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Simple Field Strength Meter

Discussion in 'Electronic Projects' started by ElectroMaster, Jul 27, 2002.

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  1. ElectroMaster

    ElectroMaster Administrator

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    This Field Strength Meter is simple and also quite sensitive. It uses an ordinary digital voltmeter to measure signal strength. The VM should be set to the lowest dc volts range for maximum sensitivity. This is normally 200mV DC for most meters.

    I have tried this at VHF and was quite pleased with the results. L1 was 7 turns on a quarter inch former with ferrite slug. This covered the UK FM band. A digital multimeter, as opposed to an analogue signal meter has several advantages in this circuit.

    First, the impedance of a digital meter is very high, around 10M / volt on most meters. This does not shunt the tank circuit unduly. Second, as opposed to an analogue meter, very small differences in signal strength can be observed more easily on the digital meter. Thirdly, used with a digital meter, the FSM will have better linearity, responding well to both weak and stronger signals, a cheap analogue meter may not respond too well to very weak changes in signal strength.

    [​IMG]
     
  2. Crofty

    Crofty New Member

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    I could do with a 2.4GHz version of this, not quite so easy i found out.

    Steve
     
  3. Hero999

    Hero999 Banned

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    Why do you need a 3M3 resistor?

    Isn't the impedance of the DVM enough to discharge the capcaitor?

    Have you tried adding a small variable capacitor in parallel with C1 so you can tune it?
     
  4. dave

    Dave New Member

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  5. Electrix

    Electrix Member

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    A couple of answers on this please,
    1."L1 was 7 turns on a quarter inch former with ferrite slug."
    What kind of coil do we use for this ? Do we get a version of this and modify it or do we build it up ?

    2. The antenna - What are the specifications for this ? Can a simple exposed wire do ?

    Appreciate your thoughts on this.

    ~NJ
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I used a resistor instead of a coil in the field strength meter that I used to tune the RF amplifier in my FM transmitter.
    The quarter-wave rod antenna was a wire about 80cm long.
     

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  7. NewcastleSAR

    NewcastleSAR New Member

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    Hi Folks..

    What frequencies can this be used with. I'm looking for some form of detector that will assist in location an emergency beacon which transmits on a frequency of 121.5MHz.

    I'm part of our local CoastGuard Rescue Team and from time to time we get called out to EPIRB activations. We get put into the the general location of the device as it also transmits on a frequency of of 406MHz (Sat link). Most tims it okay if the vessel is at sea but in a built up harbour area with loads of these EPIRB's installed on boats, detecting which one has activated can be difficult especially if it is one that hasn't been registered with the vessels name.

    UNfortunately department funds don't stretch to purchasing a ddicated unit so we on the ground has to improvise.

    Regards

    Declan
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A Field Strenth Meter doesn't have tuning so it is sensitive to all radio and TV stations and taxi cab radios anywhere nearby.
    But a Field Strength Meter doesn't have an amplifier so it is not very sensitive. A transmitter must be very close to be detected.

    Its lack of selectivity and lack of sensitivity makes it useless for detecting emergency beacons.
     
  9. bobledoux

    bobledoux Member

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  10. JimB

    JimB Super Moderator Most Helpful Member

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    Some years ago, I assisted one of the local auxilliary coastguard guys to construct an EPIRB detector.
    The first prototype used a "beefed up" field strength meter with an RF amplifier to improve the sensitivity and two tuned circuits to give a measure of selectivity.
    The second prototype was a modified Pye Reporter VHF radio Tx/Rx.
    If I remember correctly I had the two prototypes working in the 144/146Mhz amateur band where I can legally transmit to my hearts content.

    The final unit was the same as the second prototype but set up for 121.5Mhz. It even received a mention in the magazine "Coastguard" in July 1995.

    I still have the prototypes somewhere, if you want some more information please let me know.

    JimB
     
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