attenuator
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There are various online calculators that can be used, but the calculated values for an 80dB attenuation are:- | There are various online calculators that can be used, but the calculated values for an 80dB attenuation are:- | ||
- | R1, R3 = 50.01Ω | + | |
- | R2 = 250kΩ | + | |
+ | | ||
In practice, value of 50Ω are well within the tolerance of practical resistors. | In practice, value of 50Ω are well within the tolerance of practical resistors. | ||
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R1 and R3 need to be capable of handling almost all the power required, and the R2 needs to handle a percentage of it. For my construction I used a 450W resistor for R1, a pair of 3W rated 500kΩ resistors in parallel for R2, and a pair of 1.5W rate 100Ω resistors in parallel for R3. This gives a 450W capacity from the left-hand port and a 3W capacity from the right hand port. | R1 and R3 need to be capable of handling almost all the power required, and the R2 needs to handle a percentage of it. For my construction I used a 450W resistor for R1, a pair of 3W rated 500kΩ resistors in parallel for R2, and a pair of 1.5W rate 100Ω resistors in parallel for R3. This gives a 450W capacity from the left-hand port and a 3W capacity from the right hand port. | ||
- | The 450W resistor is a special part intended as an RF resistor. | + | The 450W resistor is a special part intended as an RF resistor. |
+ | |||
+ | For the same reason, wire-wound resistors are absolutely useless in this case, because they are inductors which also happen to have some resistance. | ||
+ | |||
+ | Remember, we need to keep the reactance low! | ||
+ | |||
+ | ==Parts== | ||
+ | |||
+ | * R1 = 50Ω resistor, power of the left-hand port (I used 450W) | ||
+ | |||
+ | * R2 = 250kΩ resistor, about 1% of the total power (I used 2 x 3W 500kΩ in parallel) | ||
+ | |||
+ | * R3 = 50Ω resistor, power of the right-hand port (I used 2 x 1.5W 100Ω in parallel) | ||
+ | |||
+ | * Connectors and wire (I bought a pigtail that had a BNC at one end and an SMA at the other, and an N-type chassis mount with an SMA on the back) | ||
+ | |||
+ | * Metal box that can sink the heat (I bent some 50mm x 3mm aluminium strip into half a box -- another piece of aluminium sheet can make a lid.) | ||
+ | |||
+ | You will also need things like a soldering iron, drills, spanners and screwdrivers to fit the connectors, and some way of measuring RF impedance. | ||
==Construction== | ==Construction== | ||
- | Construction should be fairly simple: each port needs a coaxial connector, | + | Construction should be fairly simple: each port needs a coaxial connector, |
+ | |||
+ | Here is my ugly soldering: | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | * R1 is the large resistor labelled " | ||
+ | |||
+ | * R2 is the pair of pale blue resistors | ||
+ | |||
+ | * R3 is the pair of black resistors numbered 100 0 | ||
+ | |||
+ | The black feeder is connected to the port on the left of the diagram; the copper-coloured feeder is connected to the port on the right of the diagram. | ||
+ | |||
+ | Even though the soldering is ugly the wires are quite short which, as you will see below, gives good performance up to 500MHz or so. | ||
+ | |||
+ | ==Testing== | ||
+ | |||
+ | Test that both the inputs produce the expected SWR measurements and impedance values (my NanoVNA said 49.something ohms and a few 100s of pF of capacitance in the 70cm band.) Also test that shorting out one port does not change the impedance values of the other by very much. | ||
+ | |||
+ | {{ : | ||
+ | If these tests pass, you have an attenuator. | ||
attenuator.1707232256.txt.gz · Last modified: 2024/02/06 15:10 by ei3jdb