Help with installing an Antenna, or just choosing the right antenna to go with your radio, or your mobile.
- Posts: 141
- Joined: March 17th, 2007, 8:32 am
- Age: 67
So much confusion on this thread. The only reason a 1/4 wave whip has a 1:1 swr is due to R (resistance) in series with the ground path. That is o.k. and normal to have a little loss there. The normal isotropic 1/4 wave antenna over a lossless ground has a radiation resistance of around 33 ohms. The shorter the antenna (with coils, etc.) the lower the radiation resistance. Therefore, the BETTER your ground system, the HIGHER your SWR will be without any matching devices (coil or capacitor) at the feed point. If you install a loaded antenna on the roof of a vehicle and it is a good low loss antenna, such as a Predator 10K, you should not expect a 1:1 match. a 2:1 match would indicate a low loss ground system. You could achieve a better match by attaching a coil or a capacitor across the feed point from "hot" to ground. I used to use a 4 turn coil about 1.5" in diameter. Using an inductance (coil) will require you to shorten the "stinger." Adjust the coil to find 1:1 swr somewhere close to 27 mhz, then use the length adjustment to center the frequency where you want the best match. You could use a high quality low loss silver mica capacitor (1000 V or better, think physically large) of around 100 pf. to start with. The difference is using a capacitor shunt will require the antenna whip to be longer rather than shorter.
I've used a variable capacitor to find the exact value of capacitance, but 100 pf should get you close. Using a capacitor is slightly more efficient than using a coil (inductor) but it is much easier to adjust the coil than to substitute capacitors to get a perfect 1:1 match.
If you were ever to take a base loaded antenna, such as a Wilson or K40 apart, you would see that the feed point is tapped into the coil a few turns above ground which accomplishes the same kind of match that I'm talking about here.
Hope the helps someone.
- 2 PILL USER
- Posts: 12
- Joined: March 30th, 2022, 12:40 pm
- Age: 56
I am planning a 10 k install on my truck this week. My Mr coiley version of it **Censored** the bed, and he is not shipping anytime soon.
Any input is appreciated. I will me mounting on my roof of 4 runner with 3 puck magnet mount
rci 2950 with texas star 500 in line