There seems to have been a revival in use of the so-called End Fed Half Wave antenna.
The prospect that a small radio such as the FT817, a magic match box and 10m of wire makes a good 20m field station appeals to many a SOTA enthusiast.
Let us model a scenario with a FT817 powered by internal battery and sitting on an insulating platform (eg a pack) 0.3m above natural ground, a 10m wire strung up into a tree at a 45° angle, and a 1m long mic cord stretched up at 45° in the other direction. The is the popular so-called ‘no counterpoise’ configuration.
A simplified model of just the current paths without regard to the bulk of the radio, or the effect of the helix of the mic cord illustrates an approximate current distribution. The model uses 1W RF input to the antenna over ‘average ground’ (0.005,13).
Above is a plot of the current distribution. Current is a minimum at the open ends, a boundary condition for the problem, and maximum in the middle of the half wave. We expect H field to be greatest near the current maximum, and E field to be greatest near the current minima.
Above is a closer look at the operator end of the system. E field is very high, around 3500V/m peak (2500V/m rms) towards the lower end.
(ARPANSA 2002) sets the permitted limits for EMR at 14MHz to:
- instantaneous: 868V/m rms; and
- time averaged: 27.4V/m rms.
The model value at 1W RF input is well in excess of the permitted instantaneous exposure and some 40dB greater than the time averaged limit.
Different limits may be prescribed in different jurisdictions.
This study is a very simple one and there are a host of issues about extension of the model to the diversity of real operating scenarios, but it sounds a warning that EMR safety of this type of configuration is a potential problem even at 1W radiated power and warrants measurement of some typical deployments with an E field probe.
- ARPANSA. 2002. ARPANSA Radiation Protection Standard (RPS3 2002) – Maximum exposure levels to radiofrequency fields – 3kHz to 300GHz (assuming plane wave conditions).
- AS/NZS 2272.2. 2011. Radiofrequency fields
Part 2: Principles and methods of measurement and computation—3 kHz to 300 GHz.
- Duffy, O. Oct 2010. Counterpoise. http://owenduffy.net/files/Counterpoise.pdf.