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End Fed Half Wave Antenna and ferrite rod matching unit for 40M

by ZS1JHG

The antenna consists of 20M or 65.5 Ft of insulated stranded copper wire about 20 AWG or

1mm, if you are using it as a portable antenna. Fit a ring connector if using a wing nut

antenna mount on your matchbox or U connector for power lug type connectors.

Most articles require an iron powder toroid as your core. I have used a ferrite AM antenna

rod which are more easily obtainable.

For a frequency of 7.100 Mhz you are looking for about 10.8 uH of inductance and 46 Pf of

capacitance according to calculations. However I used 11uH of inductance (close enough)

and found that using the ferrite rod only 6 Pf of capacitance was required. So using 22 AWG

formvar/enamel coated wire wind on about 27 turns onto the ferrite rod after having

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wrapped it with insulation tape. If you have an inductance meter (MFJ 259B will do) then

check for 11uH inductance.

You can secure your windings with insulation tape, small cable ties or hot melt glue. This is

your Secondary winding completed.

Now to make your Primary take some insulated solid copper wire (easier to wind stranded

will also work) and wind on top of your Secondary coil (27t) three turns, spread apart the

turns and secure with more insulation tape or hot melt glue. This gives you a 9.1 turns ratio

coupler.

The capacitor is made from RG58CU. 50ohm coax or RG174 if you wish to make your

matchbox very compact (100W rating)

Measure off 2.75 inches of coax and this will give you about 6Pf of capacitance based

on 30.8 Pf per foot for RG58CU and RG174, once again you can check with a capacitor meter

or your MFJ 259B. So as to make it easy to attach the coax in your enclosure prepare the

ends as per the photo. The non connected end has sealed with insulation tape, hot melt

glue or glue lined heat shrink.

Connect up as per the photo/diagram and mount into an enclosure.

A short counterpoise of about 0.05 wavelengths i.e. 6ft is attached to the earth side of the

SO-239 and a banana plug/socket can be used mounted on the enclosure for this purpose.

So the wiring goes as follows LHS of Secondary coil to earth side of SO-239, RHS of

Secondary to Antenna socket/connector.

The LHS of the Primary goes to SO-239 earth side and RHS to the centre connector of

your SO-239.

The coax capacitor is connected braid to SO-239 earth side and centre connector to Antenna

Socket/connector.

You should get an SWR of below 1.5 and impedance closer to 50 ohms with the antenna

at a reasonable height.

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Final reading with variable cap, replaced with 6Pf coax RG174 capacitor 2.75 inches long

and the same readings.

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Note the capacitor in the diagram is shown at the calculated value of 46Pf, in the matching

unit built using a ferrite rod only 6Pf of capacitance was required and 3 turns of the primary

coil were also needed.