SCARS member Conrad Warren W7WLM has been experimenting with a compact antenna that packs a big punch while using a small amount of real estate. For example, his first contact on a 12′ tall antenna, with a 27′ top element was made using 15 watts, to Australia. Conrad discussed this antenna at the July 2018 SCARS meeting. Conrad started to investigate this antenna when he couldn’t locate any recent information regarding the style of antenna. Wikipedia has a great reference on the T-Antenna, and it’s available by clicking this link.
How Does it Work?
The T-antenna acts as a vertical antenna that has capacitive top-loading and works like the inverted-L antenna. It has been around since the early days of wireless. The Titanic used a T-Antenna to transmit the emergency calls on the 200 kHz band. The top-loading allows the vertical member to be shorter than the same frequency vertical antenna. This type of antenna is slightly less efficient than the equivalent monopole antenna and has a slightly reduced bandwidth. However, Conrad reports SWR of 1.2 or less, across a given band.
Build Your Own
Construction methods really shouldn’t matter for the amateur bands. The general idea is that you need a short vertical section that is soldered or connected to the top horizontal section. Construction types would include a piece of 14 gauge vertical wire soldered to another wire that has insulators and is tied off to a couple trees. Copper pipe should work in a similar fashion. Best practices would place the top section parallel to the ground, but slight variations could be managed by trimming.
Since this type of antenna relies on a good connection to ground, this antenna works best when an 8′ ground rod is installed directly under the antenna. In our part of the country, our soil conductivity means that this is a sufficient ground radial. When connecting a PL-239 connector to the antenna, the shield must be directly connected to the ground rod. The center conductor must be connected to the bottom of the vertical section. The antenna segment must not contact the ground.
Conrad is building some additional test antennas and will be providing some more guidance on exact details, but for now, you can use these numbers to start your trimming.
|Band||Vertical Length||Top Horizontal Length|
|80||24′||Four strands of wire 27′ long with 6″ spreaders|
Tuning the wire would involve connecting an antenna tuner to the wire and the ground, and adjusting the horizontal segment to make a resonant system.