Tuning LimeSDR for 20 meters band


#1

Hello,

I’m trying to use LimeSDR for receiving 20 meters band. My antenna is a DIY receive-only magnetic loop antenna made from 2 meters long coaxial cable. Everything described below was done during weekends, at ~ 13-00 local time, when the band is open and hams are pretty active.

My Gqrx settings are the following:

Gqrx shows me the following:

I tried to use hardware AGC and increase decimation - it doesn’t seem to make any difference.

If I increase the bandwidth:

… I can listen many AM stations in nearest bands:

When I use RTL-SDR + Ham It Up with the same antenna the picture is completely different:

Here I receive a strong signal which seems to be RTTY.

And here is SSB:

LimeSDR was tuned, the firmware is upgraded to the latest version using LimeUtil --update. Previously it worked pretty well with 443 MHz and 2.4 GHz frequencies.

I’m not sure what exactly is a problem. My antenna is probably not the best one, but RTL-SDR seems to be pretty happy with it. What can I do to receive amateur radio with LimeSDR?


#2

Do you have HF modification on any of RX inputs?
If not you need them for HF with Lime.

Even if you have them the Lime with this antenna may need LNA like LNA4HF

Because: (Top RSP1, Bottom Lime)


#3

I don’t have any modifications. I tried to use LimeSDR with LNA4ALL. The noise floor rises but I don’t hear any hams.


#4

OK, I re-checked. In fact LimeSDR receives a very strong signals like the RTTY one on the screenshot above (this guy is transmitting like 100W from just a few kilometers from me). Thus it seems like I’m having issue with weak signals. It looks like there is a RTTY contest currently in progress, the whole spectrum should be filled with RTTY.


#5

Increase bandwidth to 40MHz


#6

OK, here are my new settings:

Result:

Zoomed:

I tried to increase sample rate, play with TNA/LNA/PGA, check/uncheck AGC. Doesn’t seem to change anything.

Maybe I need a filter to get rid of all these AM stations? Although there is no need in any filters according to this article maybe my antenna is too wideband? On the other hand RTL-SDR works without any filters…


#7

Any SDR wideband receiver require excellent BPF. At the moment I’m not able to show you difference between direct and filtered receiver screen but believe me it is huge!


#8

His magnetic loop act like BPF…

I am able to use Lime with tuned ferite antenna and LNA4HF on 80 m band
within HF congested urban area…


#9

@afiskon - Aleksander,

With the LimeSDR, it’s VERY important to use a Low Pass Filter (2 to 30 MHz) with the LimeSDR receive input to eliminate FM harmonics from strong nearby stations. When you use a LPF with the LimeSDR, you’ll have better receive in HF and will also eliminate the unwanted FM radio station harmonics completely. A Kenwood LF-30A or equivalent will do an excellent job.

73 de Marty, KN0CK


#10

Thanks for the advice. I tried to use 7-pole DIY Butterworth LPF with cutoff frequency 16 MHz. Unfortunately, it doesn’t seem to help much. Maybe it doesn’t work well on high frequencies. I can’t check this since I have only 100 MHz oscilloscope and 25 MHz signal generator. Waiting for a better filter from eBay.


#11

Even a cheap CB LPF should remove any FM broadcast stations. Best is to use BPFs. If you have access to old ham radios, strip out the filter section & use the filters in it. I have & can switch bands with an Arduino.

Ed