SOTA – Homebrew 23cm 12 element Yagi Antenna

Sunday 18 June 2017.  I recently purchased a new 2.5 watt 23 cm 1296 MHz transverter from SG-Lab in Sofia Bulgaria. The package includes a 2el HB9VC PCB yagi which has turned out to be a great addition for portable work, particularly from a hilltop with a good uninterrupted view of the horizon.  Today from the summit of Mt Taylor VK1/AC-037 locator QF44MP, I worked Rod VK2TWR in Nimmitabel over 130 km, not bad for 2 watts in to a tiny PCB antenna.

While the PCB antenna is a great addition to the SOTA kit, be it for local summits or perhaps on a long hike 10 km or more, what I really need is an antenna with a fair amount of ‘oomph’ that’s non-techo speak for antenna gain, whilst keeping weight and size or length within reasonable limits.

For SOTA purposes, keeping antenna construction as simple as possible is a key attribute to a successful activation. When Murphy’s Law bites and it will, simplicity in design and construction is the key to a quick recovery.

I have decided to build two 23 cm (1296 MHz) 12 element Yagi antennas (DL6WU format) one with ‘handle’ space behind the reflector and the second to mount on a camera tripod.  I figure two antenna options are better than one, plus phasing two in a stack would make for an interesting option!

To simplify the construction and to keep weight down I am using 18 mm square Western Red Cedar as the antenna boom.   I am using Yagi Calculator, written by VK5DJ to generate the element lengths and spacings.  At 1.2 GHz accuracy in element lengths and spacings are equally important. My spacing tolerances are +/- 0.5 mm while reflector and director element lengths are within 0.01 mm. Design details are shown at the end of this post.

Post update.  Since building this antenna I have constructed an all brass 6el 23cm yagi..

Inputs to the Yagi Calculator software program include:

• Frequency
• Parasitic element diameter
• Element mounting arrangement
• Boom cross-section type
• Boom cross-section dimension
• Driven element cross-section type
• Driven element cross-section dimensions and gap spacing
• Non-metal boom
• Number of director elements
• Velocity Factor of RG316 69.5%

Materials list:

• 2 * 1 metre lengths of 18 x 18 mm Western Red Cedar
• 1.2 metre length of 3.1 mm aluminium rod purchased from Alucom. (Sufficient for one set of elements)
• 100 mm length of RG316 for an 80 mm balun
• 1 * RF N Type panel mount socket
• 1 * 6 mm sprinkler riser (spacer)
• 2 brass wood screws 25 mm long
• Araldite or an epoxy resin to protect the feed point/balun
• PCB Lacquer
• Cabots Cabothane clear timber varnish 
• Sense of humour and patience  😉

Tools:

• Vernier caliper capable of measuring 0.01 of a millimetre
• Marking Gauge
• Square
• Coping Saw
• Pedestal Drill
• 2.5 and 3.3 mm drill bits
• Hobby knife
• Scribe
• Sharp pencil
• Steel Rule 150 mm (6 inch)
• Tape measure
• Hand File fine-cut
• Needle File
• Soldering iron

Boom Preparation

I used a marking gauge to mark the center line on opposite faces of the wooden boom.

marking the center line on opposing faces of the boom

Marking out element positions on each boom is a lot of fun, measure twice and mark once. I used the tip of the scribe to mark the hole positions.  Make sure you mark the position of the driven element, you will need a reference point later.

Marking out Reflector, Driven and Director element positions on each boom

Two antenna booms – left boom will have space behind the reflector as a hand-held antenna.  Right boom will have a tripod mount arrangement

Reflector and Director Elements

The 3.1 mm aluminium reflector and director elements are cut with a fine hacksaw then filed to their specified length.  Each element is measured with a vernier caliper to within 0.01 mm.  For the time being the elements are stored in a blank section while I continue work on the boom and the driven element.

Reflector and 10 director elements stored in a blank section

Folded dipole driven element

I marked out each of the folded dipole point-to-point measurements.  I had a practice session at folding a driven element of 1.7 mm soft copper wire around a short length of 23 mm dowel.  This is not the intended finished driven element, however it will serve me well for a shorter 5el yagi.

1st attempt at folding a 1296 MHz folded dipole by hand.

After three attempts at folding the driven element I have the finished product.  As mentioned above the top element in the picture is 1.7 mm soft copper wire.  Middle and bottom driven elements are each 2 mm solid copper wire found in the junk box.  After cleaning the bottom element I will mount it to a female N Type panel mount socket.   I will make good use of the spares.  🙂

thee 1296 MHz folded dipoles

Drilling the boom.  I started with a 2.5 mm pilot drill followed by with a 3.3 mm drill bit. All holes were drilled using a drill press.

view down the length of the 23cm yagi, I’m pretty pleased with the alignment of each element, my eyesight hasn’t failed me yet!  🙂

Reflector and Director elements fitted to the non-handle version

Reflector and Director elements fitted to the handle version. I plan to cut a second set of parasitic elements

Back to the driven element..

I used emery paper to remove tarnish from the folded dipole which is now ready for a drop of solder.  You will note the socket pin is soldered to one end of the open loop.  This offsets the N Type socket by 3 mm to the left or right of the imaginary center line.  It is important to remember the 3 mm offset when mounting the socket/dipole assembly to the boom, else the folded dipole will not be centered to the axis of the boom. Next fabricating the RG316 1/2 wave balun and cutting a garden sprinkler riser to make a pair of spacers to fit under the N Type socket.

Folded dipole and N Type socket ready for soldering.  Use a small needle file to remove the socket plating and expose the brass.

Oh dear (or other choice words not suitable for this blog), working with RG316 takes a lot of patience, cutting and trimming the coax is as Andrew VK1DA explains ‘character building’.  On my third attempt, that’s right 3rd attempt, I managed to not cut through the very fine multi strand center conductor. Phew, time for a stiff whisky!     🙂

If you know the secrets of working with RG316 particularly the center conductor, I’m all ears.  BTW, I am using quality 316 (silver plated copper wire with a Teflon (PTFE) dielectric) from Mini Kits in South Australia, which isn’t the copper coated steel (CCS) variety.  Now I know why the CCS version is popular with radio hobbyist like me, it’s difficult to cut through the center conductor. Ha ha..

RG174 may be a practical alternative. Remember to adjust the length based on the cable’s velocity factor (vf).

Next is the tricky part.  Without destroying the integrity of the balun with an overzealous soldering iron, carefully solder the ends of the balun center conductor (I put so much effort in to that bloody balun!) to the driven element ends, plus solder the outer braid of each end to the N Type flange. This will test your spirit and endeavour to finish the project.   Make sure you leave space around the flange mounting hole to accommodate a spacer.   Good luck!

Folded dipole and balun (RG316) secured to a N Type panel mount.  I will replace the ugly green tape with a section of heat-shrink

As mentioned earlier mounting the N Type socket requires a 3 mm offset due to the 5 mm spacing in the loop ends, if not the driven element will not be centered to the axis of the boom. Use the marker gauge to mark the offset line.

Feed point, folded dipole and balun assembly

Accurate spacing of the assembly above the boom will ensure the driven element is spaced evenly top and bottom. As best as you can have the center of the folded dipole in line with the reflector/director elements.   Align the driven element to the markings on the side of the boom.

I cut two 13 mm lengths of 6 mm garden sprinkler riser tube.  I am thankful the metal support screws have little to no influence on the VSWR reading.  Before tightening the screws check the driven element is perpendicular to the boom.  You can measure the spacing of the driven element between the tips of the 1st director and the tips of the reflector.

N Type flange mount and driven element assembly perfectly aligned to the first director and reflector elements.

Finished after 7 days of part-time construction work.  You could easily build this antenna in a day. 🙂

Finished 12el 23cm Yagi ready for SOTA action!

To help protect the copper driven element from tarnish I applied a generous coating of PCB lacquer.  To help prevent movement of the reflector and director elements, I applied a small amount of Araldite glue to each rod and mounting hole.

I haven’t decided whether to treat the wood with a coat of varnish or leave it as is.

Post update: 1 July 2017, mastering RG-316 1/2 wave balun.

Practice makes perfect, I’ve constructed a 3rd 1296 MHz folded dipole assembly.  Best attempt thus far.  Best approach to the 316 center conductor is to peel off small amounts of the Teflon dialectric until you expose the fine silver coated center conductor  🙂

Latest update: small wooden block glued to the boom

small wooden block to help protect the folded dipole and to prevent unnecessary movement

top side

Although not shown in the above picture, the boom now has a generous coating of Cabots Cabothane clear timber varnish.  🙂

Photos: © Copyright 2017 Andrew VK1AD

Published: 29 June 2017
Last Update: 5 July 2017

Output of VK5DJ Yagi Calculator

Balun design

RG316 1296 MHz 1/2 wave balun

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