Classic Hexbeam - Measurements (Part 3)

My capability to make practical measurements on the Classic Hexbeam took a step forward when I constructed a 10m monoband test-bed antenna whose critical dimensions could be changed easily and which could be highered, lowered and rotated conveniently on a fibreglass telscopic mast. This page presents some of the measurements that I made on this test-bed. Unless otherwise stated, all antenna variants were constructed with #16 wire, Reflectors were dimensioned to be resonant mid-band, and Drivers were cut 2% shorter than the Reflectors.

1. End-spacing

F/B vs end spacing SWR vs end spacing

The spacing between the tip of the Reflector and the tip of the Driver was adjusted from 6" to 12" in 1.5" steps. For each value of end spacing the antenna F/B and SWR were measured over the frequency range 28 - 29 MHz with the antenna at a height of 20ft. The charts on the right present the results:

We note that:

1. End spacings between 7.5" and 9" represent a good compromise between F/B performance and SWR.
2. On a monoband design where the SWR could be improved by external matching components, the end spacing might usefully be increased to 10.5" for improved F/B performance.

2. Beta matching

Effect on Z of varying Driver length SWR after addition if Beta coil

The length of the Driven element was reduced in 1 inch intervals whilst keeping the end spacing (8") and Reflector length constant; the antenna was up at 20 ft. At each step the Resistive and Reactive components of the feedpoint impedance were measured and are shown below in the chart on the left. Please note that the Reflector was shortened slightly before this set of measurements was taken - this raised the frequency of best F/B to about 28.75MHz.

We note that:

To confirm the utility of Beta matching, an inductor of about 270nH (Xl=50 Ohms) was added in parallel with the feedpoint. SWR and F/B measurements were then made across the band. The results are shown above in the right hand chart.

We note that:

Beta matching is an simple and effective way of improving the SWR of a monoband Classic Hexbeam

3. Effect of height on SWR

Effect of height on SWR

The antenna was raised from a height of 10ft to 22ft in 2ft steps. At each height the lowest SWR, and the SWR at best F/B, were measured. Results are plotted in the chart below.

We note that:

Height has a significant effect on the SWR of a Hexbeam; best SWR occurs at a height of about a half-wavelength.