The 10M Vertical Experiment
Someone in Lake County dropped off an old CB StarDuster antenna at Dave’s (N0EDS) shop last week, and he was so kind as to give it to me for a special antenna project.
The project was to determine if using a vertical antenna on 10M for nearly-local QSO’s would improve signal strength to other hams also on verticals. In theory, matched polarization, e.g. vertical to vertical, buys you 20db of signal. Or, perhaps better stated, cross polarization loses you 20db.
For VHF and UHF, antenna polarization is important. This was used to an advantage when the convention for ham radio used horizontal polarization for SSB (and other modes) but vertical polarization for FM. Horizontal is somewhat better for DX work and residential setups, and vertical is easier to implement on vehicles or handheld devices.
Ok, so I dusted off the StarDuster, shortened its elements to fall around 28.405MHz (a favorite local freq) connected up some old RG-8… and gave it a try.
The results were not as I expected: The antenna provided roughly the same signal strength for nearly-local contacts as a 10M trapped horizontal dipole at 25 feet. Of course, it does have the benefit of being omnidirectional, where the dipole does not.
As I thought deeper about why I wasn’t seeing 20db difference… I think it’s the nearly-local factor that’s the problem. To get to Lake County, the 10M signals are passing over a 3000 ft ridge on Cow Mountain, refracting over the top (and perhaps some ground wave in there too — it’s hard to know how far that travels around here.)
It occurred to me that refraction (and even reflection) is going to have an effect on HF polarization, just as it does with visible light. You know… why polarized RayBan’s became popular years ago… many substances polarize light, the sky, lakes, windows, and more. Glancing at the ARRL Antenna Book, it confirmed this premise.
So, the 10M signals where losing most of their polarization as they bent sharply over the top of the mountain. Perhaps even the fact that the ridge is mostly horizontal was a factor (as I remember from spatial filtering laser lab experiments back in school.)
Well, that’s why I enjoy ham radio: new lessons to learn everyday. The next parts of the project are:
- Determine what effect antenna height has in this result. The StarDuster is very low to the ground which perhaps could be part of the problem (it’s basically a half-wave dipole.)
- Determine if a different type of vertical antenna might perform better. I’ve got a lead to follow on this.
- Figure out if my old RG-8 coax is any good. Not sure yet how to do this… but would be good to know.