n4dbm
Newbie
- Joined
- Jun 9, 2018
- Messages
- 37
That is interesting on the match with the Arrow GP146. It's about as simple of an antenna as you can get, and the ground radials bent at a 30-degree angle should give a near perfect match wherever the radiating element is resonant. How much cable and what type of cable did you have between the antenna and the analyzer? The analyzer or meter should be as close as possible to the antenna as it can be. Theoretically, if everything between the analyzer and the antenna is exactly 50 ohms, it shouldn't matter. But that's never the case, and certain lengths of cable between the antenna and analyzer can and will throw readings off, sometimes badly. Regardless, the extra length in your new radials and keeping them at 90 degrees from the radiator making the match "better" is interesting. I don't have a good answer for what's going on there.
Second, the reason most 1/4 wave through-hole mobile antennas match decent with the metal being at a 90-degree angle is due to the amount of metal under the antenna, as it's not just four wires, but it's A LOT of metal. So, instead of resonant radials making up the "mirror" part of the antenna, the car's sheet metal is acting as a true ground plane.
The YouTube video link is neat, but that's a very weak antenna design. I'm certainly not throwing off on the builder at all, because I have built plenty of mechanically weak antennas for experimental and learning purposes. But, actually making this and expecting it to survive for any length in the great outdoors is ridiculous. It is beyond mechanically weak, especially with soft drawn copper. The radials at that angle with that kind of mass will droop down very quickly under its own weight. If you want to build something like this for learning purposes, then by all means do so. When you've got your initial thrill from your build, fold all the radials straight down. This will now make your antenna a half-wave. The impedance will be close to 100 ohms, and give you a 2:1 VSWR. To correct this, an odd electrical 1/4 wavelength of 75 ohm cable will transform the 100 ohms into very close to 50. You will just have made a "sleeve dipole" antenna, very similar to the popular DB Products DB-205.
If you are going to stick with a 1/4 wave, at least opt for 3/32" bronze welding rod for strength and rigidness. Also be sure to seal the SO239 connector with hot glue or silicone. If you're going to hang the antenna from a rope like the video, use a good glass insulator. The tip end of a 1/4 wave is a very high voltage point and using wood or some plastic blends will not only detune the antenna, but seriously decrease performance on transmit.
As far as match versus performance, a 2:1 match is 11% reflected power. In a practical setting, no one on the other end will see the difference in a 2:1 versus a 1:1 match. It will just be less heat dissipated by your final amplifier device.
Good luck on your project.
Second, the reason most 1/4 wave through-hole mobile antennas match decent with the metal being at a 90-degree angle is due to the amount of metal under the antenna, as it's not just four wires, but it's A LOT of metal. So, instead of resonant radials making up the "mirror" part of the antenna, the car's sheet metal is acting as a true ground plane.
The YouTube video link is neat, but that's a very weak antenna design. I'm certainly not throwing off on the builder at all, because I have built plenty of mechanically weak antennas for experimental and learning purposes. But, actually making this and expecting it to survive for any length in the great outdoors is ridiculous. It is beyond mechanically weak, especially with soft drawn copper. The radials at that angle with that kind of mass will droop down very quickly under its own weight. If you want to build something like this for learning purposes, then by all means do so. When you've got your initial thrill from your build, fold all the radials straight down. This will now make your antenna a half-wave. The impedance will be close to 100 ohms, and give you a 2:1 VSWR. To correct this, an odd electrical 1/4 wavelength of 75 ohm cable will transform the 100 ohms into very close to 50. You will just have made a "sleeve dipole" antenna, very similar to the popular DB Products DB-205.
If you are going to stick with a 1/4 wave, at least opt for 3/32" bronze welding rod for strength and rigidness. Also be sure to seal the SO239 connector with hot glue or silicone. If you're going to hang the antenna from a rope like the video, use a good glass insulator. The tip end of a 1/4 wave is a very high voltage point and using wood or some plastic blends will not only detune the antenna, but seriously decrease performance on transmit.
As far as match versus performance, a 2:1 match is 11% reflected power. In a practical setting, no one on the other end will see the difference in a 2:1 versus a 1:1 match. It will just be less heat dissipated by your final amplifier device.
Good luck on your project.