BIRD THRULINE 4450, Инструкция по эксплуатации и перечень запасных частей

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Chap ter 2
The ory Of Op er a tion
Traveling Wave
Concept
The op er a tion of the Model 4450 Wattmeter is based on the trav el ing wave con cept of RF trans -
mis sion. As RF power is ap plied to a trans mis sion line, there is a for ward wave trav el ing from the
trans mit ter to the load, and a re flected wave trav el ing from the load to the trans mit ter. The closer
the load is matched to the line, the smaller the re flected wave will be. To de ter mine the watts dis -
si pated in the load it is nec es sary to de ter mine the power of the for ward wave and the power of the
re flected wave. The dif fer ence be tween the two will in di cate ef fec tive power dis si pa tion.
Traveling Wave vs
Standing Wave
The in ter fer ence be tween for ward and re flected waves pro duces a stand ing wave in the co ax ial
sys tem. In the stand ing wave con cept, VSWR (volt age stand ing wave ra tio) is a widely used im -
ple ment. There is a sim ple re la tion ship be tween for ward power, re flected power, and VSWR.
Where: Wf = for ward power and Wr = re flected power.
Since there is a def i nite re la tion ship be tween stand ing wave ra tio and the for ward-reflected power
ra tio, the for ward and re flected pow ers may be read as in di cated by the Thruline me ter, and con ve -
niently con verted to VSWR. The charts fur nished in this in struc tion book may be used for this
pur pose.
Coupling Circuit
When the wattmeter is con nected into a co ax ial sys tem, the RF power is di rected through the me -
tered line sec tion, which is a short sec tion of 50 ohm air line. This line sec tion will not im
pair the
im ped ance of the RF co ax ial line into which it is in serted.
The main line or cen ter con duc tor of this line sec tion passes through a toroid coil, which acts as a
cur rent trans former hav ing a one turn pri mary. The sec ond ary turns of this toroid coil vary and are
de ter mined by the power range in volved.
The cur rent in duced in the sec ond ary wind ings of the toroid coil are fed to a re sis tive net work
which pro duces volt ages pro por tional to the line cur rent.
There is also a ca pac i tive di vider net work in the line that senses the line volt age. This net work
pro duces a volt age nearly equal to volt age pro duced by the re sis tive net work.
When the mode switch is in the for ward po si tion, these volt ages will add in phase. This volt age is
rec ti fied and is cal i brated to give an in di ca tion on the microammeter equal to for ward power in the
line sec tion.
Reflected Power
Reading
A cou pling cir cuit, iden ti cal to the for ward power cou pling cir cuit, is used for the re flected power
read ing. This cir cuit is po si tioned so that it will sense the power in the di rec tion op po site to the
for ward di rec tion. The me ter will in di cate this power when the mode switch is placed in the RFL
po si tion. The en ergy re sult ing from the in duc tively cou pled com po nent of the for ward wave will
bring about can cel la tion as de scribed above.
When the mode switch is in the re flected (RFL) po si tion these volt ages will op pose and al most
com pletely can cel each other when a good 50 ohm load ter mi na tion is used. This re sul tant volt age
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