Critical aspects of measurements when making a cantenna waveguide for radio astronomy. Extracts from SARA & BAA Radio Astronomy Group Mailing Lists 19/11/2025.
From Marcus D. Leech:
Probe placement with regards to the distance from the back-short, probe length, and positioning of the waveguide at the proper dish<>feed spacing. That is, if you stick to a 150mm diameter. The guide wavelength (lambda-g) grows *quickly* as you shrink the diameter of the guide. The guide *length* is not critical if it is longer than about 1 lambda-g. I generally make my circular wave-guides about 250mm long, or sometimes longer. There is very little penalty in making them longer (within reason). An important loss mechanism in wave-guides is the skin resistance of the inside wave-guide surface. So, longer guides from “improvised” materials are probably not a good idea. But anything between 1 and 2 lambda-g in guide length should be fine.
From Alex Pettit:
It is typically the case that the phase-centre of a circular-waveguide feed is just inside the entrance to the guide.
With the extended wife dish @ focal length 35cm, I could detect spacing changes in 1cm steps, which is why it is hard to guess the correct position without making operational measurements.
So, test everything, and keep re-testing until get perfect positions.
From Melvyn Jones:
The distance between the back short and probe should be 1/4 lambda-g so that the wave towards the rear short is reflected and travels exactly 1/2 lambda-g round trip. Added to the 180-degree phase shift on reflection the reflected wave adds in phase to the forward transmitted wave. Lambda-g is dependent on can diameter, but can be easily calculated, in my case using a 155mm diameter can (close to the maximum that can be used) the distance works out at 8.8cm. However, this is not that critical as these are wideband devices; (over 100Mhz bandwidth for a constant return loss of around 10dB). Adjust the probe length for the best return loss, start a little over 1/4 lambda (free space) and trim for minimum return loss. However, you will not get a sharp match on frequency; anything over 12dB is good enough.