ALL BAND HF DIPOLE
by Peter Miles, VK6YSF.
Despite the space limitations of a suburban block I wanted an antenna that would be suitable for all the HF amateur bands, including the so called WARC bands and ideally including the 160 metre band. The antenna system should also be useful for other HF services i.e. broadcast, military etc. Another aim was to develop an antenna that could be used portably, that is being able to be simply rolled out and hung up in a convenient tree or similar. What became the clear choice was a dipole cut to no particular frequency, simply cut to the maximum length for the available space and matched with an Antenna Matching Unit to the transceiver. With the antenna suspended at the centre feed point to create a low slung inverted vee configuration the antenna is very easy to install which is consistent with the second aim of portability. That is the antenna can be suspended from a single support and is therefore easy to erect from a single tree for example. I referred to the antenna as a random length dipole, but there are a couple of limitations to the ultimate dipole length. First antenna efficiency will drop off significantly at lengths dramatically less than a half wave length for the lowest frequency band to be operated. Also it is wise to avoid lengths that produce extremely high impedance to the AMU as it may be beyond its ability to match this impedance. The second example is fairly easily rectified by simple adding or subtracting some length to either the dipole or the feed line, often as little as a metre (1 Metre = 3.28 feet) will do the trick. My final antenna system consisted of a 28 metre (28 metres = 91.86 feet) centre fed dipole. (14 metres (14 meters = 45.93 feet) for each leg suspended at the centre and supported with my existing 6 metre tall (6 meters = 19.68 feet) galvanised steel pipe mast fed with 18 metres (18 meters = 59.05 feet) of 450ohm ladder line via my Z-Match AMU.
Construction
The physical assembly is extremely simple and cost effective consisting of standard electrical copper wire with the insulation removed, light weight end and centre insulators fashioned from fibre-glass rod. The centre insulators and support attachment consist of a heavy duty stainless steel key ring. See the above photo. The 18metres (18 meters = 59.05 feet) of 450ohm ladder line feed has been installed with sweeping bends and care to avoid as far as practicable any other cable and metal objects. See below stand off insulator fashioned from nylon set screw with fibre-glass spacer sleeve. One of the disadvantages of this antenna system is that it is a balanced system that is each halves of the dipole and feed-line configuration have to mirror the other. Failure to achieve this will cause the feed-line to receive and radiate energy which will result in a distortion of the radiation pattern and also allow the feed-line to pick up stray signals from computers etc as the feed-line enters the radio room. Despite this I have found that this antenna system is reasonably forgiving. Basic All Band Dipole Arrangement (1) HF Transceiver. (2) VSWR Meter. (3) Z-Match AMU. (4) 450ohm Ladder Line. (Can be any realistic length. In this case it was 18metres (18 meters = 59.05 feet) (5) Inverted 'V' Dipole. (Length subject to available installation space. In this case it was 28 metre (28 metres = 91.86 feet)
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