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Antenna No 56

 

4m Dipole [centre fed]

 

Below are construction details for a 4m open dipole intended for outdoor before-the-mast mount.  This thread is more about construction materials and techniques for such outdoor antenna than about calculating a dipole for 70MHz.

 

It can be argued that for indoor (attic) use a "piece of electrical cable", spliced and simply taped to the coax would do, but in our cold and wet climate this will not give satisfactory results when mounted outdoors.

 

Quite often it is not possible to mount an end-fed 4m antenna because the 2m/70cm collinear occupies that space and many owners do not want to put up a second mast.  In such cases a before-the-mast mount may help to install a 4m outdoor antenna on an existing mast.  The antenna can be in horizontal or vertical polarisation depending on how the U-clamp/bracket is mounted.

 

It makes a handy [test] antenna perhaps to try out a new band.  The construction techniques below are exactly the same for 6m, 4m and 2m centre-fed dipoles and can be applied for other bands e.g. air band, FM band.  I generally use 10mm diameter aluminium tubing for a dipole on VHF frequencies, however, with the saddle clamps shown below you can easily scale element diameter from 6mm to 16mm.

 

 

Principle diagram of the open dipole with before-the-mast mount:

 

 

Note: vertical axis is not to scale.  Dimensions are for 4m band.


 

Materials that I tend to use:

 

 

ABS box G366.  Dimensions 80x82x55mm.  O-seal and stainless steel screws.

Use a step drill for accurate concentric drilling.

A completed centre box for the dipole.

PP saddle clamp, M20 cable glands, 10mm aluminium tube, N-chassis socket.

M12 cable glands, 6mm aluminium tube, coax RFC made from RG-188.


 

Other materials needed:

 

 

PA [polyamide] cable glands, here M12, M20 and M25 shown.

PA [polyamide] saddle clamps. 

PP [polypropylene] saddle clamps.  M6 socket cap screws, washers and nuts.

Fiber/polyester, PMMA or POM tubes or rods used as insulator.

Alignment of the cable gland and saddle clamp is crucial.

The stainless steel brackets I use for 50mm masts.  Thread is M6.

The U-bracket in position.  Flip it 90 degrees to change polarisation.

RG-188 miniature coax [teflon dielectric] to make an RFC.

Do not forget an end cap and always mark the 'hot' side of the dipole.


 

Finally, the completed product:

 

 

The 4m centre-fed dipole ready for deployment.

The support arm is approximately 0.50m long.


 

The antenna in test position on a metal mast:

 

 

The 4m centre-fed dipole at 7m agl on a metal mast.


 

 

I hope you found the many photographs of help.  Good luck with your construction!

 

 


 


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