A preventative for the problem Ed describes is to put an ³N² barrel adapter (connector saver) on the coax-to-waveguide adapter and never remove it. That way if you have some out-of-tolerance coax cable connector, there is some ³slop² between it and the effect on the critical stub dimension within the waveguide part of the adapter. I suppose that a right angle adapter would be even better at this. Right angle adapters themselves have a frequency cutoff; beyond about 8 GHz they introduce too much vswr, but they have rounded right angle adapters which work above 8 GHz, although these are much more expensive (~$300).
Ken Javor Phone: (256) 650-5261 From: Ed Price <[email protected]> Reply-To: Ed Price <[email protected]> Date: Tue, 9 Aug 2016 02:21:55 -0700 To: <[email protected]> Subject: Re: [PSES] Standard Gain Horn Antenna Calibration I¹d like to expand a bit on horn antenna calibration durability. Most horns will show obvious damage if geometrically distorted by a collision or fall, so a visual inspection of the corners and flat planes should be normal practice every time you begin to use a horn. OTOH, there is one very vulnerable design point that will not show any evidence of damage; I¹m talking about the feedpoint transducer or wave launcher. Some antennas have a waveguide flange, so you need to use a waveguide to coax adapter (because almost nobody uses flex waveguide back to their analyzer equipment). Other horns will have this built into the antenna. Both have the same weak point, although it is cheaper to replace an external adapter than a whole antenna. Most all of the waveguide to coax adaption schemes use a small probe which extends into the waveguide and is contiguous with the center pin of the coaxial fitting. The position and length of the probe within the waveguide is critical, any variation will have large effects on efficiency (ACF) and SWR. Now, here¹s the danger; suppose you have a nice heavy coax cable, maybe like RG-213, but the tech who assembled the N connector was a bit sloppy and allowed the center coax pin to extend too far from the end of the connector. When you firmly mate that N male coax connector to the N female coax connector on the coax to waveguide adapter, you may apply axial pressure to the N female center pin. The result is that the N female coax pin pushes the little probe out of critical position, and you never notice it. Ken is right when he tells us that you can detect this by electrically looking down the coax. But how many of us regularly check our coax cable connectors for dimensions, even with a go/no go set of mechanical gauges, let alone do a VSWR/return loss test every time we hook up a horn antenna? I¹ll bet that very few engineers do a check like this (except for the ones who have already been bitten by this problem, and even then, you get complacent after a few years J). I¹m not sure how probable this scenario is, but it can happen, it happened to me and I¹ve seen this at other labs too, so it¹s something we should be aware of as long as horns are designed the way they are. BTW, getting back to horn calibration, I always left the external waveguide to coax adapter permanently affixed to any calibrated horn antenna. That way, the adapter was periodically calibrated as part of the antenna, and I eliminated the potential issues of cleanliness/conductivity of flange faces, bolt torques, and having to keep track of separately calibrated adapters. Adapters are cheap compared to the cost of an antenna, so dedicating an external adapter to a waveguide antenna makes sense to me. Ed Price WB6WSN Chula Vista, CA USA From: Ken Javor [mailto:[email protected]] Sent: Monday, August 08, 2016 6:55 AM To: [email protected] Subject: Re: [PSES] Standard Gain Horn Antenna Calibration Up-front disclaimer: I have zero knowledge of the cited IEEE standard. That being said, the title excludes antennas, and it would seem that a discussion of antenna critical dimensions would be outside its scope in any case. The critical dimensions of a horn are those affecting its gain and match to a transmission line. The gain is set by the open aperture dimensions, and the match is set by the flare angle and also (if applicable) the functioning of a waveguide-to-coax adapter. The latter can be checked measuring vswr into the horn from the transmission line, and that is easy to check measuring return loss mid-band using a directional coupler. Ken Javor Phone: (256) 650-5261 From: itl-emc user group <[email protected]> Reply-To: itl-emc user group <[email protected]> Date: Mon, 8 Aug 2016 04:30:04 +0000 To: <[email protected]> Conversation: Standard Gain Horn Antenna Calibration Subject: [PSES] Standard Gain Horn Antenna Calibration Hello All According to ANSI 63.4: 2014 Clause 4.7.3 ³Standard gain horns need not be periodically recalibrated, unless damage or deterioration is suspected or known to have occurred. If a standard gain horn is not periodically recalibrated, its critical dimensions (see IEEE Std 1309-2005) shall be verified and documented on an annual basis.² Can anyone point me in the direction of the clause in IEEE Std 1309-2005 which mentions critical dimensions? Thanks in advance. Regards, David Shidlowsky | Technical Reviewer Address 1 Bat-Sheva St. LOD 7120101 Israel Tel 972-8-9186113 Fax 972-8-9153101 Mail [email protected]/[email protected] Web www.itl.co.il <http://www.itl.co.il> <http://www.itl.co.il/> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
