Ed, Jeff, If you assume that's your second harmonic that you wish to propagate is at the waveguide cutoff frequency, fc, hence your fundamental is at 0.5 fc, then the loss of a rectangular waveguide at the fundamental will be:
Loss = (kc)(Squareroot[3/2]), in nepers/meter Where kc is (3.1416/b), where b is the width of the guide (meters) This assumes propagation in the TE01 mode, which is the dominate mode of a rectangular guide that is wider than it is high. Also assumes that the next order mode is not launched. Does this help? A good basic reference is: Harrington, Time Harmonic Electromagnetic Fields, McGraw Hill, 1961. Jim Dr. Jim Knighten e-mail: [email protected] Senior Consulting Engineer NCR 17095 Via del Campo San Diego, CA 92127 http://www.ncr.com Tel: 619-485-2537 Fax: 619-485-3788 ---------- From: Bailey, Jeff Sent: Monday, September 28, 1998 8:25 AM To: '[email protected]' Cc: 'EMC-PSTC' Subject: RE: Waveguide Propagation I don't know if any of this helps but it's my two cents worth. > -----Original Message----- > From: [email protected] [SMTP:[email protected]] > Sent: Friday, September 25, 1998 1:29 PM > To: EMC-PSTC > Subject: Waveguide Propagation > > Greetings Everyone! > > > Can anyone provide a formula for the transmission loss of a waveguide > at frequencies BELOW it's normally useful range? > >> I don't recall any formula's for this, I only remember my instructor telling me that as you go below your lower cutoff >>your losses will become very extreme (most of the power will reflect) I will try to remember to dig up my old notes if >>I can find the box at home. (typically a waveguide is designed to propogate down to 0.9*lower cutoff) > I would like to construct a high-pass filter to reject the fundamental > frequency of a transmitter, but allow the passage of the second and > higher harmonics. > >>How many higher? Once your "a" dimension of your waveguide reaches l your waveguide begins to look like two >>waveguides and you no longer propogate in TE10 mode (that's when things get too confusing for my level of >>experience with this stuff) > > I am assuming a coax transmitter output, with the coax connected to a > 6dB attenuator and then a coax/waveguide transition. Then a section of > straight waveguide, perhaps 18" long, followed by another > waveguide/coax transition. The coax would then be connected to an > attenuator and a spectrum analyzer. (The purpose of the 6dB attenuator > is to limit fundamental power reflected to the transmitter to a > maximum of 10% of the forward power. At the fundamental, the waveguide > should yield a terrible impedance discontinuity, reflecting most of > the forward power.) > > If I choose a waveguide which would normally just support the > relatively lossless transmission of the second harmonic, how many dB > of loss could I expect at 1/2 the second harmonic frequency (the > fundamental frequency)? The variables which I would know are the > frequencies involved, the physical width and height of the waveguide, > and the length of the waveguide. > > A second question would be what is the effect of the length of the > waveguide? Do I only need to provide a certain minimum length, or will > loss be strongly proportional to waveguide length? Is it possible that > the coax/waveguide transitions alone will provide enough waveguide > length? > >>I recall from previous work seeing 0.06dB or less insertion loss in WR75 (at 14.5GHz I think), the guide was only >>about 2.5" long. I your guide is very well made I don't beleive extra length up to your 18" will make much of a >>difference. > To give you a better perspective, imagine that a 4.5 GHz 100 Watt > transmitter is connected to X-band (WR-90 guide? normally used from 8 > GHz to 12.5 GHz) waveguide. What would the transmission loss be at > 4.5, 5, 6, 7, 8, and 9GHz? > > Thanks in Advance, > > Ed > -------------------------- > Ed Price > [email protected] > Electromagnetic Compatibility Lab > Cubic Defense Systems > San Diego, CA. USA > 619-505-2780 > Date: 09/25/1998 > Time: 09:29:20 > -------------------------- > > > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to [email protected] > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to [email protected], > [email protected], [email protected], or > [email protected] (the list administrators). > > > Jeff Bailey SST --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
