"Tallguide" and mode suppressors

Hi all,

Antennas for Communications (www.afcsat.com) offers waveguides taller than conventional ones, that they call "tallguides". See www.tallguide.com .
They work on TE10 mode, suppressing higher order modes with mode suppressors.
They have less losses and less dispersion than conventional waveguides.

I'd like to know:
a) Has somebody in the forum used them?
b) How the mode suppressors are made?
Thank you.

Best regards

Z

The technically better name is Overmoded Waveguides; your reference states that they are used in high-power low-loss applications in satellite Earth stations. There they are also named "beam-guides".

I have used a large-diameter metal pipe as an overmoded waveguide in a zenith-looking radiometer system, quite successfully. Instead of ~6-dB loss I achieved ~ 0.5 dB over ~3 meters, at 12 and at 18 GHz.
I used no mode suppressors b ut you can find some designs in reference papers. Try google the above names, also, how huge Earth station antennas are designed. For TE10 mode, you can use a set of coplanar lossy plates oriented to attenuate the orthopolar wave, as orthomode suppressor.

Hi jiripolivka

Thank you for your answer. Now I havesome tips.
Yes, "Tallguide" is a trade mark.
I'm wondering why you didn't need mode suppressors. Is it because you had a stright pipe with monomode transitions not exciting high-order modes?
Regards

Z

As I mentioned, I used the long stove pipes made of galvanized steel to guide the external zenith noise into underground radiometers. The polarization was determined by standard-gain horns underground. I tested the radiation patterns of both systems by moving an absorber block across the pipe openings above ground, and determined that the main lobes had the same shape as the horns alone. No more work was needed for this application.

In large parabolic antennas (mostly Cassegrain) the primary feed from a high-power transmitter and to a low-noise receiver is required to be low-loss at two separate frequency bands, either 4/6 GHz, or, 12/14 GHz. To my knowledge, overmoded waveguides (beam-guides) are typically 25-40 cm in diameter, and planar or convex 45-degree mirrors are used at bends. Antenna can rotate in azimuth and elevation while circular polarization is used. By keeping the circular shape, no special provisions are needed to keep "pure" modes.

Hi jiripolivka,

Thank you.
Then I can think that in the case of beam-guides that you describe, the waveguide size is so big (radius >> wavelength) that in the central zone ("far" from walls) we have a wave that resembles propagation in free space, a planar wave in TEM mode. In othe words, there is a combination of modes that gives that result. Do you agree with this point of view?
Regards

Z

I am not sure with that explanation. It may be true. In multimode waveguides, staggering TE10 modes in the cross section makes the low-loss point by that only the modes "touching" the metal walls do cause the loss. Other modes do not touch anything, therefore they do not suffer a loss.

Find a book on overmoded waveguides, with an exact analysis.

This is a very tricky method. In case some irregularity excites an overmode somehow within the tallguide, the mode suppressors will not help you. Instead they will block the overmode propagation and in fact form an overmode resonator. This will cause glitches in your passband because of the discrete number of resonance frequencies. The bigger the distance between the overmode suppressors, the more overmode resonances you will see.

Thank you, Radiohead.
Do you know how the mode suppressors are constructed in tallguides?
Regards

Z

Not really, but you could try to find some information by searching their patents. But it is likely that they will transform the oversized waveguide to a normal dimension for the band of interest. The larger the bandwidth requirements, the longer this transformer physically needs to be.