VE6AB Antenna Werks

CapHats Always Required

A common denominator of all my mobile HF antennas has been a caphat. I have come to realize that such a device should be part of all mobile antenna for maximum efficiency.

All of my mobile antennas have required a loading coil as part of the overall antenna assembly, however, knowing that the loading coil is the most inefficient part of the antenna, I then spent much of my time trying to eliminate at least a part of it.

The loading coil is inefficient because its resistance is in series with the antenna radiation resistance, a small value on an antenna that is small compared to a 1/4 wave monopole. All other things being equal, the more inductance that is required will result in more resistance.

The caphat comes in to play by electrically lengthening the antenna by adding capacitance above the loading coil. The larger the caphat, in terms of effevtive surface area, the greater the capacitance. the greater the capacitance, the smaller the amount of inductance nedded in the loading coil for a given resonant frequency.