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:
In order to bring the high impedance (~ 3000 Ohms) of the end fed dipole to 50 Ohms, a matching unit is required. Among the available options I chose the broadband transfomer. There are several designs available on the web: here and here.
The required transformation ratio is 64:1. The turns ratio is 24:3 with 1mm enamelled copper wire on a FT 140-43 toroid from Amidon. Primary winding and secondary winding are twisted on the first three turns (s. photo).
Experiments with a termination of 3300 Ohms and different capacitors in parallel to the primary winding show an optimum value of approx. 100 pF for frequencies below 20MHz.
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.
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.
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.
After discussing the deisgn with Heinz, HB9BCB, I made the following changes:
- Attach the Unun to the GRP- pole at a heigt of approx. 1.5m (before it was lying on the ground)
- 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.