Fiberglass Balcony Stealth Antenna for 40 m

In Singapore space is limited, and in our appartment it is difficult to setup any HF antenna. Our balcony is the only place where I have a space to mount something. Even there, officially nothing is allowed to extend out from the building.

I decided to setup a simple vertical, using a telescopic fiberglass pole, which I can pull out temporarly, when it is getting dark.  The pole has a total length of 10 m, approx. 1 m (the lowest segment) safely mounted to the balcony railing and sidewall.

vertical410 m fiberglass pole mounted to the balcony railing in the 17th floor.

holder1holder2Mounting

vertical1The actual antenna wire is fixed to the pole using Velcro strips. The horizontal wires on the picture are part of my short linear loaded dipole for 15 m and 17 m.

vertical2Antenna extended to 10 m

pipeclampsRubber protected pipe clamps to secure the segments and Velcro strips to attach the antenna wire to my telescope fiberglass pole. Allows quick extension and retracting of the antenna.

verk__00VSWR of 1.2 at resonance frequency 7.050 MHz, using only one radial around the outer wall of our appartment

Linear loaded short dipole

For my amateur radio activities on the HF bands I only have a 3.6 m wide balcony available for antennas. After some study I decided to experiment with a shortened dipole for the 15 m band. The full size 21 MHz half wave dipole would require a span of approx 7 m, therefore I searched for a solution to shorten the full size length by 50 percent.

Finally I decided for a linear loaded dipole; the layout and resulting dimensions are shown below.

loaded-dipole

First measurements at the resonance frequency showed a low input impedance at the dipole’s feed point with values around 10 Ohm. The theoretical radiation resistance of a 2 x 1.6 m short dipole would be 10 Ohm, so it is an indication of low losses in the system. In order to match the antenna to the 50 Ohm of the transceiver, I use a quarter wave transmission line transformer consisting of two parallel connected 50 Ohm coaxial cables.

50ohms_12ohms

After fine tuning the length of the antenna wires, I achieved an SWR below 1.5 for the CW segment of the 15 m band.

2016-03-26-1

The antenna’s bandwidth is around 200 kHz; I could already work DX stations around the world.

dipole-feed balun-1-1

Dipole feed point and 1:1 balun