Transcript
Moxon Antenna with Director for 88-108 MHz Looking for a compact antenna required for FM DX reception I came across the Moxon antenna and the MoxGen programme. I was also starting to learn about the 4nec2 antenna modelling programme so got into that too. The 4nec2 programme showed that the Moxon developed a pattern in the opposite direction as you went below the design frequency, the rear null was only sharp for a small part of the FM broadcast band. By comparison to the ‘wideband’ terminology for the ham bands – the broadcast band is ‘ultra wideband’. Based on what I’ve gleaned many years earlier I wondered what would happen if I placed a director close in front of the driven element. I guessed at the distance in front, and decided to just make the length of the director the same a ‘dimension A’ of the Moxon antenna. By chance I appeared to have hit on just the right thing. For a Moxgen based on half-inch tubing for a design frequency of 90 MHz adding a director 7 inches in front of it transforms it into an antenna with a good gain across the whole FM band and a reasonable front to back ratio across the whole band’.
A standard Moxon aerial from MoxGen program The SWR for the 3 element Moxon is shown below – with that of the standard 2 element version to the right for comparison. When I went looking on the Web I found I was re-inventing the wheel but I’ve not across anything specific for Band II.
SWR with director 7inches in front of D/E
SWR without the director
My other criteria was I didn’t want to mess up with wires and fibre glass poles – so I modelled it using tubing – which was help the bandwidth as thicker element = wider band width (above was 0.25 inch by mistake
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Buying a 3 element yagi is a lot less hassle – so that is what I did – but after a few weeks I wanted to install a vertical aerial – with some gain and directivity – the delta loop cobbled together worked – but was virtually omni directional. It would actually make a good aerial for domestic reception. The loft is barely 6 foot high and it would not be possible to get a yagi in. So I revisited the design for a 3 element vertical and ran the optimisation program properly – for good F/B ratio, good SWR and good gain. This gave the following dimensions – letters relate to the MoxGen diagram above. A = 47.90 inches B= 7.45 inches C= 1.75 inches D = 9.00 inches E = 18.20 inches Dir = 47.35 inches DE to dir = 9 inches
1217 mm 189 mm 44 mm 228 mm 463 mm 1203 mm 228 mm
A key difference is that I modelled elements B and D in 3/8 th tubing – although for practical purposes the outer 4 inches or so of the relefector and driven elements are in 3/8ths tubing – as the right angles were bent using a plumbers bending tool and it was convenient to use 1m (39.4 inches) lengths. The results according to the 4NEC2 model are:
Horizontal radiation pattern – no signifcant change between 96 and 105 MHz
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The aerial has been mounted in the loft on the same mast as the 3 ele antiference yagi – but about six inches below – 4NEC2 programme indicating this would not be a problem as compared to mounting on the same boom. The mast is a dry wood dowel as a metal mast would give big problems. However I have modelled the vertical aerial mounted 12 inches in front of metal vertical mast extending 4 feet (1290 mm) above, with a 3 ele horizontal mounted on the top of the mast and in actuality it looks prety good.
Mounted 12 inches in front of metal mast. Similar pattern except SWR rises abruptly below 88 MHz (below)
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Modelling different metal mast positions shows that if you go much closer that 12 inches / 300 mm separation it seriously affects the aerial performance. You do not want to put the mast through the vertical aerial itself. The f/b ratio goes down, the pattern is distorted and the SWR goes to about 2.5. In summary a fwd gain of 7.0 – 8.0 dBi, a f/b ratio minimum of 14.5 dB at band centre, and a low SWR/good match across the whole band. Placing it 12 inches in front of a metal mast adds a tiny bit of gain. In comparison a 3 ele yagi (with a parasitic element close to the dipole) models at 8.0 dB rising to 10 dB with a similar f/b ratio but needs 66 inches height compared to 48 inches for the Moxon. Performance I haven’t been able to carry out any proper tests – it certainly picks up vertically polarised stations which are absent on the horizontal. I’ve compared the signals from the various frequencies from Dover and Canterbury. Which assumes that the mixed polarisation is 50/50 MHz 90.0 92.4 94.4 97.0 99.5 101.8 102.8 104.2 106.0
Station BBC Radio 2 BBC Radio 3 BBC Radio 4 Heart BBC Radio 1 Classic FM Heart (East) BBC Radio Kent KMFM
Location Swingate, Dover Swingate, Dover Swingate, Dover Dover (500w) Swingate, Dover Dover (5kw) Canterbury Swingate, Dover C/Bury, Rough Common
power 11 11 11 0.5 11 5.2 1 10 0.1
bearing 120 120 120 120 120 120 111 120 105
Vert dB 27 34 28 20 32 28 56 35 38
Hor dB 36 34 37 23 36 25 57 34 22
pol m m m m m m m m v
I’ve also compared the signals from Wrotham at a bearing of 280 degrees (i.e. on straight of the back of the aerials – the vertical is up to 10 dB better than the horizontal – which is surprising – but there is a water tank about 300-400 mm behind the aerials and who knows what that does. Some day in the near future I’ll get a temporary pole in the garden – and see if I can make some basic directional measurements by swinging the aerial round. Construction techniques I purchased the tubing and the dipole box element from sandpiperaerials.co.uk I purchased the element mounting clamps from harrisonelectonics.co.uk – and it would probably be better to get the dipole element from them for a matched set. 3 x 1m lengths of ½ inch tubing 3 x 1 m lengths of 3/8 inch tubing. 1 x 1 m length of 1 inch tubing for the boom (make it 1200 mm if you want to mount it in front of a metal mast 6 x small hose clips from the local plumbers shop (not the big nationals - mine cost me 50p each).
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You need to cut inch long slots in the ends of each of the lengths of the ½ tubing so that you can firmly clamp the 3/8 lengths. Use a small (10mm) pipe bender – borrowed from a plumber - to bend the tubing. The trick is to bend the tubing slowly. If you make your first bend on an uncut length of tubing and aim for a short straight length of 200 mm you will be able to see how much length to allow for the bend – without wasting the tube – if you get it slightly wrong it should still give you a usable leg that you can cut off to size. The table below should be the right overall length for each piece allowing a 35 mm overlap – for adjustment. (but I don’t guarantee it!). Remember “measure twice – cut once” total lengths of 3/8th tubing to make end lengths - allows around 35mm overlap to slide into 1/2 tubing Reflector element end total (x2) 450 mm Driven element end total (x2) 410 Director end length total (x2) 140 Total 1000 (but buy 3 x 1m lengths of 3/8ths - to allow for practicing making the bends) The tricky bit is drilling the element fixing holes in the boom – you need to end up with all the elements being exactly vertical! I have a ‘workmate’ which allowed me to clamp the boom – and a drill stand. So having drilled the first hole in the boon – I fixed a 60mm bolt from one to the element clamps through it – so I could ensure that as I moved the boom to drill the next hole I could see the boom was in the correct position. Don’t forget that the driven element has to be cut into two – and my preference was to ensure that after cutting the 1 m length in two, that I trimmed a bit off – so the overall length of the ½ inch tubing was 1 m (i.e. you will end up with two 490mm lengths). Insert the bent sections into the ½ tubing – tighten the hose clamps to light grip them and then make the final placement – about 100 mm should protruding from each end. Having carefully cut and drilled and fixed – stand back and admire. Then, if it is going into a loft – disassemble. And I tell you modern lofts are terrible to work in – barely enough room to get in and to accommodate a 3 element aerial. Just a final note: The Moxon antenna was one of a number of wire aerial designs by Les Moxon which were intended for ham radio operators who needed antennas that were unlikely to be noticed by neighbours but could give good results. So most designs you see are based on thin wires supported by crossed nonconductive thin poles
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The Moxon in place in the awful loft – with 3 ele Antiference antenna a few inches above Yes – I’ve considered aerial interactions but have yet to drill the holes in the right place to mount the two on the same boom – having mis-measured one of the holes for the horizontal yagi. As far interactions between two aerials are concerned – I modelled the Moxon co-mounted on the same boom with a 3 ele yagi for FM band (using 4NEC2) – and didn’t see any difference when I deleted the yagi elements. I also modelled for the above set up – and decided it wasn’t enough to matter in the short term. Note that I couldn’t be bothered to fitted the lids to the dipole boxes either.
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