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[ Previously GW4ALG (QRT in February 2007) ]

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144 MHz Yagi Antenna

This 7 element 2m Yagi antenna uses parts from a 6 element VHF broadcast band receiving antenna (Model FM6) sold by Blake UK Limied.   The FM6 is a well-constructed antenna that uses a folded dipole driven element.  

In my Yagi design, the original folded dipole driven element used by Blake has been replaced with a conventional half wave dipole.   This change meant that I could use the unused element support from the folded dipole, plus an additional length of 13 mm aluminium tube, to add a seventh element to the antenna.

The photo below shows the general arrangement once the Blake VHF FM antenna has been modified, with elements shortened for 2m; the feed point modified; and with the reflector and driven elements modified to be adjustable in length.

 

The next photo shows the feed point.  A printed circuit board assembly that is supplied with the Blake FM6 has has been removed and solder tags used instead.  RG58 coaxial cable was used as the feeder, with a couple of FT50-43  ferrite toroidal cores to reduce current flowing on the outside of the coaxial cable. 

A Hair Pin match is used to match to the driven element to 50 ohm coaxial cable.  I probably haven't got the hair pin length or the length of the driven element exactly right, but this antenna does work well.  

 

A further choke has been provided near to the feed point by using a clip-on ferrite choke TKK Part Number SFT 72SN.

 

One problem with the design of the original Blake antenna is the way the elements are mounted to the boom.  The element is held to the boom using a two-part plastic insulator.  

The part nearest the boom is Blake Part Number: PL-2700-BLACK (where the product description is VHF 20mm SQ x1/2" ELE CLIP BLACK BOTTOM).   The top part (part number not known) is inserted between the element and the fixing.   Although the bolt is earthed at the boom, the element will have an intermittent earth connection to the boom via the threads of the bolt.  That is, in the Blake design, the element is neither reliably insulated from the bolt, nor is it reliably earthed.

My solution was to remove the top part so that the element is reliably earthed via the fixing.

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The following photo shows how a Jubilee clip has been used to create an adjustable element by using aluminium tube having an OD slightly less that the ID of the main element.   Slots were cut in the larger diameter tube to allow the Jubilee clip to positively clamp the inner tube in place.

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'. 

The elements are made from 13 mm diameter round aluminium tube which are bolted onto a 20 x 20 mm square aluminium boom.  The length of the boom is about 2060 mm.

Element Length (mm)  

Position (mm)

Reflector  

1030 30

Driven Element   

1005 286

Director 1   

940 420

Director 2   

925 800

Director 3   

915 1140

Director 4   

896 1560

Director 5   

883 2036

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