Experimental construction of vertical VHF and UHF dipole antennas

 

More details to be added soon. See a preliminary construction of my vertical dipoles for VHF and UHF transmission and reception:

VHF and UFH dipoles verticalThe UHF (top) and the VHF (bottom) vertical dipoles installed on a 40mm diameter PVC waterpipe

The top dipole is for the UHF band. The bottom dipole is for the VHF band. Both antennas are fixed on a vertical plastic PVC boom of 40mm in diameter. The dipole elements are from aluminium and have 14mm outer diameter. This design is very quick to construct and install. It’s also cheap and very few materials are needed to complete it! The only tricky part is the transition between the dipole elements and the coaxial cable. Currently, I am simply attaching the ends of the coaxial cable directly into the elements using a conductive collar. I plan to add the option for a UHF connector to each dipole to accelerate installation and improve portability.

Each of the UHF elements (top antenna), is 13cm long. They seem to be working relatively well. I haven’t measured VSWR though, I’ve only confirmed their effectiveness through local contacts. A small explanation: At the center frequency of the UHF band, i.e. 435MHz, wavelength is 68.96cm, so each dipole element should be 17.24cm. I tried this, but it didn’t work! Reason: The distance (spacing) BETWEEN the elements at the coaxial dipole transition matters too. This ‘extra’ space makes the effective antenna length from tip to tip longer than the λ/2 calculation. So, to compensate this longer length and improve performance I had to shorten a bit each element, down to 13cm. The UHF dipole is fed with a RG 58 cable. That’s a complete catastrophe for the RF power transfer at V/U bands, but I had no other cable stocked at home.

The VHF dipole (bottom) consists of two 52cm elements. This antenna does not seem to be working well, I need to shorten the elements perhaps, for the reason explained above. I didn’t do this, as I haven’t found the time, but I suspect going down to 48cm or so might improve matching. I fed it with a RG 213 cable. This makes the setup ‘heavier’ than the RG58, but the losses are significantly less.

I guess that I should also ‘swap’ cabling between VHF and UHF, to compensate losses (RG213 for UHF and RG58 for VHF).

Notice that in both cases (although not shown correctly on this photo, due to the angle of the photo taken) the cables make a U-turn before landing to the input of the dipole at right angle (with respect to the axis of the dipole). This is done to reduce the coupling between the dipole and the cable, and improve antenna’s performance (in terms of radiation pattern and input impedance).

(my note: add photos showing how to make each element tunable. Stay tuned!)