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EXPERIMENTAL HF ALL BAND AERIALS,
THE “G4TUG” AND OCFD VARIATION

An idea for a wire (low cost) aerial with horizontal and vertically polarised elements that should cover all HF bands effectively for local and reasonable DX contacts.

Unfortunately I do not have the space ( 8 mtrs by 11 mtrs) to construct one myself but feel very confident that it would give a good account of itself. I hope that somebody will try it and perhaps give a review of their findings. It could be especially useful for field days.

Since writing the above, Dave KK4PQ has started to construct my aerial design. His horizontal element is for 80 mtrs to which he has added 40 mtrs and 10 mtrs. He will send me an evaluation when completed. So far he has reported good results without any ATU although he has had to adjust the lengths to overcome interaction. That is exactly what I would expect. Anyway, Dave says it is a good idea and will give a full evaluation and results in the fullness of time.

The system encompasses a number of dipoles, fed with one 50-ohm coax, for the HF bands and containing some vertical contribution. Below is a diagram showing just over half of the complete symmetrical antenna.

 

Having trawled through many books, an aerial such as this one has not emerged, perhaps because it does not work!   The idea seems so simple that one wonders why it is not in common use. However, I welcome any comments or critique both theoretical and empirical.  It should be an interesting project.  The configuration of separate dipoles connected at a common feed point is well known (Nest of Dipoles) but the multi-band system described here uses less wire, is lighter (causing less sag), less costly and easier to construct and tune (trim).  For more information on the nest of dipoles please read "Practical Wire Antennas" by John D Heys, G3BDQ, page 12, published by the RSGB.  My idea is a development of the horizontal system described, the theory of which should be similar.

The critical dimensions are the overall lengths of each dipole, starting with the 160 metre horizontal that carries all the vertical elements of one-eighth of a wavelength for each of the other bands.  The 10 metre band may not require any elements at all because the 160 metre dipole should work well at 10 metres; at 10 metres my 160 meter dipole, 25ft high, out performs my Carolina Windom 160 metre Special, 33ft high.  Also the 40metre dipole should radiate on 15Metres. The horizontal wire should be substantial, say about 2 millimetres in diameter but the vertical wires need only be about 0.13 millimetres in diameter and virtually invisible; they will withstand 400 watts of SSB or CW.  All vertical elements for each band should be set at one-eighth of a wavelength from the feed point but if not the overall length for the band must be taken into account.  Also, not all the vertical elements will necessarily be required if they are harmonically related to the fundamental (longest ) half wave dipole.

The dimensions given are the initial starting lengths and will require trimming to the frequencies required.  There will of course be some interaction between dipoles but with a little care, preferably with an aerial analyser or just an SWR analyser, all can be brought to resonance after a few attempts.

The longest dipole need only be of a size for the lowest frequency needed; or to fit in the space available to the user.  In addition, for the bands not required, the vertical elements are not necessary.

The height available above ground dictates the longest vertical element; safety being considered because of the high voltages present at the ends of the verticals whilst transmitting.  Ensure that it is well out of reach of uninvited hands.

The polar diagrams will be unpredictable but good results could be obtained, the horizontal elements providing high angle radiation and, to a lesser degree, vertical elements contributing to low angles for long distance working.  DXers may favour an inverted V configuration because of the greater vertical propagation.  However for true DXing, A 4square or similar is required but out of reach of most HAMs and not the subject of this article.

Because of the effects of harmonic resonance, cancellation effects and additions, there could be some fortuitous gain at certain frequencies resulting in beam forming by the vertical wires; a bonus because the beams will be at a lower angle of radiation.

I hope that this project provides some fun for experimenters and will result in contributing to HAM radio generally, providing full HF coverage at the lowest cost and minimising required resources.

Open wire feeder could be attempted but only by the brave, due to the multiple effects of radiation and cancellation by the feed wires.

If fully successful, an ATU may not be required but is advisable to use one for suppression of out of band harmonic transmissions.  The transceivers internal ATU, if available, should certainly bring all the dipoles to resonance easily.  Suggested improvements welcome.

As mentioned above, a 160 metre dipole at 25ft works acceptably well without being erected at the minimum recommended height of 90ft.  Mine has worked the USA and Canada on SSB in times of the recent poor propagation; thanks to the big aerials at the far end doing the work.

An OCFD Off Centre Fed Dipole based on the above.

A development of the above antenna could be the Off Centre Fed Dipole (OCFD). The main horizontal dipole should be fed about 1/3 of the way along with ladder-line. Each subsequent dipole should follow the same rule, ie the left hand side say, should be double the length of the right hand side and all the left hand vertical elements should be double the length of the right hand vertical elements. The total length of each dipole when adding the lengths of the two vertical elements to the lengths of their distance from the feed point should add up to the theoretical thength of a half wave dipole. As above, each dipole element will require adjustment to compensate for the proximity of the other dipoles in the system. It is to be expected that the wanted harmonic performance (7 mHz and 21 mHz say) should be better than the normal dipoles described above.

Rod, G4TUG 

g4tug@talktalk.net


LINKS TO G4TUG SUBJECTS
About Me (index) Ham Radio Cars
Hobbies RodLog Ham Radio Logging System Software Reading
My Diet Magnetic Loop Antenna Sailing
Computers Noise Figure of Receivers and Pre-amps Gardening
Links to other sites Experimental HF All Band Aerial Classic Films