Design concept

The design concept for the Dual Beam Pro and I-Pro Home antennas is to give the best performance possible for a wide range of HF bands whilst maintaining an unobtrusive and aesthetically pleasing profile. The underlying design is a based on a 5m dipole which is considered acceptable for mounting on a rooftop in a residential area or erecting in a small garden.

 

A simple 5m dipole would be resonant between the 10m and 11m bands; it would, therefore, be too short for the 40m - 14m bands and too long for the 6m band. The resulting antenna is therefore non-resonant and requires additional components to optimise the design.

 

The addition of capacitive end loading elements has the effect of electrically "lengthening" the dipole. This could also be achieved by mid loading with inductance but capacitive end loading has 3 advantages:

  • It is more efficient, resulting in lower losses

  • Capacitive loading designs have a higher bandwidth

  • It increases the radiation resistance more than inductive designs

The only disadvantage is that capacitive end loading can increase the wind loading but this is not significant for the Dual Beam Pro and I-Pro Home antenna designs.

An important part of the design was to adopt a balanced, centre fed dipole with a balun to connect it to the unbalanced coax. This offers a much lower noise solution than an end fed dipole and reduces the unintended RF radiation that could affect domestic electrical equipment - both very important considerations in a residential area with restricted space. The high quality components of the balun are contained in a plastic box and encased in epoxy resin to provide stability, prevent water damage and further improve the noise performance.

The non-resonant design means that the antenna offers a wide range of impedances across the relevant bands and a suitably tolerant ATU is required to match the antenna to the transmitter. The use of a 4:1 Voltage balun brings the impedance of the antenna closer to the required 50 Ohms and reduces the workload for ATU.

 

The use of coax feeders provides a convenient method of connection to the ATU and transmitter. A low loss RG mini 8 coax is recommended and the design is optimised for a feeder length of 35m. In this configuration, the SWR presented to the ATU has been measured and the results are shown here.

 

The Dual Beam Pro and I-Pro Home have been thoroughly tested using LDG ATUs and the LDG Z11 Pro is ideal for operation up to 100W. To assess the suitability of other ATUs, a tuning range of 6 - 1000 Ohms is required for operation across the full range of bands.

I-Pro Home measurements taken at an antenna height of 1m using 20m of mini 8 coax.

Dual Beam Pro measurements taken at an antenna height of 8m using 30m of RG213 coax.

Dual Beam Pro

The Dual Beam Pro is electrically identical to the I-Pro Home but designed for mounting as a horizontal dipole. This orientation gives rise to 2 main radiation lobes at 90 degrees to the main element - hence the name Dual Beam. This antenna performs best when mounted high up on a roof or lightweight tower.

I-Pro Home

The I-Pro Home is configured for mounting as a vertical dipole. The vertical orientation makes the I-Pro Home omnidirectional with a low elevation angle. It is designed to be mounted around 1 metre above ground and can be easily removed when not in use or transported to another location.

I-Pro Traveller

The I-Pro Traveller is a smaller version of the I-Pro Home but with some significant design differences. It is smaller to make it truly portable but this means that it needs components to make it electrically longer. In addition to the capacitive end loading elements there is a mid loading inductor which has selectable tap points for the different bands.

The I-Pro Traveller is designed to be resonant and does not need an external ATU when set up correctly. The coarse band tuning is achieved by plugging the patch cables into the correct sockets and it can then be fine tuned by adjusting the angle of the capacitive end loading elements.

The manual adjustment design was chosen over the external ATU approach to reduce the need for carrying equipment on location. This is a practical proposition for a portable antenna but would not be so practical for the Dual Beam Pro or I-Pro Home.