Derek: Excellent comments on horns from Robert.
I have a couple of EMCO 3115 Double-ridged horns. This design sacrifices efficiency for bandwidth, which means time saved not swapping antennas. Although the design is rated to 18 GHz, I found the antenna factor became intolerably high (for my needs) above around 14 GHz. So I built a horn for the 12-18 GHz band. The easy technique for this is to use a section of waveguide, cut off square at one end and flanged at the other. Just use sheet metal (brass) to extend a flared horn out from the cutoff end. Use fairly heavy gauge metal sheet, and you can either solder or braze all the seams. Now, just use a coax to waveguide adapter to make your connection. There are designs available for flare angle and length of the horn everywhere from the ARRL Antenna Handbook to Reference Data for Radio Engineers to the classic textbooks. Most of us already have octave band amplifiers, so it's not that much extra hassle to change an antenna along with the amplifier for each band. If you really need to squeeze out every dB of performance from your immunity setup, then consider mounting the antenna directly off the output connector of your TWT amplifier. Zero cable loss. Of course, it's nice then to have a "stupid" amplifier, that is, one which has nothing but a TWT tube and a brute force power supply. All those fancy microprocessor controls and IO are likely to prove susceptible to the RF field. You can use a small pre-amp to boost the signal on the relatively long amplifier input cable, if you need to. Also, a long, low-loss, low-power coax is much cheaper than a long, low-loss, high power coax. And very much cheaper than a long flex waveguide. There's another benefit to using a waveguide to coax adapter to feed a horn. If you do manage to damage the system, through arcing from high SWR (like maybe the antenna falls over and lands throat down on the enclosure floor), the adapter is easily replaceable. Also, rarely, you will encounter a badly assembled Type N male coax cable connector, with the male center pin sticking out too far. It's much better to damage the female connector on a replaceable adapter, than a female connector built into the antenna. (Let's all check our coax cables today. <grin>) Now wait a minute! How big was that enclosure you described? One meter by 0.7 meters?! That sounds more like a waveguide than a chamber! Regards, Ed :-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-) Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 858-505-2780 (Voice) 858-505-1583 (Fax) Military & Avionics EMC Services Is Our Specialty Shake-Bake-Shock - Metrology - Reliability Analysis :-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-) > -----Original Message----- > From: Robert Macy [SMTP:[email protected]] > Sent: Monday, March 06, 2000 2:30 PM > To: [email protected]; [email protected] > Subject: Re: Horn Antenna > > > Derek, > > The horn antenna is an impedance matching device for getting between 50 > ohm > lines and 377 ohm free space while launching TEM mode wave fronts. > > You could use the commercially available one which has an awful antenna > factor. It's broadband as a result of the ribs placed in the horn. > > Just guessing, but if you want 1 V/m which is around 2.7mW in free space > (4.3 dBm, 0.367Vac in a 50 ohm system) and your antenna factor is around > 34dB, you'll have to power the antenna with 38.3 dBm (6.8W, 18.4Vac) > > The antenna will probably take that. > > However, if you want to get to 3V/m you'll then need (61W, 55Vac) and > might > be overpowering the antenna. > > For 10V/m you need (680 W, 184 Vac) Wow! Assuming 1 dB drop in your > cabling > and you dissipate 70W there! > > Sorry, don't know antenna design enough to tell you if you can improve > that > antenna factor but my guess is no, or the EMC supply houses would have > been > supplying those antennas already. The best bet is to go for octave band > antennas with only 10-16dB antenna factors and change the antenna a lot. > > - Robert - > > Robert A. Macy, PE [email protected] > 408 286 3985 fx 408 297 9121 > AJM International Electronics Consultants > 619 North First St, San Jose, CA 95112 > > -----Original Message----- > From: [email protected] <[email protected]> > To: [email protected] <[email protected]> > Date: Monday, March 06, 2000 12:55 PM > Subject: Horn Antenna > > > > > >Hi, > > > > I'd like to construct a wide band horn antenna covering from about 1 GHz > to 10 GHz or so. > > > > This is used only to generate a field inside a small EMC Chamber ( about > 1 > metre by 0.7 metres ), so I'm not worried about gain flatness etc.... I > just > want field;-) > > > > Any thoughts about: > > > > 1) How these things work > > > > 2) The best way to go about modelling this > > > > 3) The best way to make one > > > > Thanks in advance > > > > Derek Walton > > > > > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: [email protected] > Michael Garretson: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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