Chokes for hydrogen line cantenna feeds

A choke can be added to a cantenna feed to improve performance by suppressing unwanted common-mode currents on the outside of the coaxial cable shield. This results in a cleaner signal, reduced noise, and better adherence to the antenna’s designed radiation pattern. 

Choke Design Methods

There are two primary approaches for integrating a choke into a cantenna system: using a choke ring (waveguide choke) or a feedline choke (common-mode balun). 

1. Choke Ring Design (Waveguide Choke)

A choke ring is a physical structure added around the mouth of the cantenna feed horn or the antenna itself, typically used in radio astronomy and satellite reception dishes. 

• Purpose: It improves beam efficiency and reduces side lobes (which cut down on noise from unwanted directions) by altering the electromagnetic field at the aperture of the feed.
• Design Considerations:
  • Dimensions are frequency-dependent: The physical size (diameter, depth) of the choke ring is critical and must be precisely calculated based on the operating frequency (e.g., 1420 MHz for hydrogen line projects).
  • Calculation Tools: Specialized Excel sheets and software from sources like the SETI League or specific amateur radio operators (e.g., JA6XKQ) are used to determine these exact dimensions.
  • Construction: Prototypes can be built from sheet metal or even simple folded mesh to test effectiveness.
  • Placement: The choke ring is typically placed a short distance from the mouth of the feed element. 

2. Feedline Choke Design (Common-Mode Choke/Balun)

This method involves winding the coaxial cable that connects the cantenna’s probe to the receiver around a ferrite core or cores. 

• Purpose: It blocks current (current 3 in diagrams) that flows on the outer surface of the coaxial cable shield, which acts as an unintended secondary antenna and distorts the radiation pattern and gain.
• Design Considerations:
  • Ferrite Material: The choice of ferrite material (mix/permeability) and core size is crucial and depends on the operating frequency.
  • Winding Turns: The number of turns of coax passed through or around the core determines the choking impedance and effectiveness.
  • DIY Construction: This is a common and relatively simple DIY project in ham radio. You can build one by winding several turns of RG-58 or similar coax through a large toroid core (like an FT240-43). 

Summary

For optimal performance, the choke ring is often preferred in highly specific applications like radio astronomy feeds as it is an integrated waveguide design feature. However, a simple, effective feedline choke made with a ferrite toroid is a very common and effective way to prevent common-mode current issues in standard cantenna setups.