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Discone and feedline grounding
On Fri, 21 Mar 2014 08:14:01 -0700, Jon Danniken
wrote: Thanks Jeff, I had not considered a biconical, but it looks interesting. The discone I am building is of a "spoke" variety, with the cone built separate from the disk, so it would be simple to duplicate another cone and invert it on top of the other cone. If you want to be cheap and sloppy (like me), consider that a bowtie antenna, commonly found on broadband TV antennas, is nothing more than a flattened biconcial antenna. While it is directional, it's functions much the same way as a biconcical and is much easier to build (out of aluminum roof flashing). I do like aircraft, although I am starting to think that I might be better off with a dedicated airband antenna (looking at j-poles right now) along with a wideband antenna for general scanning. I don't have any professional experience with aircraft antennas. However, I have done some ADS-B 1090 MHz designs and tests. The basic idea is that the antenna should have the most gain at the horizon and somewhat above the horizon to get the most range. Commercial jets fly at about 5 miles altitude maximum, so gain in the upwards direction is less critical. That's quite opposite of what the discone and biconical will do. Speaking of multiple antennas, I know that some antennas use multiple elements tuned to different bands, but can you connect two antennas to the same feedline? Like, say, a discone/biconical and a j-pole? No. The best you can do is insert a diplexer at the feedpoint junction, and separate the operating frequencies. Putting two antennas in parallel doesn't work. If both antennas received the same signal, the antenna pattern would be a conglomeration of both antennas, which could just as easily result in a null as it could a peak (also known as a mess). I have such an arrangement at a site. 120ft of very expensive 2"(?) Heliax going between the tower and the building. One triplexer and three antennas, each on a different band, on top of the tower. Another triplexer and 3 radios at the other end. Works so-so as intermod and desense are a problem on some frequencies due to insufficient triplexer isolation. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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