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Yagi antenna design question
"Wes" wrote in message oups.com... On Feb 5, 7:59 am, "Yuri Blanarovich" wrote: "Wes" wrote in message ups.com... On Feb 4, 2:35 pm, "Yuri Blanarovich" wrote: [snip] Tony, by using hairpin matching, you take out portion of the highest current on the element and fold it into the hairpin where it is taken out of antenna "participation" for the price of match. You're kidding right? Disclosu I use a hairpin (beta) match on my HB Yagi. In that case I must be :-) Can you elaborate why would I be kidding? As far as I know: If you use hair pin inserted in the middle of the element, you get the shorter physical length of the element - smaller high current carrying length. I believe that your first problem is that you are considering the hairpin (Beta) to be part of the radiator instead of considering the actual case; it's part of the matching network. Do you have to shorten the (driven) element if you insert the hairpin in the middle or not (to maintain the resonant frequency)? Telrex 40m Yagis did use hairpins at the center of the element as a loading and shortening the element length. To me that is the matching network too, but at the expenses of shortening the element length - center loading, similar to base loading the resonant quarter wave vertical. Half wave resonant element has maximum current in the center, by folding portion of that element into a hair pin we take that portion "out of the picture". Just like a loading coil at the base of the vertical - current drop along the coil. op cit. I'm not "folding a portion of the element." If you subscribe to the idea that part of the antenna can be "folded into" the feeder and that by selecting the right feeder length you can "lengthen the antenna" (as shown in a lot of old ARRL literature) then I have a new limited space antenna for you. It's a one foot long radiator with a variable length feeder (a la Cecil) that "makes up" the missing antenna length. You fold the portion of the element, it's called loading, at the base, in the middle or anywhere along the element length. The folded hairpin has inductance, just like a coil, and can be replaced with the coil of similar inductance. The loading element is not the limited space antenna, look at the currents at its ends and see the difference in curent distribution along it and how it participates in the overall radiator current distribution and corresponding area under the cosine curve representative of the efficiency. We are back to the loading coil "problem" and that's why we pointed out the efect and its impact on the antenna performance. Hairpin is usually folded back on the boom, 90 deg. to radiator, with any current left, not participating in the plane of the elements. If I placed a discrete (lumped element) L-network at the feedpoint the current in it would not be "participating" either. Not much, but help with cleaner pattern. Hairpin loading stubs were proven to be worse loading elements than good quality coils. When, where, by whom, etc? Oh, BTW, did I mention "THE HAIRPIN IS NOT A LOADING ELEMENT. It's the inductance in an LC L-network. Here we go again? We are talking about standing wave circuit - antenna radiator or element. So inductance is not a loading element? As far as I know inductance (coil, hairpin) or capacitance (top hat, L, T loading) are used as a loading elements to shorten the physical length of the antenna element, while maintaining electrical length. There was an article by W6?? in CQ and other examples when they replaced hairpin loading on Yagi elements with coils and got significant improvement in the performance of KLM 3 el 80 Yagi, better gain, much better pattern due to less interference of the folded back hairpin with the element. Measurements and modeling before and after showed that. pointas mentioned before. Why? The hairpin will handle all the power an amateur can supply, it can (does in my case) form an integral balun and it DC grounds the element. No argument here, you can do that with coil and gain some edge. We might be splitting hairs here, but I hate to lose even fraction of dB if there is a better way. No gain is lost. I can model my Yagi with and without the Beta, resonating the antenna without it by lengthening the element and the gain remains the same within 0.01 dB. No gain lost would be 0.0000 dB :-) I know Beta match works, I used it in some antennas (don't like Gamma matches), even made a QSO with a ligthbulb as an antenna. It's all relative. Contesters like to chase every fraction of a dB lost, soon they can add up to some noticeable real dBs. I've been a moonbouncer, I know all about fractions of a dB. Then you should appreciate the above. 73 Yuri, K3BU |
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