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I've recently created an yagi-uda antenna for 2m and 70 cm - specifically for 145 and 433 MHz. 8 elements for 2m and 14 elements for 70 cm. I've put 2m on MMANA-GAL and optimised by myself the 70 cm band, next - I've been optimising it with built-in Optimizer.

This antenna is being fed by one connector. From the beginning, the 32 mm 1.5mm^2 rod made from copper is feeding gamma-match for 70 cm. This "copper stub" is bent in 90 degrees. Next, in shunt, I have three-element LPF Chebyshev filter, then ends with another copper stub bent in 90 degrees, to the gamma-match for 2 m. Filter is closed in aluminum can. Two dipoles, connected mechanically and electrically to the boom (which width is 20 mm). The 70 cm directors are isolated. N-connector goes from aluminum can to the center of antenna - feeding it through H1000 coax, which is placed around 32 mm away from boom, but goes straight forward it. After D4 it turns on a quite large diameter arc to the PVC-arm between antenna and rotator. https://i.ibb.co/mQ02fC2/ant.jpg this is screenshot from MMANA-GAL.

Everything went great for MMANA-GAL simulations. In real, I have strong side lobes in 70 cm band for 45 degrees and 80 degrees. Antenna is usually better than Diamond X300 in 70 cm by 1-1.5S, but this isn't enough. For comparison, 2m is better by 3-3.5S in signal. Tried to move, shrink, enlarge D1, D2, D3 and D4 for 70 cm , place dipole on 10x10x3 mm aluminum distance and only I've reduced theese lobes from -4 dBm to -8 dBm. Gain didn't get any better. That's not normal for me. Where could problem possibly be?

antenna diagram

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  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Amateur Radio Meta, or in Amateur Radio Chat. Comments continuing discussion may be removed. $\endgroup$ Commented Jul 10, 2025 at 11:11

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Based on the comment exchanges, I strongly suspect the direct interaction between the two radiators. The 2m driven element is 3/2WL at 435MHz (or whatever, depending on where you tuned the 2m yagi) and that element will act as a "dirty" director below and a "dirty" reflector above 435MHz. The word "dirty" is in there to indicate that the interaction is on an overtone basis and contributes to a lot of sidelobes.

Your beam pattern sketch already confirmed the interaction of 2m and 70cm Yagi's.

You mentioned that F/B gets worse at 430/433MHz compared to 440MHz. That is because the dirty director is competing with your real reflector. At 440MHz, the dirty reflector may be acting equally as or more influentially than the real reflector, but causing a lot of sidelobes, so still not as clean.

We also know that reflectors drop the feedpoint impedance and directors increase it. The limited information you provided so far is in agreement with this explanation.

How can you test this? Remove the driven element on the 2m Yagi temporarily. That will require you to re-tune and adjust the gamma match on 70cm Yagi. Then measure the gain, 3dB beamwidth, F/B, and a quick scan of sidelobes. I expect the sidelobes to be present due to other 2m yagi elements but will be less due to removal of the most proximal and influential element.

If that test validates this theory, then what are the practical solutions?

  1. Make the Yagi's cross-mounted. Or mount them on separate booms. You probably had a reason not to do that, because you probably knew that would avoid a lot of problems like this. But still worth reconsidering.

  2. Move the radiators farther apart. This will only lessen the interferences and will not make them zero.

  3. De-tune the 2m driven element intentionally when operating on 70cm, assuming you are not using them simultaneously.

  4. Get rid of 70cm DE and share the 2m DE for 70cm use (DK7ZB dual band Yagi). The feedpoint impedance on 70cm can be adjusted by the placement of the D1 and R for 70cm. (You might want to get rid of gamma match and use a different way to match - See W1JR 4-element yagi design from last year that achieved 50 ohm directly.)

I think 4 is still a brilliant compromise, especially with an extra 70cm parasitics placed close to the DE.

Additional insight on this antenna. You have D1-6 on 2m and D1-12 on 70cm. You probably set your expectations on forward gain based on the number of directors, and you got your money's worth on 2m but not on 70cm. That is as expected from the design. Many of your 70cm directors are not really functioning as directors, but to weaken the signatures of 2m D's on 70cm without bringing the forward gain. You probably need them if you want 6 directors on 2m, though. If you want to avoid this terrible trade-off, while taking advantage of 6 directors on 2m, I think vertical offset or putting the entire 70cm yagi ahead of 2m yagi may be the only solution. There is a lot of compromise in high-element dual-band yagi unless you cross- or offset-mount them.

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  • $\begingroup$ 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. $\endgroup$ Commented Jul 9, 2025 at 16:36
  • $\begingroup$ And what do you mean by writing "De-tune the 2m driven element intentionally"? $\endgroup$ Commented Jul 9, 2025 at 16:47
  • $\begingroup$ 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. $\endgroup$ Commented Jul 9, 2025 at 17:44
  • $\begingroup$ 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. $\endgroup$ Commented Jul 9, 2025 at 17:47
  • $\begingroup$ 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... $\endgroup$ Commented Jul 9, 2025 at 18:33

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