Building the T2FD, Folded Dipole

Building the Balanced Termination (T2FD) Folded Dipole
By Glynn E “Buck” Rogers K4ABT

The BUXCOMM model 1080T2FD, Balanced Termination Folded Dipole (BTFD)

Over the years I have built many antennas, Windom’s, Dipoles, Folded Dipoles, balanced terminated folded dipoles, BTFD or T2FD broadband antennas. I prefer to call the latter a “balanced, termination folded dipole (BTFD).” When tilted to a 30 degree incline, it is called a T2FD, or Tilted-Terminated-Folded-Dipole. It can be designed to cover all frequencies from 1.8 to 30 mHz. Our BUXCOMM cat# 1606T2FD covers 1.5 to 35 MHz, and our cat# 1080T2FD covers 3 to 30 MHz.

We can increase the Band Width of an antenna by Using a Non-Inductive Resistive Load:

We increase the bandwidth of our antenna by installing a non-inductive resistor at the far side of our BUXCOMM T2FD antennas called a BTR, or bandwidth termination resistor. The value of this BTR or bandwidth termination resistor must have the same resistance as the impedance of the BALUN attached at the feed point of our antenna.

For HF 3 to 30 MHz., applications, this BTR is usually 450 (9:1) or 600 (12:1) ohms. which requires either a 9:1 or 12:1 BALUN (respectively). By radiating all the RF energy from the transmitter, we have little or no VSWR. If all the energy is to be dissipated in the antenna, there can be no SWR, and the impedance of the antenna and feed line are matched at the feed-point by using a BALUN with the same impedance value as the resistance of the non-inductive BTR.

A well known antenna of this design is the BUXCOMM model 1080T2FD. It is a folded dipole with a non-inductive 450 ohm resistor (BTR) in the upper, center point, and a 9:1 BALUN at the feed point. The 1080T2FD is often fed with 49 or 98 feet of RG8X (Mini 8) coaxial cable.

T2FD is an acronym for Terminated Tilted Folded Dipole. This describes its general characteristics. Other authors have compared it to the diamond shape of a rhombic possibly because the T2FD uses a resistive termination; however, it is too short to be considered a rhombic.

According to consulted sources, the performance is based on the following; Depending on the line of tilt and mainly on the value of the terminating resistor and associated feedline or BALUN impedance. (Ed note: In test, the T2FD performs well (horizontal) flat-top, or tilted (30 degrees)).

Given this information; It is recommended to install it on a slant angle of inclination approximately 30° (20° to 40°) from horizontal, thereby obtaining a signal with low noise and good gain quality.

The formulas published in various sites show calculations for the T2FD for our purposes, allow the antenna to work at half the lowest frequency, increasing the bandwidth a ratio greater than 8:1.

The general formula for the construction of a T2FD for transmission is:
L (m) = 50/f MHz
D (m) = 1.5/f MHz
Where f is the lowest frequency of operation.

To convert meters to feet, multiply meters by 3.28:

Example: Length, 12.8m X 3.28 = 42 feet
Spacing, 0.385 X 3.28 = 1.2 feet or 15 inches

With regard to the broadband termination resistance (BTR), the value of the BTR is somewhat critical and depends on the impedance of the BALUN at the power input (feed-point).

Using the same broadband termination resistance (BTR) and input impedance used with the original T2FD, we will base our design of a BTR of 450 ohms (non-inductive) (R) and the feed-point impedance of 450 ohms (9:1 BALUN). or a BTR of 600 ohms, and a 9 to 1 BALUN. In this application, we will use 50 ohm coax to feed the T2FD. Thus the feed point will require the use of a BALUN with a 1 to 12 ratio or a 50 to 600 ohm (Z) BALUN. (BUXCOMM Cat.# MM121).

It was 1958 when I built a the T2FD using an 600 ohm termination, and added the Guanella version of a transmission line transformer (TLT) 12:1 BALUN. Our balanced termination, folded dipole (1080T2FD) provided an excellent bandwidth using the balanced termination, folded dipole (1080T2FD) designed for 3.8 MHz frequency with the upper frequency limit extending well above 50 megahertz. In the articles I've read indicate the Tilted-Terminated-Folded-Dipole (T2FD) is installed with the 30 degree incline, that it would exhibit an omni signal pattern. We tried the tilted version, and a flat-top version. In both tests, we found the performance to be very similar in coverage and gain.L (m) = 50/f MHz
D (m) = 1.5/f MHz
Where f is the lowest frequncy of operation.
We will design an antenna with a bandwidth between 3 and 30 MHz, then as stated; We will use 3.9 MHz for the calculation:

Therefore, using the above formula L (length) = 50/3.9 = 12.8 meters and spacing of 1.5/3.9 = 0.375 meters.

In 1966, while doing some experimenting with the balanced, termination folded dipole, I installed it using Mutt & Jeff masts, one at 30 feet, and the shorted one at 7 feet, providing approximately 35 degrees incline. After several contacts, it was soon very obvious there was a lack of back-fill in the direction back the incline. To circumvent or at least correct some of the back-fill problem, I raised the high end (tall pole) to 35 feet, and brought the low end to a similar height. In subsequent tests I raised both ends of the T2FD to horizontal, and found that it gave us a good omni pattern as well. If anything, the flat-top or horizontal installation may have displayed an edge over the sloped installation.

Important considerations:
The balanced terminating resistance (BTR) becomes more critical as the feedpoint impedance is lowered. With lines of lower impedance the BTR value becomes more critical, to within about 15 ohms. Some builders who do not know or understand this, use a low value of 390 to 400 ohms, some as low as 200 ohms. They do so in order to use a more available BALUN and/or BTR. This too adds to the problem relating to the gain/bandwidth factor of this antenna. We recommend using a balanced termination resistor (BTR) greater than 500 ohms. To be on the safe side, use a 600 ohm BTR (BUXCOMM model 600TR), and a 12:1 (BUXCOMM model MM121) BALUN.

NOTE:   We've found that a 15.5 inch spacing is an ideal "constant" and it works well on all bands and versions of the T2FD, 160 thru 10 meters.   Spacing "D" is not a critical dimension, and can vary +/- 4 inches MOL.

The balanced termination, folded dipole BUXCOMM model 1606T2FD

The balanced termination, folded dipole (1080T2FD) is an extremely broadband antenna, and it is a very quiet antenna indeed, as it is immune to terrestrial noise as compared with a vertical or a horizontal dipole. The SWR, Standing Wave Ratio, when transmitting may vary from an almost perfect match of 1.1:1 at some frequencies to 2.5:1 at other frequencies. In either case, either reading is good when you consider you are able to operate across the HF spectrum without an antenna tuner.

If you prefer building your own T2FD for 80 through 6 meters, BUXCOMM offers a kit with instructions. Wire is NOT included with the "kit."
WIRE IS INCLUDED with all factory assembled, and tested BUXCOMM T2FD series. 1080T2FD and 1606T2FD.

Advantages of the balanced termination, folded dipole (1080 T2FD) antenna:
The balanced termination, folded dipole (1080 T2FD) (Tilted Terminated Folded Dipole), originally developed by the US Navy, is an antenna still in common use by military and government receiving stations. There are good reasons for this choice by the professionals. The antenna has a balanced termination which provides it with its characteristic impedance. This terminated principle means the antenna is not prone to annoying man-made interference sources, such as fluorescent lights, dimmers, televisions etc. The antenna is also less subject to noise from likely causes, such as atmospheric static and open high-tension power lines.

The balanced termination, folded dipole (1080 T2FD) is really a "low-noise" transmitting and receiving antenna! By ensuring a constant impedance throughout the length of the antenna, the balanced termination, folded dipole (1080 T2FD) is also less prone to distortion due to multi-path fading. Our tests have shown that when compared to dipole or long-wire antennas, the background noise with a balanced termination, folded dipole (1080 T2FD) antenna is not only much lower, but allows weak signals normally not heard, to be audible and therefore legible.

One of the most desirable features of the 1080 T2FD is when using digital modes, packet radio, PSK, SSTV, MT63, etc, makes for easy recovery of their signals. The immunity to terrestrial noise reduces the number of errors in data communications simply because of its low noise figure and lower distortion.

The balanced termination, folded dipole (1080 T2FD) does not suffer from dead spots across its frequency range as we have found the specifications for the 1080 T2FD are the same for its entire frequency range. This is not only a useful feature for SWL shortwave listener who likes to listen to both the broadcast and other communications services of the shortwave spectrum. This is also ideal for the HAM who often and hastily changes frequency.

Height is not a pre-requisite: The ends of a dipole, trap-dipole, and long wire antennas have a high impedance. This is a problem when the wire runs in the vicinity of conductors such as metal roofs, trees, and similar vegetation. The balanced termination, folded dipole (1080 T2FD) has fewer of these problems because of its constant impedance at any point of the antenna. In addition, the conductivity of the ground under the 1080 T2FD antenna has little influence on its performance. The height of the lower end of the balanced termination, folded dipole (1080 T2FD) does not have to be more than 10 to 15 feet above the surface. If you hang the balanced termination, folded dipole (1080 T2FD) with an angle of 30 degrees, then the antenna pattern shows a number of lobes that it may cause you to feel the antenna is sensitive to signals from all directions, or omni-directional. This apparent "omni-directional" can be a bit misleading, as the circularity of the 1080 T2FDpattern does cover more than 300 degrees, but falls short of a full circle signal capture.

For receiving purposes the balanced termination, folded dipole (1080 T2FD) has an extra advantage. It is immune to man-made and low atmospheric (terrestrial) noise. On shortwave, this noise can be so high, that it decides the signal to noise ratio, in turn, the intelligibility of the received station.

Prep the ends of the BUXCOMM model 1080T2FD. Be sure to wrap all Ty-Wraps with black, PVC tape, supplied with 1080T2FD antenna.

The balanced termination, folded dipole (1080 T2FD) Antenna

I've spent long hours and many years experimenting with the balanced termination, folded dipole (1080 T2FD), continually improving the design. By analyzing the problems from different angles, and trying various materials, the good points of the original design could be improved upon. The new design means that common coaxial cable can be used as a lead-in to the receiver, eliminating ingressed interference from equipment such as computers, power lines, and fluorescent lights. We've found, as power is increased, the feed-point impedance is better served when lowering the resistance of the Balanced, terminating Resistors.

When increasing power level to 600 or 800 watts, a BTR of 200 ohms (BUXCOMM model BTR200-700, and a BALUN with a 4 to 1 ratio (BUXCOMM model MM41) is more efficient and more desirable with higher power levels.

    Photo 1:    With the standard BUXCOMM 1080 T2FD, 300 watt antennas, we make the characteristic impedance of the BUXCOMM balanced termination, folded dipole 600 Ohms using the BUXCOMM model BTR600. Matching the pair, we use the BUXCOMM MM161 BALUN.

LEFT, Photo 2:    Thanks to the development of our wide-band 16:1 BALUN, the antenna is matched to the 50 Ohm coaxial cable and the input impedance of most transceivers.

This BALUN not only ensures symmetry in the antenna across its frequency range, it also isolates the coaxial cable from the antenna, reducing interfering signals that might be picked up by the shield of the coaxial cable./font>

Static discharge protection

The BTR at the center of the antenna-wire at the balanced termination dissipates any static buildup during thunderstorms. This not only protects the sensitive input circuitry of the transreceiver, it reduces the atmospheric noise which is generated as a result.

Construction:

The BUXCOMM balanced termination, folded dipole (1080 T2FD) is designed to withstand harse weather conditions and has survived wind’s with speeds over 150 mph.

The higher impedances tend to balance the power in each leg more evenly. Us old timers have always used 800 ohm non-inductive balancing terminating resistors (BTR) for Rhombics, and 600 ohms for the terminated folded dipoles.

I have written several books and articles about these antennas. In my writings I have demonstrated and illustrated the advantages of using the different impedances. In tests we have found that lower feed-point impedances tend to lose bandwidth at the higher frequencies, e.g. 20 to 30 Mhz. While using BTR above 500 ohms, we've discovered that better bandwidth occurs, and less TVI. The noise figure is also better when using 600 ohms.  After a lot of trial and error, design changes, bridge, and grid-dip meter testing, we found a happy medium! Therefore, my focus has been to make these antenna(s) as broad as possible, while maintaining a relative even VSWR from 1.5 MHz to 45 Mhz. The "happy medium" is to use a 12:1 BALUN and the BTR at, or near 600 ohms.

To support the low frequencies, a BTR of 600 ohms with a BUXCOMM MM121, 12 TO 1 BALUN provides a good match over wide HF frequency range from 3.5 to 55 Mhz while still minimizing TVI, and maintaining the antenna's inherent immunity to terrestrial (man-made) noise. To optimize the 1080 T2FD for the best of all worlds, 3.5 to 55 Mhz, we use a 12 to 1 BALUN with an 600 ohm termination resistor.

Something else we've found, that by installing the 1080 T2FD, tilted at 20 to 30 degrees, no more, no less.   A single support pole, for the upper end will suffice. The 1080 T2FD will still exhibit an almost full circle, omni field of signal propagation. Another of the qualities and convenience of an antenna with a single feed line that can let you work over such a tremendous bandwidth is the feature for which the 1080 T2FD is now so popular among professional users of the shortwave spectrum.

The balanced termination, folded dipole (1080 T2FD), or Tilted Terminated Folded Dipole (1080 T2FD), is related to another well known antenna... the rhombic, known for its extraordinary performance and reproducibility of its radiation patterns. A balanced termination, folded dipole (BTFD) is "terminated" like the rhombic, a NON-INDUCTIVE RESISTOR is placed at the end of antenna, something which provides a LOAD or TERMINATION to the RF propagating along the antenna.   But, the big differences between the balanced termination, folded dipole (BTFD) and the rhombic, are that the first is much smaller, has little or no directivity and fits into a rather small real-estate space, while a rhombic antenna may be several football field sizes, and transmits a narrow horizontal radiation pattern. The balanced termination, folded dipole (1080 T2FD) is a very practical broadband antenna.

Wire size and mechanical concerns:
Building a balanced termination, folded dipole (1080 T2FD) for the 1.8 to 30 Mhz frequency range requires taking into account some mechanical design considerations. For example, you can't use a smaller wire size for the antenna, as its span is such, that number (AWG) 16, or AWG 14 can be used.

In the late 1950s, we used bamboo or cured cane poles to make our wire spacers. In 1963, some of us decided to try more spacing, different (non-inductive) resistances, and finally settling on the design with optimum performance.   Using a 600 ohm balanced termination resistor, a 12 to 1 BALUN and 1.3 ft (15-1/2 inch) spacers, a happy medium was within our grasps.   Today, upper and lower wires of the balanced termination, folded dipole (1080 T2FD) are kept at a uniform distance, we achieve this with fiber-glass spacers or spreaders.

Over the years, when I’ve had available real-estate, the WINDOM is my favorite, however when antenna property space is limited, I’ve turned to the Balanced Termination Folded Dipole (1080 T2FD). The reason these two are my favorites, I don’t need an antenna tuner to cover the HF spectrum, and only one antenna meets all my HF operating requirements. This one HF antenna will enable you to forget that collection of rhombic’s, log-periodic, wideband dipoles and similar antenna arrays! Building your own balanced termination, folded dipole (1080 T2FD) will be like having a number of dipole antennas for many bands all in a single antenna and fed with only one cable.

73 de BucK4ABT

A brief history of the T2FD, Multiband, Low Noise Antenna
The T2FD (Tilted Terminated Folded Dipole) is a multi-purpose antenna developed in 1946 by the United States Navy. The first paper describing the T2FD was written by G L Countryman W3HH, and appeared in 1949. He acknowledged that his source of information was a series of tests conducted by the U S Navy.

G L Countryman W3HH, was particularly impressed by the following observations:

1) Omni-directional.
2) Operation over a frequency range greater than 8 to 1 (eg; 3 to 30 MHz).
3) Flat impedance range
4) Support for one end of the antenna, when erected as a sloping antenna.
5) Moderate higher gain comparable to a conventional horizontal antenna.
6) Most important, The T2FD does not need an Antenna Tuner!

Later, in the G L Countryman articles published in 1951 and 1953 in the original documentation he included some test results made by the Japanese in Japan by the Kyushu Electric Communications Bureau:

The Japanese studies indicate that; “the Tilted Terminated Folded Dipole (T2FD) antenna gain exceeds the Zepp and other half-wave dipoles. In experiments with a wide range of frequencies, excellent characteristics observed with the T2FD produced an increase of 4 to 8 dB of signal in several different locations. "

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&n All text and graphics on these pages are ©®™ of G. E. "Buck" Rogers Sr K4ABT & BUX COMM Corp 1985-2010

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Thus you now have the dimensions you need to build a trong> 1080 T2FD balanced termination folded dipole (1080 T2FD).
An important NOTE to remember: These calculations are based on the 1080 T2FD using an 600 ohm non-inductive BTR, and the BUXCOMM MM121 BALUN.