Hello BJ Radionut:
Jay in the Mojave
Ok the 20 meter vertical I used was a full 5/8 wavelength at 41 feet tall. The 1/2 wavelength ground plane antenna you want to build may have to have its ground plane radials adjusted from a 45 degree down position to flat and level, as a 1/4 wavelength ground plane antenna needs the radials down at 45 degree's for a better match.
The 1/2 wavelength ground plane antenna will have a higher impedance than the 5/8 wavelength antenna, so the matching hoops will need to be longer than what was used for the 5/8 wavelength antenna. But the T1 hoop is 58 inches and the T2 hoop is 38 inches made from 1/2 tubing the Trombone 180 degree pieces made from 3/8 tubing. The lengths are measured from the ends to the center of the vertical element. Not like the I-10K at:
http://www.a1antennas.com/tuningunit.html
I am "NOT" a fan of coils or capacitors in the Matching Design (too much loss, and construction hastle), but for your 1/2 wavelength 20 meter project I would try a toroid desgn for at least the T1 Inductor. Using RG393 silver plated braid as the wire to wrap the toroid core. It will only take a few turns to get it there. You can spray it with clear vinal to seal it.
There are a few out there that will say a 1/2 wavelength vertical will not need ground plane radials. These type of antennas are NOT seen in the military, commercial, or amateur markets. Bill Or in his Vertical Antenn Book, says that ground plane radials are needed for the 1/2 wavelength antenna. And I agree. I know many have installed cheap kits that add ground plane radials, but the radials are not a 1/4 wavelength long, and are not installed flat and level.
I am sure with a little experimenting around you will find the tuning lengths.
I used the 5/8 wavelength design because it had the best pattern, that is squirting the RF Energy out at a really flat angle where the locals and the most the DX comes is, just off the horizion. So the tuning design had to match the 50 ohm coax to the high impedance of the 5/8 wavelength ground plane, as changing the 5/8 wavelength vertical element was not a option. Many antenna users think that the 5/8 wavelength antennas have more gain that others, but its the low angle pattern that makes them work so well.
Also the 5/8 wavelength will have another major lobe or pattern going out at 45 to 50 degree's up in the air. I am in the So Cal Mojave Desert, when short skip was in I was able to talk to other stations with 5/8 wavelength antennas, in Reno NV, Frensno Ca, and Az. The real odd thing was that the short skip conditions was very constant for over a half hour or so. I thought Reno Nv was a local. Oh yeah that also all had I-10K Antennas.
Good luck, please let us know how it turns out for you. Sounds like a neat project.
Jay in the Mojave
Jay in the Mojave
Ok the 20 meter vertical I used was a full 5/8 wavelength at 41 feet tall. The 1/2 wavelength ground plane antenna you want to build may have to have its ground plane radials adjusted from a 45 degree down position to flat and level, as a 1/4 wavelength ground plane antenna needs the radials down at 45 degree's for a better match.
The 1/2 wavelength ground plane antenna will have a higher impedance than the 5/8 wavelength antenna, so the matching hoops will need to be longer than what was used for the 5/8 wavelength antenna. But the T1 hoop is 58 inches and the T2 hoop is 38 inches made from 1/2 tubing the Trombone 180 degree pieces made from 3/8 tubing. The lengths are measured from the ends to the center of the vertical element. Not like the I-10K at:
http://www.a1antennas.com/tuningunit.html
I am "NOT" a fan of coils or capacitors in the Matching Design (too much loss, and construction hastle), but for your 1/2 wavelength 20 meter project I would try a toroid desgn for at least the T1 Inductor. Using RG393 silver plated braid as the wire to wrap the toroid core. It will only take a few turns to get it there. You can spray it with clear vinal to seal it.
There are a few out there that will say a 1/2 wavelength vertical will not need ground plane radials. These type of antennas are NOT seen in the military, commercial, or amateur markets. Bill Or in his Vertical Antenn Book, says that ground plane radials are needed for the 1/2 wavelength antenna. And I agree. I know many have installed cheap kits that add ground plane radials, but the radials are not a 1/4 wavelength long, and are not installed flat and level.
I am sure with a little experimenting around you will find the tuning lengths.
I used the 5/8 wavelength design because it had the best pattern, that is squirting the RF Energy out at a really flat angle where the locals and the most the DX comes is, just off the horizion. So the tuning design had to match the 50 ohm coax to the high impedance of the 5/8 wavelength ground plane, as changing the 5/8 wavelength vertical element was not a option. Many antenna users think that the 5/8 wavelength antennas have more gain that others, but its the low angle pattern that makes them work so well.
Also the 5/8 wavelength will have another major lobe or pattern going out at 45 to 50 degree's up in the air. I am in the So Cal Mojave Desert, when short skip was in I was able to talk to other stations with 5/8 wavelength antennas, in Reno NV, Frensno Ca, and Az. The real odd thing was that the short skip conditions was very constant for over a half hour or so. I thought Reno Nv was a local. Oh yeah that also all had I-10K Antennas.
Good luck, please let us know how it turns out for you. Sounds like a neat project.
Jay in the Mojave
