Category: Antennas

The OCF short vertical dipole for HF has become popular, particularly disguised as a flag pole for low impact installations and encouraged by claims of outstanding performance. The antenna was described in QST and a commercial version was available at time of writing.

The rationale for the design is that it is a short dipole, not requiring radials, and feed point offset downwards by 30% as an optimal value for performance (driven by often unsound assessments of coax loss).

Claims include:

Off-Center Fed Vertical Dipole design means no radials, 90% efficient or better across 80m – 10m

Above is the promising gain plot for one of the commercial implementations, it is only one S point (6dB) behind a quarter wave vertical with 4 buried radials. Continue reading OCF short vertical dipole for HF

Upon reading Rationale for sizing of lightning down conductor a correspondent asks whether his premises 4mm^2 main earthing conductor ok as a lightning down conductor.

Intended purpose of electrical installation ground electrode

The usual source of current on the premises main earth conductor would be a fault energised by the incoming supply. To understand the implications, lets review the supply system. Continue reading Is the premises main earthing conductor ok as a lightning down conductor

The lightning ground conductor shown at Mast ground rework might at first seem excessive, this article sets out the rationale.

The connection to a 2.4m copper clad steel driven electrode (under the green cover) is 35mm^2 copper.

The nature of lightning protection sizing

Lightning protection sizing is a risk management regime driven by the mechanisms of lightning and variation in distribution.

It is not surprising then that regulatory standards in different distributions broadly use similar design methods but set different practices for implementation in the jurisdiction.

So, let’s go standards shopping… what we are looking for is guidance on the energy (or work) that is directed to heating the down conductor, and choosing a conductor size that will sustain not just a single stroke, or an average stroke, but most events that may include many strokes in a short period of time. Continue reading Rationale for sizing of lightning down conductor

I showed in Mast ground rework the use of a Cadweld Oneshot Plus thermite weld of the ground conductor to the ground rod.

Responding to reader interest, I have made a little video demonstrating the process.

Above is a pic of the demonstration piece with crucible and slag broken away. Continue reading Demonstration video of Cadweld Oneshot Plus ground rod connection

When I moved here about eight years ago, I quickly installed a small mast and associated ground system for the station. The grounding of the mast itself for lightning protection was a temporary solution, and less temporary than planned. This article documents the rework.

Above is the temporary solution. A 2.4m copper clad ground rod was driven into the clay, and a couple of short 25mm^2 tails connected to the mast tube. The long term solution was to be tidier and allow the mower / brushcutter to be used to trim grass without fouling the earth rod or cables.

The plan is to cut to bent top of the earth rod, drive it below ground level, and make a new tail of 35mm^2 (#2) cable and Cadweld it to the ground rod. Continue reading Mast ground rework