So I have assembled most of the parts for making my arrow antenna. The arrow shafts are really nice, and come with aluminium threaded inserts.
I will upload some sketches of my design later in this thread.
Parts of the design requires me to drill and tap a hole for a bnc connector. I have the required tap (a 3/8 UNEF thread - 32 tpi) but this requires a drill size of 8.85mm.
Drills of this size are specialist tools, all i have is 8.8mm or 8.9mm. I don’t know much about this sort of thing and was hoping there might be some engineers who could set me straight…will the 8.9mm give too loose a fit, and will the 8.8mm lead to a broken tap?
Edit - 8.8mm was fine (and 8.5mm would probably be too). The aluminium is quite soft, so the tap is able to sort out any additional hole thickness easily enough.
I have always known these as UNS threads, not UNEF.
As it is an imperial thread you need an imperial drill.
looks like 11/32" is the one you want, and is readily available.
Looking forward for details of the finished item.
Well, part of the gamma match is made, image below.
Just need to make a metal bit to connect the two aluminium shafts.
The black shaft is an arrow shaft, the gold is a pole segment from a hilleberg tent that was damaged in a rather nasty storm (the arrow shafts are too narrow to insert RG213 dielectric.
The difficult part now starts of achieving an impedence match.
I would rather not use the gold coloured tube…it is a larger diameter than the black arrow shaft, whereas a smaller diameter ought to be used for the gamma match.
I shall probably try to use something other than rg213 as the donor for the conductor…just need something with a dielectric around half a millimeter thinner so that it fits an arrow shaft.
Are those aluminium tubes anodised?
Anodised aluminium is highly resistive.
So long as the current carrying pieces connect firmly with clean metal the anodising won’t do anything. It’s comparable to a thin coating of paint or plastic insulation. Different thickness and different dielectric but not a problem. New aluminium oxidises immediately it comes in contact with air and this grows progressively thicker. You can the shine getting progressively duller over many weeks. But the antenna does not degrade.
If you abrade the anodising where you need a contact it will be fine. Diameters need to be right for gamma matches.
A needle file held in a drill chuck was used to remove anodising from inside the arrow shafts…finished 2m part of the antenna pictured below, ready for tuning of the gamma match.
Next will add the 70cm elements, orthogonal to beam and 2m elements.
(Edit: 1 hour later) SWR of 1.1 at just below 144MHz rising to 1.3 at 146MHz. Not bad for first guess at gama match dimensions…will seek to improve a little.
For anyone wanting to copy the design, some images of the design of my connector block (using fusion 360) are copied below.
*Note that BNC connectors are available in two thread types - M10 and 3/8-32tpi, make sure you know which you have!
*The tapped hole for the arrow shaft uses a #8-32tpi thread to match the threaded rod that screws into the arrow shaft inserts.
*The hole for the conductor/dielectric is drilled all the way through the block, to allow entry from underneath for soldering.
Bill of materials
19mm (3/4") Aluminium Square Bar (length 1m) £12.75
10mm Aluminium Square Bar (length 1m) £4.88
19mm x 19mm x 1.6mm (3/4" x 3/4" x 16swg) Aluminium Box Section (length 2m) £7.45
Arrow shafts Mybo Cadet - Shaft Only - (size 2016) - 10 units at £1.50 each = £15.00
Easton RPS Insert - (size 2013) - 12pk [note different size to shaft, in order to give interference fit) - 2 units at £3.58 each = £7.16
Hilleberg Akto tent pole section (reuse of scrap price - Nil)
RG213 20cm from 50m roll - (price £0.25)
BNC straight bulkhead socket,50ohm * RS Stock No. 546-4465 2 units total £5.84
10 Pack Steel Rod, Coarse Threaded, 8-32 x 12-In. -11002 £16.82 (expensive, but my local steel stock holder was unable to supply)
TOTAL SPEND = £69.79 (plus around £15 in postage) with plenty of spare material to make a start on making a second one
(compared to around £150 for the arrow antennas version).