Hawaii
|
Alaska
|
Pacific
|
Mountain
|
Central
|
Eastern
|
Puerto Rico
|
UTC
|
Guam
|
Balanced Terminated Folded Dipole (BTFD)
In searching for an HF antenna that exhibits both efficient operation and broadband capability, including a capability
to operate in support of MARS activities while using Automatic Link Establishment (ALE) while avoiding antenna switching
or tuning while ALE sounding was occurring,
I came upon the Balanced Terminated Folded Dipole (BTFD).
Formulas for determining the minimum size
of the antenna are published on-line.
Consulting with several amateur radio operators who have experience with the BTFD resulted in reports
of poor performance, and included derogatory comments comparing the antenna to a ’radiating dummy load’.
These comments were very puzzling, given that the United States Navy tested the BTFD antenna in the 1940's
and those tests concluded that the BTFD was superior in performance to multiple single band antennas.
When queried, these amateur radio operators provided information on the make and model of these poorly
performing antennas. Upon further investigation, it was clear that the dimensions of the manufactured
BTFD antennas fell far short of the minimum dimensions that are determined by formula.
Placing higher credence on the United States Navy study, I decided to purchase the components to construct
the BTFD and find out for myself whether this antenna performed or not.
Using the published formulas, I designed my BTFD to operate on a minimum frequency of 1.8 MHz, with an
intended use spaning from 1.9 to 11.0 MHz in support of both amateur radio and MARS activities. The
following table discloses dimensions for antennas designed for several different minimum frequencies
of operation.
| BALANCED TERMINATED FOLDED DIPOLE DIMENSIONS |
MINIMUM
FREQUENCY
(MHz) |
LEG
LENGTH
(feet) |
LENGTH
OVERALL
(LOA)
(feet) |
W
(inches) |
TOTAL
WIRE
LENGTH
(feet) |
| 1.8000 |
91.13 |
182.27 |
18.23 |
367.58 |
| 1.9000 |
86.34 |
172.68 |
17.27 |
348.23 |
| 2.0000 |
82.02 |
164.04 |
16.40 |
330.82 |
| 3.5000 |
46.87 |
93.74 |
9.37 |
189.04 |
| 3.7500 |
43.74 |
87.49 |
8.75 |
176.44 |
| 3.9000 |
42.06 |
84.12 |
8.41 |
169.65 |
| 4.0000 |
41.01 |
82.02 |
8.20 |
165.41 |
| 5.3585 |
30.61 |
61.23 |
6.12 |
123.47 |
| 7.1500 |
22.94 |
45.89 |
4.59 |
92.54 |
The antenna that I constructed was designed for a minimum operating frequency of 1.8 MHz,
using the dimensions found in the table above. Components were purchased from
Buxcomm:
It should be noted that the spacers above are slightly short. Further, these appear to be made out
of cut-down arrow shafts and have arrow nocks on both ends, which are sealed with heat shrink tubing.
These are not guaranteed to retain the wire. I drilled through each of the nocks and then used safety-wire
to ensure that the antenna wire is retained. Now knowing how these are constructed, I would probably
fabricate my own if I were to construct another of these antennas.
My station transmitter power level is 200-watts, and the BTR450 terminating resistor is sufficient
for my operating power levels. For power levels above 500 watts, you might consider the
BTR-1KW
terminating resistor that is rated at 1000-watts.
The antenna is supported on one end by a 50-foot tower, and on the other end from a 60-foot
tower that is at the base of a slight slope, resulting in a nearly horizontal configuration.
The RG-8X feed-line results in some droop at the center point, with the feed point
at approximately 40-feet. A second antenna was constructed, to the same dimensions, and is
configured as an Inverted-V, fed with LMR-400 coax, and with the apex at 55 feet.
The following charts show antenna dimensions vs minimum frequency:
The following plots show the impedance and SWR curves for frequencies from 1 MHz to 25 MHz.
These plots were obtained using a
YouKits FG-01A Antenna Analyzer. Within these plots, the white curve represents the SWR and is
read on the left-hand scale, while the yellow curve represents the impedance and is read on the
right-hand scale. The displayed SWR and impedance (i.e. Z) are for the center frequency of the
plot.
On air reports on 75-meter and 40-meter amateur radio frequencies show the following:
-
Reports of signal strength from stations that I regularly work indicate my signal strength
is up significantly. This is reinforced by a reduction in reports of QSB or weak signal
strength, which were a regular occurrence previously, and resulted in frequent requests
to repeat a transmission.
-
Received signals are up significantly, particularly on 75-meters, 60-meters and 40-meters.
-
The noise level dropped significantly. This, in combination with the increased signal strength,
significantly improved the signal to noise ratio for received signals.
Switching between the BTFD and a FAN dipole, signals are often copied without loss on the BTFD, even if the
signals don't result in S-meter movement. Switching to the FAN dipole results in a decrease in
signal strength, an increase in noise, and loss of copy on signals of weak to moderate signal
strength.
At the time of this writing, I have been using the BTFD antenna for just over 1-month, and with almost daily
use. My personal experience with the BTFD has completely exceeded my expectations. I am completely
sold on a properly sized BTFD configuration, and highly recommend this antenna. In my 41 years
(next month) as an amateur radio operator, never have I been this pleased or excited about the performance
of a wire antenna.
73, Ray Montagne (W7CIA)