End fed HF wire antenna

For a HF antenna you can make a half wavelength dipole, but the problem is that it is only resonant at the wavelength you design it for unless you insert some loading coils. Moreover the center feeding on a half lambda dipole is not an easy option unless you have two masts and a balun at the center point. In my case I just wanted something simple to start with to see what the transceiver (a yaesu FT-991) does on the HF bands. So I installed an end-fed antenna that consists of an impedance matching circuit, essentially it is an impedance transformer called an unun that converts the high impedance of the end-fed antenna towards a lower impedance that matches your coax cable. Furthermore you need 10.1 meter of wire with a loading coil 1.85 meter away from the impedance transformer that you see in the first image.

hf_wire_antenna

Looking into the garden

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And from the tree to the house

The core of the unun is a ferrit ring that transforms the high impedance of the antenna to a lower impedance, typically around 50 Ohm which is what the coax cable has. The transceiver also performs impedance matching, about half the space inside the transceiver consists of coils and relays that do the job, but I also got an external autotuner which can perform impedance matching over a slightly wider range.

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This is what inside in the box, the transmitter input goes to the black coil that consists of two turns inside shrink tube, it has a 100 pf high voltage capacitor in parallel (I mistakenly wrote series in a previous version). Next there are 7 tight turns, you cross over, and there are 7 wide turns. Wire straps keep the turns in place. Some people prefer glue or wax to keep everything together. The ferrit ring in a material that is electrically not conductive, but magnetic field lines really like to stay in there so that it increases the inductance of any coil. This is precisely the function that it has, magnetic field lines are transferring the energy from one coil to another so that in the end the impedance of the transmitter (it feeds with 50 Ohms) is transformed to a high impedance of the end fed antenna. To avoid (or better, suppress) eddy currents in the mantel of the coaxial cable you clamp some ferrit tubes around the coax cable. For low power transmissions up to 25W this is enough to avoid complaints from you family members that they can alien voices through they headsets etc.

The autotuner  (an LDG YT-1200) gets the end-fed antenna in resonance between 7 and 30 MHz. It results in SWR’s typically around 1.5, all together this is a relatively simple solution, you don’t need a structure on the roof, and the components are relatively cheap. This shop provides you with all the required components. So far I’ve been able to establish contacts within Europe on 40m, 20m with no more than 25Watt. See also my logbook on QRZ and in particular the logbook on that site.

microkeyer2

 

This is the yaesu FT-991 transceiver, it needs a bit of current (up to 25 Amps) at 13.8 Volt and this is what the power supply on the right side does. On top of the power supply is the autotuner that talks with a cat interface cable to the transceiver.

When I showed the first image to my mother of 89 she said, tell me, what do you see in the bushes? I explained her that it was an HF antenna and that you could talk all over Europe with a wattage not enough to light up your living room, but apparently enough to talk with radio amateurs 1000’s of km away. She was raised on Java Indonesia which was a Dutch colony up to 1948. The radio receiver in those days was a box with glowing radio tubes in the corner of the room, and she recalled transmissions from a radio station in San Francisco but also the propaganda transmitter radio Zeesen in Germany. In those days the frequencies were below 300 kHz and perhaps already in the lower end of the current MW. Radio in those days was the only way to find out what was going on in a world at war where disinformation and information had to be separated. Radio San Francisco did a great job for the Dutch on Java in Indonesia, the radio system was in those days also a few meters of wire to a one of the trees in the garden. In Europe the possession of radios was prohibited, but my fathers family (like many other families) has the receivers stored somewhere deep away in closets etc. The free word, this is what you heard in the bushes.

http://logbook.qrz.com/lbstat/PD1EJO/

Last update: 14-April-2016

 

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