This page was created from a variety of information available on the internet. I often collect information that interests me, but I don't use it for a long time. Sometimes for years. So many of the links where this information came from, no longer exist. Some of the information dates back to the early 1950s, when the TFD was initially developed by the military.
|Terminated Fold Dipole (TFD)|
The Terminated Folded Dipole is a relatively simple antenna to construct. It can provide the user with wide/multi band access (160-6 Meters), without the need for a tuner. Although, some of the articles indicate that a manual tuner would be handy. Various documents list the TFD's bandwidth at from 3 to 8 times the antenna's design frequency. So a TFD designed for 7 MHz should work well from 7 to at least 21 MHz, and possibly much higher.
The down side is that, for a Minimum Operating Frequency of 1.8 MHz (160 Meters), the antenna will need a lot of room. However, if you don't want, or need, the lower frequency bands, a much shorter antenna can be constructed. Another downside might be the required height. If the TFD is erected 1/8 wavelength or less of the Minimum Operating Frequency, the vertical takeoff angle will be about 90 degrees. This is great for NVIS operation, but not for long distance.
This type of antenna is often referred to as a Broad Band Terminated Dipoles (BBTD), Tilted Terminated Fold Dipoles (T2FD), Balanced Termination Folded Dipole (BTFD), and even a Squashed Rhombic. They all essentially describe the same antenna. For example, the T2FD is a TFD, but with one side raised to say 36', and the other end at 6'. This is thought to provide some directional properties to the antenna. Personally, I have not verified this thought.
The TFD antenna is similar in construction to the Folded Dipole. The main physical difference is that, the loop is broken at the point opposite the feed point and a resistive or reactive terminator is installed. The antenna then has an essentially resistive and constant load over the entire operating range.
There are a few vagaries in the documentation I have collected. For example, there are several equations listed for determining the dimension of a TFD, but no one explains their derivation. Luckily, the actual dimensions are pretty loose. And, if you work with the equations a little bit, you find out they all have the same origin, Wavelength (λ). Some examples are listed below.
I found a spread sheet for TFD dimension calculations, created by M0LMK, that bases all the calculations on the number 984. When calculating λ, I usually start with 300/F(MHz) = λ(m). I then convert everything to feet, because that is what I understand. Well, if we take the original formula for λ in meters, and multiply it by 3.28, we get 984/F(MHz) = λ(Ft). So M0LMK simply does all the initial calculation in feet, and then converts to Feet-Inches and Metric dimensions.
In that same spread sheet, however, it is assumed that the Length (L) includes the Spacing (B). So when the Installed Length is calculated, the Spacing (B) is subtracted from the Length (L). No other document mentiones that calculation.
In yet another document, I see the formula for the length to be Length(m) = 50,000/F(KHz). But this is really a "half-length". This is derived from ((300,000/F(KHz))/3)/2 = 50,000/F(KHz) = 50/F(MHz). So the "full length" of the antenna would be Length(m) = 100/F(MHz).
The "spacing" dimension is also a bit vague. The last example describes the spacing as being Spacing(m) = 3,000/F(KHz) = 3/F(MHz). This is derived from (300,000/F(KHz))/100 = 3,000/F(KHz) = 3/F(MHz). So the spacing is assumed to be about 1% of the length. If you follow this formula you going to get some pretty wide antennas, if your designing for 160 or 80 meters. Personally, I wouldn't want to deal with spreaders that are 3-4' long. But it seem that this length is very flexible, with some commercial TFD's using only 16" for the spacing.
The TFD described in most documentation is λ/3 in total length, at the lowest design frequency. This is the original TFD that was developed by the US Navy, during WWII. There are also TFD designs that are λ/2 and λ/4. According to the documentation, the λ/2 design is a bit better than the λ/3 design and λ/4 design is not as efficient as the λ/3 design. But that is to be expected and the differences are not great. I guess λ/3 is a good compromise. The spread sheet, mentioned above, calculates the dimensions for all of these designs.
The dominant element, that sets the feed impedance, is the terminator (TERM). The terminator is simply a non-inductive resistive element that can handle the applied power. In the early development (1950s) by the National Security Agency to meet the need of the military, a 660Ω resistive element was used. Their intent was to feed the antenna with a 600Ω open wire transmission line. However, in subsequent development efforts, the resistive element was modified to different values (200Ω to 1,000Ω). That was to accomodate various open wire feed lines (e.g. 300Ω, 450Ω, etc.).
In the drawing below, the frequency entered in the space provided should be the Minimum Operating Frequency (MHz). The Total Wire Required does not include the open-wire feed line. The drawing also shows a 660Ω terminator and 600Ω open-wire line. But you could easily use a 450Ω terminator and 450Ω open-wire line.
However, some hams have a preference to using 50/75Ω coaxial cable. When feeding the TFD with coax, the high feed impedance of the antenna neest to be transformed to the low impedance of the coax. The transformation radio require is 16:1 for 600Ω feed, 9:1 for 450Ω feed, or 4:1 for 300Ω feed. Some refer to these transformers as Baluns (Balanced to Unbalanced). However, most are in fact Ununs (Unbalanced to Unbalanced). Because of this, the coax would become part of the antenna. This can cause feedline radiation and RFI. This may also skew the radiation pattern. To help prevent the feedline radiation, a choke is needed at the impedance transformer's input.