Cable RF pickup.

Useful information regarding antennas for SDR products.
Paul314
Posts: 17
Joined: Thu Nov 23, 2017 8:39 pm

Cable RF pickup.

Postby Paul314 » Sun Nov 26, 2017 10:44 am

Hi,

I just purchased two RSP2pro for the purpose of noise correlation measurements. For the price these are nice radios. Anyway, I tested a 3 foot long pice of RG58 coax where I used a SMA to BNC on the input to the radio. The other end of the coax was measured terminated, open and shorted. Even shorted this section of cable is a much better antenna than I would have naively thought. A strong station at 105MHz dropped only about 20 dB. I assume this is due to some small residual impedance across the connectors but wasn't able to find the leak by poking and prodding the various joints. Any suggestions on how I might best achieve an electrically closed system?
Last edited by Paul314 on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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g1hbe
Posts: 350
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire, UK

Re: Cable RF pickup.

Postby g1hbe » Sun Nov 26, 2017 2:41 pm

Hi Paul. RG58 comes in various flavours and some of them are terrible, with loose braid providing little screening. The other possibility is that RF is leaking into the RSP via the USB cable. A ferrite choke on this cable (as close to the RSP as possible) may help.
I'm not sure how the metal box of the Pro is grounded internally, but some people report an improvement by fitting a thick copper strap between the antenna connector and the USB metalwork.

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Andy

Paul314
Posts: 17
Joined: Thu Nov 23, 2017 8:39 pm

Re: Cable RF pickup.

Postby Paul314 » Sun Nov 26, 2017 3:37 pm

Thanks.

Removing the coax I get a decent noise floor. One can still see the station carrier peaking out of the grass from time to time. This little residual bit is likely from the USB cable. If it's shabby coax braid then I feel I got ripped off, the cable cost like $20 at Fry's. I tried a short (5") section of semi ridged with SMA connectors. Unterminated this yields little or no change compared with the open radio input. I did see some leakage with a 50 ohm SMA termination when I grab the terminator. I think I'll just switch to SMA and hopefully better quality coax.

The copper grounding strap sounds like something to keep in mind if the need arrises. I'll know more when I start measuring the correlation between the radios. The whole goal is to beat the noise floor down on the measurements.
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13dka
Posts: 136
Joined: Sun Feb 14, 2016 8:40 am

Re: Cable RF pickup.

Postby 13dka » Sun Nov 26, 2017 6:23 pm

AFAIK even the good types of RG58 provide a shielding of only 45dBm, so using this in any high-sensitivity application and/or noisy environment is simply a no-go. Use double-shielded 75 ohms sat coax or, to be on the safe side, at least H155 instead.

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g1hbe
Posts: 350
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire, UK

Re: Cable RF pickup.

Postby g1hbe » Sun Nov 26, 2017 11:01 pm

I agree with 13kda - I use satellite cable CT100 etc for all my scanning and VHF/UHF listening. It's low loss up to around 2GHz, flexible and cheap, and it has foil+copper braid. It's 75 ohms, but that makes no significant difference. The only drawback is that it's a 7mm dia cable so you may need to get a bit creative with connectors. I terminate my coaxes with satellite F-plugs and then adapt as necessary.

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Andy

Paul314
Posts: 17
Joined: Thu Nov 23, 2017 8:39 pm

Re: Cable RF pickup.

Postby Paul314 » Mon Nov 27, 2017 4:24 am

Wow, thanks. I'll look into it. 45 dB is no where near good enough for what I want to do.
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