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NanoVNA-H NanoVNA-H Documentation

Tuning an HF Antenna

This tutorial walks through the practical process of tuning an HF antenna using the NanoVNA-H. We will measure the antenna, interpret the results, and make adjustments to achieve a good match.

What You Will Learn

Section titled “What You Will Learn”
  • Setting up the NanoVNA for HF antenna work
  • Reading and interpreting SWR, impedance, and Smith chart
  • Systematic antenna adjustment procedure
  • Achieving a match across a desired frequency range

Equipment Needed

Section titled “Equipment Needed”
  • NanoVNA-H or NanoVNA-H4
  • Coaxial feedline with appropriate connectors
  • Calibration kit
  • Tools for antenna adjustment (wire cutters, tape measure, etc.)
  • Notebook for recording measurements

Example: Tuning a 40m Dipole

Section titled “Example: Tuning a 40m Dipole”

We will tune a center-fed dipole for the 40-meter amateur band (7.0 - 7.3 MHz).

Initial Setup

Section titled “Initial Setup”

  1. Set the frequency range

    Go to STIMULUS and set:

    • START: 6M (6 MHz)
    • STOP: 8M (8 MHz)

    This gives margin on each side of the 40m band to see where resonance falls.

  2. Calibrate at the feedpoint

    For best results, calibrate at the end of your coax, where it connects to the antenna:

  3. Configure traces

    Enable two traces:

    • Trace 1: S11 SWR
    • Trace 2: S11 SMITH (or RESISTANCE + REACTANCE)

  4. Connect the antenna

    Attach the coax feedline to Port 1 (CH0).

Understanding the Initial Measurement

Section titled “Understanding the Initial Measurement”

When you first connect the antenna, observe:

Look for the SWR minimum. This is where the antenna is closest to resonance.

  • If the minimum is below 7.0 MHz: Antenna is too long
  • If the minimum is above 7.3 MHz: Antenna is too short
  • If the minimum is within the band but SWR is high: Impedance mismatch

Adjustment Procedure

Section titled “Adjustment Procedure”

  1. Find the current resonant frequency

    Use a marker to locate the SWR minimum. Record this frequency.

    Example: SWR minimum at 6.85 MHz (below the 40m band)

  2. Calculate the adjustment

    The resonant frequency of a dipole is inversely proportional to length:

    • To raise frequency: Shorten the antenna
    • To lower frequency: Lengthen the antenna

    Rule of thumb for dipoles:

    • Each 1% change in length changes frequency by approximately 1%
    • For 40m (7 MHz), 1% of frequency is 70 kHz
    • 1% of a 40m dipole (about 20m total) is 20 cm

  3. Make the adjustment

    For our example (resonance at 6.85 MHz, target 7.15 MHz):

    • Need to raise frequency by 300 kHz (about 4.4%)
    • Shorten each leg by about 44 cm (total of 88 cm)

  4. Remeasure

    After each adjustment, sweep again and record the new resonant frequency.

  5. Iterate until tuned

    Repeat steps 1-4 until the SWR minimum is at your desired frequency.

Dealing with Common Issues

Section titled “Dealing with Common Issues”

Symptom: Resonant frequency is correct, but SWR is still above 2:1

Causes:

  • Feedpoint impedance is not 50 ohms
  • Poor ground or counterpoise
  • Nearby objects affecting impedance

Solutions:

  • Adjust antenna height (feedpoint impedance changes with height)
  • Add or adjust counterpoise wires
  • Use a matching network (balun, tuner)
  • Move antenna away from metal objects

Optimizing Bandwidth

Section titled “Optimizing Bandwidth”

A dipole naturally has limited bandwidth. To maximize usable bandwidth:

  1. Check SWR across the band

    Place markers at the band edges (7.0 and 7.3 MHz for 40m).

  2. Evaluate the shape

    Ideally, the SWR minimum is in the center of your desired range.

  3. Trade-offs

    • Tuning for CW (7.0-7.1): Set resonance at 7.05 MHz
    • Tuning for SSB (7.1-7.3): Set resonance at 7.2 MHz
    • Tuning for both: Set resonance at 7.15 MHz, accept higher SWR at edges

  4. Consider bandwidth enhancement

    • Larger wire diameter increases bandwidth
    • Cage dipoles or fan dipoles have wider bandwidth
    • A 2:1 SWR is acceptable for most radios with internal tuners

Recording Your Results

Section titled “Recording Your Results”

Keep a record of your antenna measurements:

ParameterValue
Date_________
Frequency range_______ to _______ MHz
Resonant frequency_________ MHz
SWR at resonance_________:1
Impedance at resonance_______ + j_______ ohms
Antenna length (total)_______ m
Height at feedpoint_______ m

Final Verification

Section titled “Final Verification”

  1. Full band sweep

    Verify SWR is acceptable across your operating range.

  2. Check with actual feedline

    If you calibrated at the antenna, remeasure with the full feedline to see what your radio will see.

  3. Compare to on-air testing

    After tuning, verify operation with actual transmissions. The antenna tuner in your radio should handle any remaining mismatch.

Other HF Antenna Types

Section titled “Other HF Antenna Types”

The same principles apply to other antenna types:

End-Fed Half-Wave

Section titled “End-Fed Half-Wave”
  • Expect high impedance (2000-5000 ohms) at feedpoint
  • Requires matching transformer (usually 49:1 or 64:1)
  • Tune transformer, not antenna length

Vertical with Radials

Section titled “Vertical with Radials”
  • Feedpoint impedance depends on radial system
  • More radials = lower resistance, closer to 36 ohms ideal
  • May need matching network for 50-ohm coax

Loop Antennas

Section titled “Loop Antennas”
  • Full-wave loops have approximately 100-ohm feedpoint impedance
  • Quarter-wave matching section or 2:1 balun needed
  • Resonance is less sharp than dipoles

Next Steps

Section titled “Next Steps”