Should coax do this (noob question)? - Page 1
Forum section: electronics / rf-microwave

author: casterle
author_rank: Regular Contributor
timestamp: January 15, 2021, 11:46:58 pm
content: I recently bought an NESDR V4, and while trying to reproduce a simple experiment whereby the strength of a received FM signal would change as I varied the length of the telescoping antenna, I found could not. Which led me to discover that, even after removing the telescoping element entirely, I received the signal at about the same strength. The SDR (thankfully) receives no signal with the antenna cable disconnected.The station I was tuning is relatively strong, probably in the middle of a local pack. My antenna is located at a little above ground level due to the location of my office within my home, so no signal is terribly strong.I had also ordered a 15' RG174 cable, so I attached that to the SDR and, with the cable alone, I received a signal comparable to my previous experiment.Finally, I grabbed the 'open' calibration attachment from my still-unused NanoVNA and installed that on the open end of the cable. If that caused signal degradation it was not apparent.Is this an indication that, while the cable I bought may have been "Amazon's Choice," it should not have been mine?If so, what should I have bought?

author: Bud
author_rank: Super Contributor
timestamp: January 16, 2021, 12:22:29 am
content: This is an indication of poorly controlled setup in terms of RF shielding. Your device receives RF with no antenna because of poor sheilding of the casing and via coupling to external wiring, i.e. power and data lines. Furthermore, coax cables typically do not provide 100 % shielding. If you connect a long coax it may well work as an antenna itself. If your SDR is a USB doungle, it may be difficult to eliminate stray coupling as some of it may be just getting through the USB connection.

author: Co6aka
author_rank: Unknown
timestamp: January 16, 2021, 04:21:39 am
content: Essentially, the shield of the coax is acting as the antenna, and the USB cable is acting as the counterpoise (which of course is another part of the "antenna.") Ferrites on both right at the SDR would likely have a substantial effect. Also, "RG-174" from the backalleys of you-know-where isn't exactly a "shielded" cable.

author: casterle
author_rank: Regular Contributor
timestamp: January 17, 2021, 07:11:37 am
content: Quote from: Bud on January 16, 2021, 12:22:29 amThe dongle is enclosed in a metal case and receives no signal with nothing attached to the antenna connector. My guess is I bought an inferior brand of cable (assuming RG-174 was a good choice for short runs to start with).Would you please point me to a good brand/suggest a better cable type?

author: casterle
author_rank: Regular Contributor
timestamp: January 17, 2021, 07:24:02 am
content: Quote from: Co6aka on January 16, 2021, 04:21:39 amI don't understand how that would work. The shield of the coax is connected, via sma connector, to the metal case which shields the receiver. Meanwhile, the center conductor is completely shielded by means of the 'open' cal plug on one end of the coax and the metal SDR housing on the other.Quote from: Co6aka on January 16, 2021, 04:21:39 amI've got a few ferrites on hand and will give that a try. I suspect the real problem is inferior (although highly rated in the back alleys of Amazon) RG-174. Would you please suggest a reliable brand and steer me to a more appropriate cable type for short interior runs (10 feet max) as needed?

author: Bud
author_rank: Super Contributor
timestamp: January 17, 2021, 07:35:58 am
content: Look for double-shielded coax such as rg-316 DS and others, but they cost more. Also there are solid shield coaxes but i do not think you need to go there.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: January 17, 2021, 01:59:54 pm
content: shielding attenuation of a common RG174 is 40dB best case; so it's very likely to have crosstalk of strong local stations

author: ConKbot
author_rank: Super Contributor
timestamp: January 17, 2021, 02:19:55 pm
content: It sounds like the sma isn't connected to the case very well. If there is a large loop area from the SMA to the case, then at high enough frequency, they aren't electrically short enough to be directly connected anymore. If they connection at the case is bad, time to use either some conductive adhesive copper tape, or some sort of EMI gasket material to ground the barrel of the connector better.

author: A.Z.
author_rank: Super Contributor
timestamp: January 19, 2021, 11:27:12 am
content: Quote from: casterle on January 15, 2021, 11:46:58 pmQuick experiment, connect the coax to the SDR SMA, pick some (possibly short) lenght of insulated wire, remove the insulation at both ends, wrap one end around the SDR SMA connector (gnd) and connect the other end to a ground (a water pipe if you don't have a better one) and check if the signal is still received even if there's no antenna connected to the other end of the coax; consider that if the FM station is a "blowtorch" it will be difficult observing much difference when changing the antenna length, also consider that to properly run the "length experiment" you may/will want to disable the SDR AGC and manually adjust the gain to a fixed level

author: casterle
author_rank: Regular Contributor
timestamp: January 28, 2021, 08:47:42 pm
content: Quote from: Bud on January 17, 2021, 07:35:58 amI've got 6 feet of RG-316DS on the way. Cost nearly $20 delivered, but well worth the price to eliminate the coax as a possible problem.

author: casterle
author_rank: Regular Contributor
timestamp: January 28, 2021, 08:58:59 pm
content: Quote from: ConKbot on January 17, 2021, 02:19:55 pmGood detective work!Upon close examination, although it *looks* like the connector is bolted to the endcap, any actual contact between the two would be serendipitous.I can fix that!

author: casterle
author_rank: Regular Contributor
timestamp: January 28, 2021, 09:07:14 pm
content: Quote from: A.Z. on January 19, 2021, 11:27:12 amGood idea, but no pipes in range. What I need to do is get my hands on a good old-fashioned solid copper ground rod, pound it into the ground right outside my lab window, and run a wire into the lab. I did that at the house I owned before I retired to make sure we had a solid ground and should do the same here.I've got some high quality coax on the way, and ConKbot lead me to find that the SDR's nice metal case wasn't actually connected to the SMA's ground, so I hope I'm good until summer.

author: coppercone2
author_rank: Super Contributor
timestamp: January 28, 2021, 09:29:40 pm
content: you can think of the SDR as a accidental fast active dipole, with the coax cable being one antenna and the other stuff being the other part of the antenna.

author: emece67
author_rank: Frequent Contributor
timestamp: January 29, 2021, 09:28:21 am
content: .

author: A.Z.
author_rank: Super Contributor
timestamp: January 29, 2021, 09:52:18 am
content: Quote from: casterle on January 28, 2021, 09:07:14 pmWell, a ground rod would give you pretty good electrical grounding, but you'll also want RF grounding, so adding some radials spread on ground will help, you may also check the discussionhereandherefor ideas (and pitfalls) in shack grounding; another thing you'll want to take care of is the nasty "common mode noise", a good cure for it are some simple chokes (at least a couple, one near the antenna and another one near the RX input) built by winding 18 turns of thin coax (e.g. RG-174) over a #43 material toroid (an FT140-43 will fit well)

author: casterle
author_rank: Regular Contributor
timestamp: January 30, 2021, 05:26:42 am
content: Quote from: coppercone2 on January 28, 2021, 09:29:40 pmI wish I knew what you just said there. I don't know much about antennas, but as I recall a dipole is basically a wire of appropriate length laid out in a straight line, divided at the center with the coax's center conductor connected to one wire and the shield to the other. If, in my case, the coax is acting as one of the wires, what 'other stuff' acts as the other? The only other connection to the SDR is the USB cable.I'll be learning about antennas along with other RF-related topics as I explore the new (to-me) world of SDR. Today I watched a video demonstrating how I can grab weather satellite images as the satellites pass above (with a very simple antenna, no less).Ain't technology grand?

author: coppercone2
author_rank: Super Contributor
timestamp: January 30, 2021, 05:34:13 am
content: Yea, I think you got that right. By fast I just mean its really high speed, usually people make these for low frequencies, from what I read online that term seems to be used for not much higher then like LF. Like two wires into an instrumentation amplifier.Not that is not natural behavior, I think some hand held radios rely on your body to form the other end, so they only work when you hold them. But I dunno if you can consider that a dipole or a counterpoise or what.I think its because you don't have a sealed metal body, and USB is weird, like often USB shield is connected through a 1K resistor (check it our for yourself, the plutosdr does it), for some kind of weird cheap-emi passing reasons.So the coaxial cable and the ground plane forms 1 part of the circuit, and the other stuff and USB shield or possibly USB wires forms the other.IIRC USB usually starts arguements regarding shield connections, I believe some systems crash when you actually ground the USB shield to the ground plane, but of course what do industrial people say for shields? connect right to the ground plane with minimum impedance.

author: casterle
author_rank: Regular Contributor
timestamp: January 30, 2021, 05:35:37 am
content: Quote from: A.Z. on January 29, 2021, 09:52:18 amNow that I'm investing in new equipment, modules and components, I should pay attention to electrical grounding. I can't say I trust the 40-year-old wiring in the little house we've retired to.Quote from: A.Z. on January 29, 2021, 09:52:18 amI've got a lot to learn! The links you've graciously provided will no doubt help.

author: coppercone2
author_rank: Super Contributor
timestamp: January 30, 2021, 05:39:40 am
content: This is common with USB-shield. So you can imagine the analysis might get weird, especially with a plastic body. I want to say I have seen everything from a direct connection to 5k to 100k there, with capacitors between 5nF-100nF, or just capacitors, or just a resistor I think. (maybe missed the capacitor). Oh right and sometimes there is ferrite added to this or just ferrite.https://www.eevblog.com/forum/projects/usb-shield-ground-connection/What you can try to do is get a optical USB cable, power it from batteries. The optical cables normally provide power but make the data lines optical (go figure), but you can connect it to a PCB which disconnects the power, but you still have a long wire in the cable... so you could cut the cable off and splice in your own fiber optics too. I bought a 50 feet optical USB cable for my plutosdr but I some what lost interest in it.

author: coppercone2
author_rank: Super Contributor
timestamp: January 30, 2021, 05:46:03 am
content: have you tried a USB cable with ferrite ? they exist.

author: casterle
author_rank: Regular Contributor
timestamp: January 30, 2021, 05:54:26 am
content: Quote from: coppercone2 on January 30, 2021, 05:34:13 amI didn't realize there might be a resistor in a USB ground, that's good to know. As for antennas and RF in general, I've got a lot to learn.

author: casterle
author_rank: Regular Contributor
timestamp: January 30, 2021, 05:55:46 am
content: Quote from: coppercone2 on January 30, 2021, 05:46:03 amNot yet. I've got some ferrites around here somewhere...

author: A.Z.
author_rank: Super Contributor
timestamp: January 30, 2021, 08:14:14 am
content: Quote from: casterle on January 30, 2021, 05:54:26 ama very basic introduction may be this onehttps://www.hamradiosecrets.com/shortwave-antenna.htmland thenhttps://on8alt.files.wordpress.com/2017/05/easy-antennas-for-the-swl.pdfhttps://kv5r.com/ham-radio/shortwave-antennas/the above should hopefully get you started, just... experiment, wire antennas may be built in different configs (endfed, dipole, loop ...) and are cheap, so just experimentyou will also need a balun, the easiest one is thishttp://www.m0pzt.com/blog/4to1-current-balun/the balun can be wired as a 4:1 (as shown) or as a 1:1 by connecting the antenna side wires in parallel (exactly as the coax side ones), such a balun will also act very well as a common mode choke, so it will be double duty, if you aren't going to use it for TX the toroids can be replaced with 140 ones (which will also be ok for TX up to around 100W)[edit #2]willing to improve higher frequencies performances, instead of #31 material use #43 one and change the total number of turns to 18 (for each core)

author: casterle
author_rank: Regular Contributor
timestamp: February 01, 2021, 06:16:38 pm
content: My RG-316 DS has arrived and WOW, what a difference pricy coax makes! Lesson learned.

author: casterle
author_rank: Regular Contributor
timestamp: February 01, 2021, 06:19:03 pm
content: Quote from: A.Z. on January 30, 2021, 08:14:14 amThanks for the references; I think I'm off to a good start.

author: A.Z.
author_rank: Super Contributor
timestamp: February 01, 2021, 08:07:30 pm
content: which frequencies are you most interested in ? Because that will condition the antenna choice