Toroid Choke Calculator

Short answer: Yes, it's highly recommended.

A dipole is a balanced antenna, but coax is unbalanced. Without a balun, RF current can flow on the outside of your coax shield, causing:
• RF in the shack (hot mic, computer interference)
• Distorted radiation pattern
• Increased noise pickup

A simple 1:1 choke balun at the feedpoint solves this. Many hams run dipoles without baluns and "it works," but adding one improves performance and eliminates potential problems.

Yes, strongly recommended — especially for fixed stations or power levels above QRP.

Understanding the Problem
Every antenna needs two "poles" for current to flow. A dipole has two obvious halves, but what about an end-fed wire? The answer: your coax becomes the other half. Without intervention, RF current flows on the outside of your coax shield all the way back to your shack. This is called "common mode current."

Why is this bad?
• RFI in your home — your coax connects to your radio's chassis, power supply, and home ground. Your appliances can become part of the antenna.
• Unpredictable performance — SWR changes when you touch or move the coax
• RF feedback — distorted audio, shocks from your microphone, equipment acting erratically

What is a Counterpoise?
A counterpoise is simply the "return path" for your antenna's RF current — the missing half of your end-fed antenna. You always have one; the question is whether you've designed it intentionally or let the antenna find one randomly (usually your coax shield and everything connected to it).

A proper counterpoise is typically a short wire (about 0.05 × your lowest operating wavelength). For 40m, that's ~2 meters. For 80m, ~4 meters.

Choke + Counterpoise: How They Work Together
A choke doesn't eliminate the need for a counterpoise — it controls where the counterpoise ends.

Option 1: Dedicated counterpoise wire
• Attach a short counterpoise wire (2-4m) to the transformer's ground terminal
• Install the choke as close as possible to the transformer (within 30cm / 1 foot)
• The choke blocks RF from continuing down your coax

Option 2: Use the coax shield as a controlled counterpoise
• Install the choke at a distance from the transformer (~0.05λ from your lowest band — about 4m for 80m)
• The coax section between the transformer and choke becomes your counterpoise
• Everything beyond the choke is isolated

9:1 Random Wire Antennas
For 9:1 unun random wire antennas, a choke is essentially mandatory. Unlike an EFHW (which is resonant), a random wire is non-resonant by design, requires a tuner, and has no natural counterpoise. Always use both a dedicated counterpoise wire (4-5m minimum) and a common mode choke.

When You Might Skip the Choke
At QRP power levels (5W or less), especially portable, many operators work without a choke. The common mode currents exist but are usually too weak to cause problems. If you experience any RFI, adding a choke is the solution.

Signs You Need a Choke
• Tingling or shock when touching your microphone or radio
• Audio reports of distortion, especially at higher power
• SWR changes when you touch or reposition the coax
• Interference with household electronics during transmit

It depends on your power level:

FT140 (1.4" diameter): Up to 200W SSB, good for QRP and 100W stations

FT240 (2.4" diameter): Up to 1kW SSB (single core), recommended for 100W+ stations

Rule of thumb: 1 FT240 core per 500W. So for 1kW, stack 2 cores. For 1.5kW, stack 3 cores.

For digital modes (FT8, etc.), size up because they have 100% duty cycle. A 100W FT8 station should use FT240.

Both work for HF, but have different strengths:

Type 31 (gray-blue): Best for 160m-40m. Better low-frequency performance. Preferred for high power because it handles heat better.

Type 43 (black): Best for 80m-10m. Good all-around choice for typical HF operation. Slightly better on the higher bands.

When in doubt, use Type 43 - it works well across all HF bands and is the most common choice.

General guidelines for HF (80m-10m):

• Type 43: 10-12 turns
• Type 31: 12-14 turns
• Type 61: 8-10 turns

More turns = better choking on low bands, but worse on high bands.

For 160m, add 2-3 more turns. For 6m only, use fewer turns (6-8).

Each "turn" = one pass through the center hole. Don't count wraps around the outside!

Depends on your power level:

• RG-174: QRP only (up to 50W). Very flexible, easy to wind.
• RG-58: Up to 200W. Common choice for 100W stations.
• RG-8X: Up to 500W. Good balance of flexibility and power handling.
• RG-213: Up to 1kW. Stiffer, harder to wind but handles high power.
• RG-142 (Teflon): 1kW+. Best for high power, heat resistant.

Avoid: LMR-400 or foil-shield coax - the tight bending radius can damage the foil.

Location depends on your antenna type and counterpoise strategy.

For dipoles: Mount directly at the center insulator (feedpoint).

For EFHW antennas:
• With a dedicated counterpoise wire: Within 1 foot of the 49:1 transformer
• Using coax shield as counterpoise: At 0.05λ distance (~4m for 80m, ~2m for 40m)

For verticals: At the feedpoint or base.

A second choke near the shack entrance can provide additional isolation, but the feedpoint/transformer choke is the most important.

Not directly - a 1:1 choke doesn't transform impedance.

The "1:1" means 50 ohms in, 50 ohms out. It won't magically fix a poorly matched antenna.

However, it can make your SWR readings more accurate and stable by eliminating common mode currents that confuse your tuner or SWR meter.

If your SWR varies wildly when you move the coax or touch it, a choke balun will likely stabilize it (by revealing the true SWR of your antenna).