The first kind: Long term aerial.
The basis is used actually my and measure, long term the optimal physics length of aerial is " the 3/4 length of the wavelengh below some frequency " , namely L (rice) =225f (Mhz) . Of course, in its the meeting on times frequency shortens a little a few, but the error won't be too big. It is with my antenna exemple, the physical length of aerial is 26.3 meters, this length is 2.85mhz
1/4 wavelengh (actual measurement standing wave is 1.25) . It is in respectively 8.770mhz (actual measurement standing wave is 1) , 15.500mhz (actual measurement standing wave is 1.1) with 21.18mhz (actual measurement standing wave is 1.7) .
These data are as follows:
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Multiple of wavelengh of length of theory of antenna of dot of frequency of standing wave comparing is actual length differs quantity
(Swr) (Mhz) (rice) (rice) (rice) (rice)
Wavelengh of 1.25 2.850 26.32 1/4 26. . .3 0.02
Wavelengh of 1.00 8.770 25.66 3/4 26. . .3 0.65
Wavelengh of 1.10 15.500 24.19 5/4 26. . .3 2.11
Wavelengh of 1.70 21.180 24.79 7/4 26. . .3 1.51
Afore-mentioned standing wave is compared is lowermost standing wave is compared -- visible, I design the antenna in 7mhz resonance this pair (as a result of the heart when debugging too urgent, be cut too much, the result is in 8mhz is syntonic) be in respectively " of wavelengh times odder on syntonic " (ah, you fasten jest I, this also is academic to aerial practice to prove, with a ha breath out) . Why to say to be in is 3/4 wavelengh resonance best? Because be in 8.770mhz when, if ask to lower a bit, measure standing wave to be in 1.6 the following frequency limits, so I tell you gladly, it is in whole 8mhz (8.000-8.970) syntonic, and times otherer what the syntonic bandwidth of frequency falls as 3/4 wavelengh however is good, length of be confined to, I not enumerate (oneself frequency
Shake all the time from 1.600mhz 30.000mhz) . Had this one experience, if you want to design frequency to some limits frequency,so I say, can place frequency first so, multiply again with 225, the electric length that the number that reach is aerial (of course, the height of aerial distance roofing does not want too low, also want prep above at least 2 meters. Referenced " shortwave antenna maintains manual " the 19th page, the ratio of height and wavelengh is in 0.4-0.5 when, aerial is spent for 40-30 in the emissive angle of elevation of perpendicular direction; When ratio is 0.1-0.25, angle of elevation is spent for 90-80; Ratio is when 0.6-0.8, deputy valve increases apparently;
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Ratio is equal to 0.8 when, begin dissension to become two wave piece; When ratio continues to increase, wave valve also grow in quantity. . . )
Attention, 1/4 wavelengh antenna needs good ground connection, if you use the line of make a present of of 50 ohm, the screen layer ground connection of line of make a present of, the antenna that and accept a place is apart from you is shorter better; If you use electrical wiring feed, so the length of aerial should be the length of all electrical wiring beyond receiver (the room interior that includes you is divided) ; Still can use parallel to adjust character of line of make a present of additionally, dan Ting is irritated, I also had not tried.
The 2nd kind: Occasionally extremely aerial
Of this kind of antenna make very simple and practical, its provenance compiles for Mr Pan Renyong " practical radio speaks " (1939 head edition, mine is May 1956 now new 1 edition) the 490th page. I feel to add ohm of a 300/75 after be used actually impedance transformer is betterer (OK and abate 77 model the sound that the monarch in receiver breaks off bang bang) . I am in 1995 erect when, the length of two bamboo pole has 3-4 rice each about. Say it is simple, because it needs the common electrical wiring with delicate stage only,be can (fine solder to the electron commonly used line) , 50 money can buy 100 meters. Additional, the characteristic impedance of whole deputy antenna is to lean " two when feed point point controls brace up child removed " will adjust. When feed the dot is in the left and right sides brace up child when interval makes an appointment with 20 centimeters, wind two strings the input indoor (cap-a-pie has two electrical wiring only, do not need to cut off) , such, in feed point point appears an edge to grow those who be 20 centimeters isosceles triangle. Because electrical wiring is very fine (of course, also can use thick, want you to have money only) , you can bind the post in both sides to go up with line of a nylon, bind aerial cent point on that nylon rope that basically gets power with fine nylon rope next. The characteristic impedance of a pair of such antenna is 80-175 ohm, establish ties again before inputting receiver the commonly used 300/75 ohm on antenna of a common television is balanced / lopsided transformer (along with you glad) , good. When be being used actually, still feel the airframe of receiver and fruit of aftereffect of crust ground connection are better. The cable that I discharged millimeter of a 10 square in 4 buildings infiltrates underground 60 centimeters. -- true hate also cannot receive lightning come down! ! ! ! Talk about bit of experience even finally, 77 model receiver itself has " receive fine tuning " , so, do not need basically to mix day of air again additionally. The attenuation pushbutton of 77-a also very the canal is used, play entirely when 4 pushbutton when going out, should be disconnect condition, after listening to an end, should play entirely piece, lest the bedspring of this switch is out of order. My 77 model receiver its are big scale platen is orchid color plastic those who make, I am added in its interior installed little electric light, such, in completely black night, also can use " back illume " . Inside tone whole airframe hind (near aerial) the pressure that screw of two pressure bedspring can adjust high frequency loop to rouse, loosen the tensile bedspring of airframe the right side can cancel wave band to choose " automatic pine " condition (below normal circumstance, press next wave band switch to turn at the same time it, right now beat of high frequency loop was not put through, until turn wave band switch again) .
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