SDR VLF, LF and MF comparison

đź’¬ Chat about SDR or radios in general.
User avatar
sm6fhz
Posts: 179
Joined: Wed Aug 19, 2020 3:21 pm
Location: JO57XJ
Contact:

SDR VLF, LF and MF comparison

#1

Unread post by sm6fhz »

Hi all.
Long message follows!
I made some comparison on VLF, LF and MF (ITU designations) on
AFERDI NET
RX-888
AirSpy HF+ (2 connector)
and AirSpy HF+ Discovery

on my 135m BOG in 0 deg (due North) direction.
I made 1 1/2 minutes IQ recordings on each covering 10 kHz to ~250 kHz.
I also compared them on MF (= MW) 1440 kHz on Radio 208 in Köbenhavn/Ishöj 0.5kW at about 202 km distance from me in broad daylight (abt 12:20Z). The antenna used here was a 40 m Beverage in 310 deg.
I do have some local QRM from self guiding lawn mowers (Tx'ing at ~50 kHz with a broadband modulation as well as plenty of harmonics) about 60 m away (closest neighbor).
It is not a very scientifically made comparison, but you can listen to the IQ recordings yourself and make your own judgement.
While listening to Radio 208, I could not detect any clear difference between the different SDR's. Very much the same readability on all four SDR's. I had not expected anything else. It might be different at night when the band is full of strong signals. I will try to make the same comparison under these conditions at a later stage.
Recording VLF and LF, I just switched the antenna connector between the different SDR's and recorded for 90 seconds.
They show quite different spectrum's, I must say. Quite surprising, actually. I have no expatiation to the difference in recorded spectrum between the different SDR's. No filters were used in front of the SDR's, just the antenna right into the SDR. If anyone has an explanation to the differences in the shown spectrum, please post it.
I enclose pictures of the spectrum in this posting and will try to up-load the IQ-files somewhere you all can access them. Is there any space on this forum where I can put them? It was not possible to attach them to this posting.
The files are:
01-Oct-2020 140906.185 0.119MHz, AirSpy HF+ Discovery (Screenshot-2020-10-02-211002.png)
01-Oct-2020 141347.908 0.119MHz, AirSpy HF+ (2 connector) (Screenshot-2020-10-02-211258.png)
01-Oct-2020 141837.636 0.125MHz, RX-888 (Screenshot-2020-10-02-211351.png)
01-Oct-2020 142336.198 0.120MHz, AFEDRI Net (Screenshot-2020-10-02-211439.png)
You can clearly see that the AirSpy HF+ (2 connector) roles off below 40 kHz while the AirSpy HF+ Discovery holds all the way down to below 20 kHz.
You can also see that the RX-888 has a big hump at 40 to 120 kHz that is not as pronounced at the other SDR's. It also has a dip at about 180 kHz that you do not see on the other SDR's.
Interesting. I do not know how to interpret it.
73 / Ingolf, SM6FHZ
Attachments
AFEDRI Net
AFEDRI Net
RX-888
RX-888
AirSpy HF+ 2 connector
AirSpy HF+ 2 connector
AirSpy HF+ Discovery
AirSpy HF+ Discovery

User avatar
sm6fhz
Posts: 179
Joined: Wed Aug 19, 2020 3:21 pm
Location: JO57XJ
Contact:

Re: SDR VLF, LF and MF comparison

#2

Unread post by sm6fhz »

Hi.

I put the IQ-files here:
https://sdr-radio.groups.io/g/main/files/SM6FHZ%20Files
Enjoy!

73 / Ingolf, SM6FHZ

User avatar
PD3LK
Posts: 517
Joined: Thu Aug 06, 2020 10:36 am

Re: SDR VLF, LF and MF comparison

#3

Unread post by PD3LK »

looking at al 4 pictures i can only conclude that your antenna is hardly suitable for the VLF frequencies.
Shouldn't the LW BC stations not be more stronger, even in Sweden? What is your antenna?
Noise at 180Kc seems to be slightly lower, i see this too with a RSP2 here in the NL. 73, PD3LK

User avatar
sm6fhz
Posts: 179
Joined: Wed Aug 19, 2020 3:21 pm
Location: JO57XJ
Contact:

Re: SDR VLF, LF and MF comparison

#4

Unread post by sm6fhz »

Hi Leon.
As I write in the first posting, the antenna is a 135m BOG directed in zero degrees (due North).
There is a certain amount of F/B-ratio so the Eu LW stations are somewhat discriminated.
I have heard NPN, Pearl Harbor, HI on 21.4 kHz using it, so it probably works quite OK in it's forward direction.
The dip in noise is visible on all SDR's and that made me curious. And the spectrum look so different on the different SDR's with the same antenna.
73 / Ingolf, SM6FHZ

User avatar
PD3LK
Posts: 517
Joined: Thu Aug 06, 2020 10:36 am

Re: SDR VLF, LF and MF comparison

#5

Unread post by PD3LK »

Ahhh ok missed that info. Maybe to much signal, did you tried a attenuator?
I'm using a vertical, Hustler-4-BTV, that also performs very well for RX on VLF and most HF BC bands.
In almost all cases it performs better then horizontal wires and several active HF antenna's.

bagmouse7
Posts: 43
Joined: Sat Aug 22, 2020 4:24 pm

Re: SDR VLF, LF and MF comparison

#6

Unread post by bagmouse7 »

Great test, it would be nice to see the radio gain settings for each of the devices under test.
It looks like the gain is set too high on the Afedri, on mine I need to set the RF gain to 23 or 26.
It is very sensitive, so the gain must be lowered (at least with my antenna).

I would love to see more results of more tests!
Thanks!

User avatar
sm6fhz
Posts: 179
Joined: Wed Aug 19, 2020 3:21 pm
Location: JO57XJ
Contact:

Re: SDR VLF, LF and MF comparison

#7

Unread post by sm6fhz »

Hi bagmouse7.
I played with the AFEDRI gain before making the recording and tried to get about the same signal strength reading on DCF77 on 77.5 kHz on all SDR's.
I can't recall exactly where I ended up with the AFEDRI gain but around 11 I guess. That should be good and I did not see any major differences when increasing the gain a few steps (enough margin). A gain setting of 11 is fine during night time in the MW band with all the big signals, and this was during day time.
73 / Ingolf, SM6FHZ

User avatar
sm6fhz
Posts: 179
Joined: Wed Aug 19, 2020 3:21 pm
Location: JO57XJ
Contact:

Re: SDR VLF, LF and MF comparison

#8

Unread post by sm6fhz »

Hi all.
I used my RX-888 on MW this weekend after refurbishing my 135 m Beverage antenna pointing in 310 degrees for the winter MW DX-season. I also made some comparison with my old faithful AFEDRI-net.
I have not been able to say that I can hear one station one one of them and not on the other.
The RX-888 seems to have a very good dynamic range, as expected. My 135 m Beverage antenna gives a quite good signal as is has a very bad ground conductivity below, <20 cm of soil (moss) over pure rock. The RX-888 has shown no signs of overloading at any time and it copes very well with a tiny signal close to a super big signal.
But, as said, I can not say that I can hear stations on the RX-888 that I would not have been able to hear on the AFEDRI. However, on the AFEDRI I need to reduce the gain at night time in order to avoid over loading it. However that does not reduce the practical / usable sensitivity due to the increased noise floor at night. With both the AirSpy HF+ and the AirSpy HF+ Discovery, I need to reduce gain to avoid over loading them at night with my 135 m Beverage.
With the RX-888 I do not need to touch any gain control nor attenuator.
I am very please with the performance of the RX-888 at MW at the moment. I will continue to explore it and compare with other SDR's in different situations and report back if anything interesting shows up.
I have some IQ-recordings (10 times 5 minutes times 2 MHz, including 160 m Ham band) from the W/E to scrutinize now :-)
73 / Ingolf, SM6FHZ.

jdow
Posts: 800
Joined: Mon Aug 10, 2020 8:17 pm

Re: SDR VLF, LF and MF comparison

#9

Unread post by jdow »

At some point it becomes sensible for Simon to adopt full scale on the A/D sine wave as an arbitrary 0 dB point, whatever it gets called. ALL front ends would have different absolute scales. So if people want to calibrate to dBm they can supply a known level signal for calibration and set the correction in with a Visual Gain control.

Hm, seems Simon already has something like that. Perhaps people should use it? (It would be nice for the a**h*l*s like me who like a closer calibration to allow 1 dB steps.)

{^_-}

User avatar
sm6fhz
Posts: 179
Joined: Wed Aug 19, 2020 3:21 pm
Location: JO57XJ
Contact:

Re: SDR VLF, LF and MF comparison

#10

Unread post by sm6fhz »

Great idea Joanne!

I concur with your opinion. It has been discussed earlier, but never came to an conclusion and implementation:
https://sdr-radio.groups.io/g/main/message/53118

73 / Ingolf, SM6FHZ

Post Reply