HB9CV Antenna
The HB9CV is credited to Rudolf Baumgartner, HB9CV, a Swiss amateur who published the design in the mid-1950s. Like the ZL Special already on this site, it's a close-spaced 2-element phased beam that packs real gain and front-to-back ratio into a very short boom — but where the ZL Special uses a simple crossed transmission line for phasing, the classic HB9CV uses a phasing harness that includes a small series capacitor, giving you a fine, adjustable way to peak front-to-back ratio after the antenna is built rather than relying purely on fixed line length and element trimming.
A true phased-driven pair, not driven-plus-parasitic
Both elements in an HB9CV are actively fed through the phasing harness — there's no purely passive parasitic element the way a conventional Yagi's reflector or director works. Power splits between the two elements through the harness, which sets both the amplitude balance and (critically) the phase relationship that produces gain and front-to-back ratio at this close spacing.
HB9CV: both elements actively fed through a phasing harness
What makes it different from the ZL Special
Both designs are close-spaced 2-element phased beams from the same general family, and either one will get you real gain from a very short boom. The practical build difference is the phasing harness: the ZL Special uses a simple crossed (transposed) line with no adjustable component, while the classic HB9CV harness includes a small series capacitor for fine-tuning the phase relationship after the antenna is assembled — genuinely useful if you want to peak front-to-back ratio precisely rather than accept whatever a fixed-length line gives you.
- ZL Special: crossed line only, no adjustable component, simpler to build.
- HB9CV (this guide): phasing harness with a small trimmer/fixed capacitor, adjustable after assembly.
Why the capacitor matters
The series capacitor in the phasing harness lets you nudge the electrical phase relationship between the two elements without physically changing the harness or element lengths — useful for squeezing out the last few dB of front-to-back ratio once the antenna is built and on an analyzer, rather than committing to a single fixed geometry from the start.
Why it's popular for VHF/UHF
Like the ZL Special, the HB9CV's short boom and light weight make it attractive for portable operation, rovers, and fixed 2m/70cm stations where a full Yagi's boom length isn't practical — with the added benefit of the capacitor giving builders a genuine post-assembly tuning knob.
Installation options
- Handheld/portable mount: light enough to hand-hold on a short mast for portable VHF/UHF work.
- Fixed mast or rotator: a compact fixed-station beam alternative where a full Yagi's boom length isn't practical.
- Stacked pair: some builders stack two HB9CVs vertically for additional gain in a still-compact package, similar to stacking ZL Specials.
| Parameter | 2m (146 MHz) | Notes |
|---|---|---|
| Reflector element | ~40.1 in (1.02 m) | Slightly longer of the two elements |
| Director element | ~38.6 in (0.98 m) | ~3.5% shorter than the reflector |
| Element spacing | ~9.7 in (24.6 cm) | ~0.12 wavelength - close spacing, similar family to the ZL Special |
| Phasing harness capacitor | ~6-20 pF trimmer (2m) | Fine-adjust for peak front-to-back ratio; start mid-range and tune |
HB9CV Dimension Calculator
Materials for HB9CV
Building the HB9CV
Mechanically similar to the ZL Special, with the phasing harness's capacitor as the extra build and tuning step.
Cut both elements
Use the calculator above to get the reflector and director lengths for your design frequency, and cut both from aluminum tubing or rod.
Mount both elements to the boom at the correct spacing
Mount the reflector and director to the boom using insulated hardware, at the spacing given by the calculator.
Connect the feedline
Connect your coax feedline (through a gamma match or other matching network as needed) at the harness's feedpoint.
Mount to your mast or handle
Attach the assembled boom to a handheld mast grip for portable use, or a fixed mast/rotator for a station install.
Sweep SWR and check the initial pattern
Sweep the feedpoint for a clean resonant dip, then do an initial front-to-back check on the air or with a field-strength comparison.
Peak front-to-back ratio with the phasing capacitor
Adjust the trimmer capacitor in small steps while monitoring front-to-back ratio, since this is the design's main built-in tuning knob for this parameter.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Front-to-back ratio won't peak no matter what | Poor connection in the phasing harness or at the capacitor | Check the harness wiring and capacitor connection for a solid joint | Re-solder for a solid, low-resistance connection before assuming the capacitor value range is wrong |
| SWR is hard to bring down | Close element spacing makes the design sensitive to small construction variances | Recheck spacing precisely against the calculator's value | Correct spacing before assuming the element lengths are the problem |
| Performance changed after handling the capacitor | Expected — the phasing capacitor is a genuine, sensitive tuning control | Check whether small adjustments produce small, trackable changes in front-to-back ratio | Normal behavior; large swings from tiny movements suggest a loose connection rather than the capacitor itself |
How is this different from the ZL Special already on this site?
Both are close-spaced 2-element phased beams from the same design family. The key practical difference is the phasing harness: the ZL Special uses a simple crossed line, while the HB9CV adds a small capacitor for fine post-assembly tuning of front-to-back ratio.
Do I have to use the capacitor, or can I skip it?
The capacitor is what defines the classic HB9CV approach and gives you the adjustable tuning advantage — skipping it and using a fixed line instead effectively turns the build into a ZL-Special-style design.
How much gain does this really give?
Commonly cited figures are around 4-6 dBd with a strong front-to-back ratio once the phasing capacitor is properly peaked, on a boom far shorter than a conventional Yagi needs.
Is it harder to build than a ZL Special?
Slightly — the added capacitor is one more component to source, mount accessibly, and tune, though the core element and spacing work is very similar between the two designs.
Can I scale this to other VHF/UHF bands?
Yes — the calculator on this page scales all dimensions to your chosen design frequency, though you'll want to source an appropriately valued capacitor for that band.
Where does the name come from?
HB9CV was the callsign of Rudolf Baumgartner, the Swiss amateur credited with publishing this design in the mid-1950s.