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MIL-PRF-15305 Coils, Electrical, Fixed And Variable, Radio Frequency General Specification ForThis specification covers the general requirements for radio frequency coils, fixed and variable, for use as simple inductive elements in radio frequency circuits. Procurement of coils of a specific design will require additional data in the form of complementary Service documents, giving detail electrical and mechanical requirements, tolerances, and applicable additions and exceptions to the general requirements and tests specified herein \(see 3.1\).

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MIL-PRF-15305F
4.6.5.4.1 Percent coupling (alternate method). The percent coupling between two radio frequency coils is to be
determined by measuring the voltage induced in the coil when a voltage is applied to an adjacent coil. In order to
maintain reasonable impedance levels, the measurement is to be performed at 100 kHz for nominal inductances of 10
µH or less, at 10 kHz for nominal inductances less than or equal to 100 µH but greater than 10 µH, and at 1 kHz for
inductances greater than 100 µH. The measurement circuit is shown on figure 10.
FIGURE 10. Test circuit for percent coupling (alternate method).
Equipment for 1 kHz and 10 kHz consists of HP Model 200 audio oscillator and Model 400 vacuum tube
voltmeter; for 100 kHz Model 606 signal generator and Model 400 vacuum tube voltmeter, or equivalent.
The coils to be tested shall be taped or otherwise secured such that the bodies of the coils are kept parallel and
in contact with each other to insure maximum coupling. The voltage levels shall be as low as possible to permit
reliable readings of V2. The inductance L should be measured at the same voltage level so that any saturation
effects are taken into account.
The percent coupling is to be calculated using the equation:
L1 V2
X 100
Percent coupling =
L2 V1
=
effective inductance of primary coil (measured at test frequency)
Where: L1
=
effective inductance of secondary
L2
=
voltage measured across primary
V1
V2
=
voltage measured across secondary
NOTE: This test may be performed using the Q meter for the oscillator at the standard test frequency allowing
simultaneous reading of L1. Reversing the coils allows reading L2 and also a second measurement of
coupling for verification or averaging.
The percent coupling is to be calculated using the equation:
When V1 = .02 Q
50V2
L1
X 100
Percent coupling =
Q
L2
= effective inductance of primary coil (measured at test frequency)
L1
= effective inductance of secondary coil
L2
= voltage measured across secondary
V2
Q
= as measured
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