Antenna Kit Building Instructions

EFHW

An end-fed halfwave antenna is a versatile and effective antenna that can usually be used on multiple bands without a tuner. This kit is a very small, lightweight and minimalistic solution expecially for portable use for POTA, SOTA, IOTA or other situations when you want to pack as light as possible and deploy the antenna in just a few minutes.


The following steps explain on how to assemble the kit. The time required to build it depends on your skills and experience but is generally less than an hour.

A bit of theory: principle of operation






A antenna wire that is a halve wavelength long has an approximate impedance of 2500 ohms - outer factors like the height above ground, terrain structure and material etc. have an impact on the exact impedance. Don’t worry to much about it as it still works just fine with SWRs below 1:2.

In order to adapt the 50 ohms impedance of the koax feedline to these 2500 ohms, an auto transformer is used to step up the impedance. An auto transformer is a transformer without galvanic separation of the two windings, which is not needed in this application. The step-up ratio is determined by the ratio of windings squared. Please refer to the table below for a few examples of how to wind the auto transformer:

# primary

2

2

2

3

3

# secondary

14

16

18

21

24

turn ratio

1:7

1:8

1:9

1:7

1:8

impedance ratio

1:49

1:64

1:81

1:49

1:64

output impedance

 2450 Ω

3200 Ω

4050 Ω

2450 Ω

3200 Ω

You might have noticed the capacitor across the input. This additional capacity is optional and might help to lower the SWR on higher frequencies. Install it if you intend to often work these bands regurarily and if you feed comfortable soldering on such a tiny capacitor. Now on to the kit build!

Step 1: Prepare magnetic wire

Straighten the magnetic wire, make sure there are no knots, loops or dents in the wire. After that, please arrange the wire as indicated in the diagram below and gently twist the parallel parts as indicated. Do not overtwist, a slight twist is enough to keep the two wires closely together.

Step 2: Winding the toroid

The auto transformer is built with a ferrite toroid, that you have to wind with enameled copper wire. Please wind them according to the diagram on the left side. Every time the wire goes through the center (hole) of the toroid is counted as a turn. Please note that the turns of twisted wire contains turns of primary and secondary windings (e.g. the picture on the right contains 2 primary and 14 secondary windings - the first two windings are shared between the primary and secondary side.

Step 3 (optional): Solder the capacitor

Please solder the included 150pF SMD capacitor onto the pads marked with C1 on the top side of the board.

Step 4: Solder BNC connector and toroid to the board

Please cut the magnetic wire on the toroid to the required length and remove the isolating coating (enamel) at the ends either mechanically (fine sandpaper) or with head (heat up your soldering iron to its maximum temperature, put some tin onto its tip and ‚burn‘ the coating - this might take 10-20 seconds. Please make sure to do this only in a well ventilated room and to not inhale the fumes).

Now solder the BNC connector and the finished toroid onto to the top side of the board at the indicated locations (see picture below).

Step 5: Connect antenna wire

Finally, put your antenna wire through the top-rightmost hole from the top side, turn it and push it up through the hole to the left and then solder the end into the pad with the antenna sign. The two loops through the boards serve as strain relieve to the solder connection. Repeat this optionally with a counterpoise wire on the left middle side of the boards.


You’re done now building the kit. Happy DX!