Cantenna (circular waveguide feed)

A cantenna is a circular waveguide. Below a certain frequency, the cutoff, a waveguide won’t carry a signal at all; above it, it will. You want the hydrogen line above the cutoff of the lowest mode (TE11) and below the cutoff of the next mode (TM01), so that only one clean mode propagates.

At 1420.406 MHz that sets a window on the inside diameter: between about 123.7 mm and 161.6 mm. Smaller and nothing gets through. Larger and the second mode can propagate, which distorts the pattern.

Worked example: a 150 mm can

From our dish and horn calculator:

• TE11 cutoff: 1171.5 MHz, comfortably below 1420 MHz
• TM01 cutoff: 1530.1 MHz, comfortably above
• guide wavelength λ_g: 373.2 mm (longer than the free-space 211.1 mm, as it always is in a waveguide)
• probe position: 93.3 mm from the closed back wall (λ_g / 4)
• probe length: 52.8 mm (λ / 4 in free space)
• minimum can length: 280 mm (0.75 λ_g)

A 100 mm can, by contrast, has a TE11 cutoff of about 1757 MHz and will not pass the hydrogen line at all. This is the most common cantenna mistake: grabbing a can that looks big enough.

Building one

The probe is usually the centre pin of an N or SMA panel connector, extended with brass or copper to the right length, mounted through the side wall. Measure inside diameter, not outside. Seams and dents matter less than people fear at 21 cm; the probe position matters more. The LNA should bolt directly to the connector, for the reasons in LNA.

As a feed vs on its own

On its own a cantenna has low gain and a wide beam. Its real job is feeding a dish: its broad pattern suits prime-focus dishes with an f/D around 0.35 to 0.45. With the open end at the focus (more precisely, the phase centre slightly inside the mouth), it gives a good fraction of the dish’s possible gain. Without a dish, a pyramidal horn is the better first antenna. LAB-010 compares all three.