Marconi “T” HF Aerial Switch

One of the greatest challenges for the typical Ham QTH is squeezing-in that low-band HF aerial for 160m or 80m.  Loading coils, loops, creative routing of the wires around trees are common methods to getting the longest wet-string in (and on) the air.

A popular aerial, especially in AM Broadcast, is the T-Antenna which can be constructed in a number of ways – Luckily, it’s quite easy to make your conventional Doublet, Dipole or G5RV become one via the flick of a switch.

In a normal situation, whether using coax or balanced-feeder – the centre-pin of your radio’s aerial connector feeds one side of the Dipole/Doublet with the other being connected to the chassis/ground.  When you switch this unit inline, BOTH sides of the feeder AND the aerial wires become the overall antenna – with the ground side now routed to the radials.

Here is a (very) simplified diagram showing a typical HF aerial in normal use:

HF Dipole/Doublet in normal use...

Here’s how things look when the T-Switch is engaged:

HF Dipole/Doublet with T-Switch Engaged...

I dislike using red+black when talking about aerials (and abhor the use of positive and negative in this scenario) – but the colours should help you understand, at a glance, what’s happening here.

The diagram shown below uses a chunky DPDT toggle-switch (which could be placed inside your Choke Balun) to configure the feeding arrangement:

  1. Conventional “straight-through” for normal operation
  2. Centre switched to both sides of the balanced-feeder and Ground routed to your radial wires

Switching to convert your Dipole/Doublet/G5RV to a Marconi T

This gives you a vertical element as well as horizontal – making it worth a try during the dark hours for some DX.

HF Aerial Switcher with 1:1 ChokeMy setup at PZT Towers uses an 80m Doublet with the feeder entering the Shack and some radials routed outside the along the floor.  It gives me a workable 160m aerial between 1930-1960KHz without needing to engage the ATU.

Shown here is my prototype unit featuring SO239 input, 2x 4mm terminals for the balanced feed, a 4mm terminal for the radials and a switch to select the operating “mode”.

A ready-made boxed version, featuring an FT240 31/75-mix toroid, will soon be available from my Ham Goodies website.