Intermodulation on RTL-SDR

[drewdavis]

The intermod on the RTL-SDRs is well documented and I was experiencing some on Friday evening. I scanned through 6m and was surprised to see a bunch of sideband convos. So I listened in and realized I was hear the 40m late net check ins. I know that having the gain turned too high will lead to intermod but I had the AGC off and gain settings all turned down. I’m using a 9:1 with a non-resonant wire as the antenna.

My question is: would a band specific antenna still act this way?

[KA1GJU]

This may come across as being harsh, but using an $11 RTL-SDR will get you the results you are seeing/hearing. I gave up on RTL-SDR's about 10 years ago, one is still used as an ADSB RXR Server for FlightRadar24 on a Raspberry Pi up in the attic of the house.
Get an Airspy HF+ and don't look back. Most of my SDRC Servers have LONG EFW's (+150') with a 9:1 unun mounted on a copper ground rod or thick walled copper pipe. You will get no issues with phantom signals appearing (on the 7th harmonic in your case). If you really need 6m, then you'll have to go to the SDR Play, but AM BC overloads those units easily and there are too many controls to fiddle with. The Airspy has fewer controls (once set you really never adjust) and all I ever do is turn the pre-amp on when on 20M and higher.
If you want to compare four diferent SDR's, I suggest you connect to my KA1GJU Super Station #2 and you sample two at the same time, the SDR-iq, Airspy HF+, SDR Play (RSPA1), and an SDR-ip. Simply drag one of the instances of SDRC over to the left edge and drag the other instance to the right edge and drop.

Attached image is Airspy HF+ on left and SDR-play on right. Settings were adjusted to give somewhat similar results.

73 Kriss KA1GJU

[jdow]

There is no effective gain control for any of the various HF paths through the RTLSDR silicon. For HF reception the expansion on the common TLA "POS" fully describes it. For frequencies above 70 MHz to 100 MHz it's usable as an introductory tool for SDR reception. Anywhere else you must add external hardware to make it even hemi-semi-demi-useful. I'd offer to send a couple your way except that I don't do something that terrible to people I'd like have a chance to get to know. Besides, my hazmat suit is at the cleaners.

{o.o}

[Max]

The intermod on the RTL-SDRs is well documented and I was experiencing some on Friday evening.

My question is: would a band specific antenna still act this way?

To answer the question you asked, yes, almost certainly. With RTL-SDR being so wide open, anything you can do that acts as any kind of bandpass filter on the front end (such as a resonant antenna) will certainly help.

However, I would say not all RTL-SDRs are born equal. Would be interested to know which version you are using? I just tried to emulate what you have shown there on my RTL-SDR Blog V4 dongle. I'm in the UK so 40m band is very strong at this time of night (about 11PM local). The antenna connected to the RTL-SDR is 132ft multiband end-fed (49:1 unun). Yes, I can see some breakthrough from HF bands, but nothing like what you have shown in your picture. If you really want to use an RTL-SDR then I would say the RTL-SDR BLog V4 (only purchased direct from them to avoid fakes) is the best of the bunch for anything like the cost:

https://www.rtl-sdr.com/product/rtl-sdr ... ngle-only/

To maximise performance you could build a front end filter unit with some bandpass/hpf/lpf filters in it. Overall I find the RTL-SDRs do work pretty well for VHF/UHF work, especially, as I said earlier, for the cost.

Just to comment on what Kris said about an alternative, I very much agree with him about Airspy HF+ being an excellent choice, but of course with the note that it has a gap at 6m band which is somewhat annoying. Of course it also has upper frequency limit of 260 MHz. However, if that caters for the frequencies you need then to me it's a "no-brainer" versus the SDR Play, which I agree as Kris says suffer to quite a large extent the same way as what you are seeing with the RTL-SDR.

If you are thinking along the lines of an effective upgrade, then wait for the Airspy HF+ Ranger. No gap at 6m band and covers up to 1750MHz:

https://www.rtl-sdr.com/wp-content/uplo ... rSpecs.pdf

However, the max bandwidth at any one time is still around 800kHz. If you want the wider sampling bandwidth (up to 10MHz) then yes, maybe consider something like SDR Play RSPdx which has better front end filtering than the earlier models. I have no idea what the cheaper new model RSP1B is like in this respect. TBH I'm not a fan of SDR Play due to my experience with the overload/cross-mod issues with my original RSP2Pro but there are loads of RSP variants online to try using SDR Console V3 Server so no excuse not to "try before you buy".

But yes, hanging a tuned VHF/UHF antenna on the front of the RTL-SDR has to be a big help at suppressing HF signals.

73

Max