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50Mhz Yagi designs for free :)

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Justin, G0KSC

Wed Nov 26 2008, 05:24PM
After building other peoples designs with great success, I decided some months ago to start designing my own antennas. As I have said in previous postings, I do not wish to re-invent the wheel but often, an antenna is desgined around other peoples requirements. With this in mind, I have taken a couple of fix parameters in whatever I do. These include the following:

    50ohm direct feed impedance
      minimum reactance
        UK available size materials
          designs made with cost bias
          (what I mean by this is that 1/2 inch tubing is around 1/2 price of 3/8 inch for example)
            sizes to fit all
            Sizes to fit all? Today as an example, I have been optimising 6 element antennas with the above parameters in mind at sizes from 5.75 metres boom length to 7.25 metres.

            Below is the examples of what I have managed to achieve with the 6 metre boom antenna.

            The SWR plot shows levels between 50Mhz and 50.5Mhz. The next 2 show gain in free space and at 10 metres above ground. Note above ground this compact antenna shows 16.56dBi gain and still achieves 25.16dB Front to Back.

            Also, I have a local supplier who is giving excellent rates for materials which I have been using for my experiments. 1/2inch 5 metre lengths of tube for just 5.00 pound each! The company is called Clifta Steels located on Canvey Island, Essex.

            If you have any interest in more info on any of the above, drop me a line.

            JJ G0KSC

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            Uffe, PA5DD
            Thu Nov 27 2008, 09:18AM
            Uffe, PA5DD
            Registered Member #218
            Joined: Wed Apr 09 2008, 08:48AM

            Posts: 9
            Thanks for sharing, Justin.

            It's not that I want to critisize your design choices, but personally I do not see the advantage of choosing a 50 ohm feed impedance in a monoband yagi. You will need a balun in any case, so why not use this piece of cable to provide a better feed impedance. Martin DK7ZB wrote a piece about it
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            Christoph, DF9CY
            Thu Nov 27 2008, 10:19AM
            Christoph, DF9CY
            Registered Member #65
            Joined: Thu Mar 20 2008, 04:27PM

            Posts: 31
            Hi Justin,
            that looks fine AND quite similar to a design I made years ago AND used for long time. The description is on my website with EZNEC files and images. (http://www.df9cy.de)
            Instead of using a 50 Ohm dipole, which needs a 1:1 BalUn anyway, I used a folded dipole (200 Ohm) and a balun.
            If you take a 50 Ohm dipole, you can take the match DK7ZB describes, as Uffe stated.
            Justin, will you post the dimensions here ??

            73 de Christoph
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            Justin, G0KSC
            Thu Nov 27 2008, 08:28PM
            Justin, G0KSC
            Registered Member #64
            Joined: Thu Mar 20 2008, 04:10PM

            Posts: 49
            Hi guys,

            Yes Christoph, I can post the dimensions here although they are not on the machine I am on. How can you say the designs look like those you have done when you haven't seen them, only the plots? It is quite funny, It sounds like something you hear kids in the streets say 'I saw it before you!'.

            Joking aside, I am sure none of us need to be Antenna engineers to know that a given antenna for a given band with a given feed impedance will be very similar to another of the same boom length, element numbers, and frequency. I am quite sure christoph, someone had similar designs to whatever you have on your website, before you.

            Uffe, the reason for using a 50 Ohm feed is for simplicity. on the DK7ZB site, did you see the rather burnt lengths of coax? Due to the exceptional high voltages carried within this matching devices (lengths of varying impedance coax cables) with a few hundred watts, BANG! your matching arrangement is dead. The power limit on my Yagi's are rated at teh same as the feeding coax. Simple.

            With regards to the second question about matching, on my antennas, you can use a simple coil in the feed coax (as Pop YU7EF does) or a lambda 1:1 balun using the feed coax type.

            Question:- Why not take a 50 Ohm patch lead from our rigs, feed that to a transformer to 93ohms, connect 93 Ohms coax for 5 Metres, then another transformer to 75 Ohms for another 5 metres, the another transformer at the antenna to 28 Ohms? Why? because A, we do not need to and B, we do not need the additional losses. So in addtion to the less stability we get in lower impedance antennas, why would we want to add a transformer that we do not need?

            JJ

            [ Edited Thu Nov 27 2008, 09:05PM ]
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            Justin, G0KSC
            Thu Nov 27 2008, 08:55PM
            Justin, G0KSC
            Registered Member #64
            Joined: Thu Mar 20 2008, 04:10PM

            Posts: 49
            Reflector at 0 (Zero)
            OK reflector to Driven is 662mm
            to first director 976mm from Reflector
            to second director 2.41 metres from reflector
            to third director 4.462 metres from reflector
            to fourth director 6 metres

            These element sizes are HALF element sizes:

            Ref 1.53 Metres
            Driven 1.492 Metres
            Dir1 1.39 Metres
            Dir2 1.344 Metres
            Dir3 1.337 Metres
            Dir4 1.33 Metres

            If anyone was the .nec file I can send it too.

            JJ
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            Uffe, PA5DD
            Fri Nov 28 2008, 10:04AM
            Uffe, PA5DD
            Registered Member #218
            Joined: Wed Apr 09 2008, 08:48AM

            Posts: 9
            But Justin, you will have the same order of magnitude potential difference between the baird of your balun (or cable coil) and for example the boom. Your cable is rated for the un-balanced potential inside the cable, not the potentials arising at the balanced feedpoint. The outer isolation of most cables were not chosen with electrical properties in mind.

            Care should be taken to keep the first couple of windings away from the surroundings.

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            Justin, G0KSC
            Fri Nov 28 2008, 04:11PM
            Justin, G0KSC
            Registered Member #64
            Joined: Thu Mar 20 2008, 04:10PM

            Posts: 49
            Aggreed. The point (missed I feel) is the increase in voltages carried in the matching arrangements on low impedance antennas.
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            Justin, G0KSC
            Sat Nov 29 2008, 07:15PM
            Justin, G0KSC
            Registered Member #64
            Joined: Thu Mar 20 2008, 04:10PM

            Posts: 49
            A Low impedance balun: the results
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            Zaba, OH1ZAA
            Wed Dec 03 2008, 10:07AM
            Zaba, OH1ZAA
            Registered Member #123
            Joined: Thu Apr 03 2008, 02:54PM

            Posts: 25
            The Collins-balun is easy to construct and very easy to package. I have used it for any band during the last 30 years. --- --- Though mechanical symmetry (in construction materials) is "elegant", I generally extend the center conductor of the coax with 60 cm (@ 50 MHz) of regular installation wire and initially fold it back onto the coax insulation to mark the grounding incision for the other end of the conductor.... Then, starting from the incision, I coil the coax into 2½ turns and continue in the same direction with the wire connected to the center conductor for another 2½ turns and solder the end at the incision to the coax braid. After that the whole balun can be covered with tape, to avoid moisture between the turns when mounted in free air. Before that connection lugs have to be soldered at the balanced end (the only two accessible points at the end of the original coax). --- 73, "Zaba" OH1ZAA/OHoMZA ---
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            Zaba, OH1ZAA
            Wed Dec 03 2008, 10:29AM
            Zaba, OH1ZAA
            Registered Member #123
            Joined: Thu Apr 03 2008, 02:54PM

            Posts: 25
            Continuing from the message above, I mean to say that the second half of the balun does not need to be constructed with coax (like in the link-pictures [green arrow]); the magnetic field coupling will take care of the symmetry, and parasitic capacitance of the "frequency centering" (if you are lucky). In any case it is wide-band enough for e.g. a 28 - 70 MHz feed. ---- I have used this Collins-balun successfully on my 50 ohms 6.5 m 6-ele 50 MHz PJ2/OH1ZAA-design for the 1992 DX-pedition. It is a very fortunate design that I have scaled for other bands too. Normally feedpoint reactive impedance grows more positive with frequency, but with this one it is turning around above resonance. This allows exceptional bandwidth with a gamma match, and a flat 1:1 SWR over e.g. 50.0 - 50.5 MHz. It is one of rear cases in antenna design where everything comes together in harmony (like Gain and F/B on top of all). Will release dimensions one day, if not done already --- Good luck! 73, "Zaba" OH1ZAA/OHoMZA ---
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