Base Loading
Coil at the bottom, whip on top. This is how most tripod whip-and-coil kits work. It's easy to reach the taps, but the coil carries the most current, so coil losses matter most.
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LOADED VERTICALS & HOMEBREW COILS
A 17-foot whip can work 40 and 80 meters if you add the right coil. How loading works, how many turns to wind on a piece of PVC, where to tap it, and how to get it tuned at the park.
THE BASICS
How a Loading Coil Works
A quarter-wave vertical on 40 m is 33 ft tall. Make it shorter and it acts like a capacitor, so it won't take power. A coil adds the "missing" length back electrically. The antenna still radiates, just less efficiently than a full-size one.
Center Loading
Coil partway up, with mast below and whip above. The current stays high along the bottom section, so it radiates better. It needs a bigger coil and is harder to tap in the field. Buddistick-style antennas work this way.
Top Hat
Spokes or wires at the top of the whip act as extra capacity. You need fewer coil turns and get better efficiency. On a whip, even 3–4 short wires splayed at the top help on 80 m.
Shorter = Less Efficient
The shorter the antenna is compared with a quarter wave, the smaller its radiation resistance. Coil and ground losses then eat a bigger share of your power. That's why a whip is great on 20 m, fine on 40 m, and a challenge on 80 m.
Coil Q Matters
A coil's "Q" is how little power it wastes. Fat wire, spaced turns and a big diameter all mean high Q. A good homebrew coil on 2" PVC can match or beat many commercial ones.
Radials Matter More
Every vertical works against the ground under it. A couple of radials on wet grass can waste more power than the coil does. More radials is the cheapest upgrade there is.
CALCULATOR
Loading Coil Calculator
Tell it your whip and your coil form. It works out the inductance you need, how many turns to wind, where to tap each band, and a rough efficiency.
Base-loaded (coil at the bottom): set the mast to 0. Center-loaded: put the coil height here.
Tap Chart
Turns counted from the bottom of the coil, with the same whip on every band.
| BAND | INDUCTANCE | TAP AT TURN | COIL LENGTH | ROUGH EFFICIENCY |
|---|
Wind a few more turns than the lowest band needs. These are starting points: fine-tune with the whip length. Very short coils (just a few turns) are the least exact, so expect to move the clip a turn either way.
STEP BY STEP
Wind Your Own Coil
An evening project: a piece of white PVC, some copper wire, two end caps and standard 3/8-24 antenna hardware.
Base-Loaded Whip
The clip lead comes from the top (whip) end and shorts out every turn above the clip. Only the turns below it are working. Clip higher for more turns (lower bands) and lower for fewer turns (higher bands). On 20 m and up you may need no coil at all.
Steps
- Test the PVC. Put a piece in the microwave next to a cup of water for one minute. If the PVC gets warm, it's lossy at RF. Try another brand. Plain white schedule 40 is usually fine. Avoid dark or gray pipe.
- Cut the form about 2" longer than the coil length from the calculator. Fit PVC end caps with 3/8-24 studs or couplers (the standard mobile antenna thread): stud on top for the whip, stud on the bottom for the mount.
- Anchor the wire. Drill a small hole at the bottom, push the wire end through, and connect it inside to the bottom stud. Use a ring terminal and solder.
- Wind tight and even. For spaced turns, wind two wires side by side, then unwind one. That leaves a perfect one-wire gap. Secure the top end through another hole to the top stud.
- Make it tappable. Bare copper can be clipped anywhere. For enameled wire, sand a small spot on the top of each turn, and mark turn numbers with a paint pen.
- Tap lead: a short wire from the top stud (where the whip screws on) to an alligator clip. Clip it on turn 12 and only turns 1–12, counted from the bottom, are in use.
- Seal it. A wrap of clear heat-shrink or a coat of clear spray keeps the turns in place and the rain off, but leave the tap spots bare.
IN THE FIELD
Tuning a Loaded Vertical
An antenna analyzer or NanoVNA makes this a two-minute job. With just the radio's SWR meter, it takes a little longer but works the same way.
Coarse, Then Fine
- Set up the whip and radials first. Everything changes tuning: radials, the whip, even you standing next to it.
- Clip the tap on the turn from the tap chart.
- Find the dip. Sweep with the analyzer, or check SWR at low power across the band.
- Dip too low in frequency? Too much coil: move the clip down one turn (fewer turns) or shorten the whip.
- Dip too high in frequency? Not enough coil: move the clip up one turn or lengthen the whip.
- Fine-tune with the whip. Get the tap close, then slide the whip in or out a few inches to land it.
- Write it down. Tap number and whip length for each band, on a piece of tape on the coil. Next time it's a 30-second setup.
SWR Stuck at 1.5–2:1?
A short vertical over good radials can have a feedpoint resistance well under 50 Ω, for example 20–30 Ω on 40 m. Then even perfect tuning leaves SWR around 2:1. That's actually a sign of low loss. Fixes:
- A shunt coil (a few µH from the feedpoint to the radials) or a tapped coil can match it.
- A small tuner at the radio is fine for 2:1 on short coax runs.
- Don't "fix" it by removing radials. SWR improves because you've added loss.
If SWR is 1:1 on every band without tuning, something is soaking up the power (bad connection, lossy coax or poor ground).
THE OTHER HALF
Radials for Loaded Verticals
The radial system is half the antenna. On a short loaded vertical it matters even more, because the feedpoint resistance is low.
More, Not Longer
Radials lying on the grass don't need to be resonant. 8–16 radials about as long as the antenna is tall is a great portable setup. Going from 2 to 8 radials makes a bigger difference than any coil.
Two Tuned Radials
Raise the base a few feet (on a tripod or a picnic table edge) and two radials cut to a quarter wave for the band work about as well as many on the ground. Slope them down slightly and keep the ends off the ground.
Radial Calculator
How many radials, and how long, on the ground or raised, for every band.
FIELD NOTES
Coil Tips
- Bigger diameter beats more turns. For the same inductance, a fat coil uses less wire and has a higher Q.
- Length about equal to diameter (or a bit longer) gives the best Q for an air-style coil.
- Make the whip as tall as you can. Every extra foot of whip above the coil means fewer turns and higher efficiency.
- Keep metal away from the coil. A mount bracket, a car body or the tripod head right next to the turns detunes it and adds loss.
- Hot coil = lost power. After a few minutes of FT8, feel the coil. Warm means it's working hard. Hot means use thicker wire, a bigger form or less power.
- Unused turns above the tap are fine on the low bands. On 10–15 m they can cause odd tuning. Use a separate small coil, or no coil, for the high bands.
- Choke the coax at the base, so the coax shield doesn't become part of your radial system. Choke recipes
- Commercial coils (whip-and-coil tripod kits, center-loaded sticks, hamsticks) use exactly this approach. The calculator works for them too: just use their whip length.
Get Outside. Get On The Air.
WIND IT. TAP IT. WORK 40 METERS FROM A WHIP.
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