Microwage RF detector
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From: tone on 24 Nov 2009 08:53 I am looking to build a simple RF detector (to prove a microwave dish is transmitting) working in the 18Ghz region. Any offers especially of the diode to use. Thanks
From: osr on 24 Nov 2009 10:14 On Nov 24, 8:53 am, tone <ax...(a)btinternet.com> wrote: > I am looking to build a simple RF detector (to prove a microwave dish is > transmitting) working in the 18Ghz region. Any offers especially of the > diode to use. > > Thanks Ouch, not a common band. This is usually done with a spectrum analyser. Unless you have a lot of TX power, you have a problem. Search for w1ghz antenna book, and look at the 24 ghz ham stuff. You need a waveguide to sma adapter and then a diode detector module. Ones that go that high are not cheap. The waveguide will probably have to be bigger then what is used at 24 so you are not "waveguide beyond cutoff" The open waveguide will be a 3dB antenna, not much gain at all. The transition from SMA to the detector will be about 2 dB of loss, So your already behind the curve. Something like this. http://www.krytar.com/html/planar_doped_barrier_detector.htm They do show up on ebay, but usually not tested. Mind you, microwaves are a RF safety issue, don't get your eyes forward of the dish. Be aware that high SWR from you sticking a probe in the wrong place could blow out your receiver front end in some cases. And even if you do have RF, it doesn't tell you how much or if your modulating correctly. This is why a RF power meter or a spec an with a calibrated antenna is used. In other words, time to find a RF technician or engineer with the correct gear. Steve
From: John Larkin on 24 Nov 2009 11:18 On Tue, 24 Nov 2009 07:14:27 -0800 (PST), osr(a)uakron.edu wrote: >On Nov 24, 8:53�am, tone <ax...(a)btinternet.com> wrote: >> I am looking to build a simple RF detector (to prove a microwave dish is >> transmitting) working in the 18Ghz region. Any offers especially of the >> diode to use. >> >> Thanks > >Ouch, not a common band. This is usually done with a spectrum >analyser. > >Unless you have a lot of TX power, you have a problem. > > Search for w1ghz antenna book, and look at the 24 ghz ham stuff. You >need a waveguide to sma adapter and then a diode detector module. Ones >that go that high are not cheap. The waveguide will probably have to >be bigger then what is used at 24 so you are not "waveguide beyond >cutoff" > >The open waveguide will be a 3dB antenna, not much gain at all. The >transition from SMA to the detector will be about 2 dB of loss, So >your already behind the curve. >Something like this. > http://www.krytar.com/html/planar_doped_barrier_detector.htm > >They do show up on ebay, but usually not tested. > >Mind you, microwaves are a RF safety issue, don't get your eyes >forward of the dish. >Be aware that high SWR from you sticking a probe in the wrong place >could blow out your receiver front end in some cases. >And even if you do have RF, it doesn't tell you how much or if your >modulating correctly. This is why a RF power meter or a spec an with a >calibrated antenna is used. > >In other words, time to find a RF technician or engineer with the >correct gear. >Steve Why not a simple dipole with a diode at the middle? You could make that on a slice of fr-4 in a few minutes, for less than lunch money. Use something like an SMS7621-079 maybe. http://www.skyworksinc.com/uploads/documents/200041M.pdf John
From: osr on 24 Nov 2009 11:30 John, at 18 on FR4? You sure? Steve
From: John Larkin on 24 Nov 2009 11:58
On Tue, 24 Nov 2009 08:30:14 -0800 (PST), osr(a)uakron.edu wrote: >John, at 18 on FR4? > >You sure? > >Steve Why not? Things scale. People use dipoles+schottkies at way higher frequencies than this. The only reason to use FR4 is because it would be an easy way to solder that _really_tiny_ part down. It would probably be easier to use real wires for the antenna itself, because they'd be easy to trim to length to tune the thing. With enough transmit power, it wouldn't need tuning at all. A patch antenna would be interesting. Googling that would give lots of hits. You could hack one on copperclad with an x-acto knife. That Skyworks diode has Xc around 50 ohms at 20 GHz, a little high but not lethal. Aeroflex has an 0402 schottky that's only 0.08 pF, which would be even better. The problem isn't quantified, so we don't know how much signal we'd have. Close to a transmitting dish, I'd assume a bunch. It's surprising what fast stuff you can do with tiny surface-mount parts on pc boards, things that used to need plumbing. John |