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matching 6 m 2 element Yagi
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I am building a 6 m 2 el Yagi dK7ZB design and it is matched with a transformer made from 75 Ohm coax. 2 stubs of 1/4 Lambda in parallel. I tohought I could ?put a 28 Ohm resistor replacing the radiator ?and then have 50 Ohms on the Nano VNA at 50 Mhz resonnance. ?But that is not, what I see. ?Resonance is 37.8 Mhz ?at 47 Ohms resitance . At 50 Mhz resistance would be 38 Ohms and SWR 1,4. ?Prior to that I had mesured the coax with a velocity factor of 1 and could calculate a Velocity factor of 0,63 for the cable which is an unknown TV cable. Then calculated the lenth with that VF so I thought I should not be too far of. ? Any suggestions ?
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Marin
DK3 UW |
开云体育Hello Marin, I assume your goal is to achieve a better match between the antenna and the feed line. Perhaps you could use a single piece of 75Ω coax, cut to 1/4 Lambda, inserted between the antenna's feed point and your 50Ω feed line.? If the dipole feed point is close to 72Ω, then this arrangement could be expected to present approximate 60Ω to the 50Ω feed line.? This should give you a 1.2:1 SWR.?? 72 x 50 = 3600, ?? (3600)^0.5 =? 60, ? 60/50 = 1.20 SWR? Not perfect, but close.? Good luck!? Tom K4AKC ---------------- On 2025-03-26 11:35 am, Martin DK 3 UW via groups.io wrote:
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You need to know what the real VF for the coax used.
That and there is no such thing as a coax with VF=1.
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Having built that transformer I found with known 1/4 wavelength?
measured using VNA the transformation was very close to the
28 ohms.? However if the VF is incorrect it will be very wrong.
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For the cable used a cable of VF=1 would be 58 inches(approximate)
but for real 75ohm RG59 VF was .68 was close to 40" and connectors
internal length counted as part of the actual electrical length.
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Generally TV cable is foamed core with VF closer to .8(not exact
and varies)?and solid insultator cored cables closer to .66 but
until you measure it will be a genralization at best.
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Allison -------------------------- Please, private email goes to bit bucket |
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well I used the VF of 1 to calulate the VF of the Coax . The NVNA showed something like 2,4 m for the real lenth of 1.5m so that calulated a VF of 0.63 for the coax. Then I calculated 75/50,1x0,63 for the coax lenght. One piece with shortened end gives the naround 50 Mhz so that fits. For the Yagi according to DK7ZB it should be 2 coaxes in parallel to tranasform the 28 Ohms to 50 Ohms. Actually I am unable to mesaure that. I would have liked to check that before mounting it to the antenna.
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Martin
DK3UW
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开云体育Personally, I don't think an SWR of 1.4 will give you much problem, but you might consider using NanoVNASaver (free application) with your VNA to sweep the frequency from, say, 30 to 60 MHz and save the result as a Touchstone file (.s1p format).? It is VERY easily done, and then you can use Zplots by AC6LA (also free) to read that Touchstone file to plot almost any parameter you want as a function of frequency ... including resistance and reactance.? It might help your investigation. Keep in mind that the reactance of a resistor doesn't change with frequency, whereas the reactance of the driven element of the antenna will.? The resistor will only be accurate at (in your case) 37.8 MHz.? Also, it takes very little difference in the length of a 1/4 wave stub to make a difference in its transformation.? You obviously know what you're doing so I'm sure that you are aware that it is a hyperbolic function in form ... and it will transform BOTH resistance and reactance.? You could possibly be off just a little in the physical length or in the estimate of 0.63 for velocity factor.? I just did a bit of playing around with TLW, the free transmission line application, and at 50 MHz a difference of only 0.5 inches in length of RG-6 coax changes the reactance of the input of a quarter wave transmission line terminated in 28 ohms by 10 ohms.? A difference in velocity factor between 0.63 and 0.64 would also be about 0.5 inches for a quarter wavelength at 50 MHz. Once you build the antenna, you could probably bring the resonance almost exactly to the frequency you want by using some sliding sections of dielectric (like nylon or HDPE).? It has a loading effect that is variable depending upon where on the driven element it would be placed ... more effect at the ends where the E-field is greater and almost no effect at the center. Just some thoughts ... Dave?? AB7E On 3/26/2025 9:35 AM, Martin DK 3 UW
via groups.io wrote:
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thanks Dave. In fact I rather don't know, what I am doing. I just did not want to get an antenna off the shelf and do something homebrew and the DK7ZB conzept looked easy. But hen on the first setup something did not match as predicted so I suspected the? matching stup so I wanted to measure it. Up to now I only have used the NVNA to trimm antennas to resonance.
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Martin
DK3UW |
开云体育Well, I think you understand more than you think, and certainly more about? antennas than the average ham. A few years ago I made a 5 element DK7ZB antenna for 6 meters.? The version I made was one of his models that gives 50 ohms without the need for the matching system, but it worked great just as I built it.? I'm a great fan of his work ... he knows what he is doing.? I'm sure your antenna will work well also, and as I said ... a 1.3 SWR is not a problem at all.? The loss in gain will not be measurable in practice and almost any transmitter can handle 1.3 even at power. Best regards, Dave?? AB7E On 3/26/2025 1:49 PM, Martin DK 3 UW
via groups.io wrote:
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just an update. I made the 1/7 wave stub again and fixed it to the Yagi. By adjusting the length of radiator and director I ended up with a resonance at 50.190 and 50 Ohms. SWR is below 1.5 from 50.08 to 50.350 so perfect for CW and FT8 The antenna is up 10 m now and reports for a station abt. 80 km away are abt. 10 db better than on the? 1 element quad.? As the signal is not stable it is difficult to get exact figures but F/B is around 2 S units and at 90 ° its even less. So that looks like a good? match with the expected performance.
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Martin
DK3UW |
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