HFU HF Underground

Technical Topics => SDR - Software Defined Radio => Topic started by: Elf36 on October 27, 2021, 1327 UTC

Title: SDR issue with new antenna (RSP2)
Post by: Elf36 on October 27, 2021, 1327 UTC
So I finally put up a decent antenna dedicated to my RSP2 on HF. It is a 9:1 balun with 70' of main line and about 25' of counterpoise. So before I was just using an MLA-30 and I was so excited to compare the 2 antennas. I'm sure the new longer antenna will pull in weaker signals, but the noise it brought was bad too. It seemed like most all of the time frequencies 5000/10000/15000 etc were coming in clearer with the MLA-30. The signal may have been larger on the wire, but the MLA-30 was actually more clear? It seemed similar when I hooked up my 135' OCF dipole. I first had it hooked up to the second ANT port (Non bias T) then I even tried making a jumper for the HI-Z port with wire and a SO239. The HIGH-Z connection is supposed to be optimized for HF, but it seemed to make matters worse? Maybe its settings in SDRuno that I need to change or learn to adjust? I was pretty deflated with my results. Any advice would be greatly appreciated.
Title: Re: SDR issue with new antenna (RSP2)
Post by: Elf36 on October 27, 2021, 2124 UTC
Just got home and have messed around with the High-Z side, hooking up earth ground etc. No luck. Currently going back and forth between the MLA-30 and my 135' OCF dipole. The MLA-30 seems to work just as good as 135' of wire up high? I'm at a loss

The MLA-30 is hooked to port B with the Bias-T amplifier and the OCF is on Port A

I also noticed when I was tuned to one of the time stations 10.000/15.000, the OCF was picking up something completely different. Spanish music? I hooked up a NooElec AM broadcast filter, even though I'm certain there are no AM broadcast transmitters anywhere near me. Did seem to help get rid of the music though?
Title: Re: SDR issue with new antenna (RSP2)
Post by: ~SIGINT~ on October 27, 2021, 2246 UTC
Here are my settings in the image below if it helps. No issues with my unit and and it has been running for several years now and I live in the middle of RF city. I primarily use a Barker & Williamson antenna (T2FD) in an inverted-V configuration followed by an end-fed longwire, a Wellbrook loop and a mini-whip. All 4 antennae exhibit different receive characteristics. After installing the loop I had to add a Flamingo AM filter as the loop was too sensitive and would overload the SDR's input. I now also have a Flamingo FM filter in-line notching out both broadcast bands. All 4 antennae are connected to an external coax switch and to the A input on the SDR. My B input is used for ADS-B, VHF and UHF.

(http://www.milspec.ca/board/img/2021-10-27_183425.png)
Title: Re: SDR issue with new antenna (RSP2)
Post by: Elf36 on October 27, 2021, 2328 UTC
Thanks for the reply. I really hope I get it worked out. I was really looking forward to doing more listening. Especially Utility type stuff. I've turned knobs for many years and enjoyed it, but I also realize that I could hear much more being able to see more spectrum at a time. I live in a fairly rural area and don't have a lot of high tech devices in my house etc. I just thought I'd be blown away by my new large wire antenna. I typically use my HF rig for listening for the most part. I'll keep plugging along, but I just don't get it?
Title: Re: SDR issue with new antenna (RSP2)
Post by: Josh on October 28, 2021, 0039 UTC
On the bigger antenna, might have to reduce gain in the sdr to make sure you don't max out the adc. With so much surplus gain designed in most in modern sdr you can stand to add some decimation as well as gain reduction, both will increase dynamic range.
Title: Re: SDR issue with new antenna (RSP2)
Post by: Elf36 on October 28, 2021, 1105 UTC
Yes. I will just have to keep trying with gain adjustment etc.
SIGNIT- For some reason I didn't see the screenshots for your settings last might on my PC, but now I do. I'll do a comparison and match and see how that works.
Title: Re: SDR issue with new antenna (RSP2)
Post by: ChrisSmolinski on October 28, 2021, 1116 UTC
I'll echo Josh's suggestion to check the gain - the SDRPlays can overload.

Regarding overall noise / performance between the two antennas: What is the ambient RFI situation like at your QTH? If RFI levels are relatively high, then an outdoor antenna is not going to perform significantly better than a simple indoor loop. And could even be worse if the outside antenna is closer to the noise source(s). You said you're in a rural area so the chances of high ambient RFI are fortunately low, but not zero.

I assume you have coax from the balun to SDR. Have you tried a common mode choke on the coax? They can work wonders, I use them here. Wrap lots of turns (15 or more) around a large toroid ferrite core.  I use the Fair-Rite 5943003801 which you can get from Mouser, etc.  Search the Equipment board here on the HFU and you can probably find some before and after waterfall screenshots I have posted.
Title: Re: SDR issue with new antenna (RSP2)
Post by: NJQA on October 28, 2021, 1234 UTC
I think everyone has hit the most likely problems:

signal overload (do you have a nearby AM,FM, TV, Public Service, etc xmtr)
SDR settings (critical - one incorrect setting can cause all sorts of problems - especially gain and decimation)
common mode (which I feel is greatly under appreciated as a potential problem)

I would tackle the SDR settings first, common mode next, and then worry about adding external filters.
Title: Re: SDR issue with new antenna (RSP2)
Post by: Ray Lalleu on October 28, 2021, 1444 UTC
The OCF antenna is a bad idea, prone to enhance the local noise, and also the signals from the nearby MW stations. You may have almost no local noise from neighbors, but your line is carrying the noise from your own house to the antenna, and as the antenna is unsymetrical, the noise go back with wanted signals to your receiver.

The simplest way to get by : changed your OCF in a center fed antenna with both arms same height above ground, and run the line in the center plane as far as possible from the antenna, as least a quater wavelenght for any band of interest, so the common mode noise on the line is just coupled equally to both arms of the antenna, and as the arms are connected in opposition of phase to the line, the noise is cancelled. BTW, the strong MW signals are also cancelled, as the antenna becomes insensitive to vertically polarized waves on lower frequencies.

The OCF is just a concept for transmitting in the harmonically related ham bands. Nothing good for receiving, as the SWR is not a problem, specially if you use a high impedance antenna input!  You even have no necessity to use coax cable. Any twisted line is enough. Use a twisted line from any scrap of phone or network cable to go trough your window sill. The outdoor part of the line can be made with widely spaced wires (lower losses when the whole antenna+line is tuned), with just plastic spacers cut in plates for flower boxes (or any plastic good enough for sunlight exposure). The impedance of such a line is not important. For the best results, you need an ATU including a balanced input.

A center-fed antenna with tuned line is known as 'doublet' in England and 'Levy antenna' in France and elsewhere. A doublet apparently sized for 6MHz can give good results on any frequency between between 3 and 12 MHz, and even more. Lower, the tuning becomes very sharp, higher the receiving pattern has more and more lobes. I don't understand why that antenna is so badly understood in the USA.
Title: Re: SDR issue with new antenna (RSP2)
Post by: Elf36 on October 28, 2021, 1830 UTC
I could easily make a dipole for 6Mhz. should be about 38/40' per leg. It would likely need to be skewed on one side though. It would not be perfectly flat-topped with both legs being at equal height. I guess I wouldn't need a balun, but would would prob try to get some coax wraps, preferably with some toroids or wrapped around a large one , as Chris pointed out. The frequency range you mentioned should cover most all of my listening needs. I guess I thought it would be just like using my HF rig for listening. I don't have any issues with it. Good RX, little interference, no overload etc. It will be worth it though, to get it figured out. The SDR should help me cover larger bandwiths quicker etc and all the other many features. I will try most everything mentioned by everyone. Lot's of great feedback from the group. Thank you all for your help. I'll keep you posted.
Title: Re: SDR issue with new antenna (RSP2)
Post by: ~SIGINT~ on October 28, 2021, 2117 UTC
I highly recommend the SDRUno Cookbook. It has saved me many times, and still does.

The latest version is available on-line:
https://sdruno.nn4f.com/ (https://sdruno.nn4f.com/)

And an older copy in PDF format is still availabe here for download:
http://n6pet.com/wp-content/uploads/2016/06/SDRuno-cookbook.pdf (http://n6pet.com/wp-content/uploads/2016/06/SDRuno-cookbook.pdf)
Title: Re: SDR issue with new antenna (RSP2)
Post by: Elf36 on October 28, 2021, 2245 UTC
Thanks. I just opened the link. I'll check it out after dinner.
Title: Re: SDR issue with new antenna (RSP2)
Post by: RobRich on October 29, 2021, 1346 UTC
Assuming it is not local EMI/RFI causing your issues, have you considered a different antenna deployment? An end-fed inverted-L could offer DX benefits, especially since you already have a decent horizontally-polarized antenna. An inverted-L would add vertical polarization into the mix.

Feed it at the base with your 9:1 transformer. Use a few short ground radials for the RF ground side, though alternatively you could opt for just a ground rod since it is only a receiving antenna.

Look here from some ideas on antenna length:

https://palomar-engineers.com/tech-support/tech-topics/best-hf-end-fed-antenna#:~:text=Suggested%20wire%20lengths%3A

You are not transmitting, so you can disregard the coax length recommendations. The designs linked above are using coax losses to mask impedance mismatch, which is not a huge concern for your receiving setup.



Back to the dipole idea, if you can not fit an "ideal" flat-top, then perhaps an inverted-v would fit? Again, that would get you a little vertical polarization as well.




If you do mind experimenting at little, you might find your MLA-30 preamp would work decently with a loop-on-ground antenna. Even just a 60' LoG should suffice with a preamp.
Title: Re: SDR issue with new antenna (RSP2)
Post by: ~SIGINT~ on October 29, 2021, 2107 UTC
I have 2 x Bullet-9LF (.02-30 MHz) matching transformers from Palomar Engineers and I like the units. I have 1 in service as and end-fed antenna and the second as a spare to experiment with. I am also using the CMNF-500 as the common mode noise choke. As mentioned, the counterpoise is optional as the Bullet uses the coax's shield as the counterpoise. You can see that specified in the "suggested wire lengths" table. What is not so apparent on the web site is that you have to install a choke at the "Minimum Coax Length" or else the entire length of the coax becomes part of the antenna and ultimately will bring noise into the receiver on the shield. The choke is shown in one of the diagrams. This is well detailed in the instructions that ship with the Bullet.
Title: Re: SDR issue with new antenna (RSP2)
Post by: Elf36 on November 07, 2021, 2253 UTC
I've had more time to compare the antennas and definitely notice a difference now. I think the first compares were similar due to the fact that the signals were strong, such as AM broadcasts or time stations. Listening to ham radio or pirate stations show much more gain on the wire vs the MLA-30.