tag:blogger.com,1999:blog-4774014561040227748.post7253358516479618262..comments2024-03-22T10:42:05.161-06:00Comments on KA7OEI's blog: "TDOA" direction finder systems - Part 1 - how they work, and a few examples.Unknownnoreply@blogger.comBlogger6125tag:blogger.com,1999:blog-4774014561040227748.post-26282026620808030652018-09-06T17:17:42.022-06:002018-09-06T17:17:42.022-06:00Hi,
Well, the drawing is correct - the text is wr...Hi,<br /><br />Well, the drawing is correct - the text is wrong, and I will correct it immediately.<br /><br />Because the diode "disconnects" the element when no current is passing through it, that end of the coaxial cable is open. If it were a 1/4 wave, the impedance would be "inverted" and the open at the far end would cause a short at the other, effectively shorting out the signal from the "other" element, causing the entire thing to be very deaf.<br /><br />A 1/2 wave line will preserve the impedance, so an "open" at the far end is also an "open" (very high impedance) on the other so the piece of coax on the de-energized diode's antenna element will be (more or less) invisible.<br /><br />On where to measure, if the leads are kept short (1/2"/1cm or less) the broadband nature of the coaxial stubs means that they are quite forgiving in measurement error - but I typically measure from the ends of the center conductor. On something like this, the actual dimensions aren't super critical (as velocity factor of coax is quite variable) but it's more important that where multiple stubs are involved - particularly in antenna phasing - that be made identically.<br /><br />Good catch - thanks!<br />KA7OEIhttps://www.blogger.com/profile/01153508130273704727noreply@blogger.comtag:blogger.com,1999:blog-4774014561040227748.post-48381595092954734792018-09-06T15:32:55.289-06:002018-09-06T15:32:55.289-06:00Great writeup!
However I got a bit confused abou...Great writeup! <br /><br />However I got a bit confused about the L2 length numbers. If I understand it correctly from the TDOA switch antenna picture (picture 4) L2 is the length from the "center joint" to the antenna and should be 0.5 electrical wavelength (26" for 2 meters and RG-58x with velocity factor 0.66). In the text you state: "L2 = 13.5 inches (34cm) for cable with a solid polyethylene dielectric". This would mean that the feedlines from the "center joint" would be about 1/4 wavelength?<br /><br />Batilanhttps://www.blogger.com/profile/10765551905609132586noreply@blogger.comtag:blogger.com,1999:blog-4774014561040227748.post-13257588912613943792018-04-16T18:05:05.738-06:002018-04-16T18:05:05.738-06:00The most important thing with a "rotating&quo...The most important thing with a "rotating" antenna type of array is that ONLY the antenna that is "ON" should be "visible" in the array. In other words, if you have four antennas in an array, only the one currently selected should have a low impedance - the other three should be "open" or else they will act as parasitic elements. The original "Dick Smith" RDF array worked miserably in its original configuration as they shorted the "unused" antennas to ground to stop the RF - but instead, the other three elements acted as parasitic elements, messing up what should have been the omnidirectional pattern of the the one antenna that was activated.<br /><br />There are three common ways to do this:<br /><br />- A diode switch AT the antenna being switched with little/no feedline: When "on", the diode connects the antenna, when "off" this antenna is, for all practical purposes, floating an not longer resonant. If the feedlines connecting the antennas to the common point are the same length and multiples of 1/2 wavelength, no diodes are required at the junction point - but any other length will require diodes at the junction, too.<br /><br />- Use ODD multiples of feedline to connect the antenna to the diode switch and short out the antennas NOT being used at the end of that feedline, at a common switching box. Because of the odd 1/4 wavelengths, the "antenna" end of the feedline will appear to be open. Practically speaking 1/4, 3/4 and 5/4 will work pretty well at VHF and UHF - but if you go too long, "open-ness" will gradually diminish. This method typically requires two diodes - one on the "antenna" side of the feedline to short it out and another one on the "radio" side of that first diode to open the circuit so that the other antennas connected at that common point weren't shorted out.<br /><br />- Use half-wave multiples of feedline to connect the antenna to the switching box and inside this box, the series diode is turned off when the antennas are NOT being selected. This has the advantage of needing only a single diode to "open" the circuit. 1/2, 1 and 1-1/2 wavelengths seem to work out OK - but going much longer than that reduces the effectiveness.<br /><br />I personally use the last method - 1/2 wavelength multiples on both ground plane vertical antennas (on a vehicle) and full-sized vertical dipoles (on a roof). On my rooftop antenna I didn't bother putting baluns or chokes on the coax so one side of the dipole is connected to the shield all of the time - but contrary to what others might say, I can't say that I've seen a problem with doing that.<br /><br />As long as one has at least 15dB of "off" isolation, it might as well be "infinity" isolation. A single PIN diode in series works just fine - and that means that at 2 meters, practically any PIN diode will work. At UHF the self-capacitance of the PIN diode (and other circuitry/layout) starts to reduce isolation unless a low-capacitance diode is used with good construction techniques. Of course, when the diode is "off" one should design the circuit to put several volts of reverse bias across it to reduce its capacitance.<br /><br />I hope that this answers the questions.<br />KA7OEIhttps://www.blogger.com/profile/01153508130273704727noreply@blogger.comtag:blogger.com,1999:blog-4774014561040227748.post-67434674817663716842018-04-14T10:27:17.171-06:002018-04-14T10:27:17.171-06:00Great write up (you explain a lot and in detail; m...Great write up (you explain a lot and in detail; more than others do).<br />I've seen setups like this where the diode rf switches were both at the antenna elements and also at the T junction of the coax. I've pondered this and have to think that by having them only at the antenna elements means that the side that, at a given moment, is "off" leaves one with a open-ended coax stub on that side.<br />Not being really sure if that affects performance but when I had previously pondered the need for two sets of diodes on EACH SIDE, I concluded that reducing down to just one diode on each side would best be attempted with the diodes at the coax T junction. That way you don't have, electrically, an open coax stub to the side that is currently "off".<br />Wondering if you concur? I haven't built one of these DF setups YET... but thought that when I do I'd use four diodes, two on each element and two at the T junction, then try reducing to just the couple diode at the T junction. And I'd try similar but with the diodes at the antenna elements too - same as what you have depicted. Not sure if I'd be able to get definitive results but it could prove educational.<br />Thanks again. Great write-up that is pretty darn complete.!!Paul-e-wogghttps://www.blogger.com/profile/13634137808795782106noreply@blogger.comtag:blogger.com,1999:blog-4774014561040227748.post-45934766534986622522017-04-03T16:07:21.577-06:002017-04-03T16:07:21.577-06:00I checked the contact info on my main ARDF page an...I checked the contact info on my main ARDF page and it worked - try it again.<br /><br />Information on the processors may be found at the bottom of the side-bar titled on my main ARDF page, "How Do I Get Programmed Chips?"<br /><br />73KA7OEIhttps://www.blogger.com/profile/01153508130273704727noreply@blogger.comtag:blogger.com,1999:blog-4774014561040227748.post-74242912352168494052017-04-03T13:14:31.701-06:002017-04-03T13:14:31.701-06:00I am interested in purchasing your programmed PICs...I am interested in purchasing your programmed PICs for the Doppler III kit and the contact page on KA7OEI must be old. Give me a shout!Anonymoushttps://www.blogger.com/profile/16776383219731916094noreply@blogger.com