This compact dual-band Yagi antenna was built for portable use and has 4 elements on 2m, 7 elements on 70cm, and a single 50 ohm feed point for both bands. 

On 2m, the antenna provides an estimated gain of 9 dBi, with an SWR of less than 1.3:1 between 144 and 146 MHz. 

On 70cm, the antenna provides an estimated gain of 10.5 dBi, with an SWR of less than 1.5:1 between 430 and 437 MHz.

50 ohm coaxial feeder terminates on the 2m driven element via a choke balun.  The 70cm driven element is mounted close to the 2m driven element and acts as a parasitic ('open-sleeve') radiator.

All the elements are made from 0.5 inch OD (13 mm OD) round aluminium tube.    The boom is 1 m in length and is made from 20 x 20 mm square aluminium tube.   This underside view of the antenna shows the general arrangement.

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 M5 x 50 mm zinc plated bolts and wing nuts, with a star washer under the bolt head. 
 

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, rubber plastic'. 

The following photo shows the RG58 coaxial 'tail' exiting the underside of the feed point via a sleeved grommet.  To reduce RF current flowing along the outside of the feeder, the RG58 tail is used to form a choke balun by making two turns around a snap-on ferrite core.  A plastic tube filled with white CT1 sealant provides weather protection to a spliced joint between the RG58 tail and the main length of RG213 feeder.

The snap-on ferrite core used for this project was a 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 antenna dimensions were initially estimated through the iterative use of EZNEC antenna modelling software.  I am very grateful to Roy Lewallen W7EL for generously making his excellent modelling software freely available.  Some final adjustments to the 2m Reflector and the two Directors were then required to set the resonant frequency to 144.5 MHz.  The following table details the 'as built' dimensions.

The EZNEC files that I used to help me determine the dimensions of this antenna can be downloaded from here:
https://www.4alg.uk/radio_g/qrp/2m70cm_4+7_2m.ez
https://www.4alg.uk/radio_g/qrp/2m70cm_4+7_70cm.ez
Note 1:  The above files show the 'as built' dimensions in the 'Wires' table.  
Note 2:  I was unable to use EZNEC to precisely model this VHF/UHFantenna.   Although the program put me on a springboard to creating this fine antenna, the EZNEC SWR predictions do not agree with the SWR measurements indicated on my Welz SP400 SWR meter.

The photo below shows the feed point at the 2m driven element.  The plastic enclosure is a grey project box Model PX-2 having overall dimensions of about 130 mm (L)  x  65 mm (W)  x  45 mm (H).  The enclosure is mounted on the boom using screws and some large 'penny washers' (sometimes called 'repair washers').   A length of wood dowel was used to secure the two halves of the driven element to each other using super glue.  The element was then held in position using more super glue where the element passed through the enclosure.  Once the super glue had set hard, the RG58 cable was passed into the enclosure via a sleeved grommet before being carefully  terminated onto the driven element using solder tags.  Plenty of white CT1 sealant was then used to secure the element to the enclosure, and allowed to set fully.

The lid of the enclosure was then screwed in place and several layers of insulating tape were applied to ensure that the lid was splashproof.

Results - 11th June 2024

My first contacts with an earlier version of this antenna were on 11th June 2024.  Despite the flat conditions, I had a great time operating /P during the 'Two Metre Tuesday' CW activity.  

Operating from Gloucestershire (IO81RU), accompanied by Tony G4WIF, I started the session running QRP at 5 watts.  I put out a few tentative CQ calls, and back came Marc F6IIV (near Lannion, 334 km).  Marc is a keen supporter of Two Metre Tuesdays and was running 10 watts to a 9 el Yagi.  This QRP QSO was my first QSO using the antenna, and I was delighted!

I then increased power to 40 watts and had some fine 2-way QSOs with Ken G4RHR (Felixstowe, 265 km); John G3YPZ (Long Sutton, Spalding, 210 km); and Dave G0DJA (Bolsover, 175 km). 

Finally, I was delighted to make my first ever CW QSO on 70cm; and my first 70cm QSO using the new dual-band antenna. John G3YPZ (Long Sutton, Spalding, 210 km) and I had arranged a sked for 70cm during our earlier 2m contact and, at 20:04 UTC, I could hear John on 432.050 Mc/s.  Running 10 watts, I sent John 429, and received 519. 

That day, the final 70cm CW QSO was the icing on an already rich cake!

Results - 18th June 2024

During this Two Metre Tuesday operation on the evening of 18th June 2024, CW contacts were made on both 2m (QRP) and 70cm with Paul M0GSX located near Dudley (90 km).   On 2m CW,  I also worked John G3YPZ (Long Sutton, Spalding, 210 km); Phil G4WUX (Stafford, 120 km); Dene G4ZOI/P (Grizebeck, Cumbria, 267 km);  Dave G0DJA (Bolsover, 175 km); and a 2-way QRP contact with Ken G3LVP (Cheltenham, 34 km).