How the HB9CV works
Unlike a Yagi, where only one element is fed, the HB9CV feeds both elements through a phasing line that crosses over between them, giving them a phase difference of about 225°. Combined with the eighth-wave spacing, that makes the signals add up towards the front and cancel towards the back. It was designed by Rudolf Baumgartner HB9CV in the 1950s. The dimensions here follow the formulas published by DK7ZB, with a rear element of 150 ÷ f metres and a front element of 139 ÷ f metres.
Building it
- The phasing line is a rod or stiff wire about 5 mm from the elements, connected at the tap points and crossed over in the middle.
- A small variable capacitor in series with the feed matches it to 50 Ω. Adjust for the lowest SWR, then replace it with a fixed one of the same value if you like.
- Expect about 4 dBd of gain and a front-to-back ratio of over 20 dB.
Foxhunting
The deep rear null of an HB9CV is ideal for direction-finding: point the back at the signal and the null shows its direction precisely. For more gain on a single band, see the Yagi and Moxon calculators.
Questions
How much gain does an HB9CV have?
About 4 dBd, similar to a three-element Yagi in a much smaller space.
Which way does an HB9CV point?
Towards the shorter front element.
What are the HB9CV dimensions for 2m?
At 145 MHz, about 1.03 m for the rear element, 0.96 m for the front, and 26 cm spacing.
Why is the HB9CV good for foxhunting?
Its rear null is deep and sharp, which pinpoints a signal's direction.