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The TFH (Top Fed Helical) is a multi-band, HF TX antenna only 2 metres long, designed for indoor, discrete outdoor, portable or-holiday use.

The conventional short vertical helical HF Antenna is well documented. It is usually 6-8ft high and helically-wound. Invariably it has an umbrella type top hat, of several feet diameter, and an elaborate groundplane system is required. There is an even distribution of voltage/current throughout its length, and it is a very efficient short radiator.

However, you do not get something for nothing, so let us look at the drawbacks:

  1. It is invariably a single band affair.
  2. It is very narrowband, but very efficient within that narrow band.
  3. Having a very low impedance it requires a matching unit located at its base.
  4. It requires an extensive λ/4 diameter radial groundplane system, which usually means a lot of digging up of lawn and garden to accommodate it.
  5. Since the antenna with its ATU must be situated in the middle of the groundplane, it will be located out of doors, whilst you are comfortably located indoors with the Tx/Rx, the two being joined together with coax.
  6. Because of 4 & 5, above, any ATU adjustment means rushing out of doors, and back indoors repeatedly to make normal loading adjustments.

So what is the answer? Well, the ideal seemed to be a short vertical helically-wound HF antenna, between six and eight feet high, using a normal ground system, usable over several HF bands, not narrowband, and with loading adjustments carried out near the operating position. In addition, an even current/ voltage distribution throughout the whole antenna, which the conventional short vertical has proved to be effective.

The TFH fulfils this specification. It is a multi-band antenna; can be used indoors on a single simple base, or discreetly erected on the wall outside, with wall pipe clips. It is ideal for portable use or can be taken on holiday, with a small rig.

Fig 1 shows the antenna configuration. L1 is the short helical winding, and L2 is the single wire feeding from the ATU. The ATU, (Fig 2) is an 'LC' type. It is invariably used in the configuration shown, with the option in certain cases of reversing it to CL configuration. The ground system I use is a convenient waterpipe. A metal window frame has been used as an experiment.

Fig 1
Fig 1: Basic configuration of the top loaded helical antenna.

Fig 2
Fig 2: A simple 'LC' or 'CL' aerial tuning unit, which is invariably used in the configuration shown.

Construction

Antenna

The antenna is wound on a 2m length of 2in inside diameter (i/d) (2.25in outside diameter (o/d)) grey polypropylene downpipe using 106ft of single strand 1/0.6mm grey PVC-covered 1.2mm o/d wire.

At the top of the pipe, fit a 4mm socket as shown in Fig 3a. Drill a small hole adjacent to the socket, push through the wire end and solder to the socket. Wind the wire, evenly spaced, to a total winding length of 45in using approximately 4 turns per inch. It is convenient to put pieces of tape around the coil at intervals to hold the wire temporarily. The last few turns at either end should be secured to the pipe with cyanoacrylate adhesive. This adhesive should be handled with extreme care, as it sets in about 3 seconds, with an indestructible joint. After final testing, the pieces of tape are removed, and the turns coated with a clear lacquer.

Fig 3
Fig 3: Construction details of the TFH antenna (a) self supporting or (b) wall mounted.

The simple wooden base (Fig 3a) is made of 12in x 12in x 0.5in heavy wood with a 12in long square section 'prong' on which the antenna pipe can be slipped. For outdoor use, the wire turns should be coated with marine varnish, and the 4mm socket replaced with an insulated hard soldered connection. For mounting on a wall outside a window (Fig 3b), two plastic pipe brackets are used, making an inconspicuous antenna.

The ATU

The ATU consists of a tapped coil and 500pF variable capacitor as Fig 2. This is mounted inside a convenient metal case.

The inductance L3 is wound on a 4.5in length of 1.25in o/d paxolin tube using 95 turns of 18 gauge enamelled copper wire, tapped at 12 - 12 - 12 - 12 - 20 - 12 - 5 - 5 - 5 turns. The taps on the prototype are adjusted with a clip, but a rotary switch could be used, or the tap soldered for monoband use. Good clearance is required between the coil and the metal box, but otherwise the layout is not critical. A short length of RG58 50Ω coaxial feedline connects the ATU to the Tx/Rx.

The feedline L2 is a 10ft length of PVC covered 24/0.2mm stranded wire. One end is terminated with a 4mm plug (to antenna), and the other a coaxial plug (to ATU).

Operation

The antenna should be located clear of metal objects, and electricity supply wiring. It should be in a position to enable feedline L2 to be taken in the clear to the ATU, eg the centre of the room. If mounted on pipe brackets outside the window, as Fig 3b, it should be kept clear of any metal pipes or in-wall metalwork.

The antenna is usable on 80, 40 and 20m over the whole of each band. It has been loaded successfully on 160m though not tried on the air.

It is essentially a low power device (up to 25W or so), and in the interests of safety it is suggested that a maximum of 10W output should be used indoors.

In operation, connect the ATU in the LC mode; select the band, and then adjust L3 tap and C1 for maximum received signal strength. Only a minor adjustment of C1 may be necessary on transmit for Tx loading.

The antenna has been used mostly on 80m CW, obtaining excellent results with a 10W transmitter using a Pi output.

Setting up at 3550kHz gives a usable bandwidth of over 100kHz, from 3500 to 3600, without readjustment of C1. Further small adjustments of C1 enable the whole band 3500 - 3800kHz to be used in approximately 100kHz segments. No harmonic content or TVI has been detected.

Careful checking indicated that the voltage/current distribution on the helical coil L1 is even over the whole length, as per target specification, and this holds good on 80, 40 and 20m. On 160m, the ATU should be tried in both the LC or reversed CL modes.

The antenna is a very low cost, convenient, transportable device breaking down into Helical L1; wood base and plug-in feedline L2, plus ATU. It takes only a couple of minutes to set up or break down. High rise building dwellers, with antenna restrictions or difficulties, should find the configuration of Fig 3b useful, with the rig and ATU just inside the window. If the window frame is metal, it can be tried as the ground (it worked at this QTH), or a convenient water pipe could be used. Otherwise, a U4 insulated counterpoise lying on the floor would suffice. Metal gas pipes or electric conduit should not be used as a ground. An existing good HF all band ATU can be tried in lieu of Fig 2.

Partlist

L1Single strand 1/0.6mm PVC covered 1.2mm o/d wire: (eg 1 kV RMS, 1.3 amps - grey - Marco Trading CBL/EW1/Grey) wound on 2m of polypropylene downpipe 2 inch i/d, 2.25in or 57mm o/d, (eg grey - OSMA20MF282 or similar)
L210ft of 24/0.2mm PVC covered stranded wire 2mm o/d
L318-gauge enamel covered wound on 1.25in, o/d x 4.5in long paxolin former
C1Airspaced good quality 500pF variable capacitor with knob
Optional Wall Clip Pipe brackets, (eg grey OSMA20MF281 or similar)
SK1/SK2Coaxial sockets (identibal)
4mm plug and socket.

G2BZQ, Richard Q Marris.