Timeline for answer to Dual band yagi-uda radiation problem by Ryuji AB1WX
Current License: CC BY-SA 4.0
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25 events
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| Jul 11, 2025 at 18:03 | history | edited | Ryuji AB1WX | CC BY-SA 4.0 |
Realized an error in my thinking.
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| Jul 10, 2025 at 12:56 | comment | added | LaCrimen | okay, I've found you. I'm currently writing you a message on forums.qrz.com. Thank you! Everything you said can solve this problem, but let's talk about details at qrz.com. | |
| Jul 10, 2025 at 12:54 | vote | accept | LaCrimen | ||
| Jul 10, 2025 at 12:38 | comment | added | Ryuji AB1WX | If you place 2m R1 right next to 2m DE, the feedpoint impedance drops sharply. If folded DE, that may bring 200ohm to 75ohm range. You could add R2 to bring it down further and also improve F/B. That's a part of the advantage of 5.2.1. Conversely, if you place 70cm R1 right behind 70cm DE, you are minimizing the impact of 2m DE/R1, but at the same time, the feedpoint impedance of the 70cm DE may be more like 20 ohm. Study W1JR 4 element Yagi published in 2024; he used an extra director to adjust the impedance. You can use the same technique to adjust the feedpoint Z of DK7ZB on each band. | |
| Jul 10, 2025 at 9:00 | comment | added | LaCrimen | I think, the best for me is to replace DE 2m some centimeters behind DE 70 cm for about 6-7 cm, place long and resonant RE 70 cm near DE 70 cm and see what could possibly happen. With folded dipole, I would need to move it also to the rear and check influence on parasitic elements - folded dipole would be more influential on directors than normal dipole, which antenna was calculated for. Again, more problems - on folded - 4:1 balun, on normal DE - transmission line from filter to gamma-match. Thank you again for help. Maybe you could know what is better, considering mechanical aspects. | |
| Jul 10, 2025 at 7:43 | comment | added | LaCrimen | Will give you a credit. My yagi on 2m is DL6WU long-yagi solution. Rethinking of all the problems is crucial now. | |
| Jul 10, 2025 at 7:37 | comment | added | LaCrimen | #5 is brilliant idea. I don't like DK7ZB solution - because of no impedance transformation. I would like to have 70 cm and 2m separately feeded, which I could tune up to my best frequency and get rid of reactance. You're right about trusting simulation. But folded dipole makes another problem - where would be 4:1 transformer? 4:1 from coax seems ok, but it's quite big. Will reconsider two options: 1. 60 + old 44 mm in distance from dipole, place resonant, but big RE behind DE 70 cm (with problem - how to make transmission line from filter to gamma-match on 2m? By coax?), 2. folded dipole. | |
| Jul 10, 2025 at 3:09 | history | edited | Ryuji AB1WX | CC BY-SA 4.0 |
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| Jul 10, 2025 at 2:46 | history | edited | Ryuji AB1WX | CC BY-SA 4.0 |
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| Jul 10, 2025 at 2:24 | history | edited | Ryuji AB1WX | CC BY-SA 4.0 |
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| Jul 10, 2025 at 2:03 | history | edited | Ryuji AB1WX | CC BY-SA 4.0 |
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| Jul 10, 2025 at 1:43 | history | edited | Ryuji AB1WX | CC BY-SA 4.0 |
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| Jul 10, 2025 at 1:14 | comment | added | Ryuji AB1WX | I have a lingering question as to whether you can trust simulation, at least the engines you are currently using, since they failed to see this problem. | |
| Jul 10, 2025 at 1:13 | comment | added | Ryuji AB1WX | @LaCrimen Given your results and DK7ZB result, I think some significant sidelobes on 70cm are unavoidable because 2m elements are excited in overtone mode. DK7ZB took his approach by tolerating the sidelobes coming from the DE but then suppressed them with 70cm-dedicated parasitics. Yours has the disadvantage of 2m DE adding big side lobes. You still have some room to make your design unique, but I think DK7ZB's idea of sharing the driven element is one brilliant solution. The simplest effective solution I can think of at the moment. | |
| Jul 9, 2025 at 23:38 | comment | added | LaCrimen | Okay, but something doesn't suit for me in DK7ZB antennas... I know this solution by having one dipole for 2m and 70 cm. Maybe placing DE 2m around 5-7 centimeters before long (something around 350 mm in length) RE 70 cm could work. Side lobes at 45 degrees are the problem - if it could be reduced to -13 dBm at least, compared to main lobe, I would be very happy. By repositioning DE 2m by 10 centimeters, nothing bad happening in simulation, at 145 MHz. Maybe F/B worsen by 1.5 dB, but gain remains unchanged. I will check out this W1JR yagi. | |
| Jul 9, 2025 at 21:46 | history | edited | Ryuji AB1WX | CC BY-SA 4.0 |
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| Jul 9, 2025 at 21:43 | comment | added | Ryuji AB1WX | DK7ZB dual band Yagi "solved" this problem by sharing the same driven element on both 2m and 70cm and added a 70cm-dedicated radiator and directors. That would work. Still a compromise, but I can see a better compromise than having two driven elements close by and interacting. You can adjust the feedpoint impedance on 70cm by adjusting the reflector and director placement. | |
| Jul 9, 2025 at 21:35 | comment | added | Ryuji AB1WX | One thing worth considering is to move DE 70cm well behind DE 2m so that there won't be anything between DE70cm and R70cm, and nothing is placed close to DE70cm. That's the most critical space in Yagi. | |
| Jul 9, 2025 at 21:31 | comment | added | Ryuji AB1WX | If other parameters are comparable, a reflector interacts more strongly with the driven element than a director, given the same distance. If your simulation fails to capture the cross-band interaction, the whole optimization and the design become a question. You might need to manually tweak the 70cm yagi, especially the driven element and its nearby elements (including those of 2m). Your 70cm Yagi's reflector most likely needs a tweak. After all, if you care about performance, it may be best to use a sub-boom to "float" the 70cm yagi. | |
| Jul 9, 2025 at 18:33 | comment | added | LaCrimen | Thank you very much for all advances. Maybe I just move the DE 2m 5 cm away (it will be 94 mm in distance between DE's) and place wide reflector, 50 mm before DE for 70 cm? Is this 94 mm could be enough with reflector behind them? Or maybe more distance? I will need to reposition D1, D2, D3 and maybe D4 afterwards. Moreover, will need to reconsider feeding this 2m DE. This will be around 14 cm distance between N socket and dipole... | |
| Jul 9, 2025 at 17:47 | comment | added | Ryuji AB1WX | If you somehow fix or optimize away from this problem, it is likely that you want to reoptimize 70m reflector and D1 length/placement as they are probably strongly influenced by the presence of 2m driven element. You run optimization that probably did something given correct or wrong constraints (since the sim apparently failed to capture the interaction) and then when you fix that, the design is certainly not optimized, without re-work. It may simply that your 70cm yagi is not properly optimized due to failure to consider the interaction. | |
| Jul 9, 2025 at 17:44 | comment | added | Ryuji AB1WX | Just quickly for now. The impedance looks consistent with what I anticipated, given the narrowband nature of Yagi. If you electrically de-tune the driven element of 2m Yagi with a reactance, that interference may be lessened. It may or may not be a good enough solution by itself. Yeah the simulator may be a disappointment. If your simulation was run on a wider frequency range from 130 to 500MHz or something (just on the 70cm Yagi part) it may give some more insight, but may not be very fruitful. But regardless of how simulation gives us, I think the interaction is a good suspect in theory. | |
| Jul 9, 2025 at 16:47 | comment | added | LaCrimen | And what do you mean by writing "De-tune the 2m driven element intentionally"? | |
| Jul 9, 2025 at 16:36 | comment | added | LaCrimen | So, I could do the measurement earlier than I thought. So, here's the Smith chart: freeimghost.net/images/2025/07/09/vna2.png Do you need anything more? Why in MININEC3 and NEC2 there's no impact on far-field? Are theese programmes poorly simulates current distribution along antenna? I knew the fact, that MININEC3 takes it better than NEC2, and sometimes even NEC4. The 2m DE is 986 mm long. 70 cm DE is 330 mm long. D1 70 cm - 319 mm. D1 2m - 925 mm. | |
| Jul 9, 2025 at 14:46 | history | answered | Ryuji AB1WX | CC BY-SA 4.0 |
