QRP End Fed for 40m/20m/10m

This antenna build was strongly inspired by the well known PAR end fed antenna.   The matcher box uses a T82-43 Fair-Rite core.  The primary side (50 Ohm) should have an inductive reactance of at least 4 times the input impedance for a broadband transformer.  With the chosen core, a primary winding of 3 turns has an inductive reactance of 183 Ω @ 7MHz.  This is a little low, but as the design has to work on 10m, its an OK compromise.  The secondary winding has 24 turns, and the 3 turn primary is wound bifilar with the secondary .  The turns ratio of 1:8 gives a secondary impedance of 50 x 8² or 3200 Ω.  A 150pF capacitor was put in parallel with the primary.

The secondary connects to a 10.4 metre length of wire which then connects to a choke / 40m loading coil.  The other side of the  coil connects to a 2 metre length of wire.  The coil tunes the 40m match.

A short length of 20mm OD PVC electrical conduit was used for the coil former.  52 turns of 24 gauge wire is wound on the former.  At each end of the winding, two small holes were drilled to anchor the winding ends.  A plastic plug was fitted into each end of the conduit to terminate the coil for the antenna connections.

The plastic plugs were drilled in the centre so that a 3mm screw 25mm long would fit tightly.  The plastic plugs were made from Polymorph Pellets from Jaycar.  The pellets were simply heated with a heat gun until they were soft and clear and then pushed into a short length of 20mm conduit.  When they cooled, the resultant plug was pushed out of the conduit and sawn off to the required two lengths. The plug sections were then fitted with screws.  The coil is encased in heatshrink.  The ends of the antenna wire that connect to the coil were soldered to 3mm brass standoffs and then covered in heatshrink.  The wire ends can then be screwed onto the loading coil.

The whole thing was connected up and the shorter wire end connected to an 8m squid pole as a sloper antenna.  The VSWR was tuned on 20m by adjusting the length of the 10.4 metre section.  Once this was OK, the VSWR of the shorter 2m length was adjusted for best VSWR on 40m.

Matcher

The antenna matched very well on 7.090 MHz, 14.2MHz and 28.5MHz.
To check the effects of the antenna positioning, it was then set up as an inverted V with each end about 1 metre off the ground.  The tuning was still good.  The antenna was then set up using a 4.5 metre squid pole, again as an inverted V.  The VSWR was slightly worse, but still fine.

Curious to see what the power handling was, the matcher high impedance output was connected to a similar matcher using a larger FT140-43 core that transformed the impedance back to 50 Ohms.  The power was measured from this matchers 50 Ohm side connected to the transceiver while the larger core 50 Ohm side was connected to a power meter.  The power was increased from 10W to 50W in 10W steps at 7MHz.  There was very little compression of the output at 50W, but the smaller core was heating up.  The loss was more or less constant up to about 40W.  The power transfer was then tried at 14 and 28MHz.  28MHz had a little less loss.

Sweep using K6BEZ Sweeper

Sweep from 1-10MHz using the homebrew K6BEZ antenna sweeper.

Version 2

Here is another version of the antenna.  This time I used the electrical tubing for the coil former and the winding real.  It came out fairly well.  To keep the bulk down, I used lighter 17 strand hookup wire that has an outside diameter of about 1.14mm and 0.14mm strands.  The coil was wound with 51 turns of 0.7mm wire as I had this on hand.  The inductance must be a bit less as it needed about 2.2 metres for the short antenna length on the far side of the coil.

Coil in centre of tube

Antenna wrapped up

20m sweep

40 m sweep

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