Sometimes a summit offers little space to deploy an antenna. Either because there isn’t much room or because there are many people at the top – or both. Then an antenna with a small footprint is required.
Some time ago I had a S2S-QSO with N1ZF. In the following e-mail contact we exchanged information and photos about the antennas used for this QSO. This is when I became aware of the „Random Wire Antenna“ that Paul had successfully used. It consists of a 12.8 metre long wire as radiating element and an equally long ground radial. Matching is done with the KX3’s internal ATU. Since the ATU cannot match any impedance, it is important to choose a wire length that is not a multiple of half a wavelength. This way the „odd“ length of 12.8 meters emerges.
A delta loop antenna – especially for the 40 metre band – is not exactly suitable for SOTA. It is huge and requires a lot of space. Furthermore the set up is time-consuming. Nevertheless I’d like to show this design here because:
During sunspot minimum 40 m still offers the opportunity to work a lot of DX.
The delta loop provides the required take off angle to work the DX.
Sometimes space (and time) don’t matter.
It’s fun building it 😉
A possible application could be intercontinental S2S events.
Here is the Link to the 20 metre version of the delta loop. During our DX-pedition (Link) to the Principality of Liechtenstein in Sept. 2013. DL4AAE worked hundreds of JA’s on 20 metres with this antenna and 100 W output power.
Recently I came across an article from Andrew, VK1AD, in which he describes a QRP L- tuner for 3.5 MHz to 52 MHz. It can be found here.
Andrew’s article is based on an essay by Peter, VK3YE, published in VK’s Amateur Radio magazine, Edition 5, May 2017.
I really wanted to try this easy to build L- tuner, because I thought it might perform even better than my EFHW- transformers (UnUns) and the EFHW- dipole I’m currently using for my SOTA activations.
The impedance of an End Fed Half Wave (EFHW) dipole lies in a range of approx. 2k Ohm to 4k Ohm. In order to achieve a reasonable matching at 50 Ohm, a transformer can be used.
As a starting point I simply copied the design of a broadband transfomer, using a ferrite toroid FT 140-43 from Amidon with a turns ratio of 3:24. The transmission ratio, which is the square of the turns ratio (1:8), is 1:64. This means an impedance of 3200 Ohms will be transformed to 50 Ohms. The above mentioned range of 2k to 4k will fall in a range of approx. 30 to 60 Ohms which results in a slight (but acceptable) mismatch.
There are several manuals available, e.g. here or here.
A delta loop antenna is not particularly suitable for SOTA. It requires much more space than a vertical and the set-up is more complex.
Nevertheless I’d like to show this design here because sometimes space (and weight) don’t matter and a low take off angle is welcome.
If you are looking for a delta loop for the 40 metre band, try this Link.
A possible application could be the VK/ZL/JA <> EU S2S event or the Trans- Atlantic S2S QSO party.
During our DX-pedition (Link) to the Principality of Liechtenstein in Sept. 2013. DL4AAE worked hundreds of JA’s on 20 metres with this antenna and 100 W output power.
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.
Another very good manual, written by Heinz, HB9BCB, can be found here.
Nachdem ich einige SOTA- Referenzen mit Monoband- Antennen aktiviert hatte, entstand der Wunsch nach einer Mehrbandantenne, denn der Bandwechsel war eine zeitraubende Angelegenheit. Die Antenne sollte mindestens die Bänder 40m, 30m und 20m abdecken, optional 15m. Der Umbau sollte nicht länger als drei Minuten dauern.
Die Idee stammt aus der CQ-DL 06/2014. Dort wurde ein Artikel mit dem Titel „Endgespeiste λ/2 Koaxantenne“ veröffentlicht.
Beschrieben wird ein endgespeister Halbwellendipol, dessen Fußpunkt- Impedanz mittels Viertelwellentrafo in die Nähe von 50Ω gebracht wird.
Die Antenne ist einfach in der Herstellung und leicht im Freien aufzubauen, da sie – vertikal betrieben – nur einen Aufhängepunkt benötigt. Nachteil dieser Antennenform: Es handelt sich um eine Monoband- Antenne.
Eine gute Erklärung zur Wirkungsweise des Viertelwellen- Transformators findet man im Praxisbuch Antennenbau von Max Rüegger, HB9ACC, Kapitel 7.1.3.
If a symmetrical antenna (e.g. half wave dipole) is fed with coaxial cable, a common mode choke (or current BALUN) is required to suppress unwanted currents on the outer side of the braid.
The mechanism is described nicely in the antenna book of HB9ACC (Chapter 3.7.4).
It is also recommended to read the ‚Transmission Line Transformers Handbook‚ of W2FMI.