M6SIG Posted March 27, 2014 Share Posted March 27, 2014 I notice there are some confusion in the Ep. The "ring" on the yagi is actually a folded dipole. The purpose of a folded dipole in a yagi is to match the antenna. In theory, the impedance decrease as the number of directional element increase. Because we need a 50 ohm antenna, and we want a lot of directional elements, so that we need a folded dipole there. Quote Link to comment Share on other sites More sharing options...
M6SIG Posted March 27, 2014 Author Share Posted March 27, 2014 Just watch the whole ep. I notice you guys forgot to put a matching resistor at the end of the collina. Since you want a 75 0hm antenna, which is good for receiving only, you need to put a 75 ohm resistor between the center conductor and the outer conductor at the end of the last element. After all, it is a very good ep! Quote Link to comment Share on other sites More sharing options...
jobdone Posted March 27, 2014 Share Posted March 27, 2014 (edited) I have built several coaxial collinears for 70cm usage , well done for trying a 1090Mhz , this is very hard to get right due to the size needing to be perfect! I normally add a quarter wave (in air) section to the base of the antenna , then solder 4 horizontal ground planes to the outer of the coax at this point. this has 2 effects, for transmitting it decouples the coax from the antenna (as the coax will run hot) and should aid the antennas performance. I normally use RG58 for these antennas, however RG58 requires soldering and Darren's method seems great for spacing. Be careful what plastic pipe you use as some can add characteristics to your antenna! try to check whats in the plastic.... I normally leave the end of the antenna open, however I have seen the other designs with the antenna 'shorted' at the end or a quarter wave added. I am guessing that a 16ele like Darrens will have a gain of 10-12Dbd and a fairly low angle of radiation, sometimes a 4ele(with about 5Dbd) can work better in some locations due to the angle of radiation. So the pros are: omidirectional antenna with a nice bit of gain however the cons are:narrowing the angle of radiation, it is possible especially at high frequencies to TX underneath or over the antenna! and it still does not outperform my biquad with a reflector... Edited March 27, 2014 by jobdone Quote Link to comment Share on other sites More sharing options...
cooper Posted March 27, 2014 Share Posted March 27, 2014 Got some pics of your antenna? I wonder what those ground planes would look like. Won't a biquad with reflector be directional and as such a poorer match for the stated usage scenario: receiving ADS-B? Any suggestions about what would make a good reflector for a biquad? Most examples use any type of flat surface coated with a metal layer but some say you should get more of a dish/parabole shape. Something like a wok or a metal colander. Does the reflector size matter much? I've googled this stuff, but it seems nobody wants to claim one is better than another and thus they also never explain WHY something is better or even large enough. Quote Link to comment Share on other sites More sharing options...
jobdone Posted March 28, 2014 Share Posted March 28, 2014 You are correct about the biquad being directional and yes it's unfit for this purpose. The biquad's reflector is wire mesh (flat) which is fencing wire over a metal frame, the mesh size is proportional to the frequency it's being used on as is the spacings in the mesh, however tinfoil over a frame could be used on all frequencies it's just not very permanant . As stated I use this is for 70CM usage , not for 27CM but scaled it wil work ok , just directional (about 70 degree beam I hear). I have a picture of the coax collinear out on a field trip with me somewhere. it is strapped to a cheap fishing pole with wire ties. the whole thing is made from RG58. I used about 2M for feeding the antenna, the ground planes are just made from pieces of 450 OHm feeder cut at just over 1/2 wave then stripped in the middle , bent into a v shape and soldered straight the the outer braiding of the coax (done twice into a cross shape which is also soldered together for strength) then waterproofed. The reflector size of a biquad I believe is about 3/4 of a wavelength square, however this size is not critical as long as it covers the biquad. What is critical is the spacing between the biquad and reflector this is altered for VSWR but is typically about 1/7 of a wavelength Quote Link to comment Share on other sites More sharing options...
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