Building the Balanced Termination Folded Dipole, (BUXCOMM 1606T2FD-1400)
By
Glynn E “Buck” Rogers K4ABT
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The T2FD, modified to a Balanced Termination
Folded Dipole (BTFD)
All text and graphics on these pages are ©®™ of G. E. Rogers Sr and BUX COMM
Corp 1958 - 2012
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 for any number of frequencies between 1.8 and 30 mHz.
The original Tilted Terminated, folded dipole (T2FD) was the design of amateur radio operator (An Experimental All-Band Non-directional Transmitting Antenna" by Gil L. Countryman, W1RBK, (W3HH), QST, June 1949), the antenna was first used for maritime and naval communications.
It was 1958 when I built a modified version of the T2FD. Instead of using the 800 ohm, non-inductive termination, I used an 450 Ώ Non-Inductive termination, and added the Guanella version of a transmission line transformer (TLT) 9:1 BALUN. Our balanced termination, folded dipole (BTFD) provided an excellent bandwidth using the tilted terminated, folded dipole (T2FD) designed for a low frequency with the upper frequency limit extending well above 50 megahertz. The articles I've read indicate the Tilted-Terminated-Folded-Dipole (T2FD) installed with the 30 degree incline, would exhibit an omni signal pattern..
This is not quite the situation, as I've found the direction of the slope is favored as the direction of the main lobe. Still the tilt or slope of 30 to 45 degrees is the recommended installation method.
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 of 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 slightly over 6 feet. After all the raising and lowering of the ends, the antenna’s signal still favored the slope side or direction of the low end. In subsequent tests I raised both ends of the T2FD to horizontal, and found that it gave us a good omni pattern, but was not as effective as when it was installed as a sloped antenna. SEE FIGURE 1;
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 25 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 450 ohm BTR (BUXCOMM model 450TR), and a 9:1 (BUXCOMM model B15C91) BALUN.
To determine the dimensions of a BTFD or T2FD using 450 ohm termination and 9:1 BALUN use the following formulae to calculate the dimensions. Use 50.000 divided by F (frequency in Mhz). To make the calculations linear when computing dimensions for bands other than the bands used in the examples below.
Example, to calculate a T2FD for 80 through 10 meters; divide 3.7 Mhz into 50.000. 50/F then to convert from meters to feet, multiply by 3.28:
Length = 50.000 divided by 3.7, = 13.5 X 3.28 = 44.4 or approximately 44.5 ft.
Another example: if you wish to build a T2FD to cover 160 thru 6 meters, for the length, use the above formula as follows:
50.000 divided by1.9 Mhz = 26.3 meters, multiplied by (meter conversion to feet) 3.28 = 86.3 feet.
Spacing =(L feet x 0.2 = inches), thus 86.3 X .2 = 17 inches inches... Spacing is NOT critical, and can vary 4 inches either way.
Spacing = 2/100 of length, or Length in feet times 0.2 = inches
Thus you now have the dimensions you need to
build a T2FD balanced termination folded dipole (BTFD).
An important NOTE to remember: These
calculations are based on the T2FD using an
450 ohm non-inductive BTR, and the BUXCOMM B15C91 BALUN, or the
1606T2FD-1400
Figure 1: Several installation configurations for the The balanced termination, folded dipole (BTFD) BUXCOMM model 1606FD-1400
The tilted terminated folded dipole (T2FD) 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.
* Frequency range 1.8 - 55 MHz
* Can be used as an SWL monitor or transmit antenna from 1.8 to 30 MHz
* Low-noise design, reduces sensitivity to terrestrial man-made noise and atmospheric static.
* Constant sensitivity over the entire frequency range without an antenna tuner.
* Coaxial cable between antenna and receiver.
* Length, 25 meters
* Passive, therefore no inter-mod
* Antenna is completely factory assembled and, ready to erect.
* Heavy duty construction, both wire and fiberglass.
Advantages of the balanced termination, folded dipole (BTFD) antenna
The balanced termination, folded dipole (BTFD) (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 (BTFD) 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 (BTFD) 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 (BTFD) 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 BTFD 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 (BTFD) does not suffer from dead spots across its frequency range as we have found the specifications for the BTFD are the same for its entire frequency range. This is not only a useful feature for shortwave listeners (SWL) 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 change frequency's.
Example:
To design a T2FD for the middle of the 80 through 10 meters (BUXCOMM
model 1080T2FD-1400 3.7 Mhz to 30 Mhz):
|
|
Desired frequency |
|
|
Length Total |
(50 / 3.7) x 3.28 |
44.40 feet |
|
Element Spacing |
(3/3.7) x 20 |
16.2 inches |
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 (BTFD) 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 BTFD antenna has little influence on its performance. The height of the lower end of the balanced termination, folded dipole (BTFD) does not have to be more than 10 to 15 feet above the surface. If you hang the balanced termination, folded dipole (BTFD) 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, however the circularity of the T2FD pattern does it over 300 degrees, but falls short of a full circle, signal capture.
This back-fill null can be the result two properties:
1) Lack of back-fill aft the support mast,
2) Poor capture by the antenna in the E plane of the slope toward the low end.
Figure 2: Properly installed, balanced termination, folded dipole (BTFD) is omni-directional over most of its operating range.
Another technique used to determine the dimensions or length of the T2FD is: 328/F = total length in feet of both sides, divided by 2,
resulting answer is length "A" to "B" ("L") of the T2FD.As an example; we will use a frequency of 1.9 Mhz (center of the 160 meter-band) we divide 328 by 1.9 = 172, divided by 2 = 86 feet.
To determine the spacing "D" 9/F = 4.7, to convert to inches, multiply 4.7, times 3.28 = 15.5 inches. Spacing "D" is not a critical dimension, and can vary +/- 4 inches, MOL.
This 30 to 35 degree angle enables the antenna to be sensitive for horizontally polarized, as well as vertical polarized signals. This feature is where the T2FD exhibits one of its inherent properties; Reduced signal fading.
Although the 1606TR is designed for transmitting and receiving, for reception the balanced termination, folded dipole is incomparable:
For receiving purposes the balanced termination, folded dipole (BTFD) 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.
FIGURE 3: 16 inch spacing is not a
critical dimension, and can vary +/- 2 inches MOL.
FIGURE 4: Capture methods used to hold T2FD wire elements in place.
The balanced termination, folded dipole (BTFD) Antenna
BUXCOMM has experimented with the balanced termination, folded dipole (BTFD) for several years, continually improving on 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.
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 transceiver, it reduces the atmospheric noise which is generated as a result.
Construction: The BUXCOMM balanced termination, folded dipole (BTFD) 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. However since 1965 I have found that better bandwidth and VSWR is obtained when using an 450 ohm termination resistor, and a 9:1 BALUN.
With our 1606FD (T2FD) at 25 feet, using our Kenwood TS-450 (sat) at 100 watts, we made the following VSWR measurements:
Soil and environment may produce a variation in VSWR.Frequency Power VSWR
1.83 100W 1:2
3.57 100W 1:2
7.05 100W 1:3
10.1 100W 1:4
14.1 100W 1:5
18.1 100W 1:6
21.2 100W 1.8
24.9 100W 1:4
28.5 100W 1:4
29.5 100W 2:1
To support the low frequencies, a BTR of 450 ohms with a BUXCOMM B15C91, 9 TO 1 BALUN provides a good match over wide HF frequency range from 1.8 to 30 Mhz while still minimizing TVI, and maintaining the antenna's inherent immunity to terrestrial (man-made) noise. To optimize the T2FD for the best of all worlds, 1.8 to 55 Mhz, we use a 9 to 1 BALUN with an 450 ohm termination resistor.
Something else we've found, that by installing the BTFD, tilted at 20 to 30 degrees, no more, no less. A single support pole, for the upper end will suffice. The BTFD 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 Balanced Terminated Folded Dipole is now so popular among professional users of the shortwave spectrum.
The balanced termination, folded dipole (BTFD), or Tilted Terminated Folded Dipole (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 (BTFD) is a very practical broadband antenna.
Wire size and
mechanical concerns:
Building a balanced termination, folded dipole (BTFD) 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 450 ohm balanced termination resistor, a 9 to 1 BALUN and 1.4 ft (15-1/2 inch) spacers, a happy medium was within our grasps. Today, upper and lower wires of the balanced termination, folded dipole (BTFD) 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 (BTFD or 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 (BTFD/T2FD) will be like having a number of dipole antennas for many bands all in a single antenna and fed with only one cable.
The BUXCOMM 1080
and 1606 T2FD antenna series:
Features:
* Frequency
range 1.8 - 55 MHz (- 3 dB points)
* Low-noise design, reduced
sensitivity to man-made noise and atmospheric statics.
* Constant
sensitivity over the entire frequency range
without an antenna tuner.
* Use
standard coaxial cable between antenna coupler and transceiver.
* Length
just 25 meters (85 feet)* Passive design prevents intermodulation
* Heavy duty
construction in PVC covering, and SuperFlex wire.
By installing
the BUXCOMM T2FD at a 30 degree angle ensures the antenna will be receptive
to horizontal, as well as vertically polarized signals.
Installing the high end of the T2FD at 30 to 40 feet and let it tilt
at a 25 to 35 degree angle to a low end height of 8 to 10 feet
(approximately 3 metres), provides a signal capture that eliminates fading
and enables both path (E and H plane), diversities.
The antenna pattern displays many lobes, so many, that you can
consider the antenna open to signals from all directions.
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, vegetation, and wires. The T2FD has less trouble because of
its constant impedance at any point of the antenna.
DO NOT INSTALL NEAR POWER LINES!
into the insulator wire-guide. The insulators shown below are for example only. The actual insulators supplied with BUXCOMM T2FD's are large, heavy-duty, light-weight, Delrin (tmEI DuPont), UV resistant material.
To view the selection of BUXCOMM T2FD antennas, go to:
www.PacketRadio.com/catalog
73 de BucK4ABT
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