16-01-2018, 08:55 PM
Good evening all.
I'm currently in the process of building an LFA yagi.
http://hh.vk7.com.au:82/blog/index.php?c...id_post=39
I've done basic measurements with the antenna placed slightly above ground, pointing into the sky, using a half wave length of coax to avoid any impedance changes from what is at the feedpoint. Measurements seem good. Return loss is about 27dB, not bad.
I've now terminated the coax tail (RG-213) which runs down around the rotator (via preamp actually) to heliax. The feedpoint connection is as short as I could get it.
I've placed one ferrite core right close to the feedpoint (about 20mm).
https://www.mouser.com/ds/2/150/2643102002-346453.pdf
These ferrite cores have an impedance of approx 250 ohms at 144MHz. I seal them from moisture using sealing tape with electrical tape over the top for UV protection. I then route the feedline at a partial 90 degree bend straight down to under the boom, where there is a small loop and about 200mm further down the coax is three additional ferrite cores, so hopefully this will be enough to stop common mode current.
I've seen several designs of these yagis, and some have 1/4 "stubs" connected to the feedpoint at one end, then shorted to the boom. These allegedly create a DC short, and a "pseudo" BPF effect on the antenna. I'm doubtful of this effect, however, it is what it is. The DC short is of more interest to me, however, my second, and preferred option is to directly connect the centre of the far end of the loop driven element to the boom. As voltage is at a minimum here, we can do this. We get the DC short, and possible other advantages when it comes to static/noise.
I'm wondering what others have on their LFA yagis balun/stub etc wise, and interested in anyone who has built their own such as I.
Regards
Hayden VK7HH
I'm currently in the process of building an LFA yagi.
http://hh.vk7.com.au:82/blog/index.php?c...id_post=39
I've done basic measurements with the antenna placed slightly above ground, pointing into the sky, using a half wave length of coax to avoid any impedance changes from what is at the feedpoint. Measurements seem good. Return loss is about 27dB, not bad.
I've now terminated the coax tail (RG-213) which runs down around the rotator (via preamp actually) to heliax. The feedpoint connection is as short as I could get it.
I've placed one ferrite core right close to the feedpoint (about 20mm).
https://www.mouser.com/ds/2/150/2643102002-346453.pdf
These ferrite cores have an impedance of approx 250 ohms at 144MHz. I seal them from moisture using sealing tape with electrical tape over the top for UV protection. I then route the feedline at a partial 90 degree bend straight down to under the boom, where there is a small loop and about 200mm further down the coax is three additional ferrite cores, so hopefully this will be enough to stop common mode current.
I've seen several designs of these yagis, and some have 1/4 "stubs" connected to the feedpoint at one end, then shorted to the boom. These allegedly create a DC short, and a "pseudo" BPF effect on the antenna. I'm doubtful of this effect, however, it is what it is. The DC short is of more interest to me, however, my second, and preferred option is to directly connect the centre of the far end of the loop driven element to the boom. As voltage is at a minimum here, we can do this. We get the DC short, and possible other advantages when it comes to static/noise.
I'm wondering what others have on their LFA yagis balun/stub etc wise, and interested in anyone who has built their own such as I.
Regards
Hayden VK7HH