Highfields Amateur Radio Club
Constructors Corner.

450Ω Ladder Feeder Dual Band Antenna.
Design: Dave, GW0ROL.
Article: Brian, MW0GKX.

This design (in diagramatical form) landed on my desk courtesy of Dave, GW0ROL, who has used one of these to good effect while holidaying in the South of France some time ago. On 2 Metres, running 5 Watts, he worked a through a repeater in Sardinia, over 300 kM away across the water.

It is made from 450Ω Ladder Feeder and, as such, is very slim in profile. There is a phasing link, but don't be put off by that, the construction is simple.

Brian.

Parts List:

Method: (Please note the diagrams are not to scale)

diagram of feeder layout

Take the ladder feeder and, in the first "rung", make a hole in the centre large enough to take the nylon string.

Measure 1470 mm (57 7/8 inches) from this end "rung" At this point remove the insulation from both side rails, bend the 2 feeder wires inward and solder together.

Measure back from the joined end 67 mm (2 5/8 inches) and strip about 10 mm (3/8 inch) of insulation from one rail only at this point.

On the other rail measure back 457 mm (18 inches) from the joined end and cut the rail the oposite rail to the little stripped "window" you just made. Remove about 25 mm (1 inch) of the rail (you could remove the rest of the rail from this point but you would lose strength for the mounting hole).

See detail diagram on the right for how it should look.

detail of phasing link

Take the piece of stiff wire and bend and trim to fit as per detail diagram (left) and set this aside for now.

Prepare the one end of the coax by removing about 25 mm (1 inch) of the outer sheath.

Tease out the braid into 2 equalish amounts. Twist the 2 braid wings to consoildate the loose strands.

Position the coax to the centre of the joined ends of the ladder feeder, wrap the twisted braid wings around the joined wires and carefully solder the braid to the ladder wire.

Wait for it to cool, trim any loose whiskers and cover the bare soldered joint with insulating tape, leaving a space to work with the centre conductor.

detail of phasing link

Remove the dilectric (centre conductor insulation) from the coax centre conductor to a point just above the joined ends/braid (you need it to be close but not shorting), solder the phasing link to the coax centre conductor and trim off any excess centre conductor.

Solder the other end of the phasing link to the stripped "window" 67 mm up the feeder, with the 67 mm part of the link laying along the ladder rail without the "window" (see detail diagram, left, the red is the braid of the coax) and cover with tape when cool.

Affix the phasing link to the ladder rail opposite the "window" with the small cable ties (or some tape). Cover the coax end completely with tape.

Attach the nylon string to the hole in the top.

Fit a coax plug suitable for your rig to the other end of the coax.

Hang, using the string, from a branch, curtain rail or other suitable support, but not too close to metal objects.

detail of hook

Tuning should not be required for use on the 144 - 146 MHz or 430 - 440 MHz amateur bands if the measurements are followed.

If using outside some varnish on the top ends and the cut out section of the ladder feeder would be a good idea, as would varnishing the coax end and the area around the "window", this should stop water ingress.

As made the antenna is very portable, lending itself to holiday use or stored ready for emergency use.

Optionally, for full time use at your home station, you could make a PVC pipe cover (Dave suggests 1 1/4 inch waste pipe) and seal it in there.

Dave tells me that he doesn't use nylon string. What he does is start with ladder feeder longer than required, trims the used rail down to size, trims out the rungs and bends the unused rail over to form a hook. See detail picture left.

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