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ALL BAND HF DIPOLE

The All Band HF Dipole was constructed and refined for use with my Z-Match Antenna Matching Unit at my previous residence in Melbourne, Victoria from 2004 - 2007


The centre insulators and support attachment consisting of a heavy duty stainless steel key ring.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, simple cut to the maximum length for the available space and matched with an AMU (Aerial 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 there for 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 of 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 will do the trick.

My final antenna system consisted of a 28 metre centre fed dipole (14metre for each leg) suspended at the centre and supported with my existing 6 metre tall galvanised steel pipe mast  fed with 18 metres of 450ohm ladder line via my Z-Match AMU

 

Stand off insulator fashioned from nylon set screw with fibre-glass spacer sleeve.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 consisting of a heavy duty stainless steel key ring.  See the above photo.

The 18metres 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

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 18Mtr)

(5)  Inverted 'V' Dipole. (Length subject to available installation space. In this case it was 28Mtr total length)

 

Performance

The antenna has been installed roughly in a north-south alignment. When the antenna configuration is modelled with MMANA-GAL Antenna Analyser, the following radiation pattern for the 10mtr band is produced. I have over laid the results on the great circle map centred on Melbourne, Australia. Now I know this is not to be taken as gospel however it gives some clues of what is going on and the results have been born out to some degree by the stations that I have contacted.

Great circle map centred on Melbourne , Australia.

The radiation pattern for this antenna is both and advantage and disadvantage based on how they form with relation to the various bands. The 10metre band experience seems to be for the most part a good outcome.

Considering the above radiation plot for 28.5Mhz with the first 10metre band contacts on the 8/12/2004 Allan VK4HUX on 28.47Mhz North of Brisbane, and on the 11/12/2004 David K8CC Missouri USA and Theodore N6IIU Sonora California  USA also on 28.47Mhz.

See below plots for comparisons with other bands and as I said these results are not to be taken as gospel, but do give clues about what patterns can be expected with this sort of antenna system.

The first HF antenna system that I erected in early 2004 was a simple 40 metre band dipole in a similar height and alignment. This antenna was operational on both the 40 metre and the third harmonic band of 15 metres and provided a good operational comparison with the replacement all band dipole system and gives almost identical operating characteristics on the two above mentioned bands.

The random length all band dipole represents some clear advantages in cost and operational flexibility within the limitations of the average Australian suburban block. There for if you can have only one HF antenna the random length all band dipole would be a pretty good choice.

Radiation plot for 7.050MHz

Radiation plot for 14.150MHz

Radiation plot for 28.500MHz

The The very interesting 3 D radiation plot of the antenna at 28.50MHz.very interesting 3 D radiation plot of the 

antenna at 28.50MHz.

 

 

 

 

 

 

The above radiation plots were produced using MMANA Antenna Analyser software by JE3HHT, Makoto (Mako) Mori. 

References

The ARRL Antenna Book.

The 1990 ARRL Hand Book.

 

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Page last revised 28 January 2008 
 

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