I was curious how well traps work in vertical antennas. So I decided to look for a not to big design and build one and do as usual a lot of measurements !
My eye has fallen on a design from KG0ZZ, see the site where I found most info. Even more info is here.
Because I had not all the parts exactly following the video, the results are good but the dimensions of the aluminum pipes are different and the traps are made of other type of coax cable.
Let us start with the result first for a change !
This picture shows a RigExpert antenna analyzer. I had connected the antenna and after a LOT of tuning, which is very easy and fun to do by the way, I got this result. Al three bands (20/15/10m) gave a deep dip in the SWR. So the traps were working splendid, and the dimensions of the tubes were also nicely tuned. The second picture shows a detail of the 20m band. Very happy with it ! The whole band under 1:1.3.
The 15m band is tuned a bit to high but overall under 1:2. Getting the dip in the middle is not difficult, but it is dependent of the exact lay out of all the earth radials too. So I decided to let it be for the moment. If I am going to install the antenna I can fix the radials better than in my back yard. After installing I do a re-tune. How I did that will be discussed later on. The 10m band is ok with me.The whole band 1:2 and the dip of 1:1.3 a little under half way the band.
If you are not satisfied with this result, you can play with the length of the resonating piece of the antenna and the radials, in order to get the feeding point exactly on the point where the impedance is 50 Ohm. However this will never be over the whole band.
Conclusion of this build:
- Easy to get all the parts together, junkbox, alu shop, some sheet metal screws, a piece of coax, some glue (I used hot glue, which is ok but not super,...)
- Making the tubing on lengths is simple with a iron saw, and I like the sheet metal screws to connect all the pipes. Making the sleeves in the pipes is a bit of a thing because you need a grinding wheel, or a hand saw for aluminum. But all in all very well to do.
- Making the traps is a bit more work, and I used thin, high quality Teflon coax (5mm thick, silver coated, military stuff,...). Getting them exactly on the right frequency is for me trial and error and feeling of course. There are programs on the internet to calculate it precisely if you know the type of coax. The 10m trap was right the first time !! but the trap for 15m I had to make it twice, and than I still needed some corrections by spreading the windings a bit.