Weird RXR action when changing center of band

[KA1GJU]

While investigating the QRM generated by a local AM BC station that jumps to 5KW at local sunrise, I found the following action with the Radio Airspy Discovery using my
normal settings of:
AGC High (or low) doesn't matter
Attenuator OFF
Pre Amp OFF
Visual Gain 0dB

The signals on 1930kHz go from unreadable to readable, along with all the QRM from AM 1540 (and AM 1540 mixing with AM 1270 and or AM 1450) when right clicking on the band scope to "slide" the view left or right of the tuned signal. Is this the action of the band pass filters in the Airspy or something else?

See attached movie.

[KA1GJU]

Another brief movie of the ever shifting (image?) of AM 1540 station using the slider at the bottom of screen.

73 Kriss KA1GJU

[Max]

The signals on 1930kHz go from unreadable to readable, along with all the QRM from AM 1540 (and AM 1540 mixing with AM 1270 and or AM 1450) when right clicking on the band scope to "slide" the view left or right of the tuned signal. Is this the action of the band pass filters in the Airspy or something else?

Kriss, I've seen exactly this sort of behaviour with the SDR Play RSP2Pro and also the RTL-SDR i.e. shifting "ghost" images when moving the waterfall centre frequency. I have not yet seen it on the HF+ Discovery but my most powerful local TX is about 7 miles away with ERP no more than 1kW and as far as I can see the Airspy copes with that with no issues, unlike the RSP2 where that AM station crops up all over the place even at much higher frequencies!

Out of interest have you tried the same thing with one of your RSPs?

What immediately comes to mind is that as the Airspy has a local oscillator and mixer, that the spurious mixing products caused by the very strong local AM signal will change as the LO frequency is changed. Every time you move the waterfall span you move the LO frequency (from Airspy notes it's not at all clear what frequency this is but apparently it's quite a lot higher than the signal frequency (I have no idea how a "polyphase mixer" works) hence you also change the output frequency of the non-linear mixing products. My guess is you would not see this on a direct sampling receiver. The Airspy is excellent but maybe the strength of you local AM 5kW station is too much even for the HF+? Maybe see what happens with input attenuator on the antenna socket? Pain, as I know your receivers are remote, bit would be an interesting experiment.

As can be seen on the Airspy block diagram, the AGC acts in three places, and this seems to be borne out as effectively, on the AM band at night here in the UK, the HF+ does not seem to be usable without the AGC on even with adjusting the front end attenuator setting. Because of this I'm not at all sure what value there is in having an AGC-Off mode at all for the HF+. I wonder if anyone ever uses it at all?

https://airspy.com/airspy-hf-plus/

Max

[KA1GJU]

Max, the RSP1A does the same thing, even worse with other AM BC showing up in the 120m band (2400kHz +/- 100 kHz), even after local sunset here after my local station went QRP. You can chase them up and down the band while centering the scope/water fall on other areas of spectrum.

[Max]

Max, the RSP1A does the same thing, even worse with other AM BC showing up in the 120m band (2400kHz +/- 100 kHz), even after local sunset here after my local station went QRP. You can chase them up and down the band while centering the scope/water fall on other areas of spectrum.

Yes, exactly what I have observed. Has to be spurious mixing products. That's the joy of an analogue mixer on the front end or a low-priced receiver. But from my observation, the HF+ does very well, and considering it's in a tiny box smaller than the size of a matchbox, I just don't know how they do it. How far away is the 5kW station? My guess is it must be presenting itself to the front end of the HF+ as a real flame-thrower.

No surprise whatsoever on the RSP1A....... I always feel I'm being brutal when describing their shortcomings (especially as they are a UK company and I'm in the UK..... I'd much rather be able to praise them) and from my own experience I can just never understand all the super-glowing reviews on YouTube and press, but prepared to believe more recent models maybe have better performance than RSP1/RSP2? Then again, to be totally fair, it's a different tool for a different purpose, with hugely wider sampling bandwidth than the HF+ if needed and full VHF/UHF/SHF coverage. But a serious HF DXing tool they ain't, at least of you have any local very strong MF/HF stations like we both seem to. (Again, IMHO).

Max

[jdow]

As I recall you tend to use very big antennas. Overload is a very real problem in very large signal AM BC settings. Do remember that practical filters do not have very high (60 dB or higher) image or alias rejection. The numbers on your screen to the extent they mean anything suggest you have VERY strong AM stations present. Just for grins and giggles try a smaller antenna or placing a step attenuator in line with the antenna set to 30 dB. I bet the effect changes. If it does you can adjust the attenuator until the effect is at acceptable levels. A 30 dB broadcast band rejection filter might do the trick. Maybe even 20 dB. BUT, this is where it can get weird. If there are any fairly long wire fences around dirty connections can give this effect with really strong signals. This is a serious problem in mountain top repeater arrangements around here. Tower maintenance must also concern itself with preventing loose connections. A loose connection plus rain and time can lead to really nasty hard to track down interference problems. (I have often mused about getting #@*&*& with the police and building a super simple jammer - a beach ball with a dipole antenna for the police radio band with a diode at the feed point.)

{^_^}

[KA1GJU]

It turns out, I can hear the same crap on my rice box radios too. Today, while replacing the antenna on my server at the PCARC club station, I could hear the 1st and 2nd harmonic of the 5KW AM radio station (AM1540) on 3080kHz and 4620kHz both on the SDR on a 175' EFW and on a 80m doublet using an IC-7300. Two not so identical radios and antennas separated by about 300'.
I'm beginning to wonder if the Airspy HF+ firmware update from Dec 11(?) with it's "gain distribution" has anything to do with it. But I'm hearing the 1st harmonic on 3080kHz on my home stations too, which are also only 5.1 miles from the TXR.
I have a high end multistep attenuator to test with tomorrow and my DIY AM BC filter. If the AM BC filter doesn't remove the 1st and second harmonic, then that would tell me the station has some issues that need to be brought up to the engineer. The mayhem all starts at 0640 or so, or that when they bumped the power from 1kW to 5kW this AM while I was monitoring for products.

[jdow]

In ham and RF engineer terms, as opposed to musician terms, the first harmonic is the fundamental frequency, 1540kHz in this case. The 3080 kHz is the second harmonic. There is logic to both ways of thinking just as there is with floors labeled G, 1, 2, 3.... in Britain.

Anyway - almost any rectifying junction, rusty fence wires maybe, can create and reradiate harmonics. Seriously, dirty metal in contact with other metal, particularly long pieces of metal, can cause this sort of a problem. So can an overload. It'll be interesting what you dig up.

BTW - "ntia" is the agency you want to visit as well as the FCC sites for specification various services must meet in terms of harmonic and other spurious emissions. You might find that if you are close enough to a fire breathing dragon its off spring (2nd and 3rd harmonics as it were) might be legal yet still high enough for you to detect. It takes real work to attenuate a harmonic of a 10 kW signal (+70 dBm) down to 1 mW (0 dBm). Up stream cables may leak energy at that level. And 1 mW can be a usable signal at HF. And no shield is perfect.

{^_^}

[KA1GJU]

@JDOW

The issue with this radio station appears on all my servers that are down here in the lower part of NH. The club station (KA1GJU@PCARC) is 5.2 miles NE and my QTH servers (KA1GJU Super Station #1 thru 4) are at 5.1 miles SSW of the offending station.
I placed an adjustable attenuator in line, and it did do what it's supposed to do. When I set it to from 0 to 10dB, I witnessed a drop in all signals by 10dB, when on 20dB I saw a 20dB drop on all signals, etc. The harmonic from 1540kHz on 3080kHz was still there, just reduced by the amount of attenuation applied.
As for my AM BC filters, they are currently MIA, so I ordered some more and are due to arrive in a few days... pending the incoming snow storms.

I have an IC-7300 on Remoteham's on a 80m flat top doublet at the club station, it hears the harmonic at S9+13dB of the end of the doublet (not broadside to it).
Attached are what I see/hear via two platforms. The SDR antenna is 300' away from the doublet deep in the woods and has a similar signal strength on it.

[jdow]

If you can go man portable with a tuned ferrite rod antenna testing to see if the second harmonic's relative power changes as you wander around the property. Or simply drive half the distance to the station. If the station's primary power and the second harmonic change comparably then present your findings to the station's chief engineer. Show him how he could monitor for such problems with an SDR in his car for cheap. He might appreciate it even if it is not NBS traceable and all that stuff.

{^_^}