Skew Planar Plane Antenna for 435MHz. October 2024. 

The skew-planar antenna was first introduced to the amateur radio community by Robert H. Mellen, W1IJD, and Carl T. Milner, W1FVY, in a 1963 QST article, yet it has received little attention since then. The only other notable discussion in amateur radio appeared in the November/December 2006 issue of TCA: The Canadian Amateur, in an article by Graham Ide, VE3BYT, and David Conn, VE3KL. http://www.ve3byt.com/SkewPlanarAntenna/

The skew-planar antenna features a set of elements arranged in a tilted or skewed plane, typically forming a triangular or rectangular pattern. This configuration produces an efficient radiation pattern with omnidirectional coverage, making it suitable for various applications, including amateur radio, satellite communication, and other wireless systems.  

Photo 1. Skew-Planar Antenna assembled for testing.

The antenna is especially popular among RC (Radio Control) enthusiasts, who use it on aircraft due to its omnidirectional pattern and circular polarization. Circular polarization helps maintain a stable signal despite rapid changes in the aircraft's orientation, which is crucial for reliable communication and video transmission in FPV (First-Person View) set-ups. The circular polarization reduces signal loss caused by cross-polarization issues with other antenna types, thus enhancing video quality and consistency. For optimal performance, both the transmitting and receiving antennas must use the same circular polarization rotation—either right-hand or left-hand—to avoid significant signal loss.

In amateur radio, the skew-planar antenna's omnidirectional coverage and circular polarization characteristics make it well-suited for Low Earth Orbit (LEO) satellite operations. Its ability to maintain consistent signal strength despite variations in satellite orientation provides an advantage for reliable communication.

Photo 2. Top view of Skew-Planar Antenna showing the right-hand rotation 

Photo 3. Top view of Skew-Planar Antenna showing the element attachments 

Antenna description

The antenna is a 4-element skew-planar antenna for 438 MHz constructed from all aluminium around a standard male N connector.  

Construction

 
The antenna is constructed around a standard male N connector with a 100mm square aluminium radial mounting plate with a 16mm centre hole to attach to the N connector with the standard coax gland screw cap as shown in Photo 4. The elements have been attached with 3mm diameter pop-rivets to the mounting disks

 
The smaller upper disk is fitted to a standard male N connector's coax centre pin with a 6mm diameter stainless steel stud as shown in Photo 5 and positioned in the N connector and secured with two part epoxy. A small rubber grommet has been fashioned to fit tightly in the end of the N connector body with a smearing of marine grade silicon to make the assembly water proof. Photo 6 shows the final assembly.

Photo 4. Element attachment plates. 

David Conn (VE3KL)'s analysis of the Skew-Planar antenna confirmed that the element lengths should be longer than a free-space wavelength at the chosen design frequency by a factor of about 4.5 percent. 

Note: I have determined the factor as being 5.5%. 

The element lengths for various bands are shown in the below Table 1 for Fig 1.

Each element is shaped as shown in Figure 1 & 2 and in Photos.  On each end of an element at the 1/4 length point, make a bend around a fairly tight radius - for the 435 MHz antenna this was around a radius of about 20mm.  The half element portion is curved out so that the straight 1/4 length sections meet and form an angle with each other of about 100 to 110 degrees.  If necessary, adjust slightly the curvature where the one-quarter and the half element lengths meet.

Photo 5. Centre pin of the Male N connector soldered to the copper sleeve. Copper sleeve Silvered Soldered to the 6mm stainless threaded stud.  

Photo 6. N Connector, lower plate and threaded stud assemble.

Fig 1. Element layout. See Table 1 element lengths for various bands.

Table 1  Dimensions for some bands  

Fig 2. Antenna general layout. 

 
The antenna mounting is a standard antenna mirror mount bracket with a female to female N connector bulkhead socket fitted for a range of similar antennas that are constructed around male N connectors. See: Generic Antenna Mount.

Photo 7. Element being shaped.

Photo 8. Element angle rotation being shaped.

Adjustment 

While the Skew-Planar Antenna is quite forgiving due to its broadband characteristics, there are limited opportunities for easy adjustments to its elements. However, one useful adjustment is the separation between the upper and lower attachment plates along the threaded stud. This adjustment allows you to minimize SWR and tune out any matching reactance

Photo 9. Upper attachment plate allows some adjustment along the threaded stud to minimise SWR

Antenna testing  

Photo 10. NanoVNA displaying the antenna feed point SWR and Impedance.

The antenna electrical matching characteristics were measured with a NanoVNA and were recorded as a more than suitable SWR from 420 MHz to 450 MHz with a minimum or best match at approximately 443 MHz. The Smith chart also indicated a near ideal impedance match of 49.2 +3.49j at 442.8 MHz.

Modelling

Fig 3 Azimuth and Elevation Radiation pattern of the Skew Planar Wheel Antenna at 438MHz.

MMANA-GAL Antenna Analyser predicted the following results from the model.

Gain : 5.71 dBi = 0 dB (Vertical polarization)

F/B: -0.19 dB; Rear: Azim. 120 dg, Elev. 60 dg

Freq: 438.000 MHz

Z: 58.186 + j6.409 Ohm

SWR: 1.2 (50.0 Ohm),

Elev: 0.0 dg (Perfect GND :1.50 m height) Based on that the antenna is likely to be mounted over a steel roof.

Fig 4 A three dimensional view of the radiation pattern of the Skew Planar Plane Antenna at 438MHz. Radiation plot was produced by MMANA-GAL Antenna Analyser software.

Antenna Gain Range Testing

This is the most important antenna measurement because even if all other measurements such as SWR and resonance are satisfactory, however if the antenna fails to achieve at least an approximation of the desired or predicted gain, it can be considered a failure. Measuring antenna gain is perhaps one of the most challenging tasks to accomplish successfully, as it requires a large and unobstructed area, especially free from metallic obstacles that can significantly distort the antenna's ideal radiation pattern. Figure 5 below illustrates an example of an antenna gain range test using the NanoVNA.

Fig 5 Shows the basic antenna gain range test set-up.  

Figure 4 shows the basic set up with d indicating the distance between the Source dipole antenna and the Reference dipole antenna and while not critical needs to be between 2 and 3 wavelengths apart. In the set up the two antennas were placed 2mtr (approximately 3 wavelengths) apart.

The ideal separation for antenna gain testing depends on various factors such as the frequency of operation, the type of antennas being tested, and the testing environment. Generally, a separation of at least 2-3 wavelengths is recommended between the transmitting and receiving antennas to minimize interference and achieve accurate measurements. Larger antenna separations can give false readings due to ground reflection and other multi-path effects. 

The suitable height above ground for antenna gain testing depends on various factors, including the type of antenna, the desired testing accuracy, the operating frequency, and the testing environment. As a general guideline, a height of at least 1 to 2 wavelengths above ground is recommended to minimize ground effects and reflections.

The antennas in this set-up are positioned 1.5 meters above the ground, which is slightly over 2 wavelengths at 435 MHz.

Source Antenna is a 435MHz Source dipole antenna.

Reference Antenna is also a 435MHz Reference dipole antenna. A measurement will be taken with this antenna to determine the base line. This antenna is replaced with the Skew Planar Plane Antenna and the return loss measured that will show the gain/los in dB with respect to the Reference dipole antenna. 

NanoVNA set to LOGMAG with a display of  -24 ~ +6dB and calibrated to remove the lead characteristics from the measurements and with the reference antenna and set the base line to 0 as per Fig 4.

Photo 11 NanoVNA showing the Fig 5 set-up and calibrated for the base line to be zero.  

The first test was to rotate the reference dipole 90° for cross polarization measurement to benchmark cross polarization immunity of the Skew Planar Plane Antenna. With the reference dipole rotated at  90° to the source dipole a path loss of approximately -28dB was recorded.

Photo 12 NanoVNA showing the Fig 5 set-up with the reference dipole rotated 90° for cross polarization measurement.  

Fig 6 Shows the basic antenna gain range test set-up with the antenna under test in place.

Target performance.

The expectation is that the Skew Planar Plane Antenna will have a high degree of cross polarization immunity from high path loss and that the antenna a more or less all of sky view.

Test Results.

The Skew Planar Plane Antenna was tilted in a range form 0.0° to 90° in 5° to 10° increments and with source antenna polarization changed by 90° for each tilt angle.

Photo 13 The Skew Planar Plane Antenna set up at 45° angle 

Table 1  Recorded data from Skew Planar Plan Antenna range testing.

Fig 7 Shows range testing data overlayed on the MMANA model's predicted slice radiation pattern.

Photo 12 NanoVNA showing the Fig 5 set-up and displaying Skew Planar Wheel Antenna at 45 with the gain compared with the reference dipole antenna.

Conclusion

The range test of the Skew Planar Plane Antenna confirmed that its performance closely matches the MMANA-GAL model. The antenna demonstrates strong immunity to path losses due to cross-polarization. Except for directly above the antenna, the radiation pattern is generally uniform and, again, closely aligns with the model.

It is important to note that there are trade-offs: achieving a more omnidirectional radiation pattern and immunity to cross-polarization losses comes at the cost of a gain reduction of approximately -1 to -8 dBd compared to a reference dipole.

The antenna, as used by model aircraft enthusiasts in the 2.4 GHz and 5.8 GHz bands for FPV (First Person Video), is well suited for this purpose and may also be beneficial for LEO (Low Earth Orbiting) amateur satellite communication-particularly on the uplink, where additional power can compensate for the slight loss in gain.

MMANA-GAL model file for the 438 MHz Skew Planar Wheel Antenna: skew_planar_wheel_antenna_Mk2_20241022_mmana.zip

Video of the construction and testing of the 435MHz Skew Planar Plane Antenna.