figure 8 loop reflector antenna? - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: December 17, 2018, 03:20:19 am
content: I saw an antenna in a European store today, it looked like a parabola made of mesh with a figure 8 in front of it. I did not really understand how it works because it looks like the loops that made the figure 8 were almost as big as the parabolic reflector so the focal points did not really make sense to me.What is this type of antenna called? The parabola was like 10 inches wide.

author: OwO
author_rank: Super Contributor
timestamp: December 17, 2018, 06:06:19 am
content: biquad antenna?Also a reflector doesn't need to be huge to be effective. The reflector in a yagi is only a piece of wire.

author: coppercone2
author_rank: Super Contributor
timestamp: December 17, 2018, 06:13:27 am
content: its like a biquad but its round not square and i think it was a bit bent like tear drops and the reflector was the same size as the 8 figure diameter and the loops were only a few inches behind the parabolic reflector.How does that work with the parabolic reflector? I thought you need a feed horn at a focal point why would you make it parabolic in that case?

author: hagster
author_rank: Frequent Contributor
timestamp: December 17, 2018, 07:52:48 am
content: Ibdont think it makes much difference if the loop is square or round.As far as I can tell the biquad is a loop antenna. The second loop reduces the input impedence and creates a second in-phase radiating element. The 'reflector' should in theory be either quarter wave away, or as close as possible to have a constructive reflection. Of course the closer the reflector the more it 'shorts out' the loops and reduces the input impedence. The exact size of the loop and distance to the reflector can be optimised to tune the antenna bandwidth and input impedence.

author: coppercone2
author_rank: Super Contributor
timestamp: December 17, 2018, 10:36:35 am
content: why is the reflector parabolic ?

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 17, 2018, 10:52:59 am
content: Quote from: coppercone2 on December 17, 2018, 10:36:35 amTo make it look like something it isn't? My guess is that it is fake.

author: coppercone2
author_rank: Super Contributor
timestamp: December 17, 2018, 11:09:27 am
content: Quote from: PA0PBZ on December 17, 2018, 10:52:59 amyes its that thing without the bunny ears in front pretty muchI thought it might be a scam I can't figure out what its doing

author: kruszi
author_rank: Newbie
timestamp: December 17, 2018, 03:56:35 pm
content: I don't think that the mesh is a parabolic dish. Usually, dish directivity work with a dish diameter >5*lambda. I guess it's a mechanical thing to have the reflector formed as a dish. This might heavily improve the stability of the reflector. A strictly rectangular reflector might be not as robust as a spherical cap with enforcement around its edge. After all, a sphere is an ideal form to deflect applied forces in all directions.

author: hagster
author_rank: Frequent Contributor
timestamp: December 17, 2018, 07:31:22 pm
content: That looks like garbage to me. The reflector is probably just for show.

author: coppercone2
author_rank: Super Contributor
timestamp: December 17, 2018, 09:05:00 pm
content: lol

author: cdev
author_rank: Super Contributor
timestamp: December 19, 2018, 03:12:31 am
content: The TV antenna in your picture is a rabbit ears (dipole with balun usually) antenna and the figure 8 is likely for UHF.The 'reflector' may just be for show. If its is actually metal, I cant see its purpose. The figure 8 may be a biquad without a reflector on one frequency but it wouldnt function as a biquad at others. Performance may be okay, in close in areas the main concern is multipath, not signal strength.