Dual Band VHF/UHF single 50Ω feed

Shack in the box radios have not only all of our HF-Band available but also are able to use the VHF and UHF band. And that not only for FM but also CW and SSB. One draw back though is that the antenna port for VHF/UHF is combined into a single antenna port. To use such a radio on VHF/UHF for CW/SSB we would most likely use some form of horizontal polarized Yagi-Uda antenna (Beam). This would mean that we would need to use a diplexer for VHF/UHF to split the antenna port into two ports, one for our VHF Antenna System and one for the UHF part.  There is quite a bit of additional hardware required. So what about a dual-band Yagi-Uda antenna system?

There are multiband Beams e.g. antennas with multiple interlaced elements for the different bands like the antennas from Opti-Beam.  Multi-band Beams for the HF bands cover a frequency ratio of 2:1 whereas the frequency ratio for the 2m/70cm  (144/432 MHz) band is about 3:1. We can feed a 20m and a 10m dipole with at a common feedpoint. The reason is that if we transmit on 10m the 10m dipole will show a low impedance and the 20m dipole will show a high impedance. And if we transmit on 20m, the impedances will be swapped, i.e. on 20m the impedance will be low and on 10m it will be high. Of course there will be interaction but we can cancel that by adjusting the length of either dipole. This is quite easy due the the 2:1 frequency ratio. But, if we look at dipoles for 2m and 70cm with a frequency ratio of 3:1 the situation for the higher frequency band is different.  At resonance on 70cm both dipoles, the 2m and 70cm dipole, will show a low impedance. As such, there will be currents on ALL elements. The question is how will this affect the overall antenna? Will it even work? From memory I've had a few issues back when I modelled a multiband RC dipole for HF trying to add 30m into the mix., see here.

Anyway, the below is a sketch of the antenna, a Radiation Coupled Yagi-Uda Antenna (RC-Beam) for 70cm and 2m.  The idea is to make a lightweight antenna for portable SSB operations using a single feedline. Saving either a COAX cable or a Diplexer, to operate in the field with a single antenna for 2m and 70cm. I believe that this could be a nice addition to a shack in the box radio like my ICOM IC-7100.

The below should give a bit of an impression of the antenna radiation pattern for the VHF part of the antenna.

Antenna with 10mm element modeled data at a frequency of 145 MHz with 10mm elements:

• GAIN: 11.01 dBi

• -3dB Beamwidth: 50 degree

• F/B: 16.6 dB

• SWR: 1.05:1

And the 70cm part of the Antenna, modeled at a frequency of 434 MHz with 10mm elements:

• GAIN: 15.02 dBi

• -3dB Beamwidth: 30 degree

• F/B: 16.1 dB

• SWR: 1.08:1

Not to bad, but needs a bit more work to get the 2m SWR down. The modeled 2m SWR using 3.2mm elements was below 1.2:1. But it is a good start and the concept seems to be working. Not sure if the boom is sturdy enough though. (The issue was the reflector element, which consisted of two individual element halfs and did not make electrical contact)

Materials I used: 

• 8x 1m 3.2mm Al Welding rods (5356)

• 1.7m 20mm PVC pressure pipe

• 13x 25mm conduit holder

• 3mm Stainless Steel (317) fasteners

• bit of Delrin for the driver mounting plate (ymmv)

• VHF/UHF Choke