A few days ago I cut the aluminum tubing of my home brew vertical antenna down to the dimensions specified by W9SR in the plans for the top loaded low band antenna. After I established the fact that the resonant point was so far off the mark in my initial testing I thought it best to get all my ducks in a row and set up the antenna closer to the receipe outlined in the ARRL handbook.
Reducing the length of the antenna mast by 24" makes it possible to load the disassembled parts into the passenger compartment of my car and close the rear hatch. This was a major factor that led to my decision to cut the length down. I want to be able to travel with the antenna secured inside the vehicle instead of lashed to the cargo rack on the roof exposed to possible damage or vandalism.
Wednesday afternoon I had some spare time so I loaded everything up and headed to my favorite QRP portable operating sight. This is the same park just outside of town where I held my earlier testing sessions but I set up in a different area to take advantage of some shade trees.
I erected the antenna in a steady wind I estimate was blowing about 10 mph. At 13 feet tall the radiator was much easier to hold steady while inserting the fiberglass foot section into the tripod base. I deployed the four elevated radials same as before. I left the loading coil where it was from the last session at 16 turns. Once the radial ring terminals had been connected I raised the telescoping section of the tripod to position the base of the antenna at approximately 8 feet. I have been careful to duplicate the conditions of the test sessions from one to the next for the sake of accuracy.
After connecting my 50 foot coax and the MFJ-259B analyzer I noted that the SWR reading had dropped another full point to 6.5 but still unusable on the 40 meter band. I did not add the four turns back to the loading coil because I was pressed for time. That will have to be an experiment for another time.
What I like about the 259B is how I can sweep with the tuning control across huge expanses of HF spectrum and watch how the antenna system reacts to the tiny signal generated by the analyzer. I have been doing this each time I have set up and tested this antenna and last night was no exception. I was surprised to discover a large dip in the SWR at approximately 18 MHz that was not present during my last test.
1.2 : 1 SWR Bandwidth: 18.088 - 18.220 MHz
2 : 1 SWR Bandwidth: 17.700 - 18.585 MHz
This means that my homebrew 40 meter vertical would be an excellent radiator when used for the amateur allocation of the 17 meter band (18.068 - 18.168 MHz). Definitely an interesting finding and now I'm curious to cut a set of resonant radials for 18 MHz and see what happens. My first contact using the antenna could very well be on 17 meters instead of 40 as I have planned.
While frustrating at times this project has been very entertaining and a learning experience as well. The farther I go the more questions seem to pop up but therein lies the fun of experimentation. For now I am going to take a break from it and consider what I have accomplished so far. If there are any antenna gurus out there that have been following along I would love to hear any input. This can probably be considered an advanced antenna project and it certainly is to me. Figuring this thing out is not as easy as trimming the ends of a simple dipole.