Club Presentation Topic – Doublet Aerial & Balanced Matcher

At our meeting of the 21st September, I delivered a power point presentation on my experiences with building an 80m Doublet Aerial and Balanced Matcher.    The Power Point File is too big to import so I will summarise the presentation and include some helpful information.

80m Doublet Aerial

This is a basic representation of the 80m Doublet I erected.

The Doublet in it’s perfect form is way too big to fit on my suburban block, so I had to bend it a bit (It turned out to be roughly “E” shaped without the centre bit).

The centre span and feeder.  The 450ohm flat feeder is guided down to my shack through 32mm conduit which keeps it clear of the metal roof etc and allows gentle bends

The Northern end. A 2 metre dogleg here. The aerial wire is ordinary black plastic covered 7 x .023 copper wire.

The Southern end.

Please refer to these links for further information on the Doublet Aerial:

Balanced “S” Matcher

I have been intending to build this project for some time now and with the need for a balanced feed for the Doublet it was a good reason to go ahead with it instead of using a balun.    It is based on the “S” Match ideas of PA0FRI and it’s application by RZ3AE.    Instead of using the recycled ferrite tubes from printer cables I have used the items noted in the “Tube Baluns” article by OH1AYR.  These tubes, Wurth 74270057, are available from RS Components.  The following images help explain how to build the “S” matcher.

The binocular transformer. Two ferrite tubes are bound together and two lengths of RG213 are fixed inside each tube.  The exposed outside cover of the RG213 is stripped off leaving the braid exposed.

The exposed braid is separated and joined at this end of the transformer. The centre conductors of the RG213 are connected to the Inductor binding posts.

The exposed braid is separated at this end of the transformer and joined to two wires connected to the PL259 socket (Tranceiver). The centre conductors of the RG213 are connected to the Capacitor binding posts.

A surplus aircraft radio Rotary Inductor is connected across the left hand binding posts. A 500pF variable capacitor is connected across the right hand binding posts. This completes the build.

Note that the capacitor body is live and needs to be isolated with an insulated shaft coupling and plastic bolts.

The Rotary Inductor’s body needs to be earthed.

All done. Found that I could match all bands, including the WARC bands, on the Inductor terminals. The capacitor terminals could be needed with a shorter aerial.