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  • Originally posted by lutorm View Post
    Heh, I just pulled out my copy of Jackson's "Classical Electrodynamics" to see what he says, and he seems to split things up into "electrostatics" which is what you are describing (stationary charges), "magnetostatics", which is what I'm thinking of (stationary currents and fields), and finally "electrodynamics", which is the treatment of time-dependent fields and the link between the electric and magnetic fields. So you're right, it seems there's no clear definition.

    The key change in behavior happens when the wavelength (ie speed of light / frequency) becomes small compared to the size of the system. For these purposes, 60Hz AC is like DC, because the wavelength is 5 million km. That's what makes it possible to use Kirchoff's law's, for example to assume that the current along a conductor has the same value everywhere. Once the wavelength becomes comparable to the size of the system that is no longer true, because now things change on the same time scale it takes the current or voltage to propagate from one place to another.
    Hi lutorm
    You are right. Very low power frequency AC (60 Hz), like DC, follows Kirchoff's law for the practical line lengths. However at RF level, wavelengths being less than few hundred feet, Kirchoff's law starts become invalid at wavelength scale, but if the line/antenna is split into very small segments, it still holds good at segment level. Current & voltages on a wire vary from zero at nodes to maximum at intemediate peak points, following the shape of a sine wave.

    Thanks for raising this issue, as it prompted me to study further and discover new facts and clear any misundetstandings I had.
    Best Regards
    Last edited by abcd567; 2014-04-24, 00:28.

    Comment


    • @lutorm
      @Birdie
      I feel that we are cluttring this forum by discussing fundamental theories, in which most forum members are not interested. Most members are interested in easy DIY instructions to make a good antenna/amplifier sytem.
      If you want to continue the theoretical discussion, I feel it is better we do it by email. My email is:
      abcd567@hotmail.com

      Comment


      • Yeah, good point. But I'd say we should just start another thread, that way the discussion is there in case others are interested.
        Patrik J. / KB1VGP / FR24 T-PHTO1

        Comment


        • Originally posted by lutorm View Post
          Yeah, good point. But I'd say we should just start another thread, that way the discussion is there in case others are interested.
          Good idea!

          Comment


          • It meeds one of you 'Antenna' guys to start it and take the first post or two - so you can start the thread with notes for a good antenna and go back and tweak [edit] that first post as the good design becomes a great design. Two threads :
            * One for mounting on a pole by the base
            * One for centre mounting / primary use at a window for high rise dwellers (Hong Kong, Shanghai, Toronto )

            Comment


            • Originally posted by SpaxmoidJAm View Post
              yep here ya go.
              [ATTACH=CONFIG]3949[/ATTACH]
              the lengths are from where the dielectric clears the bnc connector (I've written 136 on the drawing should 138 finger trouble). i imagine the gap between the wire might matter but considering how simple it is. ....
              Here are simulation results of your J-Pole:
              Gain = 3.76 dB; SWR = 4.47, Mmismatch loss (due to SWR higher than 1) = 2.23 dB.
              Net Gain as seen by receiver = 3.76 - 2.23 = 1.53 dB

              SpaxmoidJAm's J-Pole - simulation output.png


              SpaxmoidJAm's J-Pole - simulation input.png

              Comment


              • Here's the range on the 3 and 1/2 shorted top
                3_5_Shorted.jpg

                I'll take the j-pole to work tomorrow, i also made a 8 section open top today using the soldering method i used before on the 3 1/2. They just seems to have less resonances when soldered.

                my south east view is marred by the very hill that F-EGLF1 sit upons
                Last edited by SpaxmoidJAm; 2014-04-24, 17:04.
                T-EGLF8

                Comment


                • Originally posted by alexsiong29 View Post
                  Is anyone here know how to build a simple airband antenna?
                  118-136mhz...
                  Best and easy to make for less then $10 a 1/4 wave Ground Plane antenna.

                  ground_plane.jpg

                  See these HAM links for more info:

                  http://www.ccars.org/projects/2mgp/tech_2mgp.htm

                  http://www.hamuniverse.com/kc0ynr2metergppvc.html

                  Now you're going to want to center it around 122 MHz for the VHF Air Band so use this calculator: http://www.wavelengthcalculator.com if you want to tune it to a specific frequency like the Tower.

                  Take the wavelength and divide by 4 to get the 1/4 wave of which is what you cut the radials length.

                  122MHz = 152.69mm for the 1/4 wave.
                  www.ADS-B.ca

                  Comment


                  • E Mail I received from the 'FR24 Box' developer ! Ham Op' (Smart Guys, eh !) Guenter, DL4MEA..
                    Hello Jerry,

                    Yes, I designed and developed the FR24 receiver, which is sold with just different Linux software as the Radarcape product.
                    And inside it is basically a Mode-S Beast, but with some improvements found with the Radarcape.

                    Some advice for the more range? Well, it is LOS and you know it is mostly dependent on the quality of LOS and nothing else. Then, you should have less than 3dB cable loss, and not just a rubber duck antenna. The antenna delivered with the FR24 box is pretty good, it is around 5dBi gain and very good for that size.

                    With that set up, you should see a range of around 250nm distance. If you don't check antenna position and cable loss, etc... if not, then let me know.

                    I reach 250nm with that set up at home, and nothing else than the equipment described.

                    73, GŁnter

                    Guenter's Papers are here;
                    http://www.modesbeast.com/resources/...jekt-ADSB3.pdf

                    Interesting Read ! 73 (Ham Op for 'Best Wishes')
                    Jerry
                    VE6CPP
                    F-CYQL1

                    Comment


                    • So..with the addition of a Pre Amp Help ? Lets Experiment ! Guenter says
                      "• MatLab processing of captured data
                      • Cancelled:
                      I do actually not believe that there is any possible
                      gain from this in aspects like
                      performance increase and saving FPGA resources
                      • Further improvement of RF receiver
                      • faster detector using a diode
                      would drastically help decoding overlapping frames
                      • lower noise figure 'cancelled'
                      since there is no further improvement from 2dB down
                      to 1dB on a terrestrial antenna.
                      So.. 'Let's Keep On Experimenting' ! Tell us your Data..and I will Share with Guenter ! 73
                      Jerry
                      F-CYQL1

                      Comment


                      • THE SQUEEZED FRANKLIN
                        ....THE SQUEEZED ANTENNA CAN BE FITTED INSIDE A PVC PIPE
                        ....THE MATCHING STUB IS STILL AN OBSTRUCTION AND REQUIRES A BOX

                        Gain = 7.26 dBi, SWR = 2.18 causing mismatch loss = 0.64 dB
                        Net Gain as seen by receiver = 7.26 - 0.64 = 6.62 dBi


                        Squeezed Franklin-6 elements+Matching Stub-Simulation output.png......Squeezed Franklin Dimensions.jpg

                        Later Addition:
                        In above Sketch, one dimension of Phasing Stub was missed out (the longest part of Blue-Colored Phasing Stub).
                        It is λ / 4 = 68 mm. Please see sketch below:

                        Squeezed Franklin - STUB Dimensions.PNG
                        Last edited by abcd567; 2014-04-27, 14:11.

                        Comment


                        • Originally posted by abcd567 View Post
                          THE SQUEEZED FRANKLIN
                          ....THE SQUEEZED ANTENNA CAN BE FITTED INSIDE A PVC PIPE
                          ....THE MATCHING STUB IS STILL AN OBSTRUCTION AND REQUIRES A BOX

                          Gain = 7.26 dBi, SWR = 2.18 causing mismatch loss = 0.64 dB
                          Net Gain as seen by receiver = 7.26 - 0.64 = 6.62 dBi


                          [ATTACH=CONFIG]3970[/ATTACH]......[ATTACH=CONFIG]3971[/ATTACH]

                          How would it work if made from copper brake pie (3/16")? - easy to find on ebay ... it would probably flatten making the bends though.

                          Comment


                          • Peter - filling a pipe with sand before bending it will help to keep it from getting squashed at the bend.


                            Sent from my GT-I9500 using Tapatalk
                            T-VABB7 | RTL dongle + Raspberry Pi + dump1090 + Bulgarian 5dBi collinear

                            Comment


                            • I inquired a while back regarding the Receivers Noise Floor Specs.. which I now see is '2db'..so changing the 'supplied feedline'.. It is H155 cable, 29.6 dB/100m or 9 dB/100ft and 30 feet is 2.7 dB loss.

                              If you replace with LMR400 which for 50 feet is about 2.1 dB loss..Surprising as it sounds, you will not see much, if Any, improvement. You would be hard pressed to detect distance change related to a a 0.6 dB change in front end NF of 2db.

                              Seems to me your only avenue to greater distance is a higher antenna or an antenna with 2 or 3 dBi more gain.
                              Guenter mentions 5 dBi as the gain of the standard FR24 antenna, and you'd need one at least twice as long to see 3 dBi gain improvement.
                              Put the supplied antenna as 'High as Possible'..with the supplied co-ax, probably the 'surest way' to increase your distance...or build a 'larger antenna' I guess !
                              Jerry
                              F-CYQL1

                              Comment


                              • Originally posted by peterhr View Post
                                How would it work if made from copper brake pie (3/16")? - easy to find on ebay ... it would probably flatten making the bends though.
                                I will have to simulate again using 3/16" copper pipe to get the results and do the fine-tuning tweaks.

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