Part of our Building Your First Ham Radio Station guide.
A vertical whip is a great space-saving HF antenna, no need for two supports like a dipole, just one point going up. But verticals get a bad reputation for being fiddly to tune, mostly because people chase the whip length first when the ground system is usually the real problem. Here’s how to actually approach it.
The starting length
For a basic quarter-wave vertical, the formula is:
Length (feet) = 234 ÷ frequency (MHz)
For example, a 20m quarter-wave (14.2 MHz) comes out to about 16.5 ft. If you’re using a shortened, coil-loaded mobile whip instead of a full-size quarter wave, this formula won’t apply directly. The loading coil does part of the electrical work, and tuning becomes about coil position/tap point rather than raw length (more on that below).
Get the ground system right first
A quarter-wave vertical over a perfect, infinite ground plane has a feedpoint impedance around 35-37 ohms. In the real world, your radial system stands in for that ground plane, and how you build it changes that impedance more than almost anything else:
- Elevated verticals (radials up off the ground): 2-4 resonant quarter-wave radials, drooped 30-45° below horizontal, works well. Drooping the radials down actually raises the feedpoint impedance closer to 50 ohms; it’s a legitimate tuning tool, not just a mounting choice.
- Ground-mounted verticals: the more radials, the better, since you’re fighting real ground loss. 16-32 radials, even short and laid loosely on the surface, meaningfully outperform 3 or 4.
If your SWR is stubbornly high and won’t come down no matter how you trim the whip, look at the radials before you cut more wire off the antenna.
Tuning a full-size whip
- Set up the antenna and radials in their final position, height and radial layout both affect resonance, so tune it where it’ll actually live.
- Connect an SWR meter or antenna analyzer and sweep across the band.
- If the lowest SWR point is below your target frequency, the whip is too long, trim a small amount off the tip (or retract a telescoping section) and recheck.
- If the lowest point is above your target frequency, the whip is too short, you’ll need to add length or switch to a taller section.
- Work in small steps: a few millimeters at VHF, 5-10mm at a time on HF whips. It’s easy to remove metal, much harder to add it back.
Tuning a loaded/mobile whip
Shortened whips with a loading coil (common for mobile HF) are tuned differently. You’re not trimming a simple quarter-wave length, you’re adjusting how much of the coil is in the circuit, or moving a tap point along it. Most coil-loaded whips have either a sliding tap or a telescoping tip section above the coil for fine adjustment:
- Extending the tip section (or moving the tap toward more coil turns) lowers the resonant frequency
- Shortening the tip (or moving the tap toward fewer turns) raises it
- Make small adjustments and recheck SWR each time, loaded whips tend to have narrower bandwidth than full-size verticals, so precision matters more here
Add a choke at the feedpoint
Because a vertical is inherently unbalanced against ground, common-mode current on the coax shield is a common problem. It shows up as unstable SWR that changes when you touch the coax, or RF feedback in the shack. A 1:1 common-mode choke at the base of the antenna, before the coax runs off toward the radio, usually clears this right up.
Troubleshooting checklist
- SWR won’t come down anywhere in the band → check radial count and connections before trimming further
- SWR changes when you touch the coax or move it → add a common-mode choke at the feedpoint
- Resonant point is in the right place but SWR still isn’t great → check your radial droop angle, or verify the antenna isn’t too close to metal structures, gutters, or a mast it’s mounted to