Does anyone have schematic of VHF power meter?

[Abu Ammora]

vhf power meter

Does anyone have VHF power meter Schematic ?

I found this power meter on the Net , but Can anyone explain to me how does it work

 

[flatulent]

vhf powermeter schematic

The two diodes and capacitors on the left are a voltage doubling rectifier. The output turns on the emitter follower.

This circuit should not be used on sources of over a few mW or it will be damaged.

 

[snake]

Re: VHF power meter

it just rectfy the antenna wave then a capacitor charges with the peak voltage, than that level is amplified with a transistor to turn on a led.
these circuit just show you that you have some power on the antenna you can use a bether one if you want to know how much energy you have.
the formula to know how much power you have is :

P = (v + 0,3) ^2 / 2*R

R= 50 Ohm

P = (V+0,3)^2 / 100

by

 

[Abu Ammora]

Re: VHF power meter

it just rectfy the antenna wave then a capacitor charges with the peak voltage, than that level is amplified with a transistor to turn on a led.
these circuit just show you that you have some power on the antenna you can use a bether one if you want to know how much energy you have.
the formula to know how much power you have is :

P = (v + 0,3) ^2 / 2*R

R= 50 Ohm

P = (V+0,3)^2 / 100

by

Thanks Snake , But what about the 0,3 . do u mean 0.3 , if yes from where you get this value ?

 

[Abu Ammora]

Re: VHF power meter

P = (v + 0,3) ^2 / 2*R

Thanks snake , but from where 0.3 you obtain

 

[biff44]

Re: VHF power meter

Analog Devices and Linear Technology both make cool log amplifier chips that would make an excellent power detector.

 

[g579]

Re: VHF power meter

Hi Biff,

Could you please give some type numbers to start with for AD and LT ?
Do you have perhaps some price guess ?
Thanks
g

 

[snake]

Re: VHF power meter

sorry, i don´t had time to write here, it is 0,3V or 0.3V here in Brazil we use the , inted of . to show de decimal mark, sorry.

these value are the voltage drop that we loose when the germanium diode are in conduct mode and whit low corrent across it.

so we can just use these value. if you have an rf generator you can make a correction curve for that. look at the net to ham + power + meter.

the analog part number is ad9307 it is an log amplifier that make messuriments about rssi, it is much more acurate but much more expensive.

if the power you are looking to mesure is very week you have to use it but if not you can use de diode.

sorry about my poor english.

 

[Abu Ammora]

Re: VHF power meter

Thanks Snake , but Can u PLz suggest another Diodes to use instead of 1n60 , For ex 1n4007 or 1n44441 , and is the voltage drop will change ?

 

[snake]

Re: VHF power meter

yes it will change, if you use a silicom diode de voltage drop will be 0.7V insted of 0.3V that a germanium diode have.

you can look for a rf schottky zero bias diode. then in theier datasheet you will find voltage drops that are even less then 0.3, you can look the voltage drop puting a variable power suply at the input of the circuit then with another meter ( not that you are using to mesure the power) you can mesure the voltage drop in the diode at varios values and make a calibration curve.

try to look at avago, analog devices and philips for that diode type.

or just look for a germanium diode, its bether than a silicon one.