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:
Here’s how things look when the T-Switch is 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:
- Conventional “straight-through” for normal operation
- Centre switched to both sides of the balanced-feeder and Ground routed to your radial wires
This gives you a vertical element as well as horizontal – making it worth a try during the dark hours for some DX.
My 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.