Just a quick comment, the relationship between AF(dB/m) and G(dB) is
AF(dB/m)=20*log(f_MHz)-G(dB)-29.79 As you can see, if you have an antenna with a constant gain, it is a physical law that the AF goes up with frequency. Now, of course you can design an antenna with a extremely large gain (small AF). But, for most cases, it is not desired, because the higher the gain, the narrower the beam. For immunity test, you really want a moderate gain antenna to cover a reasonable area. EMCO 3115 has a gain between about 7 and 14 dBi accross the band. I do not know if you really want the gain to go any higher than that. Zhong Chen > -----Original Message----- > From: Price, Ed [SMTP:[email protected]] > Sent: Tuesday, March 07, 2000 9:03 AM > To: [email protected] > Subject: RE: Horn Antenna > > > 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. > > 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. 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]
