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The TFD or T2FD NVIS antenna

When Alison, G8ROG, gave her talk on NVIS one of the antennas she mentioned was the Terminated Folded Dipole design - also known as the T2FD (tilted terminated folded dipole) or BTFD (balanced terminated folded dipole). Although is seems to have a little less gain than a dipole it is quite an appealing antenna as it allows operation on a wide range of HF bands (or any frequency between) and use of an ATU is allegedly not essential. It is also claimed to pick up less noise than other wire antenna types - possibly because the main lobe points upwards.

I spent a lot of time looking at this antenna design a year or two ago before concluding it didn't really match my needs at the time. What follows is based what consultants call a 'literature review' ie I haven't done anything useful myself but have read lots by others who claim to have. The dimensions seem to vary widely, even on diagrams from the same source, so it may be a matter of 'suck it and see'.

The basic design can be seen in the diagram below. There are many versions of this design online, some replacing the balun with balanced feeder. I doubt they all work equally well, so experimentation may well be needed. (One of the joys of the hobby of course!) Buxcomm in the US (see link below) sell a commercial version of this antenna as well as a range of parts for those wanting to make their own, including terminating resistors. Full details of their design is available on their website. It appears that their version is tried and tested so could be well worth trying.  BTW if you have 83ft to spare and want the easy route, just buy a £200 Yaesu YA-30, which appears to be much the same antenna. Another similar antenna, is available from Barker & Williamson who were awarded a US patent in 1983 (see link below)..
Diagram of the TFD antenna
Many designs, including Bixcomm's use a 9:1 balun and a 450 ohm terminating resistor, R1 (450/9=50 ohms, which matches 50 ohm coax of course). Other versions features baluns from 4:1 to 16:1 and terminating resistors from 300 to 800 ohms, depending on the match. Some designs use 75 ohm coax, which would probably require an ATU in the shack. One source suggests the exact value of the resistor and antenna dimensions are more critical at the lower resistance values.

Dimensions A and B both vary, depending on the frequency range needed and the need to match the antenna to the terminating resistance correctly. Typical values for the length, A, are between 20 and 26m. The amount of spread, B, ranges far more widely - between 0.4m and 3m. A number of sources provide a formula to work out A and B.

A is typically put at around 100/f metres, where f is the frequency of the lowest band of operation in MHz.

B is sometimes put at about 3/f metres, although both B&W and Buxcomm seem to use a value of about 1.66/f

For operation from 80m upwards (based on a mid-band frequency of 3.65MHz)

A = 100/3.65 = 27.4m and

B = 3/3.65 = 0.8m (or about 0.45m if you believe B&W and Buxcomm)

The tilted version of this antenna has one end higher than the other and may have some gain in the direction of tilt, which does not seem to be a particularly useful feature if you want it to radiate vertically upwards so you can work someone nearby via NVIS propagation. Those with telescopic masts might want to do some experiments to see if a tilt converts it from an NVIS to a DX antenna. Any tilt will introduce a degree of vertical polarisation, which may make electrical interference pickup and TVI risks a bit higher.

The fact that the antenna has a terminating resistor built-in makes some people to think that a lot of power is wasted. In fact, depending on the dimensions and frequency of operation, the terminating resistor may be doing very much other than helping the SWR match slightly - most if not all power will be radiated. The antenna was originally designed for the US military and there are reports that the original dimensions were optimised for their military bands. This would mean that the resistor would be more likely to be absorbing power at amateur frequencies. There are now designs around which have been optimised for the amateur bands. If used for transmit it is important to fit a high power resistor that can take at least half of your TX power as, in a fault condition, it may end up absorbing a fair chunk of it. Fireworks may entertain the neighbours but are probably best avoided.

More information:

The website of US company Buxcomm has a strange domain name but they not only provide full antenna design details but also sell hard-to-source parts like high power terminating resistors and complete antennas.

Military grade antennas from Barker & Williamson big bucks!

The 1983 Barker & Williamson patent, which has interesting SWR curves