I am building a parabolic antenna for 2 meters. I do not have an etched antenna so am thinking to wind a dipole around a toroid. I am wondering what effects the ferrite toroid will have? I am contemplating the toroid because it is small and thus be concentrated near the focus rather than spread out on a rod.
If anyone has some input on the idea of the toroid I will be very interested.
You have posted this in the “Receive Antennas (below 30 MHz)” forum, but since you are talking about an antenna for 2 meters did you mean to post it under the “Amateur Radio Antennas” forum?
Is this a school assignment, a mental exercise, or actually something you want to do? Or is there a bridge involved someplace around here?
A parabolic dish on 2 meters? What size is this thing? I mean assuming 50% efficiency you need to get into the 4 meter diameter (~13 feet) area before you start getting to the gain levels of a good, and simple, 8 element Yagi. At 144 MHz a 4 meter dish with a 50% efficiency yields about 12.6 dBi of gain, while M2’s 2M7 antenna, on a 2.7 meter (8 feet 10 inches) boom yields 12.3 dBi. The 2 kg Yagi is going to be a much easier structure to work with than the 100+ kg of the 4 meter dish. Even pushing the efficiency up to 70% with a well designed feed you only end up with about 14 dBi in a 4 meter dish, still just short of the gain of M2’s 4.6 meter long, 3 kg weight, 2M9SSB Yagi.
I don’t see why you would not be able to wind an antenna on a toroid, however I have to ask what kind of losses will be present do to the lumped inductance. Also have to wonder how this will affect bandwidth (Q).
On the subject of a feed, how are you going to shape the pattern of the feed? A simple dipole or single element wound feed will likely over illuminate (causing spillover) the dish. Have you looked at illumination loss and spillover loss? How will your proposed toroidal based feed address these issues?
While wanting a feed to be the best theoretical point source possible is an admirable end goal, I am not sure the losses and inefficiencies caused by the reduced element size are worth the effort other than as a mental exercise. Sure, the feed blockage might be less, but I don’t think that will make up for the other shortcomings. And don’t forget you still have to address the illumination issues, regardless of the size/form of the feed.
T!
