Want to do a few each frequency of late paragraph 8 wood are directional aerial, saw a few about 8 wood the book of directional antenna, summed up the following experience (the rule) , it is the thing on the book, the likelihood is not certain and correct, hope everybody is the same as good point out mistakes so that they can be corrected.
Because be brought to aerial,each index is in low frequency paragraph low end is satisfied more easily, and change in high end faster, so its design frequency to be chosen normally in more tallish than central frequency some frequency.
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１ . Zhen Ziyuan the make choice of of several
Brace up child amount basically is a basis given gain will decide, and the gain that is made to aerial basically decides λ of L/ of length of Yu Tian line, accordingly, can decide aerial length above all according to what asking gain is worth, decide next according to the commonly used span of director and reflector brace up child amount; Perhaps choose directly according to experience data. Normally, the brings Xiang Zhenzi number that is brought to aerial is 6 ～ 12, if increase the amount of director to do not have distinct effect to increasing gain again, to the requirement of tall gain, can use antenna blast method will solve.
2. Passive brace up child the choice of dimension
Passive brace up child the alternative of dimension is main should from directional with impedance character two respects consider. Make the choice of Xiang Zhenzi span. Because gain basically decides at L/ λ , if bring Xiang Zhenzi's amount to already was decided, criterion span is obtained a few bigger relatively advantageous, but when ＞ 0. After 4 λ , aerial gain drops quickly instead, take commonly so (0. 15 ～ 0. 4) λ . The first director and active brace up child span should be less than other slightly brace up child span, for example ≈ 0. When １ λ , gain rises somewhat. Normally the length of director is in ≈ (0. 42 ～ 0. 46) λ limits. Brace up child amount heals much, the optimal length of director also heals short; When be like Duan Jiaokuan of requirement job frequency, the length of director also should be obtained some shorter.
Reflector and active brace up child span is in normally (0. 15 ～ 0. 23) inside λ limits, the around radiate that meantime is apart from pair of direction to pursue is compared and input impedance impact is bigger, span is lesser can restrain effectively backward radiation, but input impedance is inferior be hard to match with impedance of line of make a present of, want eclectic alternative consequently. Reflector length normally (0. 50 ～ 0. 55) λ . If be opposite from beginning to end is higher than the requirement, can use the form of emissive net.
Passive Zhenzi chooses aluminous control to become commonly, conductor radius chooses to be in normally (１ / 200 ～ １ / 300) inside λ limits. Brace up child tropism of the other side of degree of finish does not have too big impact, and have certain effect to impedance character, zhen Ziyu is thick, characteristic impedance heals low, it is advantageous to spreading impedance bandwidth.
Bring to aerial to have even structure and inhomogenous structure two kinds of forms, former it is to show director is the circumstance that waits to grow and wait for span, and latter is to show director is to grow and differ the circumstance of span, be apart from active Zhen Ziyue's far director, its length is shorter, interval is bigger. They can adjust position of respective " optimal " . Both is compared, say even construction commonly advocate valve is narrower, deputy valve n is taller; and inhomogenous structure advocate segment or section is broader, deputy valve n is inferior, but if be adjusted appropriately, also can make apparent depress deputy valve and do not send too earth spreading advocate valve or reduce aerial gain. Relative to character, latter for emergency use is flexible.
3. Active brace up child the choice of dimension
To active brace up child main demand is can have with line of make a present of match goodly, for this, active brace up child should design for syntonic length, alternate its input impedance to the numerical value that be equal to or approachs impedance of character of line of make a present of.
8 wood are directional aerial is to use coaxial cable feed commonly. When active Zhenzi uses half semmetry brace up the period of the day from 11 pm to 1 am, because suffer passive brace up child influence, value of its input impedance is inferior, because this needs to try to raise active brace up child input resistor, commonly used method is to convert amount to brace up child. Proper choice amount to brace up child length, the diameter of two conductor is apart from than reaching meantime, raise active effectively brace up child input resistor, be united in wedlock adjust reflector to reach a few around measure that cite Xiang Zhenzi, the standing wave that can gain satisfaction is compared. Next, as a result of amount to brace up child equivalent radius is added thick, advantageous also to bandwidth of spreading impedance frequency. Of course, active brace up child also can use add match implement symmetrical brace up child form.
About be being brought to the feed problem of aerial, if use coaxial cable feed, to assure the symmetry of aerial, need in make a present of line and aerial interface place add a balance - lopsided converter, for example U form canal matchs implement, T matchs and transmission transformer of broad frequency band.
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