N5ESE's Portable VHF Ground Plane
(for use with mag-mount antennas, with pictures)by Monty Northrup, N5ESE
| NOTE: 'N5FC' is my former call. This project was constructed while that call was valid, and you may observe references to it. |
I needed a simple portable ground plane for the local ARES Y2K deployment, where I would be stationed at one of the local fire stations. The building had a flat asphault roof, about 15 feet up, and I wanted to use my 2-meter 5/8-wave magnetic-mount whip. I needed something in a hurry, and I also wanted something that was portable enough to put in my suitcase when I traveled on business, so I could use my 1/4-wave mag-mount in my hotel room.
The pictures which follow depict the results of my efforts.
I used a very shallow 4" junction box cover (at least, I guess that's what it is) as the mag-mount base. As you can see from the pictures, it's about 4 inches in diameter, and 1/2" deep. Perfect for mounting a mag-mount antenna, and readily available for about $1 from your local hardware store or electrical supplier. While I was there, I also bought (2) 36" lengths of 10-32 threaded rod (the thread doesn't really matter). The threaded rods would comprise the radial elements of the ground plane. Cutting them in half and deburring the cuts with a hand file, I ended up with (4) 18" lengths of threaded rod. I drilled 4 holes (equidistantly spaced) in the perimeter of the junction-box cover, so that the threaded rods could be mounted radial using two nuts and two external-tooth-washers on each rod. What I ended up with is a 4-radial ground-plane that can easily be taken apart and transported compactly. I later expanded it to 8 radials, with a slight improvement in SWR (and hopefully some improvement in radiation efficiency).
When tested with my 2-meter HT and VHF wattmeter at 147.0 MHz, my 5/8 wave mag mount shows 1.05:1 SWR mounted on the roof of my car. When mounted on the 4-radial portable ground plane (described above), the same antenna shows 1.15:1 SWR. When 8 radials are used instead of 4, the SWR improves to 1.10:1.
The ground plane as described should as easily be used on 220 or 440 MHZ without modification, although if you use it only on those bands, you could save some real-estate by by shortening the radials. Keep in mind that it's important to get good solid electrical continuity between the center mount (i.e., the junction-box cover) and the radials.
I throw the whole she-bang in my suitcase or my duffle bag when traveling, re-assemble it at my hotel room, and get much better performance than I would with just my rubber-ducky or my mag-mount antenna without a ground plane.
Be sure to check out the following images:
73,
Monty N5ESE
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