What it is:
This antenna is sometimes called a Tuned Transmission Line Trap or T2LT. It is essentially an end-fed vertical half-wave dipole which can be made entirely out of coax.
Theory:
The top half of the dipole is usually made from just the dielectric center conductor with the shield removed. The bottom half of the dipole is unmodified coax with a choke to isolate the antenna from the rest of the coax which serves as a feedline.
Both halves should be approximately a quarter wavelength (¼λ) long using a Dipole Antenna Calculator (or 234/MHz = feet).
It seems a waste of coax to strip off the shield for the top half when you could simply attach a piece of wire instead, which is what I did. Start with an extra foot which you will trim to resonance.
The choke can be formed with 8 to 12 turns of coax on a suitable form between 3 and 5 inches in diameter. Some experimentation is to be expected, adding or removing turns for lowest SWR.
Coax Choke
sometimes called an “ugly balun”
TIP: an empty plastic peanut butter jar makes a strong and lightweight coil form.
Of course, an analyzer will make tuning easy and precise, but you can use the SWR meter in your rig if you don’t have one.
Advantages of this design:
• excellent on-air performance
• low radiation angle good for DX
• no transformer required = efficient
• no radials = tiny footprint
• RF energy well out-of-reach = safe
• easy to build with commonly available materials
One disadvantage is the requirement for a support. While a suitable tree limb could easily work in many parks, some operators will need a mast to deploy this antenna.
I installed a ring terminal at the top for hoisting via an arborist throw line. I also used ring terminals at the midpoint between the upper wire and coax center conductor joined with a machine screw and hex nut.
Midpoint Connection
TIP: You could make this antenna out of 75-ohm CATV coax if that’s what you have.
TIP: A better way to make the T2LT is to use RG58 coax with PL-259 connectors on both ends. Add a PL-259 to the bottom of the wire element. Then, simply join the wire to the coax with a standard double-female barrel connector.
Performance:
Click the map to see QSOs
during August using T2LT antennas
Click the Table to see Countries Contacted
Discussions:
M0MCX
Videos to Watch:
20m version (M0DQW)
20m version (G5TM)
15m version (G5TM)
10m version (G3OJV) |