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The terminated folded dipole (TFD) is an HF folded dipole antenna with a non-inductive terminating resistance opposite the feedpoint. This scheme provides for a broadband no-tune antenna over a wide range of frequencies. Commercial versions maintain an SWR of 2.5:1or less from 80M (3.5 MHz) to the 10M (28 MHz) ham bands. Both my 66 ft. and 133 ft. long TFDs work very well with <1.5:1 SWR most bands. (see below).

No antenna is the perfect antenna.

A TFD (or T2FD) is not for everyone. You'll need some back yard real estate. Not for HOAs. It is harder to build over most other wire antennas. It does take some piddling with wire lengths and the terminating resistor for lowest SWR on all bands. Once properly built and tuned, it is a a very good

no-tune antenna for quick band changes with no antenna tuner. With mine I have worked Europe to Asia and Oceana countries with 5 watts QRP.

But ... If you want to read how worthless a TFD antenna is — read about them on the internet. All these antenna gurus have computer modeled them to claim a useless antenna, a glorified dummy load or worse, and certainly worthless for QRP. Of course, they've never actually used one. TFDs have been used by the military, U.S. Navy, submarine service, and embassies, but for some reason, claim they are worthless for ham radio.

For a different perspective, see L.B. Cebik, W4RNL (SK) analysis:

https://antenna2.github.io/cebik/content/wire/t2fd.html

Discussion. Building a TFD is not for the faint of heart. It takes considerably more effort and hardware than building a simple dipole. Especially if you don't have tall trees. I've built several of them experimenting with different dimensions. My first TFDs were based on the commercial lengths, such as the Yaesu YA–30 (length 83 ft.) or Bushcomm SWC-100C (78 ft., or an astonishing 157 ft. length). I made QRP QSOs with these lengths, but SWR minimums were near the 4, 8, 12 and 17 MHz maritime bands, not ideal for the ham bands.

One of the major complaints of the TFD is the amount of RF power dissipated in the terminating resistor (Rterm), common at these non-resonant lengths on the ham bands. My first TFD was 66 ft. long, resonant for 40M. The maximum current and minimum voltage at the feed, I reasoned, would also be seen at the terminating resistor, opposite the feed. Maximum voltage is minimum power dissipated by Rterm, confirmed by building an RF detector and LED to visually show the power across Rterm. Turns out making the TFD resonant for minimal Rterm loss is the approach B-Squared Engineering uses on their amateur radio TFDs.

See: http://www.b-squareengineering.com/TerminatedFoldedDipole.htm

The 66 ft. length, and later a 133 ft. length TFD, moved the SWR minimums to the desired ham band frequencies. I reduced the power dissipation and lowered SWR further by raising Rterm to 820 ohms as B Squared found best, and building a 16:1 balun at the feed for the 50-to-800 ohm matching. Most commercial TFDs use a 300 ohm feed line and 390 ohm termination, or with 50 ohm coaxial feedline, a 9:1 balun for a 450 ohm terminating resistance.

Current configuration and results. My current TFD is 133 ft. long and 28 ft. above ground. It has low SWR, 1.5:1 or less, on the ham bands, and reasonable on the WARC bands except 24 MHz and upper end of 10M and 160M. It's advantage over the 66 ft. TFD is better performance on 80M. With "more wire in the air," there is some improvement in weak signal sensitivity. My transceivers are connected to the TFD directly, no antenna tuner is used. Signals are about the same on the TFD as a dipole or G5RV, if not a bit less. However, the TFD is a much quieter antenna, usually about 2 S-units less noise than the dipoles or vertical, yielding a better signal-to-noise ratio on receive. The two wire runs on my TFD are 16" apart using 1 inch O.D. PVC spacers. I found little difference experimenting with different spacings, (tried 24 and 32 inches) or height above ground. In the New Mexico desert, I have no idea where electrical ground is. I'm on well water, with water 220 ft. down the hole if that's any indication of electrical ground.

I am 99% QRP and 100% CW. Been a ham since 1964. My modest station consists of a modified Ten Tec Scout (purchased from the Ten Tec
factory 1994) still going strong and simple, a joy to use, and a great receiver. Set at 5 watts most of the time, though I turn up the wick to its QRO 50W max on rare occasions.

See: http://www.zianet.com/qrp/SCOUT/tt_002.htm

Antennas are the 133 ft. long TFD and a 5BTV trap vertical with 36 radials. This combination of horizontal and vertical polarized antennas is sometimes very advantageous as one can have a 3 S-unit difference switching between polarizations. The ionosphere does that.

I am active in various QRP events and SKCC, the Straight Key Century Club (I am #10073S). My favorite mechanical key is my 1938 McElroy bug or "Mac key."

72/73, Paul NA5N

NA5N@zianet.com

History. The TFD was developed by the U.S. Navy during WWII. During the war, the Germans and Japanese became quite efficient in locating allied ships and planes using HF direction finding systems. A transmitting vessel could be located by triangulation in about 15 minutes. To counter this threat, transmissions were kept short (5-10 minutes) and changed frequencies often. The problem with this frequency-hopping tactic was the time it took to "tune" antennas from one frequency to another. Often several minutes ... time that pertinent communications could not take place.

Submarines were particularly vulnerable since most operated "lone wolf" with no escorts for protection. To avoid visual detection, they charged their batteries only at night. While surfaced, they also transmitted required position and operation reports risking radio detection. Locating a submarine surfaced with depleted batteries meant certain destruction. To avoid direction finding detection, rapid frequency changes were enacted while transmitting messages. However, it took time, several minutes in poor weather, to retune their short end-fed wire antennas. The Navy developed the broadband TFD for transmitting that allowed almost instantaneous frequency changes without retuning, even hops of many MHz. Separate antennas were used for transmitting and receiving on separate frequencies for additional security.

After the war, one of the Navy TFD developers, Capt. Gil Countryman, W3HH, published an article in the June 1949 QST presenting the design and the suitability for amateur radio use. It worked reasonably well as a no-tune antenna on the ham bands, even though dimensions in the article were based on the Navy design optimized for military, aeronautical and maritime frequencies. Unfortunately, these military dimensions have remained prevalent in most amateur radio TFD literature, forcing many to criticize their reduced efficiency on the ham bands.