EFHW- Dipole with Traps

While searching for an alternative to my Linked- Dipole I tripped over the posting of Pedro, CT1DBS, illustrating an end-fed half-wave (EFHW)- Dipole with Traps. Here is the Link to his article. With this design changing bands can be done in no time thaks to the traps. Lowering or modifying the antenna is no longer necessary.

If set up as inverted L this antenna has even more advantages compared to a centre fed Dipole:

  • Significantly less weight as only a short piece of coax is required.
  • Lower footprint as only one additional anchor point is needed. This is a huge advantage on alpine summits with limited spaces.

My EFHW- Dipole consists of the following:

1. Transformer

In order to bring the high impedance of an End Fed Half Wave (EFHW) dipole (~ 2000…3000 Ohms) to 50 Ohms, a matching unit is required.
Since this chapter has grown over time, please follow this Link to a separate essay!

2. Traps

Following Pedro’s example I purchased a Pico Trap kit from Sotabeams. The components used on the PCB can handle up to 10 Watts which is sufficient for the MTR3B or the KX3 @ 12V.

pico_traps_kit

Soldering of the surface mount capacitor requires a calm hand and a pointed soldering tip. The number of windings on the toroid can be found in the online instructions. It also describes how the traps can be adjusted. An antenna analyzer or something similar is required.

trap-tuning

3. Wire

The wire is DX- wire UL from http://www.dx-wire.de/

4. Set- up

The antenna is usually set up as inverted L on a leightweight GRP- pole from Lambdahalbe http://www.lambdahalbe.de/. Other setups, like inverted Vee etc. are possible.

Edit (28.05.2017):

After discussing the deisgn with Heinz, HB9BCB, I made the following changes:

  1. Attach the Unun to the GRP- pole at a heigt of approx. 1.5m (before it was lying on the ground)
  2. Tune the wire length for minimal SWR (instead of resonance, i.e. XL = XC = 0)

The wire length for the 40m- „tail“ is now about 2m shorter. SWR on 20m is 1.15 : 1 and on 30m 1.5 : 1. On 40m it’s slightly higher: 1.9 : 1, because IZI is in the range of 30 Ohms. This behaviour is still not fully understood and requires some more investigation.
Slight deviations in wire length (compared to HB9BCB’s design) are most likely due to different L- values of the inductors used in the Traps.

EFHW-Dipole_w_Traps_3

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4 Gedanken zu “EFHW- Dipole with Traps

  1. vk5bje 14. Mai 2017 / 23:49

    Hi Roman, a very neat 9:1 Unun and I like the antenna very much. Thanks, John D, VK5BJE/VK5PF

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