Rf amplifer and near field probe black magic - Page 1
Forum section: electronics / testgear

author: tekman11
author_rank: Contributor
timestamp: November 27, 2014, 03:01:37 pm
content: hi all, I looking to do some EMC goofing around (I have not done this before)how much bandwidth is required and are there any cheap near field probes available?Ebay sell RF amps for $20-30 50mhz to 1ghz would these be suitable?Once I have all the stuff and measure FreqX @ xxdbm, how do I know if its a problem?thanks guys

author: alex.forencich
author_rank: Frequent Contributor
timestamp: November 27, 2014, 10:20:59 pm
content: Don't buy the probes; make 'em:http://www.compliance-club.com/archive/keitharmstrong/emc_testing1.html

author: VK5RC
author_rank: Unknown
timestamp: November 27, 2014, 10:31:27 pm
content: In the amateur radio publication Radcom, from the radio society of great Britain there are a series of articles on EMC, they might provide some background information / reading . The author is highly regarded in EMC I understand.

author: tekman11
author_rank: Contributor
timestamp: November 27, 2014, 11:47:04 pm
content: Quote from: alex.forencich on November 27, 2014, 10:20:59 pmWhat is the material used to make the Loop, is this an item available at rat shack?Great infoKeep it coming

author: alex.forencich
author_rank: Frequent Contributor
timestamp: November 28, 2014, 07:44:25 am
content: Looks like those loops are just regular wire or magnet (enamelled) wire. So you probably have what you need in your junk box. However, get some pieces of hardline coax if you want to do it right - there are some diagrams on that page for how to make a better one out of that.

author: tautech
author_rank: Super Contributor
timestamp: November 28, 2014, 07:56:54 am
content: Quote from: tekman11 on November 27, 2014, 11:47:04 pmCoax cable

author: Howardlong
author_rank: Super Contributor
timestamp: November 28, 2014, 09:51:13 am
content: It's semi rigid coax, instead of braid it's solid copper.I made some of these a couple of years ago, actually they can be quite handy. I wrapped mine in kapton tape, as it's too easy for the exposed copper outer of the semi rigid to touch your circuit inadvertently while you have your eyes turned away looking at the spec an or whatever.Rather than terminate it yourself, see if you can pick up some ready-terminated semi rigid at a radio rally/ham fest/ whatever it's called in your country. Usually, but not always, they're terminated with SMA connectors, so you'd need an interseries adapter or cable to the N type on your spec an.Alternatively, RS for example stock RG402 in 1m lengths if you're desperate. Farnell have RG401, RG402 and RG405. Soldering it, particularly the inner, can be an experience by the way, as the dielectric has a nasty habit of melting away, but there maybe someone here who has some tricks to divulge that I'm unaware of.Howard

author: biot
author_rank: Regular Contributor
timestamp: November 28, 2014, 10:37:08 am
content: There seems to be quite a gap between the cost of making one of these for what amounts to change, and the lowest price you can buy these for. The cheapest I've found is Saelig's 4-piece set for $100.I'd love to have a play with one of these -- seems like an interesting learning opportunity -- but I can't see myself making one of these. Still have a scar from the time I tried to make a 1GHz antenna (which didn't work).So does anyone know of a low-end near-field probe that's good enough to get a feel for things, but cheap?

author: tautech
author_rank: Super Contributor
timestamp: November 28, 2014, 10:22:30 pm
content: Quote from: Howardlong on November 28, 2014, 09:51:13 amYou can make them out of ~3 mm 50 Ohm Coax.Those I have are of a stiffish clear coated Coax, I have no idea which type.Being insulated they will be much more convenient to use.While they won't be as good as an EMI probe kit, they will still work fine for simple investigations.The 2 I have have different diameter loops, 20 mm & 50 mm with ferrite beads "shrink-sleeved" in place on the scope/SA side of the loop close to where the Coax inner is soldered to the braid.At ~ 1 m long terminate to the input connector of your choice.BNC seems the logical choice then use BNC to F type adapters should you need to connect to a SA.As you say: Handy as.

author: Howardlong
author_rank: Super Contributor
timestamp: November 28, 2014, 10:35:12 pm
content: The electric field and 10pF probes are most useful for me for non-EMC testing, just normal development work, although I substituted a 2pF as most of my stuff would be swamped by 10pF.The electric field one and the loops let you know something's there but not very quantitatively. The capacitive probe however lets you tune stuff relatively.I guess that's a key point about RF, to test almost anything quantitatively, repeatably, absolutely, and accurately is going to be an intrusive test. By that I mean you will need to have a known signal source physically connected to the DUT and a a detector physically connected to it too. As soon as you're dependent on the ether, all bets are off. It's not unusual for certified test labs to be +/- 6dB in over the air tests, that's quite a bit!