A Four Element Six Metre Yagi for 50 to 52.5 MHz

By Paul McMahon VK3DIP

Amateur Radio Magazine Dec 2011 pp 32-40

– Errata and pictures.

This antenna continues to perform well more than 2 years later, and after all that weather the VSWR and performance doesn’t seem to have changed. Obviously it could still fail in the future but so far so good. The major special bits here were the lifting of the driven element to sit on top of the boom (approx. 35mm above the plane of the rest of the antenna) to make construction easier, the use of a capacitive/DC connection between the driven element and the coax, using copper strips for a more reliable connection without weakening the element, and the use of plumbing/water conduit in telescoping sizes as the insulated boom, for cheapness.

Below are the original pictures and figures from the article as well as a couple that didn’t make the original. Hopefully they are a bit easier to see in this format.

73 Paul VK3DIP

Base Design

Element	Length (m)	Position.(m)	Diameter.(mm)
1	2.902	0	16
2	2.788	1.076	16
3	2.636	1.769	16
4	2.567	2.928	16

Table 1. Base design, position measured from element 1.

At a frequency of 51.0 MHz this gave:

· Input Impedance = 52.74 J 12.76 Ohms

· Forward Gain = 9.4 dBi

· Front-to-Back Ratio = 12.9 dB

The pattern obtained is shown in Figure 1.

Figure 1/. Antenna Pattern

Along with the overall response in Figure 2/.

Figure 2/. Overall Response.

To check the effect of displacing the driven element I exported the design to a .nec file and ran the basic single frequency pattern analysis in 4NEC2 . The Non displaced results are shown in Figure 3a, and 3b.

Figure 3a. Base Yagi Far Field Pattern 4NEC2. Figure 3b. Base Yagi main display 4NEC2.

Raising the driven element by 35mm can now be easily done in the 4NEC2 editor and the program re-run. The results for this can be seen in Figures 4a, and 4b.

Figure 4a. Modified Yagi Far Field Pattern 4NEC2. Figure 4b. Modified Yagi main display 4NEC2.

Comparison of Figures 3 and 4 shows luckily only very minor differences. The pattern is now slightly asymmetric but the numerical values of input impedance and gain have varied only by the smallest amounts.

For this run I have set the height above ground at 9 metres (because that is about where it will be in my case) and the results are shown in Figures 5a, and 5b.

Figure 5a. Modified Yagi, 9 metres above real ground, Far Field Pattern 4NEC2. Figure 5b. Modified Yagi , 9 metres above real ground, main display 4NEC2.

Building it.

Parts List

Item	Description	Quantity	Comments
Boom (part1)	40mm DWV pipe	3 Metres	Should fit neatly inside the class 9
Boom (part2)	40mm Class 9 pressure pipe	1.5 Metres	Eg. Half a 3 metre length, neat fit over 40mm DWV
Glue	Plumbing pipe glue	some	Either red or blue, I used blue.
End Caps	40mm DWV cap	2	Optional, fit last if required
Elements	16mm diam. 1mm wall, Al tube.	4 by 3 Metres	Cut to lengths given in text
Inspection Tee	20mm Inspection Tee	1	Eg. Clipsal 246-20
Hose joiner	13mm barbed pipe joiner	1	Typical black plastic, cheap irrigation fitting. Check inner diam to fit 10mm fiberglass rod.
Fibreglass Rod	10mm diam. fibreglass rod	Approx. 750mm	Off cut from earlier project originally electric fence support sold in 1 or 1.5M lengths
Conduit	20mm (gray) electrical conduit	3 by approx. 75mm	Off cuts from earlier project. A 5M length is cheapest. Caution, must fit neatly over 16mm Al tube, some do some don’t, so check first.
Shim	Copper Shim 20mm wide.	2 by 60mm	Sold in plastic packet as long life plant labels, or alternately EMC /leadlight tape.
Coax (Tail)	RG58	3 Metres	Good quality is best
Connector	N (RG58) Inline female socket	1	Could use PL259 if desired
Balun	Ferrite tubular cores	6	Jaycar part number LF1258 pkt of 6
Filler	Silicon filler etc.	Lots	If Silicon get the neutral cure type, much cheaper by the large tube even if you have to buy the gun also.
Bolts	75mm Quarter inch galvanized bolt nut washer set	2	For mounting the DE to the boom
Epoxy Putty	5minute Epoxy putty	Approx. 50mm	Sold in stick form eg. Selleys KNEAD IT

Table 2 Parts List

A cross section diagram of the centre of the driven element is shown in figure 7.

Figure 7. Driven element assembly cross section.

Figures 8 through 14 show the driven element and its components at various stages of construction. One key thing to remember is to always try fitting the pieces together without glue or silicon first, it is a lot easier to adjust spacings or hole diameters with the separate components rather than with bits glued together.