Central Idaho Amateur
Radio Club
     
CIARC - Balanced Terminated Folded Dipole
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)
L
(feet)
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 65-foot tree that is at the base of a slight slope, resulting in a nearly horizontal configuration. The LMR-400 feed-line results in some droop at the center point, with the feed point at approximately 40-feet.

The following plots show the impedance and SWR curves for frequencies from 1 MHz to 40 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.


BTFD IMPEDANCE AND SWR PLOTS
1 MHz plot 2 MHz plot
3 MHz plot 4 MHz plot
5 MHz plot 7 MHz plot
9 MHz plot 11 MHz plot
13 MHz plot 15 MHz plot
17 MHz plot 19 MHz plot
21 MHz plot 23 MHz plot
25 MHz plot 27 MHz plot
29 MHz plot 31 MHz plot
33 MHz plot 35 MHz plot
37 MHz plot 39 MHz plot
40 MHz plot

The following plots show the impedance and SWR curves for frequencies from 1 MHz to 40 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.


BTFD IMPEDANCE AND SWR PLOTS
1 MHz plot 2 MHz plot
3 MHz plot 4 MHz plot
5 MHz plot 7 MHz plot


BTFD IMPEDANCE AND SWR PLOTS
9 MHz plot 11 MHz plot
13 MHz plot 15 MHz plot
17 MHz plot 19 MHz plot


BTFD IMPEDANCE AND SWR PLOTS
21 MHz plot 23 MHz plot
25 MHz plot 27 MHz plot
29 MHz plot 31 MHz plot


BTFD IMPEDANCE AND SWR PLOTS
33 MHz plot 35 MHz plot
37 MHz plot 39 MHz plot
40 MHz plot

On air reports on 75-meter and 40-meter amateur radio frequencies show the following:

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)

Posted 12 October 2014


Download a .pdf copy of this article by clicking here.