6E beam for 70 MHz

70 MHz has become increasingly popular since Icom added it to some of their new transceivers. Since owning an Icom 7100 and enjoying the UKAC and portable events I decided to make a beam for the band.

I had some aluminium from a previous order at aluminium warehouse, and so decided to look around the web for a suitable design. Martin K7MEM website takes you to a javascript page that allows you to specify what material you have available. This is quite a flexible approach and gives you the ability to make something without being too prescriptive in the early design.

In my case, I had some lengths of 8mm and 10mm tube and a 25mm box for the boom. The website allows you the flexibility to specify the desired frequency and then select the required gain or boom length. Thankfully the design can be created in both metric and imperial measurements, you just specify the sizes in the design.

Program output.

The current design is an antenna for 70.2 MHz. It has 1 Reflector, 1 Driven, and 4 Director Elements. Estimated Gain is 8.917 dBd.

Antenna Dimensions

Cumulative
Spacing (mm)

Element

Element
Length (mm)

Zero

REFL

2094.58

854.11

D.E.

2068.64

1174.4

D1

1933.49

1943.1

D2

1918.83

2861.27

D3

1901.45

3928.91

D4

1884.12

Insulated elements

This is specified at the time of the design, it is interesting to see the design with and without insulated elements. Insulating the elements from the boom has the effect of shortening them slightly, but why not have a play with the calculator and see the difference. The insulators I used were 4mm ABS plastic, cut into strips but you can use anything to hand.

Driven element

Often the most difficult parts to complete, as it requires a split and insulation from the boom.

Options.

1/ Place a small plastic tube inside both parts of the driven element, thus making them rigid and then mounting this to the plastic insulator on the boom.

2/ Fix the driven element directly to the plastic insulator using 2 plastic clamps on each section. This should provide enough support to stop the element sagging or twisting.

3/ Commercial VHF style dipole centre.

Parts list / Supplier options.

Aluminium warehouse can supply lengths of aluminium in both imperial and metric sizes.

Black Plastic Nylon P Clips Mounting Cables Tubes Pipe Brake Motorcycle Car. These can be used to fix the elements to the insulator, they are available in various sizes. The clips allow the elements to be removed by simply pushing/pulling the element into the clip, and are therefore ideal for portable use.

Something more robust would be needed for a permanent solution.

 

 

 

Stauff clamps can be sourced online. These are more expensive than other options, but ideal for a permanent antenna build.

 

 

 

 

VHF dipole centre available from your local rally, eBay or junk box

 

 

 

Element insulator, search using 4mm ABS plastic sheet.

 

 

 

Closing thoughts

The antenna is a little quirky, the dip shows a good match at 70.200 and the bandwidth is wide enough to cover the entire 70 MHz band. The dip shown on the RigExpert is a little nonconventional, but it certainly has gain and good F/B ratio from the testing on site.

Alternative designs by DK7ZB

Audio clips from testing.

GM4NFC Alex 520 KM

GI4SNA David @530 KM

July 2017 70 MHz Trophy Cup.

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© 2015 Dave, M0TAZ