Quarter Wave Ground Plane Antenna Guide.en
Quarter Wave Ground Plane Antenna Guide
Sources
This article is based on the UK High Altitude Society guide: UK High Altitude Society - Payload Antenna Guide.
See the original article for additional photos of the antenna!
Introduction
The 1/4 wave ground plane antenna is a popular choice for radio communications due to its omnidirectional radiation pattern, which emits equal radio power in all horizontal directions. This makes it suitable for applications where you need consistent coverage regardless of the receiver's position.
Understanding Wavelengths and Quarter Wave Antennas
A wavelength is the distance a radio wave travels during one complete cycle. The wavelength (λ) is calculated using the formula: λ = c / f, where c is the speed of light (approximately 299,792,458 m/s) and f is the frequency in hertz.
A quarter wave antenna uses a radiating element that is exactly one-quarter of the wavelength long. This length is optimal because it creates a natural impedance match with 50-ohm coaxial cable and provides efficient radiation. The antenna also requires a ground plane (typically consisting of radials) that acts as a mirror, creating the electrical equivalent of a half-wave dipole antenna.
ISM Bands
ISM (Industrial, Scientific, and Medical) bands are radio frequency bands reserved internationally for unlicensed use by equipment that generates radio frequency energy for industrial, scientific, medical, or similar purposes.
This guide covers construction of quarter wave antennas for two ISM bands:
- 433 MHz - Commonly used for short-range wireless devices
- 868 MHz - Used for European ISM applications
Antenna Specifications
For 433 MHz:
- Radiating element length: 17.3 cm (quarter wavelength)
- Radial length: 17.3 cm (each radial)
For 868 MHz:
- Radiating element length: 8.6 cm (quarter wavelength)
- Radial length: 8.6 cm (each radial)
1/4 Wave Step by Step Build
You will need:
- Wire cutters
- Sharp blade
- Ruler
- Single core wire (1/0.6mm) for the radials
- 50-ohm coaxial cable "pigtail" with SMA plug on one end
- Soldering iron and solder
Step 1: Trim the insulation from the coax
Measure the appropriate distance from the bare end of the coax based on your frequency:
- For 433 MHz: Measure 180mm from the bare end, slightly longer than the target of 170 mm
- For 868 MHz: Measure 90mm from the bare end, slightly longer than the target of 85 mm
Cut the insulation without damaging the braid inside. Gently roll the blade over the cable.
NOTE: The exact length of the coax is not critical yet, we will trim it shorter later in step 8.
It is important at this point not to cut the braid inside.
Step 2: Carefully Remove the insulation back to the end of the coax
The easiest way to do this is to roll the blade around then carefully cut lengthways and remove the insulation in segments.
Step 3: Leaving 5mm of braid cut the braid and remove
Step 4: Tin the Remaining Braid
Make the remaining braid into two stumps and tin:
This is the radiating element. Put this to one side.
Step 5: Prepare the radials
Cut two lengths of single core wire. The length should be approximately double the quarter wavelength:
- For 433 MHz: Cut two lengths about 350mm long
- For 868 MHz: Cut two lengths about 175mm long
Mark the center point with a pen and then remove about 25mm of insulation from the center.
Again this is easiest by gently rolling the wire on the edge of a blade and then cutting lengthways and removing the insulation.
Step 6: Twist Together
Using a pen or large nail as a former place the wires around and twist together. Then apply solder to secure them.
Step 7: Connect the two sections
Slide the coax part through the small hole in the center of the radial section. Solder in place.
Step 8: Trim Radiating element and radials
From the point where the shield ends, measure and trim the radiating element (the exposed coax center conductor, also called the driven element).
Important: When cutting the radiating/driven element, it is safer to leave it slightly too long initially. You can always trim more material off, but you cannot add it back. Start with approximately:
- For 433 MHz: Leave about 180 mm (slightly longer than the target 173 mm)
- For 868 MHz: Leave about 90 mm (slightly longer than the target 86 mm)
Repeat with the radials, trimming each to approximately the same length as the radiating element:
- For 433 MHz: Trim each radial to approximately 173 mm
- For 868 MHz: Trim each radial to approximately 86 mm
At this point, you have a basic antenna structure. The next step will help you fine-tune it for optimal performance.
Step 9: Tune the antenna using a NanoVNA
To achieve optimal performance, you should measure and tune the antenna using a NanoVNA (Nano Vector Network Analyzer) to find the lowest SWR (Standing Wave Ratio) at your target frequency.
9.1: Prepare the measurement environment
- Place the antenna in free space - ensure there are no metal objects, walls, or other conductive materials nearby
- Keep your hands and body away from the antenna during measurement, as they will affect the tuning and SWR readings
- Ideally, suspend the antenna in the air or mount it on a non-conductive stand
- Ensure the radials are properly spread out and straight
9.2: Connect the NanoVNA
- Connect the NanoVNA to your antenna using a short, high-quality coaxial cable
- Use appropriate adapters if needed (e.g., SMA to your connector type)
- Power on the NanoVNA and allow it to warm up for a few minutes
9.3: Set up the measurement
- Configure the NanoVNA to display SWR (Standing Wave Ratio)
- Set the frequency sweep range around your target frequency:
- For 433 MHz: Sweep from approximately 420 MHz to 445 MHz
- For 868 MHz: Sweep from approximately 850 MHz to 885 MHz
- Set the center frequency to your target:
- For 433 MHz: Center at 433.0 MHz
- For 868 MHz: Center at 868.0 MHz
9.4: Perform initial measurement
- Observe the SWR curve on the NanoVNA display
- Note the frequency where SWR is lowest
- If the lowest SWR is below 2.0:1, the antenna is reasonably well-tuned
- If the lowest SWR is above 2.0:1, proceed to fine-tuning
9.5: Fine-tune the radiating element
- If the lowest SWR frequency is below your target frequency (e.g., 430 MHz when targeting 433 MHz), the radiating element is too long - trim off 1-2 mm and remeasure
- If the lowest SWR frequency is above your target frequency (e.g., 436 MHz when targeting 433 MHz), the radiating element is too short - you may need to start over, or if only slightly off, adjust the radials slightly shorter
- Trim in small increments (1-2 mm at a time) and remeasure after each adjustment
- Continue until the lowest SWR occurs at or very close to your target frequency (433.0 MHz or 868.0 MHz)
9.6: Fine-tune the radials (if needed)
- If adjusting the radiating element doesn't achieve the desired result, you may need to trim the radials slightly
- Generally, shorter radials will raise the resonant frequency, longer radials will lower it
- Make small adjustments and remeasure
9.7: Final verification
- Once tuned, verify the SWR is below 2.0:1 at your target frequency
- An SWR of 1.5:1 or lower indicates excellent antenna performance
- Record your final measurements for reference
Note: Remember that any changes to the antenna's environment (metal nearby, different mounting, etc.) will affect the SWR. Always measure in the same conditions where the antenna will be used.
Congratulations! You've made and tuned a lightweight 1/4 wave antenna. It is recommended you secure and straighten the radials and radiating element using straws or similar supports.
You can trim the straws down but take care not to snip the elements inside. The radials form a ground plane. The radiating element should be oriented vertically. The length of the coax from the connector to the start of the antenna isn't critical, though it is suggested it's kept as short as possible.
You don't need a coax with an SMA plug on it - you can solder the other end directly to your circuit board.
Regulatory Information for Finland
ISM Band Restrictions in Finland
In Finland, ISM band equipment is regulated by Traficom (Finnish Transport and Communications Agency) under the Radio Frequency Regulation. For the 433 MHz and 868 MHz ISM bands, typical restrictions include:
- 433 MHz ISM band: Maximum power is typically limited to 10 mW ERP (Effective Radiated Power) with duty cycle restrictions
- 868 MHz ISM band: Maximum power is typically limited to 25 mW ERP with duty cycle restrictions (often 1% duty cycle for certain applications)
These restrictions are designed to prevent interference with other users. Always consult the current Traficom Radio Frequency Regulation (most recently updated March 31, 2025) for the exact power limits and duty cycle requirements applicable to your specific use case.
70cm Amateur Radio Band in Finland
The 433 MHz frequency also falls within Finland's 70cm amateur radio band (432-438 MHz). If you hold a valid Finnish amateur radio license, you can use a 433 MHz antenna for amateur radio communications with significantly higher power limits:
- Basic Class license: Up to 25W maximum power
- General Class license: Up to 100W maximum power on certain sub-bands
Note: Amateur radio operation requires proper licensing and adherence to amateur radio regulations, which differ from ISM band rules. Operating in the amateur radio bands requires a valid amateur radio license issued by Traficom. Unlicensed operation in amateur radio bands is illegal, even if using ISM-compliant power levels.