900 MHz 9 Element Plumberís Delight Yagi Antenna

Picture of 9 element 900MHz antenna

NOTE: This antenna differs in element mounting, in that elements are solid rods and mounted through the boom. Refer to other areas of the Antenna area for construction of the mounting plate. These are common to all my antennas and so are only described once.

All rods used as elements and the aluminum boom can be cut with a sharp hacksaw.

All aluminum that I used is T6 or T5 hardness.

I purchased my aluminum at Ridalco, 1551 Michael Street (near St.Laurent and Belfast). Total aluminum cost was under $25 (incl tax).

This is a fairly straight forward antenna to make, if you have the tools and you measure, mark, and cut very carefully. It does take a little time but it's is a fun project.

Boom length is 71cm or about 28". I actually used 30".

Boom is made from 3/4" T5 square aluminum tube. T5 is more than adequate for this short and light weight antenna.

Elements are 3/8" T6 round aluminum rod. You will need approximately 48" of rod.


Element dimensions and Spacing (all in cm)

Element Name
Element Length
From Front
From Last Element
D7
11.65
3.00
-
D6
11.8
9.70
6.70
D5
12.10
16.40
6.70
D4
12.20
22.50
6.10
D3
12.70
28.70
6.20
D2
12.10
34.80
6.10
D1
12.40
40.80
6.00
Driven
14.60
46.25
5.45
Reflector
17.10
53.80
7.55

Mounting the Elements

The elements are mounted thru the boom. Drill the boom carefully using a squared up drill press. Use a grinder to lightly dress up the edges of the elements or they will not fit into the hole in the boom. Once inserted into the boom, carefully centre the element. Drill a small 7/64" hole thru the boom and into the element, deep enough to accept the screw. Using a tap set, tap the boom and element to accept a 6-32 stainless steel screw. I cut the screws to 5/16" length. Carefully insert the 6-32 screw into the newly tapped hole. Tighten but be careful not to overtighten- they snap.

Note: When you are drilling and tapping the holes to accept the screws, if you choose not to drill all the way through the element, be sure that you allow for the nose of the tap. Be careful not to force the tap too deep or it will break off and you will have to build another element.

Antenna Interfacing

NOTE: Fabrication of the gamma tube arrangement and mounting of the N-type connector can be a slow process - work carefully, be patient, and it will probably work fine.

This antenna uses a gamma match. The gamma tube is exactly 1" centre-to-centre (ctc) below the driven element. The tube is 4.3cm long and nicely accepts the gamma element. The gamma rod (which actually forms a capacitor in series between the tube and the connector) is precisely 2.2cm long, from the centre of the hole (which connects to the N-type connector) to the end (however I used 4.4cm from RG8, because my centre conductor is smaller diameter, hence the capacitance would be smaller). The gamma rod was originally 3/16"d or .5cm (again - the diameter of mine is smaller than the original). The gamma rod is insulated from the gamma tube by a dialectric which nicely fits inside the tub and which accepts the gamma rod. In my situation the dialectric was actually the material around the centre conductor of RG8 coax.

900 Mhz gamma match

The gamma tube is held to the element by a mounting plate which starts out as a piece of 1/4" thick aluminum flat plate, 3/4" wide. Measure 3/8" from the end of the plate and at the mid-width of the plate place a dimple for drilling a 3/8" hole. This will eventually slip over the element and hold the gamma tube at precisely 1" from the element.

For ease of handling, the gamma mounting plate should not be cut off from the aluminum stock until all other work with it is complete.

At 1" from the dimple for the hole for the element, place another dimple for the hole which will hold the gamma tube.

The gamma mounting plate, element and gamma tube must be held rigid. Rigidity is accomplished by installing 2 set screws: one for the gamma tube and one for the element. Using a drilling clamp and drill press, carefully drill a set screw hole through the 1/4" aluminum until it intersects the hole for the element. Do the same for the gamma tube hole. Tap the 2 set screw holes. I used 4-40 hex head screws that I bought at the hobby shop for $1.79 (for 4).You may have to touch up the holes for the gamma tube and the element as it does not take much to distort these holes and make the fit difficult. Try using a fine round file, or fine metal sandpaper wrapped around a drill bit, if the fit is too tight to work with. Ideally you want a snug but workable fit between the mounting plate, tube and driven element.

Once all the drilling is completed, you can cut off this mounting plate with a hacksaw, at exactly 1.75" from the end.

The gamma tube protrudes (is tightened so that it protrudes) 9mm or .9cm from the mounting plate, towards the connector.

The gamma mounting plate is fastened on the driven element, 2.9cm ctc from the boom, for starters.

Mounting the N-type connector

This is the messiest part of the construction. I think the easiest approach would be to start with some 1/16th inch thick, 2" angle aluminum. Mark things out, do the drilling , then cut it down to size. I fastened the mount to the boom with 2 pop rivets and then fastened the connector to the mount with 4 small machine screws.



Closing Comments:

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.

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

This design is cloned from a commercial antenna which is robust, and has a broad bandwidth. Tuning range is approximately 900-950MHz and the gain for 9 elements is 10dBd.

Not field tested yet but I don't expect any problems. It tuned up in the basement in a few minutes.

 

Frequency/SWR curve supplied by Gary N1TLL

Frequency/SWR curve of 9 element 900MHz antenna

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This page was updated on March 4th, 2003