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144 & 432 MHz Yagi Antenna

This small dual-band Yagi antenna was built for portable use.  It provides 3 elements on 2m, and 5 elements on 70cm, with a gain of about 9.3 dBi (7 dBd) on both bands.  The SWR in the CW and SSB segments is very low on both bands.

In this Yagi design, two unbalanced delta matches are used to match the RG58 50 ohm coaxial feeder to the two driven elements.   The 70cm elements are positioned above the boom, and the 2m elements are positioned below the boom.  The centre of each 70cm element is 20 mm above the centre of the boom, and the centre of each 2m element is 20 mm below the centre of the boom.

The elements are made from 0.5 inch (13 mm) round aluminium tube.    The 1 m long boom is made from 20 x 20 mm square aluminium tube. 

Each element is fixed to the boom using a plastic moulding that I 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 60 mm zinc plated bolts and wing nuts, with a star washer under the bolt head. 

Note that plastic end caps have been fitted.  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'. 

 

 

Initial 'in the field' tests confirmed that my innovative use of the twin unbalanced delta match provided acceptable results.  The final element dimensions were optimised 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.

Element Length (mm)  

Position (mm)

 2m Reflector   

1000 25

70cm Reflector   

312 195

 2m Driven Element   

932 352

70cm Driven Element   

294 378

70cm Director 1   

288 637

70cm Director 2   

280 828

 2m Director 1   

888 895

70cm Director 3   

260 975


The EZNEC files that I used to model this dual band Yagi antenna can be downloaded from here:
https://www.4alg.uk/radio_g/qrp/2m70cm_3+5_2m.ez
https://www.4alg.uk/radio_g/qrp/2m70cm_3+5_70cm.ez
https://www.4alg.uk/radio_g/qrp/2m70cm_3+5_2m.txt
https://www.4alg.uk/radio_g/qrp/2m70cm_3+5_70cm.txt

When running SWR plots using the above .EZ files, be sure to select the Alt(ernative) ZO option on the results screen.   It is the purpose of the separate unbalanced delta matches (one for each driven element) to transform the low feed impedance at each driven element to 50 ohms.

 

The photo below shows the general arrangement for feeding the two driven elements: one driven element for 2m, and one for 70cm.  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 photo below provides greater clarity about the two unbalanced delta matches.  Note that an aluminium grounding strap is used between the centre of the 70cm driven element and the boom.  The outer conductor of the coaxial feeder is bonded directly to the strap and 70cm driven element. 

Within the enclosure, the centre conductor of the coaxial feeder is connected to two home made capacitors.  The capacitor for 70cm is about 3 pF, and the one for 2m is about 12 pF.  Both capacitors are made from double-sided copper clad board where the substrate is fibre glass.  Capacitors made this way are not suitable for high power transmitting applications because they will get warm -- even at 40 watts (100% duty cycle).   But they will be fine for my QRP purposes.  If I were to use high power with this antenna, I would use air spaced trimmer capacitors, such as 'bee hive' trimmers.

The antenna side of each home made capacitor is connected to the appropriate element using a self-tapping screw and star washer via a short length of wire.

 

In this arrangement, about 12 mm of unshielded centre conductor terminates on the home made 3 pF capacitor.   The length of wire from the capacitor to the 70cm driven element is 62 mm.  The connection point is 25 mm from the centre of the element.    

On 2m, the length of wire from the home made 12 pF capacitor to the driven element is 135 mm.   The connection point is 86 mm from the centre of the element.    

No attempt has been made to prevent currents flowing on the outside of the coaxial feeder through the use, for example, of a choke balun.   To be honest, it would not concern me if some radiation did take place from the coaxial cable, or if the radiation pattern wasn't quite symmetrical.   I'm delighted with the performance of this simple dual band antenna. 

 

Results - 11th June 2024

My first contacts with 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 432.051 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).


 

 

5 Element 70cm Extension

I have now built an extension to this dual band antenna by adding another 5 elements for 70cm (making 10 elements for 70cm in total).  When in position, the extension performs very well indeed, increasing the forward gain on 70cm to about 12.5 dBi (10 dBd) without a significant change to the SWR on either 2m or 70cm.



 

The photo below shows the five additional elements for 70cm which have been mounted on 18 x 18 mm wood moulding purchased from my local DIY store.   The plastic spacers between the wood boom and each element were obtained from Blake UK Limited.  I believe that the product description is 'VHF 20mm SQ x1/2" ELE CLIP BLACK TOP'. 

 

The left-hand side of the wood boom has been planed down so that it just fits within the square section aluminium tube of the basic antenna.   Three holes have been drilled in the wood boom such that the holes line up with the positions of the front three elements in the basic antenna.   With the 70cm extension in position, the resulting dual band antenna has the following dimensions:

 

Element Length (mm)  

Position (mm)

 2m Reflector   

1000 25

70cm Reflector   

312 195

 2m Driven Element   

932 352

70cm Driven Element   

294 378

70cm Director 1   

288 637

70cm Director 2   

280 828

 2m Director 1   

888 895

70cm Director 3   

260 975

70cm Director 4   

258 1240

70cm Director 5   

256 1510

70cm Director 6   

254 1770

70cm Director 7   

252 2000

70cm Director 8   

250 2200


The EZNEC files that I used to model this extended dual band Yagi antenna can be downloaded from here:
https://www.4alg.uk/radio_g/qrp/2m70cm_3+5_2m_extended.ez
https://www.4alg.uk/radio_g/qrp/2m70cm_3+5_70cm_extended.ez
https://www.4alg.uk/radio_g/qrp/2m70cm_3+5_2m_extended.txt
https://www.4alg.uk/radio_g/qrp/2m70cm_3+5_70cm_extended.txt