here to go to our main page on waveguide
here for more waveguide equations
here to go to our page on transmission line loss
This picture of
crossed waveguides came from JB. It depicts a sorry situation
that happens often in a microwave lab, especially in (but not
limited to) setups by newer engineers. The flanges on many waveguides
will allow you to connect them in a "cross-guided" fashion,
resulting in a mysterious higher-than-expected loss (usually around
20 dB). The resulting horrendous VSWR plays havoc as well. If
you click the picture you'll access a larger image, suitable for
framing, we recommend you display it over the waveguide cabinet
in your lab. Thanks, JB! Nice
Loss due to metal
The ideal metal loss for rectangular
waveguide boils down to some neat closed-form equations. For this
calculation we used Pozar's book, which covers the subject.
The first two things to calculate
are the RF sheet resistance (function of the waveguide material's
conductivity, permeability, and frequency).
Next we need to calculate the
imaginary part of the propagation constant, beta. We're only going
to consider the dominant TE10 mode. First we consider the cut-off
Now we look at the wavenumber
within the waveguide fill material (which is usually air):
Then the phase constant is:
Need to post the loss equations...
The loss goes to infinity at
the lower cutoff frequency. Here's a plot of the loss of WR-90 X-band
waveguide. Note that it blows up at the lower cutoff frequency of
6.557 GHz. In practice, the generally accepted frequency band limits
for rectangular waveguide are between 125% and 189% of the lower
cutoff frequency, in this case 8.2 to 12.4 GHz.
While we are waiting for equations
to develop, here's a plot of eight different waveguide sizes, using
the conductivity of pure copper:
This is one of the coolest plots
in microwave engineering, waveguide vendors have been known to make
it into a poster which hangs in hundreds of laboratories around
the world. We did this one in Excel, if anyone wants a copy of the
spreadsheet, it's in the download
Because it's hard to read exact
numbers from the chart, here are the losses at selected mid-band
WR-90: 0.108 dB/m (10 GHz),
WR-62: 0.168 dB/m (15 GHz)
WF-42: 0.370 dB/m (21 GHz)
WR-28: 0.576 dB/m (32 GHz)
WR-19: 1.04 dB/m (48 GHz)
WR-15: 1.51 dB/m (60 GHz)
WR-12: 1.97 dB/m (75 GHz)
WR-10 2.69 dB/m (90 GHz)
Remember, this is for ideal copper
waveguide. Expect to see "real" losses slightly higher!
How about another Microwaves101
rule of thumb?
There is considerable overlap between waveguide
standards, you can almost always find two types that will work at
one frequency. In order to get the lowest loss, choose the waveguide
that has the largest dimensions.
Loss due to dielectric loss
Ordinarily waveguides are filled
only with air, so the loss tangent loss is essentially zero. But
once in a while you'll find a reaon to consider a dielectric-filled
waveguide, then the properties of the dielectric must be considered.
This topic is covered on a separate