marty, KN0CK


@martywittrock did you have a chance to go back and “measure and report ALL bands from 160m to 70cm later today for the USB version of the LimeSDR and the Mini if I have time” ? Interested. thanks.




Sure did - you can see that data in this post for the LimeSDR USB version. I am in the midst of getting my Lime Mini back on the air once I do a manual reload of the FPGA using a ByteBlaster and then I’ll characterize that one, too. Here’s the link for the LimeSDR USB version (1.4) RF Power Levels for 1.8 MHz to 1.28 GHz for the Tx1L (Tx Lowband) port:

More as I have it, stay tuned…

73 de Marty, KN0CK


I have issues transmitting WSPR. I am using Windows 7, WSJT-X and SDR Angel with Virtual cable between them.
However there is interference in the audio and audible chirps in the transmitted signal (i use my HF radio for monitoring). And then no spots on the wspr map. Any ideas how to deal with this?
73 de LZ1CLA


@kiromn - Kiril,

The most important accessory you need for WSPR to work under WSJT-X is a time-sync app that’s called Dimension 4 that you can download at the following link:

Download Dimension 4 and install it. Then time sync to one of the servers of your choice. this is an important step because if you’re not time sync’ed (and the Windows time sync isn’t accurate enough) then no matter if you have it all set up right you won’t get any responses to your transmission - - YOU MUST BE SYNC’ed. Once you’re time-sync’ed, then try another WSPR transmission…Make sure that you’re not overdriving the audio either through the Windows sound panel, or through SDRAngel’s modulator - and you should be using USB modulation (use the SSB Modulator but tune it to the USB when transmitting - you MUST use USB modulation for WSPR - it won’t work on anything else). The ‘clicks’ in the audio are not capable of being remedied…You’ll have to live with those, but I can tell you that it doesn’t make that much difference to the overall transmission - it will still work. It’s an anomaly of SDRAngel and WSJT-X…Interrrupts between the two apps will create those ‘clicks’ in the audio - disregard them, it worked for me fine.

Hope this helps - let us all know your progress, Kiril,

73 de Marty, KN0CK


Ok, so the solution was to boost the LP filter to 130khz in SDRAngel. I dont know why, but this made the Lime transmit on full power. I had timesync installed already and i know it is mandatory
Now it works. Keep in mind that my HF antenna is just a ham stick on my Balcony and it is far from optimal.
I have installed 30x30x10 Suono 12V FAN to the PA, it works like a charm and is quiet enough.

Here is my config that actually works. This is LimeSDR Mini and 3-4W Leory 2-700MHZ Alliexpress $9 PA


@kiromn - Kiril,

GREAT NEWS!! And great pics to go along with it, too…! There are very few of us that have attempted WSPR in terms of transmit, I know of not many other Hams that applied this to transmitting and successfully gathering reprots on WSPR - less than 6 - maybe even less. Glad you used yours on 20m - now I have to get mine a little more ‘together’ and let mine loose there, too…!

It’s true - I didn’t know that your LPF wasn’t set to 130 MHz. At 130 MHz there is no filtering (technically) when used in the HF bands…Glad you found that. The filter is ‘wide open’ (as Mike @Kc7noa would say). So it will allow your transmit signal to be full output (meaning, you’ll drive at around +7dBm to +12dBm on the 20m band) and do well for your PA.

I am anxious to see more of your WSPR reports and effort. Mine is to get the T/R switch solid (hopefully this weekend) and then I can use mine more than I am now.

Again, congratulations, Kiril, on joining the ranks of Hams that have applied their Limes on the air…!

73 de Marty, KN0CK


I’ve ran ft8 for rx only … letting it report spots to pskreporter … 20m …


I will leave WSPR TX for Today, to see how far it will go. You can monitor my callsign at
Tomorrow i will try FT8.
Marty, what kind of T/R switch you are planign to use?
I want to make a portable digital hf rig with the Lime.
Another question. Is it possible to receive signals with the PA attached (for my DMR/DSTAR hotspot)?


@kiromn - Kiril,

I am planning to build a hardware VOX switch that will also switch between receive and transmit. Nothing from SDRAngel or SDRConsole has been developed to deal with this elegantly, so I’m going to sample the audio output from the PC (Line Out or PC speakers) with a VOX switch I’ve bought and tie that to a relay driver that will switch the RF paths between receive and transmit. That way I know that I can run SDRAngel or SDRConsole and the LimeSDR will have the receive and transmit ports switched appropriately when audio becomes present in the PC’s speaker or Line Out path(s).

I’m going to work on this through the week and will disclose what I did hopefully by Sunday night or through the following week - stay tuned…

73 de Marty, KN0CK


Nice, i will be looking forward to see the implementation :slight_smile:
Meanwhile i have managed to reach Japan at ~9000km from my QTH. My SWR meter shows only 0.5W of power coming from the PA. What is your power rating on SSB?


So, after leaving the WSPR TX for 24 hours here is the final picture.
In the end the $9 PA burned a transistor. I suppose it is due to the very bad soldering of the board. I recommend a presoldering of the SMA connectors and the output transistors.


@kiromn - Kiril,

Absolutely remarkable and a great mission accomplished…! The fact that you have Japan, Iceland, and the US in the list from your QTH with the Lime is incredible…Even the contact in the Canary Islands is great…! I can attest that these dirt cheap wideband PAs run as hot as a steel forge when they run…Replacing that transistor in that amp is relatively easy and you get a pack of them for the same money as you paid for the PA itself. Enough to rebuild that PA 2 or 3 times - look up the part number on EBay.

Great stuff, Kiril…You are the winner on this HANDS DOWN (a folksy term we use in the US to mean you win without any doubt). I’m looking forward to running my Lime on HF WSPR on a 24 hour stretch, too, soon. I just have to set some time aside today to get it together if I can get past my home projects (I’m finalizing some renovation work in the house) - but I have to get going on this…Your efforts are greatly encouraging for the Lime to be a real viable small form factor setup for quick deployment in the field.

Keep the reports coming, Kiril - this is great stuff…!

73 de Marty, KN0CK


Thanks for the kind words Marty. It seems that an inductor has fallen out of the PA board. Can you please tell me what is the value of L1? As i see from google the output transistor should be RD01MUS1. Can you please confirm?
If it is so the PA should be arround 1W and not 3.5W The former is the power dissipation of the transistor not the output power :slight_smile:
Now here is one WSPR session without PA, only the LimesdrMini


@kiromn - Kiril,

That is OUTSTANDING…!! Hat’s off to you, Kiril, for getting your LimeSDR barefoot and STILL getting some good WSPR calls into the UK and Northeastern Europe…! THAT is incredible - I didn’t even have that good of luck doing that here in the states when I tried…

Great news again and keep us updated, Kiril,

73 de Marty, KN0CK


That is awesome!!



The RD01MUS1 is still used and is a viable gain block for V/UHF and even good to HF - here’s the datasheet for it:

…And getting this gain block is not hard either, other than it comes from China (EBay):

In terms of the coil, for that wideband PA, substituting a 500nH coil for the one in the design or just using the same form and winding about 2 turns on it should get you there. I will take a harder look at mine this evening and let you know what I find out.

73 de Marty, KN0CK