¼ Vertical, if the coax runs between radials it can’t bypass the choke
Reality check: Coax coupling and stray capacitance still matter — even between radials.
There’s a persistent belief that a feedline lying between or under the ground radials of a ¼-wave vertical can’t possibly “bypass” the choke because the radials are at RF ground potential. Unfortunately, physics disagrees.
Any coax shield running close to the radials or the soil is capacitively coupled to that return system. That stray capacitance sits directly across your choke if the choke isn’t mounted at the feedpoint, giving the common-mode current a convenient shunt path to sneak around it. Even a few picofarads make a big difference.
Why even small capacitances ruin your choke isolation
Let’s take a simple example: a few tens of picofarads between the coax shield and the radial field.
| Stray capacitance | Reactance (7 MHz) | Reactance (14 MHz) |
|---|---|---|
| 20 pF | ≈ 1 137 Ω | ≈ 568 Ω |
| 50 pF | ≈ 455 Ω | ≈ 227 Ω |
Now put that in parallel with your “great” 5 kΩ choke. The effective isolation drops to just a few hundred ohms — barely better than no choke at all. The feedline is now radiating, the current distribution shifts, and your pattern and noise pickup suffer.
The fix: choke right at the feedpoint
The first half-meter of coax matters more than most think. If you put the choke even a meter below the feedpoint, that un-choked section becomes part of the antenna system — acting as a stub or extra radial. Modeling and field measurements confirm that this short section can carry significant current, distorting the pattern and bringing RF back into the shack.
Proper placement: mount the choke directly at the feedpoint. Route the coax straight down and away at 90°, and bury or elevate it quickly to reduce near-field coupling. A second choke at the shack entry is optional but useful as an extra “stopper.”
Why this myth persists
It sounds intuitive: if the coax lies among the radials, surely the radials will short out any field around it. But remember — the field is strongest right at the feedpoint, and capacitance exists even between insulated conductors. A mere 20 pF of coupling, as W8JI notes, equals around 800 Ω at 10 MHz. That’s plenty to compromise an otherwise excellent choke.
Practical takeaway
- Install the choke at the feedpoint itself — for a ¼-wave vertical, the choke is the feedpoint.
- Keep the feedline away from the radial field — or bury it quickly.
- Add a second choke at the shack entry for extra protection.
- Don’t assume radials eliminate stray coupling — they don’t.
You can have the best choke in the world, but if it’s more than 35 cm (14 in) from the feedpoint, stray capacitance will undo its work.
Mini-FAQ
- Why does the choke have to be right at the feedpoint? — Because any coax length above it becomes part of the antenna and radiates, defeating the purpose of isolation.
- On a ¼-wave vertical, where exactly is the choke? — At the feedpoint itself. The choke defines the electrical boundary between the vertical radiator and the feedline.
Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.
Questions or experiences to share? Contact RF.Guru — we love real-world data.