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Antenna No 63

 

2 element Yagi for 4m

 

I had a requirement to build another 2 element 4m Yagi based on the excellent model by DK7ZB, see Antenna No 45 for my earlier prototype.  I extrapolated his measurements to suit 8mm aluminium tubing and I used a suitable ABS project box to house the coax impedance transformer.

 

This antenna will be used in HP for field portable operations and in VP for tests with hams in my region.

 

 

Simulation of the antenna's horizontal pattern.  F/B is about 15dB.


 

 

The element diameter is 8x1mm and the boom measures 20x20x1.5mm.  The design is of an open dipole with Z +- 28 Ohms so it will require impedance transformation to a 50 Ohms feedpoint.  A quarter wave parallel section of 75 Ohms teflon coax (RG-179) is used for that purpose.  The RG-179's Velocity Factor is 0.70 so two sections of  0.75m (750mm) are required for 4m band frequencies.  Note that I added 10mm either side to solder the center coax conductor (stripping 5mm tip leaving 5mm isolation).

 

The high-impact ABS project box (model G331) measures 115x90x55mm.  I needed a rectangular model to fit the coax impedance transformer inside. 

 

To achieve a good looking construction the trick is to precisely align the [centre of] dipole tubing with the [centre of] saddle clamp inside and furthermore position the saddle clamp to fix the box onto the boom.

 

I used a reclaimed professional N-socket.

 

The elements are held in place by polypropylene (green) saddle clamps matching the 8mm tubing diameter.  The clamps are held onto the boom by stainless steel M6 hexagonal socket head cap screws.

 

Two sections of RG-179 are held parallel by miniature tie-wraps.  I used plenty of miniature tie-wraps about every 25mm.  I originally planned to use a plastic tube to support the coax but this was not necesssary, just carefully loop the coax to fill the gap between the open dipole and the N-socket.

 

The following dimensions were extrapolated from DK7ZB's original design, adjusting for 8mm tubing:

 

 

Element

Position (mm)

Length (mm)

Dipole

0

2095

Director

440

1950


 

Note: my dipole is actually 15mm shorter because of the construction method which uses a section of Cu wire to reach the open dipole in the PP clamp.

 

 


 

 

Antenna test / measurements 14/04/2016.

 

 

The 2 element 4m Yagi fully assembled for testing.

Antenna with centre mount on a fiber mast, ready to erect.

ABS box with coax impedance transformer (RG-179) and polyethylene saddle clamp.

Antenna in H polarisation clamped to a fiber mast at 5m height, test position.

SWR curve in H polarisation.

Antenna in V polarisation clamped to a fiber mast at 5m height, test position.

SWR curve in V polarisation.  SWR shifted by 50kHz flipping from H polarisation.


 

 

Construction:  the dipole elements were initially 25mm longer than calculated.  I mounted the antenna in horizontal polarisation at about 5m above ground level on a fiber mast.  A low loss test coax ran "behind" the antenna.  Initial resonance frequency was around 66.70MHz well below the 4m band, as might be expected.  I cut back the dipole element taking about 5mm off each side at a time whilst re-checking SWR.  This resulted in measurements of 69.050, 69.200, 69.500, 69.950Mhz for lowest SWR.  I cut a final 5mm off which yielded in a nicely symmetrical SWR curve on 70.150Mhz, SWR of 1.06.

 

 


 

 

Update 24/04/2016.

 

The 2 element Yagi is now in use for daily QSO's on 4m FM.  The antenna is mounted onto an insulated mast to avoid unwanted reflections.  I am using an 38x2mm diameter fiber/polyester tube 3.0m long, inserted 0.5m into an aluminium tube.  The antenna height is now 10m agl.  SWR on 70.400MHz was measured at 1.2

 

 

The 2 element Yagi in VP, about 4m above the roofline.

Using a 38mm PP saddle clamp to mount the Yagi onto a fiber/polyester tube.


 

 

Initial observations are positive: one ham about 45km away (ondulating terrain over two hill crests) is up by 4dB and the FM signal is now almost hiss free.  Other hams about 30km away (ondulating terrain over one hill crest) report my signal up by 2dB to 4dB. 

 

 


 


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