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Yagi antenna design question
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. 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. 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. 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. You would be better off by inserting coil at the feedpoint instead of (Beta match) hairpin, or use Delta match, or offset feeding at the 50 ohm 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. 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. 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. |
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