Monday, May 07, 2007

Homebrew Screwdriver Antenna

After searching the internet for a homebrew antenna, I stumbled upon this interesting antenna called "screwdriver antenna". It is basically a multi-band antenna that willcover 10m to 40m if build properly and down to 80m with added ground plane. There is so many comercial unit available such as the YAETSU ATAS-100/120, High Sierra, Tarheel. But the original was designed and built by DK Don Johnson W6AAQ. His fellow ham, KC6FRY have a website featuring the DK-3 screwdriver antenna by Don Johnson. Check it out here! Apart from this, there are other website and article discussing about the "screwdriver antenna".

As for myself, I am building this antenna for the 6m and 10m operation. But it's sad that this band is not active here in Peninsular Malaysia.

Below is the picture of the loading coil. The coil is wound on a 27mm PVC thick wall water pipe. The groove will provide a seat for the 1mm bare copper wire. How to make the groove??? Go and find your electrician friend and borrow his conduit threading machine.

Check out the photo below for the matching section. The base of the matching section is salvaged from a wi-fi antenna. Cheapskate! I also choose to use the matching coil at the base of the antenna where the RF is fed in. Some people use UNUN. The matching coil is made out of 13 turn of copper wire on a 19mm delrin rod with 3 silver mica capacitor connected from the center pin of the SO-239 connector to turn 6 (18pF), turn 11 (18pF) and turn 13 (39pF). The first point of the coil is connected to the SO-239 ground and the last point of the coil is connected to the holding section made out of 35mm copper tube. The center pin of the SO-239 connector is then connected to the motor control board which will turn the threaded rod to bring the loading coil sliding in or out of the copper tube. This is actually like a telescopic action. The matching section is then assembled into the PVC pipe socket. Cheapskate again???

Then this is how it looks like when it is assembled and connected to the copper tube.

The other end of the copper tube has copper finger soldered to it to provide a connection to the loading coil that will slide in and out of it.

I can't describe how to make the mechanical system that will make the loading coil slide in and out the copper tube. But I honestly think that most amateurs is exposed to DIY and they are very handy. I must also admit that some are $$$ loaded and buying a commercial unit is not a problem (no pun intended). But for me??? Definately not $$$ loaded but I consider myself is handy. All that matters is satisfaction that I 'll have when the antenna works. Even if it is just in 6m and 10m band. That's because my current license only allow me to transmitt in these band. Below is the photo of the mechanical item. The threaded rod is at the center of the assembly. The assembly is then connected to the loading coil assembly. The other end of the threaded rod is connected to the driving motor.

Below is the photo of the whole assembly. At the top is where I connect a whip. I have tested the antenna indoor at 5W on 10m and the SWR meter is 1.25:1. It is possible to get 1:1 SWR since this antenna works as "Actice Tuning Antenna System" I will conduct further test on other band with antenna analyzer belongs to a friend. Hmmmm... when will I get to see him??? The whole assembly is without the motor and the control board. This item is still in a thinking process and it is currently drawn on my scratch notepad! Once everything is in, I will polish the copper tube and spray a layer of protective lacquer to preserve the beautiful color of the shiny copper. The PVC will be painted with gun metal color. Or maybe all in gun metal color... what do you think???

Till then, keep visiting my blogpage for further update on this project and also some newer project that is currently in my head. I need coffee and I am addicted to hombrew!!!
Maybe I should consider homebrewing "moonshine" next time....... :)