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stadl

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stadl last won the day on October 29 2017

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About stadl

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  • Birthday 19/12/1973

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  1. stadl

    What's on your mind?

    Don't under estimate the number of 60+ middle class suburban women, and the ease of buying a subscription for a mother's day gift And to be fair, with the 3-4 different titles, all for the same market getting passed around the circles of friends, they probably have more repeat readers of a publication per subscription than almost any other print media - so the advertising success might be better than you think.
  2. stadl

    Very unwell family

    Rough set of conditions there. Hope things work out ok. Yell out if you need help with anything - or just need a distraction for a bit.
  3. I'm with Chaos on this. When I first looked at that syllabus, I saw the date is 2003 and thought, this is guaranteed to be shit. But when you read it from the perspective of learning outcomes (I'm not a qualified trainer, but I have produced enough training packs to comprehend the value of understanding the LOs) it's not so bad. A little dated, hard to make compelling.exciting, but if you focus on the LOs rather than the 'Abouts' the syllabus is a reasonable framework. So you don't need to spend an eternity covering appletalk and IPX/SPX, but they need to know they existed in a timeline/evolution of networking technologies, reached their limits (for commercial or technical reasons) and got passed by. The fundamental architectures of networks, the building blocks - why is a switch better than a hub? why is a hub better than a switch? routing vs bridging etc. All good to know. Duplex/simplex modes, and it's always a good thing if there is 1 less person in the world who can't differentiate baud rate from bit rate - dialup modems might be dead, but radio codecs are not, nor is quantum computing. As for a project, I'd be tempted for build networks using different technolgies/architectures measure and assess - make it relevant, because the performance can be ping times/latency for gaming, or data transfer rates for streaming videos. Understand protocols - then transcode streaming music/video and examine the bandwidth.
  4. stadl

    Smallest, most direction antenna

    Really not sure what that would do - That is a question for people with more brains than me.
  5. stadl

    Smallest, most direction antenna

    Just consider the back of an envelope calculation for what the ToA difference is for a pair of antennas 30cm apart: 1 nanosecond - and that is for a signal in line with the antennas - so you are measuring angle by comparing differences between 0 and 1ns - needing you need to measure many times faster than 1ns - or downconvert in a phase matched way. Possible, just not simple or cheap
  6. stadl

    Smallest, most direction antenna

    ToA systems are non trivial. The correlation algorithm is a brute force of Multiply-Accumulates that's pretty easy to implement. The issue is what are you performing the analysis on? Raw RF or downconverterd/IF. There are SDRs around these days that will output the I&Q samples of the IF, so that is best. Sample time and rate are a balancing act - bigger sample set (increased by sampling faster or taking more samples - longer capture) will give more data for correlation to reinforce. But you ideally want the Rx and TX stationary during the measurement, so short capture time is desirable. A preference to higher sample rates if you can get there. The best outcome is from adding channels - but every channel is an antenna, phase calibrated feeder, high quality phase aligned receiver an ADC channel, more data bandwidth, and more processing - so more $s and more size/weight & power - TANSTAAFL Triangulation is not going to happen in the DF receiver. That is a higher level tracking function performed by the person or software that extracts multiple Lines of Bearing from different locations. The underlying DF technology is independent of the higher level tracking systems. I've worked on systems (as a hobbyist) combining doppler receivers and small beam antennas being swept. Include your GPS position on a map, and you have a simple but effective mobile vehicle mounted DF system. The 2D plane part is not really an issue. Yes, your Rx Antennas need to be in the same plane - but easily achievable. The Tx antenna, assuming a single antenna, won't make much difference, other than 3dB loss if it's alternate polarisation (horizontal vs vertical). It won't mess with the phase detection/ToA calcs that much.
  7. stadl

    Smallest, most direction antenna

    Bearing accuracy will be dependent on a lot of things. The 2 most common ways of configuring/implementing that sort of approach are: 1. Switched antennas and a single receiver - look at 'Doppler RDF kits' for details - I've DFed with the equipment - and got solid bearings to around 10 or 20 deg with 4 antennas on a larger scale (approx λ apart) - But never looked much into the maths/implementation details to understand how it works. 2. Multiple receivers, with time of arrival calculations / beamforming. Antennas need to be rigid (wobble relative to each other = errors), phase matched/calibration on all cabling and synchronised receivers required. My dated and edge experience* was with acquiring the signal with ADCs and processing in the digital domain, but I believe there are also options for analogue mixing of the signals to get a blip. * worked as a software dev on a few Radar and DF systems, in the late 90s - not in signal processing subsystems, but needed to understand the basics of the physics, system design and software concepts. I'm not qualified to answer the details, but have a good generalist/systems engineering view of these sorts of systems to be able to identify the challenges based on past work with a bunch of people far smarter than me.
  8. stadl

    Smallest, most direction antenna

    If 35x20cm is too large, you're going to have a hard time with DF. A mangentic loop antemnna may get the size a little smaller, but best DFing is with the null, which is supposedly counter intuitive for beginners. Without knowing the usecase, I can't say of there are other technologies applicable to your task, that may not even need to use DFing technology.
  9. stadl

    Smallest, most direction antenna

    Longest element will be half a wavelength - so approx 35cm, and element spacing will be approx 20cm. So a 2 element is 35x20ish, and a 3 element is 35x40ish - + extera boom length of you want a handle I have seen some nice designs using a rudimentary pistol grip under the boom, which reduces the boom grip and can be prefereable for some people holding, while others just extend the boom 10-20cm behind the reflector to grip it that way. - Lots of personal choice on construction. Some of the simplest designs use tape measures (cheap flat spring steel) and a bit of broom handle or PVC pipe/conduit There are a several designs around - googling for '70cm 2 element beam' or '433Mhz 3 element yagi" or even "UHF direction finding antenna" or combinations of whether you want to work on 'beam vs yagi' frequency vs wavelength etc. http://www.csgnetwork.com/antennae3ycalc.html While the theory can calculate element lengths and spacing, the element material and thickness , what the boom is made of (conductive or non-conductive) etc, all vary the system, and lots of people have through trial and error created common designs that work, and some are sold as kits, but many are published with design notes and pictures for you to build at home. The good thing is that the Amateur radio 70cm band covers 430-440MHz (with extensions to down to 420 or up to 450 in some parts of the world). The 433MHz ISM band commonly used by garage door openers and other low power devices is in that band that all have to work or not work on their own merits, in a generally unlicenced maship. But , the band overlap, means that an amateur radio design for 70cm beam should work for 433 ISM band devices without modification With most single band designs you can elect to delete director elements from the front if you need to modify the design. It WILL reduce the gain/widen the lobe, and it might impact other characteristics, but it should still be resonant - just don't push it too far. Chopping a 4 element design to 3 is probably manageable, chopping a 13 element design back to 3 is asking for problems. But it's worth a try if you find a design. Also be wary deleting elements for dual-band antennas (e.g. 2m/70cm) , as you may not know which elements are shared between bands, or have some complex interaction. https://en.wikipedia.org/wiki/Yagi–Uda_antenna
  10. stadl

    Smallest, most direction antenna

    That's a 90 degree model - has negligible applicability to a 45 degree model. 6dB is not bad, but it's also identical front and back, which is not ideal for DF You could DF with it - but I still don't know why you are obsessed with an L when a Yagi would probably be better. Less gain, and some front-back ratio is more useful.
  11. stadl

    Smallest, most direction antenna

    A single element won't act as a reflector unless it is placed at the correct spacing. A few mm away won't act as a reflector, it will just act as a large capacitive element and mess up the antenna. Placed at the right distance from the main element it will have the correct parasitic resonance and cancel out signal in a particular area, and reinforce it in other directions - that is the fundamental design of a 2 element beam/Yagi antenna. And yes, the antenna elements are not required to be straight - so you could have L shaped elements - but you would need to model it to determine the variations to length and spacing, as it won't be exactly the same. If you want to take that to extreme, look at the Hexbeam/Spiderbeam antennas that (on larger wavelengths typically 10-40m) bend the elements one or more times in Ls, Ws and Cs to allow longer elements to fit in a smaller space - but they are not simple.
  12. stadl

    Smallest, most direction antenna

    Yes it sort of works that way, but remember the scale, and the need to fully enclose the receive antenna except for the aperture - otherwise, wile you will have a small peak surrounded by a big null for maybe 30 degrees either side, the back side, that sheet will act as a huge reflector and give the antenna pattern a fat arse for 180 degrees at the back In terms of scale for the aperture - The cutoff frequency for a rectangular aperture is where the wavelength is less than double the longest side. So to pass a 70cm signal, the aperture needs to be no smaller than 35cm That gives you a view of the scale you are building - a pinhole camera style setup with a 35cm+ pinhole. Then you have diffraction and reflections, as you have realised - several nested boxes - or at least stacked aperture plates could prevent this with this - but then you're probably just better using a tube as the aperture - but size will need to be probably a wavelength long. You usually then need conductive stuff far enough away from the antenna that it does not form part of the antenna - several wavelengths would be nice. I'll confess I'm reaching the limits of my current knowledge and understanding, on this topic. I understand the basics, but have forgotten the details - it's been 25 years since I studied, and antenna design and RF black magic haven't been much of my dabbling in amateur radio since.
  13. stadl

    Smallest, most direction antenna

    Not practical for the sizing he wants unfortunately. This sort of concept works like a scaled up version of a pinhole camera, or a spotlight with baffles. But consider the scale difference between the wavelengths - Light is ~700 nm, while UHF (433MHz) is ~700mm. UHF is on a scale 1,000,000 times larger,. You can build a box/baffle, but it will be km across, and directional on a solar system scale.
  14. stadl

    Smallest, most direction antenna

    The L dipole won't really give you enough directional to be useful for DFing. They are more of interest when you want to reduce height/ground real-estate in n a HF antenna when the lengths are 10s of metres. You get a longer resonant length in a shorter space - lose some omni, but at least gain a little by pointing the small gain in a useful direction, and the weak end where there's nobody you want to communicate with. The shielding/aperture concept not so easy in reality - unless you are building a dish. But in the case of a simple 2 element beam (e.g. yagi) antenna you have the reflector at the back which does give some front-back, while being about 35 wide, 20cm long. A 3 element beam, would be adding one director element in front of the driven element, and up the gain/push the pattern a bit more to the front, and double the length - with the right construction either option can be very portable. Having a significant back lobe is not the end of the world. If you stand in one spot and spin 360, you'll get two directions, move a few times , and repeat, and quickly you find the geometry only works for one of them, as one way direction your Line of Bearings will cross, while the otherway they won't
  15. stadl

    Smallest, most direction antenna

    Sounds like the requirement is pretty close to a ARDF/radiosport configuration, on 70cm/UHF. Don't be thinking that a narrow beam is always what you want. Narrow Beam antennas are rarely a nice clean pattern. Expect several side lobes, and some of them are quite distinctive - so while they might be 10dB below the main lobe, they are still 10+db above the nulls on either side of them. Assuming you are manually sweeping the antenna around, you might get a ...oo...oOo...oOOo...oOo...oo... sort of result as you sweep past. Very easy to end up chasing a lobe off the boresight even worse if you are in a location with lots of reflections. A lot of people use a relatively low gain, one wider smooth lobe at the front, and enough front -back ratio to be able to tell the difference. Then you sweep the single lobe and listen/watch for the peak gradual rise then fall, and you can get the peak within +/- 10 degrees, which is good. An alternative is a loop, that has a very narrow null, and then you steer for the sileence. In terms of 'pocketable' does it need to be that size in use, or in transport. There are several designs for collapsible antennas - and cutting up a cheap tape measure for metal elements that roll up - and flex/bend when you bang into things is a classic design for ARDF antennas. Also be aware with dishes - you end up with a beam in azimuth and elevation - doubles the complexity, because you can't hunt in one dimension at a time, you have to always hunt polar-2D.
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