Balanced Terminated Folded Dipole (BTFD)

In searching for an HF antenna that exhibits both efficient operation and broadband capability, including acapability to operate in support of MARS activities while using Automatic Link Establishment (ALE) whileavoiding antenna switching or tuning while ALE sounding was occurring, I came upon the BalancedTerminated 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 ofpoor performance, and included derogatory comments comparing the antenna to a ’radiating dummyload’. These comments were very puzzling, given that the United States Navy tested the BTFD antenna inthe 1940's and those tests concluded that the BTFD was superior in performance to multiple single bandantennas. When queried, these amateur radio operators provided information on the make and model ofthese poorly performing antennas. Upon further investigation, it was clear that the dimensions of themanufactured 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 toconstruct 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, withan intended use spaning from 1.9 to 11.0 MHz in support of both amateur radio and MARS activities. Thefollowing table discloses dimensions for antennas designed for several different minimum frequencies ofoperation.

BALANCED TERMINATED FOLDED DIPOLE DIMENSIONSMINIMUM

FREQUENCY(MHz)

L(feet)

LOA(feet)

W(inches)

TOTALWIRE

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 thedimensions found in the table above. Components were purchased from Buxcomm:

BTR450, 450 ohm non-inductive resistor, 550-watts (SSB)

B15C91, 9:1 BALUN, rated at 1500-watts

6FGSPCR, 16.5 inch spacers (set of 6)

Insulator (2 needed)

#12 stranded copper wire

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. Theseare not guaranteed to retain the wire. I drilled through each of the nocks and then used safety-wire toensure that the antenna wire is retained. Now knowing how these are constructed, I would probablyfabricate 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 myoperating power levels. For power levels above 500 watts, you might consider the BTR-1KW terminatingresistor 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 isat the base of a slight slope, resulting in a nearly horizontal configuration. The LMR-400 feed-line resultsin 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. Theseplots were obtained using a YouKits FG-01A Antenna Analyzer. Within these plots, the white curverepresents the SWR and is read on the left-hand scale, while the yellow curve represents the impedanceand is read on the right-hand scale. The displayed SWR and impedance (i.e. Z) are for the centerfrequency of the plot.

BTFD IMPEDANCE AND SWR PLOTS

BTFD IMPEDANCE AND SWR PLOTS

BTFD IMPEDANCE AND SWR PLOTS

BTFD IMPEDANCE AND SWR PLOTS

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 upsignificantly. This is reinforced by a reduction in reports of QSB or weak signal strength, which were aregular occurrence previously, and resulted in frequent requests to repeat a transmission.

Received signals are up significantly, particularly on 75-meters, 40-meters and 60-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 ifthe signals don't result in S-meter movement. Switching to the FAN dipole results in a decrease in signalstrength, 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 almostdaily use. My personal experience with the BTFD has completely exceeded my expectations. I amcompletely sold on a properly sized BTFD configuration, and highly recommend this antenna. In my 41years (next month) as an amateur radio operator, never have I been this pleased or excited about theperformance of a wire antenna.

73, Ray Montagne (W7CIA)

Posted 12 October 2014