VE3CVG's 900 MHz 7 Element Plumberís Delight Yagi Antenna

NOTE: Refer to other areas of the Antenna area to describe fastening of elements to the boom and construction of the mounting plate. These are common to all my antennas and so are only described once.

All round tubing elements were cut with a plumberís tubing cutter available at most hardware stores. The aluminum boom can be cut with a sharp hacksaw.

All aluminum that I used is T6 hardness.

I purchased my aluminum at Ridalco, 1551 Michael Street (near St.Laurent and Belfast). Total aluminum cost was under $20 (incl tax).

The dimensions that I used are:

900 MHz dimensions

This antenna is short and light. Considering its frequency, you should end-mount it. You can use exactly the same boom-to-mast mounting arrangement as for centre mounting. When cutting the boom, be sure to leave enough square boom, at the director 5 end, to allow you to fasten the element to the boom. I suggest about 20mm in this case. At the reflector end. I'd suggest a little over 75mm spacing between the reflector and any mast pipe or tower leg (that way, the mast or tower acts as a 2nd reflector when positioned for vertical polarization (my theory anyway). An overall boom length of about 610mm or about 2 feet should work OK. Always start measuring from the farther most director and work towards the reflector. Use a marker pen to carefully mark everything first.

Use 3/4" square tubing for the boom. A nice feature of 3/4" square tubing is that it slides just perfectly into a 1" square tube. This is not a requirement in this design but you may find it useful to note.

The parasitic elements are 1/2" round tubing and are fastened to the boom by attaching to 1" square plates using pop rivets. You experiment with the right material for the driven element.



The details of the driven element

This little antenna uses a trombone feed driven element. The driven element mounts on the opposite side of the boom from the other elements, with the boom centred inside the C. This ensures that the business part of the driven element is nearly in line with the parasitic elements. After construction, the driven element is fastened directly to the metal of the boom either using rivets directly, or using a mounting plate just like the parasitic elements.

I suggest making the driven out of 1/4" copper rod which is a lot easier to bend than T6 aluminum. Using copper also moves the point of corrosion and electrolysis away from the feedline/element junction and you can solder the feedline direct to the copper (centre conductor) and the boom. You can also use rigid plumbing copper (buy the thin stuff) and 1/2" fittings. Better yet, you can use 3/8" copper tubing if you can find some 3/8" elbows, but you may have to use smaller rivets for fastening to the mounting plate. Unless you know how to bend tubing, it is really difficult to make the element out of any kind of soft tubing, it collapses at the ends. Aluminum rod works too, but you are back to a corrosion problem with dissimilar metals.

The easy way is to connect the feedline is to solder the centre conductor to one of the open ends of the C shaped element; and the shield to the element at the boom. Tie-wrap the feedline to the boom.

Whichever polarization you will be using, if you end up using tubing rather than solid rod, you must drill a small drain hole at the lowest point of the element.

900 driven element details

This antenna should look like an electrical short when you measure across the feedline.

NOTE: Some of the metal experts out there may know how to bend copper tubing into a nice C. The use of aluminum for the driven would present a potential connection problem but there are plenty of solutions to this.




Closing Comments:

This antenna design is cloned from a commercial antenna which is robust, easy to build, and has a broad bandwidth.

Not tested yet.

 


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This page was updated on August 28th, 2002