Some Quick Thoughts on HF Wire Antennas
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 non-resonant 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.  My version is 1.5:1 or less on 80-10M.
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 networks. 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. They surfaced daily at sundown to charge their batteries, taking all night long. Locating a submarine surfaced with depleted batteries meant certain destruction. Rapid frequency hopping was imperative to avoid submarine detection as they transmitted and received battle, position, weather and other tactical messages. It took time, several minutes in poor weather, to retune their short end-fed wire antennas. Several broadband HF antennas were developed, including the TFD, that allowed almost instantaneous frequency changes without retuning, even hops of many MHz. Separate antennas were used to allow 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 discussing 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 and maritime dimensions have remained prevalent in amateur radio use forcing many to criticize their reduced efficiency on the ham bands.
 
 
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. ... acceptable for the ham bands, but not ideal.
 
One of the major complaints of the TFD is the amount of RF power dissipated in the terminating resistor (Rterm), common at these random lengths. I then built a TFD 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 current at minimum 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 133 ft. length, moved the SWR minimums to the desired ham band frequencies.  I reduced the power dissipation and lowered SWR further by raising Rterm to 800 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 9:1 balun for a 450 ohm Rterm.
 
Current configuration and results.  My current TFD is 133 ft. long and 30 ft. above ground. It has low SWR, 1.5:1 or less, on the ham bands, including the WARC bands except 24 MHz and upper end of 10M.  It's only advantage over the 66 ft. TFD is slightly better performance on 80M.  With "more wire in the air," there is some improvement in weak signal sensitivity on the other bands.  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, 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. Of course, 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.
Testing.  I used various methods testing the TFD other than measuring the SWR and reactive components with an antenna analyzer (a Rig Expert AA-35 Zoom).  I found RBN (Reverse Beacon Network) and the host of remote SDR receivers to be invaluable in comparing signal strengths between the TFD and my other antennas.  I plotted on a map 100 QSOs on 40, 30 and 20M to see where my signals went, all at 5W QRP, to plot the radiation pattern, which seemed very similar to a tune dipole on those bands. My TFD runs almost due north-south with excellent coverage along both U.S. coasts and a slight null to the north and south.  Many of those QSOs were kind enough to give me A/B antenna comparison reports. Of course, this varied due to daily propagation changes.
 
The real test was the CQ Worldwide DX CW Contest (CQWW) in November.  I used a KX2 with the TFD exclusively. The 50 stations I worked yielded 44 countries, stretching from the Canary Islands to the east to VK, ZL and JA to the west on 20M and 40M.  Granted, some of those guys have impressive antenna farms and good ears, but still, some of that 5W had to still get there.
 
In short, for those who claim the TFD is just a fancy dummy load antenna and unsuitable for QRP is clearly bunk.
 
TFD on the Internet.  You will find plenty of information on the internet, including many ham radio sites and forums.  READ WITH CAUTION.  It seems those who have used a TFD were favorably impressed, including for Automatic Link Establishment or ALE installations requiring quick and rapid frequency changes.  See  http://hflink.com/antenna/.  Others claim they are absolutely worthless antennas.  Of those, most base their opinions on computer modeling the TFD, rather than any real-world experience.  Apparently, the TFD does not model well, but a real one in the air works well.
 
Another reference worth reading is by antenna guru R.L. Cebik, W4RNL (SK) "Notes on the Terminated Wideband Folded Dipole" http://on5au.be/content/a10/wire/wbfd.html
 
 
66 foot version
This page in primarily intended for those new to HF or QRP. What is the best antenna for HF?  Should I purchase an antenna, or build my own? 
 
The configuration I have used for years with good results, primarily with QRP, is a multiband dipole (the TFD) and a multiband vertical (5BTV). The two polarizations ensures best selectable performance under varying band and ionospheric conditions.
 
I'm not an antenna guru, though I have built and employed many antennas over my 50+ years hamming, the last 30 primarily QRP, to form a few conclusions.  Here's a few points to consider, especially if your relatively new to HF ham radio.
 
There is no perfect antenna. The tuned resonant antenna is hard to beat. It is the standard against which all antennas are compared and relatively easy to construct.  It has an impedance of 73 ohms, meaning it can be fed directly with 50 ohm coax for an acceptable 1.5:1 SWR or so.  A 1.5:1 balun can improve this match. The main disadvantage is a resonant dipole is a single band antenna plus the 3rd harmonic (i.e., 3.5/14MHz or 7/21 MHz). Most hams prefer multiband antennas to overcome this disadvantage.
The fan dipole is a good option for a multiband antenna. The most common are two dipoles from a common feed point,  – one tuned for 40/15M and the second for 20M, or 80/20M if you have the room, with the the two dipoles separated by a 15 to 30 degree angle.  A fan dipole is relatively easy to build, though the mutual coupling between the two dipoles often requires a little trimming of the wire lengths for to achieve resonance.
Inverted vee is simply a dipole erected with a single supporting mast at the center feedpoint and often used when there is limited space.  I have had good luck with fan dipoles constructed as an inverted vee for multiband use. 
Trap vertical antennas can be very effective multiband antennas if properly installed.  By all means, install some radials, at least 16 with 32 radials even better for a lower SWR, good virtual ground, a lower radiation "take off angle" for DX, and lower noise.  The Hustler 4BTV (40/20/15/10M) and the 5BTV (adds 80M) is a good "bang for the buck" option.  Other verticals include the WARC bands.  Such trap verticals are relatively easy to install and tune for low SWR and thus a good no-tuner antenna.  Most verticals are a self supporting, though non-conductive guy wires are prudent if you live in a windy area.
 
The best combination is to have two antennas — a multiband vertical antenna AND a multiband horizontal (dipole) antenna.  Switching between the two can often present a 2 or 3 S-unit difference depending upon the polarization of the signals. Theoretically, you want the same polarization as the transmitting antenna (which you don't find out until well into the QSO).  As stated before, the ionosphere has a profound effect on propagation and this includes modifying the polarization of the transmitted signal.  Whether signals are primarily horizontal or vertically polarized can change from band-to-band and day-to-day, and can often change quicky around local sunrise/sunset, following a solar flare changing the ionization of the E/F layers, and other factors.  Hams have no control over the polarization phenomenon, except to have both horizontal and vertical antennas to optimize current conditions.
 
I have used both horizontal and vertical antennas together for years and find it to be a great advantage, especially when working QRP.
 
 
 
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The NA5N Terminated Folded Dipole (TFD)
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