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.20140831_160423

20140908_204710The 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.  20140831_160531The 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.



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.20140831_160942

20140831_160352Curious 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.

The matcher box was also shrunk down to fit into a 50 x 38 x 20 mm enclosure.

Coil in centre of tube

Coil in centre of tube

Antenna wrapped up

Antenna wrapped up








New compact matcher box

20m sweep

20m sweep

40 m sweep

40 m sweep

This entry was posted in Projects, radio communications, SOTA. Bookmark the permalink.

34 Responses to QRP End Fed for 40m/20m/10m

  1. asphotos says:

    Very neat! 3bq

  2. Hi Glenn, looking forward to reading about the antenna’s performance on a SOTA activation. The practical design permits band changes without lowering the squid pole. 🙂 The design is ideal for SOTA and QRP activations, well done! Good luck hope the antenna performs well. Incidentally where are the black cases from? The compact size is ideal for a QRP ATU.
    Andrew, VK1NAM

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  12. Martin Hepp says:

    Hi, thanks for the nice description” I designed a nice case and accessories for this design for 3D printers. See the materials here: http://www.thingiverse.com/thing:1948711

  13. ve3ips says:

    Check out the 3D printed stuff for this antenna….http://www.thingiverse.com/thing:1948711

  14. Hi Glenn

    I have had a play around with the design using a Jaycar Toriod L15. Is this acceptable toriod to use?

    There is not a lot of data on the Jaycar web site.

    I get a good SWR on my MFJ meter and wire lengths seem to be about the same as in your article.


    • vk3yy says:

      Hi Malcolm, without knowing the specs of the Jaycar toroid, its hard to say. I would stick to the known toroid to reproduce the original performance.
      Cheers, Glenn.

  15. Paul Taylor says:

    Glenn, could you double all dimensions, so that the first piece is a half wave on 40 (20m), increase the tail (double?) and experimentally adjust the turns on the loading coil to get it resonant on 80-m, and have it work on 3.5, 7 and 14? That would make an 80m antenna not much longer than about 24.4m long, Would the amount of L required to resonate on 80 be too much? VK3HN.

    • vk3yy says:

      Hi Paul, yes, it should work as an 80/40/20 configuration, itmight take some experimenting to get the choke/loading coil working properly!

  16. Martin Hepp says:

    Paul, PD7MAA has described an 80m version similar to your proposal at http://pa-11019.blogspot.de/2012/04/149-transformer-for-endfed-antennas-35.html.

    My files for 3D-printed parts include this option:

    One caveat: When building the 80m-version, I found it much more difficult to get a good SWR for 20 and 10m as compared to the 40-20-10m version. This might be due to effects of the actual configuration (I tried a sloper on a 10m mast), or that slight variations in the wire length have a stronger effect on the higher bands.

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  18. Paul Taylor says:

    Glenn, what radials or counterpoise did you use? Diagram shows something, times 2. Are they full 40m radials? Or shorter? What would you consider to be the minimal counterpoise for this antenna?

    • vk3yy says:

      Hi Paul,
      In most cases I don’t bother with a purpose counterpoise. If the setup is raised off the ground or dry and rocky, I might throw out a couple of metres of wire. The reference to X2 is the socket on the matcher. cheers….

      • Paul Taylor says:

        Tnx, discussing this antenna with Owen VK3EAR, who is planning on trying it out this weekend down the beach. I advised him to run a wire into the salt brine 😁 I think it will work for him regardless.

      • Owen Mehegan says:

        I ended up needing to add about two meters to the short side of the antenna to get it resonant on 40. Without it, the dip was at about 8.800 MHz for some reason. 20 and 10 were fine more or less, and didn’t change much when I lengthened it. Works a treat now!


      • vk3yy says:

        Ho Owen, great to hear that it works! wonder if you used the same coil former that I used, ie 20mm conduit?

      • Owen Mehegan says:

        Yes, Paul actually assembled this for me months ago, but I hadn’t had a chance to put it to the test until now. I checked, and he did use 52 turns on 20mm conduit. I’m not sure what else would explain the difference in performance. I’ll measure exactly what lengths I have ended up with and report back. Maybe there is some variation in the match box construction.

  19. vk3yy says:

    OK Owen. I have made three so far with very similar results, maybe the conduit varies in it’s properties a bit?. Anyway, great to hear it is working!

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  21. John Dorson says:

    I am very interested in following your design of the QRP EFHW antenna for 40-20-10 meters but do not find any information for the T82-43 Fair-Rite core. Where can these be ordered from. I do see FT82-42 cores however. Thank you for your help.

    John K2JHU…

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