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
> --------------------------
>
>
>
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>
Jeff Bailey
SST
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