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.
.
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 |
.
|