Technical Topics > SDR - Software Defined Radio

SDR issue with new antenna (RSP2)

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Elf36:
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.

ChrisSmolinski:
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.

NJQA:
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.

Ray Lalleu:
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.

Elf36:
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.

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