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 are able to
increase the Band Width of an antenna by Using a Non-Inductive Balanced
Termination Resistor (BTR):
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
balanced 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). We have found that
as we increase the BTR value, and the BALUN ratio, the bandwidth decreases
slightly.
During our experimenting with the T2FD
antennas, we’ve determined that a 450 ohm BTR, and a 9 to 1 BALUN provides
optimum performance, or the “happy medium” between the
gain and bandwidth factors of our T2FD.
Thus the choice for our commercial product is a BTR of 450 ohms, and a
1:9 BALUN.
We build both a 3 to 30 MHz version
(cat# 1080T2FD), and the 160 through 6 meter T2FD, (cat# 1606T2FD).
The length of our 1080T2FD is approximately 47 feet long (including
ty-tails). Our cat# 1606T2FD is
approximately 90 feet, including ty-tails.
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 (9:1) BALUN with the same impedance (Z) value as the resistance of the (450 ohms) non-inductive BTR.
The BUXCOMM model 1080T2FD is a well known antenna of this design. 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 formulae for use when constructing of a T2FD :
L (m) = 50/f
MHz
D
(m) = 1.55/f
MHz
Example:
Length = 50/3.7MHz = 13.5 meters, convert to feet, multiply by 3.28, = 44.4 ft.
Example: Spacer
Dimension
Dim = 1.55/3.7 MHz= 0.418 meter,
convert to feet, multiply by 3.28, = 1.37 ft., convert to inches,
multiply by 12, = 16.5 inches.
Where
f is the lowest frequency of
operation.
With regard to the
broadband (balanced) termination resistance (BTR),
the value of the BTR is somewhat critical and depends on the impedance of the
BALUN employed at the power input (feed-point) of the T2FD.
Using the same broadband (balanced)
termination resistance (BTR) and input impedance used with the original
T2FD, we will base our design on a BTR of 450 ohms (non-inductive) (R) and the
feed-point impedance of 450 ohms (9:1 BALUN). In
practice, we use 50 ohm coax to feed the T2FD. Thus
the feed point will require the use of a BALUN with a 1 to 9 ratio or a 50 to
450 ohm (Z) BALUN. (BUXCOMM Cat.# B1K91 or B15KC91).
It was 1958 when
I built my first 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 3.7 MHz frequency with the upper frequency limit extending well
above 50 megahertz. In the articles I've
read, the writer indicates their Tilted-Terminated-Folded-Dipole (T2FD) is
installed with a 30 degree incline, and 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.
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 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 (version) installation.
Important considerations:
The
balanced terminating resistance (BTR) becomes more critical as the
feedpoint impedance is increased. With a higher impedance the BTR value
becomes more critical, to within about 15 ohms.
Some builders who do not know or understand this, use a high value of
600 to 800 ohms. They do so in order to use a more available termination
resistor, or BTR. This adds to the
problem relating to the gain/bandwidth factor of this antenna.
We recommend using a balanced
termination resistor (BTR) between 300 and 500 ohms, 450 ohms being found to be
optimum performance. The happy
medium is to use a 450 ohm BTR
(BUXCOMM model TR450), and a 9:1
(BUXCOMM model B1K91) BALUN.
NOTE: We've found that a 16.5 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
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 has all the parts needed to build your T2FD.
BUXCOMM also has both models
complete, factory
assembled, and tested BUXCOMM T2FD catalog numbers; 1080T2FD and 1606T2FD.
Some background and advantages of the T2FD antenna
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 BUXCOMM 1080T2FD pattern does cover more than 300 degrees, but falls short of a full circle signal capture.
For
receiving purposes the balanced termination, folded dipole (1080T2FD)
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 destroys the signal to noise ratio, in turn, the intelligibility of the
received station.
Important Note:
Do not exceed the power
rating of this antenna. Failure to do so will damage the Balanced Termination
and void the warranty!
Tips for assembly of the
BUXCOMM 1080T2FD and 1606TG2FD:
Although the wire will “grab” when inserted into the slots of the NOCs, you can
add ty-wraps as shown to further secure the wire elements in place. Photos below
illustrate three methods to secure wire element(s) to spreader
Prep the ends of the BUXCOMM model 1080T2FD. Be
sure to wrap all Ty-Wraps with black, PVC tape,
supplied with 1080T2FD
antenna.
Photo
1:
With the
standard BUXCOMM 1080
T2FD, 1400 watt (SSB) antennas, we make the characteristic impedance of the
BUXCOMM balanced termination, folded dipole 450 Ohms using the BUXCOMM model
TR450. Matching the pair, we use the BUXCOMM B15KC91 BALUN.
Photo
2: Thanks to
the development of our wide-band integrated BALUN and TR450, the antenna is fed
with 50 Ohm coaxial cable and the matches the input impedance of our
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.
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. The
BUXCOMM balanced termination, folded dipole (1080
T2FD) is designed to withstand harsh weather conditions and has
survived wind’s with speeds over one hundred miles/hr.
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.
73 de BucK4
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
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Thus you now have the dimensions you need to build
a BUXCOMM 1080 T2FD
balanced termination folded dipole (BUXCOMM
1080 T2FD).
An important NOTE to
remember: These calculations are based on the BUXCOMM
1080 T2FD using our
BUXCOMM 450 ohm, non-inductive cat# TR450, and the BUXCOMM B1K91 or B15KC91
BALUN.