Wednesday, September 27, 2017

Projects - 4m slim jim antenna

As I wrote before I now have a working 4m set-up using the 4m transverter kit from DF2FQ (XV4-40). I also have a compact 2/4/6 meter beam that I will describe in more detail in a future post.

Although the beam is compact, I cannot use it at home as it is too large. How can that be? Well, I don't have a fixed mast - all my antennas start from my attic. I use a 20 meters long sloping end fed wire from my attic into the garden in an L shape for 80m-10m (resonant on 40/20/10m - works OK for 40-30-20). For 6m I use a vertical end fed wire that I put on a fiberglass pole in the attic and then push up through an attic window (I sometimes use this construction for lower bands as well). For 2m I have a compact 4 element beam that I can just push through the same attic window. I attach that one to an aluminium push up mast.

The 2/4/6m beam is just too large to fit through the attic window. So for working Es - one of my favourite sub hobbies - I needed a solution for 4m (having 6m and 2m covered). With Es you can get good results even with a "simple" vertical - as I have already experienced using my end fed wire for 6m. So, it was time to create a 4m vertical. Using a bit of ladder line PG8M had lying around and a nice online calculator by M0UKD (with construction tips), I created a slim jim for 4m.

Ladder line as the basis for the Slim Jim antenna

The online calculator and all the other info on John's page give you a good starting point but then you are tasked with finding the exact feed point position. As John M0UKD notes, it is important to perform the final tuning in a set-up that resembles your target situation. I stuck the ladder line to my fiberglass pole in the garden and pushed that up a couple of meters to make sure the antenna was away from obstacles (as I will be using it when I push it out from my attic). I tried different feed point positions, each time making measurements with my VNA.

Locating the feed point
Finding the feed point position is a trial and error process. This is slightly complicated by the wires being insulated. I didn't like the idea of having to remove the insulation on a large stretch to find the right spot, so I used the set-up in the picture above: two needles connected to my VNA. That way I could try different points and check the effect on the antenna SWR curve.

Finally I added a W2DU style current choke on approx 1m of coax (soldered to the feed point) that conveniently terminates with a SO-239 connector: ready to go.

The end result gives me an antenna with an SWR of 1:1.6 on 70.200. I might improve that a bit more by tweaking the stub length in the future but as it is I am pleased to know I am ready for next year's Es season.