Another look at the Tilted Terminated Folded Dipole
TTFD or T2FD Broadband Antenna System
What is a TTFD ?
The antenna is basically a folded dipole of one third of a wavelength, overall length, at the lowest frequency you want to operate, with maximum efficiency. The antenna will perform with lower efficiency at frequencies lower than the above, and will retain its excellent low noise pick-up characteristics.
How to Build the TTFD
To calculate the length of your TTFD,in meters, use the following formula: 100 divided by the lowest frequency in megahertz
Here’s an example
For general short wave listening, a good low frequency cutoff is 7 megahertz. So 100/ 7 = 14.28 meters. For all practical purposes, your antenna length can be 14.3 meters. This antenna length is an excellent compromise, as it is able to provide excellent performance on frequencies from 6 megahertz up to 30 megahertz.
The second calculation that you will have to perform is the one that will provide the separation between the upper and the lower portion of the folded dipole, using the formula : 3 divided by the frequency at which we want maximum efficiency
Some recent experiments show that the TTFD tends to perform better increasing the separation between the upper and lower wires. Instead of the 3 divided by the frequency in megahertz standard formula, you may try a design using about twice more distance between the two wires and see what happens. Experimenting with antennas is a lot of fun, and this one is a very easy to perform experiment, as you will only need to build a new set of insulated spreaders for the antenna using the wider separation.
Let’s run the standard formula first.
For our 7 megahertz example, it’s simply: 3/7 (MHz) = separation in meters
Calculating 3/7 = 0.42 meters, which for all practical purposes is 40 centimeters. You may try using 5/7 (MHz) = separation in meters, and then compare your results. My experiments show that the wider separation is useful especially in versions of the TTFD cut for a minimum operating frequency of 20 megahertz and above, but its nice to try the approximate 70 centimeter separation on the antenna cut for a 7 MHz low frequency cut-off. If you don’t have a measuring tape or ruler with metric measuring units, then simply multiply the results obtained from the above two formulas by 3.28 to obtain the length in feet.
For example: 14.3 meters x 3.28 = 46.9 feet, or for all practical purposes = 47 feet. The separation between the upper and lower sections of the antenna = 0.4 m x 3.28 = 1.31 feet or 16 inches in case you wanted to use the wider separation between the upper and lower wires, then 0.71 x 3.28 = 2.34 feet or 28 inches in round numbers.
Material Needed
To build your TTFD, Figure 1, you need copper wire, of the hard drawn type, but you may also use standard antenna wire #16 or #14, or you can use PVC insulated household type wire, typical stuff for home electrical installations. The TTFD is a special kind of folded dipole, so you must realize that the top portion of the antenna is divided in two by an insulator located exactly at the middle of the top section. That insulator is bridged by a carbon resistor of 390 ohms and 4 watt dissipation, for an antenna that is to be used only for receiving purposes or QRP work. The 390-ohm carbon resistor must be carefully soldered to each side of the antenna and protected from the weather by placing it inside a piece of PVC spaghetti tubing sealed with epoxy resin. Of course, you will not make the small 390 ohm 4 watt carbon resistor hold both wires together, you must use a high quality antenna insulator to hold the two upper wires together, and then bridge the insulator with the 390 ohm resistor. My advice is that you protect the 390 ohm 4 watt resistor from the effects of the weather inside a small section of PVC plastic pipe, seal the ends with silicone and let only two flexible leads come out of the section of PVC tubing. If you cannot find a 390 ohm 4 watt carbon resistor, then you may use a pair of 680 ohm 2 watt resistors connected in parallel to substitute for the 390 ohm 4 watt resistor.
If you are thinking of ever using the TTFD as a transmitting antenna, with powers above 10 watts, then the terminating resistor must be modified accordingly. More about using the TTFD for transmitting later.
The lower portion of the antenna is also interrupted exactly at its center with an insulator. The antenna feedline connects to each side of this insulator.
How to feed the TTFD antenna system
You may use 2 types of downleads with the TTFD.
1. Balanced high impedance lines, like open wire, 300 ohm or dual-coax shielded balanced feeders
2. Coaxial cable fed via an appropriate balun located at the antenna terminals
Easiest of all, you may use heavy-duty 300-ohm dual balanced transmission line, which may be available under the name of “low loss TV foam twin lead.” Then you must use a balun and antenna tuner combination to connect to your radio. If you are enthusiastic about saving costs and enjoy homebrewing to its maximum extent, they you may build your own balanced transmission line, like our grandfathers did. Any impedance between 200 and 600 ohms will work beautifully with the TTFD antenna. But, do remember that in this case as with the 300 ohm balanced line, you must use either an antenna tuner with a balanced input, or a 4 to 1 balun connected between the open wire line and the tuner.
Then there is a little known and not very much used, but nevertheless quite nice way of feeding this antenna system. You can use a homemade parallel feedline of very low loss made from two 75 ohm coaxial cables. This low loss line shielded line is made in the following way: The outer shield or braid of the two parallel coaxial cables are joined together at the antenna end, right next to the antenna insulator, and are left floating without any connection. But please, do remember to seal it from the adverse effects of the weather by using some kind of silicone caulking compound as a sealant. No water may seep into the coaxial cable or it will be damaged permanently. This will happen in a short time after improperly installing it without complete sealing from the adverse effects of the weather. The center or inner conductors of the two coaxial cables are connected, one to each side of the antenna.
This is, technically speaking, your homebrewed 150 ohm dual balanced and shielded transmission line. It is taken to the shack, where you connect the center conductors to a balun or to the balanced input of your antenna tuner. It may be dealt with like a coaxial cable, because it is a balanced + shielded transmission line, enjoying the advantages of both a balanced and a shielded feeder. The outer shields or braids of the 2 cables are also joined together at the end of the line, right next to the antenna tuner. They are then soldered, and here they are connected to your shack’s ground.
There is another way to feed the TTFD with just one single 75 ohm coaxial cable , and this is done by connecting a special 6-to-1 balanced to unbalanced (balun) transformer right at the antenna terminals. The 6-to-1 stepdown balun will provide a match to the 450 ohm nominal impedance of the antenna while at the same time matching to the 75 ohm coaxial cable.
For receiving purposes you may substitute the 6 to 1 balun by a more readily available 4 to 1 balun without experiencing significant losses. I have tried it with the 4 to 1 balun for transmitting and it seems to work quite well too. I must emphasize that proper use of the TTFD involves working via an antenna tuner. While others may disagree, proclaiming that a TTFD should behave like a better than 2 to 1 SWR antenna over its design frequency range, my experiments show that this does not hold in practical installations, making it necessary to use the antenna tuning unit to provide an appropriate match. Plus, another very important function is as the TTFD is an aperiodic or broadband device, any non essential emissions, like harmonics , spurious and mixing products reaching it, will be radiated. Hence, the need of using any broadband antenna with a tuner.
Spreaders are important to keep it nice looking ! The TTFD construction involves the use of at least six fiberglass or PVC spreaders that keep the upper and lower wires separated from each other at the calculated distance, which in our example is 40 centimeters, or 70 centimeters. At the end sections the antenna is supported using high quality dacron or nylon fishing lines which act as very long insulators. The TTFD antenna must be placed with one end at the top of the mast and the other end at a lower height so that the antenna slopes at an angle of 20 to 40 degrees, from the horizontal.
Using it for transmitting with the typical 100 watt transceiver
If you want to use the TTFD for amateur band transmitting purposes, you must replace the 390-ohm 4-watt carbon resistor with a non-inductive resistor of 390 ohms capable of dissipating at least one fourth of the transmitter power on the SSB mode, or one third of the transmitter power in the CW or RTTY mode. The resistor can be assembled from a series parallel combination of two-watt carbon units. for a typical amateur band 100-watt transceiver the resistor must be capable of dissipating about 25 watts.
Some notes about the TTFD
The TTFD antenna is essentially a vertically polarized omnidirectional radiator. It will pick up less noise than a random wire, a dipole or a vertical, outperforming the typical wire antenna especially in noisy downtown locations.
P.S. A shorter TTFD, for example one about 8 meters overall length will still be an excellent performer for the 8 to 30 megahertz range. If you are short of space, try this short version of the TTFD!
Originally posted on the AntennaX Online Magazine by Arnie Coro, CO2KK. Host of Radio Havana Cuba’s ” DXer’s Unlimited Radio Hobby Program.
Last Updated : 16th March 2024