This 12 element Yagi antenna for 70cm, shown here under test, has been designed and built for fixed station use and provides an estimated gain of 14.4 dBi, with an SWR below 1.5:1 between 430 and 438 MHz.   Anyone with a basic tool kit can use the details provided below to make an excellent antenna having a wide bandwidth; low SWR from 430 to 438 MHz; and high gain.

This antenna uses parts from an 8 element VHF broadcast band receiving antenna (Model FM8) manufactured by Blake UK Limied and sold by CPC. The FM8 is a well-constructed antenna that provides many useful parts for making this 70cm beam, including the mast clamp, boom, elements, and some of the plastic mouldings required to mount the elements.

All the elements are made from 12.7 or 13 mm outside diameter (OD) round aluminium tube.    The boom is 1640 mm in length and is made from 20 x 20 mm square aluminium tube.   50 ohm coaxial feeder terminates directly on the driven element via a snap-on choke balun (further details below)..

The following underside views of the antenna show the general arrangement, including the waterproof connection between the main RG213 feeder and the RG58 tail from the driven element.  The driven element is mounted within a waterproof box, which also contains a choke balun.

Each element is fixed to the boom using a plastic moulding obtained from Blake UK Limited, Blake Part Number: PL-2700-BLACK.  The product description is 'VHF 20mm SQ x1/2" ELE CLIP BLACK BOTTOM'.   The elements and plastic mouldings are fixed to the boom using stainless steel fixings, comprising M5 x 50 mm bolts, M5 shake proof washers, M5 x 20 mm washers, and M5 wing nuts.
 

Note that plastic end caps have been fitted to the ends of each element.  I found these on eBay via the following link:
https://www.ebay.co.uk/itm/254693938971
Or just search by:
'1/2 inch End Caps, End Covers for Tubes, Rods & Threads'. 

The antenna dimensions were determined through the iterative use of 4nec2 antenna modelling software.  I am very grateful to Arie Voors for generously making his excellent modelling and optimisation software freely available.  

Table 1

Note 1:  Although this antenna was modelled using a Driven Element length of 318 mm, it was found during final testing that the length needed to be reduced to 288 mm to achieve the low SWR values predicted by the modelling exercise. 

Note 2:  This antenna was modelled using a Director 2 position of 327 mm.  However, it was found during final testing that the position needed to be increased  to 352 mm to achieve the low SWR values predicted by the modelling exercise. 

The 4nec2 files that describe this antenna can be downloaded from:
https://www.4alg.uk/radio_g/qrp/432_12el_G4ALG_20250213.nec
https://www.4alg.uk/radio_g/qrp/432_12el_G4ALG_20250213.nec.txt
 

The photo below shows the feed point at the driven element.  The plastic enclosure is an off-white IP66 waterproof box having overall dimensions of about 115 mm (L)  x  90 mm (W)  x  55 mm (H). 
I purchased my enclosure via eBay for about £5.  This link might still work:
https://www.ebay.co.uk/itm/404718453634
Or search by 'IP66 Electronics ABS Plastic Waterproof Project Box Enclosure'
 

The enclosure was drilled to accept the outside diameter of the driven element at 10 mm above the bottom outside surface of the box, and 32 mm from the front outside edge of the box.   For mounting the box to the boom, three 4.5 mm holes were drilled along the centre line of the enclosure at 10 mm, 50 mm, and 73 mm from the front outside edge of the box.  A 10 mm hole was drilled at 22 mm from the two outside surfaces of the box for the sleeved grommet, and the hole opened up slightly with a round file so that the grommet fitted snugly into the hole.

The enclosure was then mounted on the boom so that the centre-to-centre spacing between the Driven Element and Director 1 was 57 mm.   The box was fixed in position using three No. 8 x 1/2 inch screws, with an M5 x 20 mm 'penny washer' (sometimes called 'repair washer') under the middle screw.  

To allow for termination of the RG58 tail, the feed point provides a gap of 20 mm between the two halves of the driven element.   Note that the overall length of the driven element needs to be as specified in Table 1 (above). 

A length of wood dowel was used to secure the two halves of the driven element to each other with a gap of 20 mm using super glue.  The driven element was then held in position using more super glue where the element passed through the enclosure.  Once the super glue had set hard, RG58 cable was passed into the enclosure via a sleeved grommet.   

To reduce RF current flowing along the outside of the feeder, the RG58 cable is used to form a choke balun by making one pass through a snap-on ferrite core (Model TKK  SFT-72SN - Type 43 ferrite material).  

A toroidal core made of 43 material could also be used, and may be easier to obtain.

The RG58 was then carefully terminated onto the driven element using solder tags and No. 4 x 1/4 inch stainless steel screws (2.9 mm head x 6.5 mm thread).  Plenty of white CT1 sealant was then used to secure and seal the element to the enclosure, and allowed to set fully.

The RG58 tail was joined to RG213 coaxial cable using couple of PL259 cable connectors and an adaptor.

The connectors were then wrapped in a small piece of 10 mm foam (I used carpet underlay) and the connectors and foam were inserted into a plastic tube 150 mm in length.  The plastic tube had been cut from a length of plumbing pipe marked 'FloPlast 1.25/36mm ABS 1.8 mm'.    More 10 mm foam was then wrapped around the coaxial cables and pressed into the tube to partially fill the space within the tube, leaving about 20 mm clear space inside each end of the tube.   The foam provides a friction fit for the join so that the tube is held in place while reducing the amount of sealant needed to fill the space.  The two ends of the tube were then filled with white CT1 sealant to provide weather protection.

The 20 mm x 20 mm aluminium boom was reinforced at the point where the mast clamp passes through the boom using a pair of 2 mm aluminium plates held in position using No. 8 x 3/8 stainless steel screws.    The plates also deal with a design or manufacturing problem with the boom supplied by Blake UK.  This problem results in oversized holes being drilled through the boom, and a sloppy fit when using the supplied mast clamp. 

4nec2 Predictions

Further experimentation
The maximum gain obtainable from a conventional Yagi antenna is largely determined by the spacing between the Reflector element and the final Director element.  Therefore, the longer the boom, the greater the maximum gain that can be achieved.

By inspection of the above dimensions, it did cross my mind that some of the element spacings, and the element lengths, are quite short.  Therefore, I wondered whether it would be possible to create an antenna having fewer elements (but the same boom length) as the above 12 element antenna, and end up with a new design that offers similar characteristics in terms of forward gain, wide bandwidth, and low SWR.