On my tower at home I have two dual-band VHF/UHF antennas. One is an Arrow Ground Plane @ 55ft, the other is an Arrow J-Pole @ 45 ft. Both are fed by the same type coax.
One evening I was trying to use the W9BIL D-STAR repeater in Cadwell, IL which is 49.1 miles from my house according to QRZ.com. The ground plane is normally used by my Winlink RMS Station and the J-Pole is normally on my shack’s dual band radio.
I was pleased when I was able to key up the repeater operating with the J-Pole although the signal strength meter barely moved. I switched to the ground plane antenna thinking the extra 10 foot or so of height might help a little but to my surprise I could not hear W9BIL at all. I never thought much about the antennas being all that much different and assumed that height would trump anything else.
Some research was in order.
Discounting the difference in height which should have helped if everything else was equal there are two other factors that come into play:
- Antenna Gain
- Angle of Radiation
Let’s look at three antenna types: a simple ground plane, a J-Pole which is a 1/2 wave antenna and a 5/8 wave antenna which is another popular style for VHF/UHF.
| Antenna Type | Reference Gain | Angle of Radiation |
| 1/4 Wave Ground Plane | 0.0dB | 25° |
| 1/2 Wave J-Pole | + 0.85 dB | 20° |
| 5/8 Wave | + 2.35 dB | 16° |
The lower radiation angle puts more of the power you’re radiating at the horizon. The higher gain means the antenna pattern is also focusing the power in the most desirable direction.
For the curious, going longer than 5/8 Wave tends to hurt your cause. Beyond that length the radiation angle begins to increase.
You may be wondering why I included the 5/8 Wave antenna when I didn’t have one to compare. Well, it turns out several companies like Comet make nice 5/8 Wave antennas for Amateur Radio use at fairly reasonable prices. You never know when an upgrade may be in order!
73,
Tim N9PUZ
No comments:
Post a Comment