430 MHz DIY Yagi problem. Ideas needed.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by BX2ABT, Dec 2, 2018.

ad: L-HROutlet
ad: l-rl
ad: Subscribe
ad: Left-2
ad: L-MFJ
ad: DLSpec-1
ad: Left-3
ad: MessiPaoloni-1
  1. KD6RF

    KD6RF XML Subscriber QRZ Page

    As KC8VWM said above, a good known good design has to be copied exactly. Including detail of boom and element size, attachment method, insulators, etc.... And feedpoint geometry, spacings, interconnect lengths, etc... --- all very important on the higher bands like 70 cM.

    IOW, if you are spending a lot of time simply trying to get the SWR down, you can end up with a yagi with low SWR but that is way down on gain for the SAME reason(s) that the SWR didn't match the original design in the first place. Chances are that whatever is different from the original design to give you high SWR is ALSO going to hurt the yagi's gain.

    It may be a good idea to step back and review the design and your implementation and try to figure out WHY it isn't duplicating the results of the original design.... rather than cutting and pruning and changing feed methods to simply get the SWR down.

    I love homebrewing and think it's a great way to learn!......but don't get in too much of a hurry and end up fixing only the SWR issue, while possibly masking other issues. It can be a lot of work, but once you get it right you'll know you have an antenna with good match to 50 ohms AND high gain.

    Last edited: Dec 6, 2018
  2. KM3F

    KM3F Ham Member QRZ Page

    Han's , I think you did get closer to the actual impedance the driven element offers.
    Transforming from a higher impedance of the folded dipole to 50 ohm coax always adds complexity and losses.
    This is why I chose abandon these feed types.
    After your accumulation of tests, it becomes more clear about what you have to work with.
    Now it's just a matter of fine tuning all the dimensions until you get the match down.
    As for the Gain, you have little to go with for that performance parameter unless actual Gain tests are done in the field.
    Band width usually is more influenced by element diameter. A change in this size usually results in length change to arrive back to the starting resonance desired and sometimes a staggered configuration can be used to attain wider band width.
    It's all inter reactions.
    SWR is a measure of system feed matching and losses.
    It is not the total indicator of efficiency for the whole system but is a larger part.
    The antenna loses are actually very low. It involves element 'ohmic' resistance and 'radiation resistance'. These are two different kinds of losses that involve electron movement effects in metal. You can't do anything about them.
    All you have to work with is forming/shaping the beam and the best and lowest loss feed system plus of course antenna height.
    Good luck.
  3. N3DT

    N3DT Ham Member QRZ Page

    The only good way to resolve this is to get hold of an analyzer that you can either attach right at the antenna or use one that will measure using the OSL method, which is essentially removing the effect of the trans cable to the antenna. The other thing is if there's a balun or impedance transformer, it too has to be correctly made and tuned. At 432 it's easy to be off by 1MHz depending on the VF of the balun coax. I doubt you will resolve this by making adjustments with different lengths of coax, all it will do is confuse the issue. You really need to know the exact VF and characteristics of the coax and try to subtract that out from the system and at 432 it needs to be precise, not ballpark. Another option that may be available is to make sure you are using an exact half wave multiple of coax for the feedline. Then what you see at the end of the coax will be a close representation of what the antenna is doing (less the loss), but then again you need a method to accurately measure the half wavelengths of coax. Again, the analyzer is good for that. I'm betting that your antenna is close, but no banana, so reactance plus the effect of the trans line is very much confusing the issue. BTDT. It's very hard to build a uhf antenna and reproduce the same effect, too many variables. Good luck.
  4. BX2ABT

    BX2ABT Ham Member QRZ Page

    Thanks for the feedback, guys. I appreciate it. First off, this is what I build.....
    All dimensions are within the millimeter compared to the design as it rolled out the VK5DJ's yagi calculator software, except the dipole which might be a couple of millimeter short depending on how you measure it. Every element completely isolated from the boom. The RG-58 (vf 0.66) 4:1 balun is also made exact up to the millimeter (22.7) and looks like this:
    I use a 2x3 cm piece of single-side copper PCB with 3 holes. The middle one holds the pigtail, the outer ones the 22.7 mm long RG-58 loop. Braids are all soldered on to the copper and the ends go through and are connected to the loop. Looks like this:
    Maybe you got me wrong, but I measure at the end of the pigtail, not with 75 feet of coax attached (I did that once for fun to see what would happen, but as you pointed out, you only get false readings). The SWR meter is attached directly at the end of the pigtail and from that a short coax to my Kenwood HT putting out 5W.

    I'm not striving for lowest SWR, more for resonance at my desired frequency (center of the sat band: 436.5 MHz) and an efficient total system.

    @KD6RF: I am not sure if the yagi is performing as it should and I want to be as sure as I can by measuring what I can. You all are right if you say a design has to be copied 100%, but then how to check that? And if you measure, how to interpret the results. And if the results are off, then how to find the cause and correct/adjust that cause? Apart from an antenna analyzer there must be other methods. 20 Years ago there weren't these fancy boxes and people still build stuff that worked, right? So, how to measure and interpret what I can measure with my cross-needle SWR meter?

    @KM3F: you say "Transforming from a higher impedance of the folded dipole to 50 ohm coax always adds complexity and losses. This is why I chose abandon these feed types." So if you don't like transforming with a 4:1 balun, hence don't like folded dipoles, then what is your preferred method? I find it confusing anyway: a folded dipole has an impedance of 288 ohms, so to connect to a 50 ohm feedline there should be a transformation of 1:6, not 1:4, but all designs I've seen use a 1:4 balun to step down the impedance to 75 ohm because that seems "good enough".

    @N3DT: yes, I'm slowly finding out that building a UHF antenna is not for the faint of heart, but I'm not giving up easily.
    KD6RF likes this.
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Looks like you did a terrific job.

    However, it's important to remember that "SWR" and "performance" are not related.:)

    My Bird dummy load shows VSWR = 1.02:1 at 3000 MHz or any frequency below that. But it doesn't radiate.

    The best 432 MHz antenna system I ever had was eight stacked K2RIW 16 element Yagis in an large "H"-frame (for e.m.e. work) and although its SWR was about 2:1 at 432.100 MHz, it created signals strong enough to reach the moon and back (almost 500,000 mile round trip) and I could hear my own echoes.

Share This Page