First Tower Part 8 – VHF/UHF Antenna System Design

M2 2M Beam

M2 Antenna Systems 2m Yagi

We have an interest in weak signal work on the 2m and 70cm bands. We currently have a vertical ground plane antenna installed at about 50 ft for accessing our local repeaters on these bands but the surrounding tree cover and lack of gain have prevented us from pursuing our interest in VHF/UHF contesting, Grid Square collecting, etc., on these bands. We designed our tower to accommodate long-boom Yagi’s on 2m and 70cm for these purposes, and we have decided to install them as part of our project. I spoke with Jason at M2 Antenna Systems this evening. After some discussion about our goals and the tower and station equipment we have, we decided upon the following antennas and related equipment for VHF/UHF weak system work:

Both antennas have good gain and F/B ratios and very good patterns. Initially, we will drive the system with our Icom IC-9100 Transceiver. This rig provides 100W on 2M and 75W on 70cm barefoot. This should be enough power to have some fun on these bands initially. We plan to add an amplifier on 2m and 70cm, so we selected antennas capable of handling 1KW+ power. We are also having M2 Antenna Systems assemble a custom pre-amp and high-power switching system to mount at the top of the tower, which will accommodate our planned power levels. Both antennas feature rugged construction to help them withstand the winters that we have here in New Hampshire, USA.

M2 70cm Beam

M2 Antenna Systems 70cm Yagi

It is important to pay careful attention to the feedline losses at these frequencies. This is especially important for us as we need 200’+ of coax feedline to get from our shack to the top of the tower/mast where the antennas are located. To ensure good performance, we set a target for the total losses in the feedlines to be no more than 1.5 dB. To meet this goal, we settled on the following configuration:

Both feedlines will have about 1.2 dB of loss end to end using these components. We will place both beams on the 15′ mast above our upper SteppIR DB36 Yagi. We plan to place the 70cm Yagi at the top of the mast and the 2m Yagi between the DB36 and the 70cm Yagi in the middle. This configuration should minimize any interactions between the three antennas.

You can read more about our tower project via the articles which follow:

– Fred, AB1OC

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