Let’s start by explaining what T2LT means. I visited several sites online that use the term without explaining it. T2LT or TTLT is an abbreviation of Tuned Transmission Line Trap. It sounds complicated but it really couldn’t be simpler to build, erect and use.
It can be compared to a regular dipole in that it is one half wavelength long. Even though most reports suggest it should be used as a vertical antenna it requires no radials or counterpoise wires. Just like a regular dipole it can be erected vertically, horizontally, as a sloper or even an inverted-V.
The T2LT is fed in the centre but the feedline actually connects to one end! How does that work? Let’s look at a diagram of the antenna to see.
As the diagram shows, there are three components to the T2LT: a quarter wave wire, a quarter wave section of coax and a choke. Some sources suggest building the T2LT from a single length of coax but I found that approach to be undesirable due to the need to remove the sheath and braid from a quarter wavelength of coax.
The quarter wavelength of wire forms one leg of the dipole. The other leg of the dipole is the outer skin of the braid of the coax. The trap (choke) terminates the coax braid at the quarter wavelength point. The whole antenna can be fed with any length of coax.
My build of the T2LT antenna for the 20m band used an air core choke that I had previously built as a Common Mode Current choke. It was tested using a nanoVNA and found to have significant choking effect on 20m. It is 30 turns of RG-58 coax wound on a plastic tube with a diameter of two and a quarter inches and has a PL-259 plug on each end.
I had a length of coax that is 16.5 feet long and it already had a PL-259 connector on one end. I soldered an 18ft length of 18ga wire to the far end of the coax. A length of 18 feet was chosen to allow for adjustment for minimum SWR and for attachment of an insulator and support rope.
To erect the antenna I simply attach a barrel connector to each end of the choke. The feedline attaches to one side of the choke and the antenna coax/wire attaches to the other side.
Initial testing reveals that the antenna has a fairly flat SWR of around 1.5:1 from 14.000 to 14.350 MHz and it is very easy to tune. The length of the wire is not particularly critical. It is certainly a very good receive antenna but its performance as a transmitting antenna has not been tested yet. This post will be updated when further testing has been performed.
UPDATE: The antenna has now been field tested and QSOs made with it. Setting it up as a sloper hanging from a tree and using only 35W, I made my first T2LT QSO with a station in Texas from my home QTH in Ontario.
During the QSO I noticed serious, distracting key clicks coming from my radio’s speaker. A sure sign of RFI due to the feedline braid picking up some of the radiated signal. Moving the feedline so it was oriented perpendicular to the antenna cleared that problem.
Looking at how the antenna works I realized that it is important to keep both arms of the dipole equal in length. I trimmed the wire to exactly match the coax half of the antenna. Now the antenna is balanced. The SWR improved to 1.3:1.
Another method of supporting the antenna was also devised. The antenna was loosely coiled around a 31ft fibreglass telescopic pole. The pole can be ground mounted with a suitable guying arrangement, or truck hitch mounted.
I am going to have fun with this antenna. It is fast and easy to erect and uses very little space making it perfect for POTA deployments.