question about field strenghth meter

[darklordchris]

well lets see if this works.

i had a question about this fm field strenghth meter design. why is their a red circle around the capacitor by the antenna. thats not some new electronics symbol i'm not familiar with is it?

 

[Pommie]

Your attachment isn't showing. I think you may have to add it as an image,
**broken link removed**.

Edit, it's originally from this post.

Mike.

 

[darklordchris]

it shows up for me. anyway its audio gurus design from that thread, and i just want to know why is their a red circle around 22 to 100pf capacitor. is that a special capacitor? or did he just for some reason draw a circle around it?

**broken link removed**

 

[JimB]

Nothing magical about that capacitor, to function correctly the circuit does not even need it, but under some conditions such as probing a live circuit it is nice to have.

As for the red circle, I think AG just got out of bed that morning and thought - "I will do something with a red circle today!"

JimB

 

[audioguru]

As for the red circle, I think AG just got out of bed that morning and thought - "I will do something with a red circle today!"

I used the red in another thread where somebody used an electrolytic capacitor that did not pass radio frequencies.

 

[darklordchris]

wait....so will this thing work if i leave the capacitor out altogether?

 

[audioguru]

wait....so will this thing work if i leave the capacitor out altogether?

Yes, if the antenna does not touch anything conductive.
But it will pickup low frequencies without the capacitor like mains hum.

A 22pF capacitor feeding the 10k resistor reduces pickup of frequencies below 727kHz so it will pickup strong AM radio stations.
If the capacitor is only 10pF and the resistor is 1k ohms then frequencies below 16MHz will be reduced.

 

[darklordchris]

Yes, if the antenna does not touch anything conductive.
But it will pickup low frequencies without the capacitor like mains hum.

A 22pF capacitor feeding the 10k resistor reduces pickup of frequencies below 727kHz so it will pickup strong AM radio stations.
If the capacitor is only 10pF and the resistor is 1k ohms then frequencies below 16MHz will be reduced.

i'm not having any luck with this thing. can this meter use a nte109 diode? its suppose to be an equivalent for a ECG109 or SK3090?

 

[Mikebits]

How are you testing the circuit? ie. how is it hooked up, what are you using to test it such as a known good signal source?
AG would be best to tell you what power level it will detect, but I suspect at least +13dBm near the meter.

 

[audioguru]

I used the Field Strength Meter to peak the output tuned circuit of my FM transmitter to the center of the FM broadcast band.
My transmitter had an 80cm antenna and the FSM also had an 80cm antenna, the antennas were parallel and they were about 2m apart. The output voltage was about 0.25VDC.

 

[darklordchris]

How are you testing the circuit? ie. how is it hooked up, what are you using to test it such as a known good signal source?
AG would be best to tell you what power level it will detect, but I suspect at least +13dBm near the meter.

i've been using a couple of small cheap fm transmitters for car sterios. they each have about a 30ft range, and i've tested all over the fm band. i have also tested for conductivity with my multi meter by using the settings that test resistors....i'm still kind of a beginner, if anyone was interested

 

[audioguru]

My FM transmitter is much more powerful than a cheap car FM transmitter. I don't think the simple Field Strength Meter will detect such a weak transmitter.

 

[darklordchris]

My FM transmitter is much more powerful than a cheap car FM transmitter. I don't think the simple Field Strength Meter will detect such a weak transmitter.

hmmm

well ok. what would i need to build for something like that. because basically these small cheap ones are the only stuff i'm going to have access to test with until i can build a larger transmitter.

 

[mneary]

It may detect it at close range. It's not cheating to connect them together, just use the capacitor so you don't get the DC component.

 

[audioguru]

FM stereo transmitters made to broadcast an MP3 player in the car to the car radio have an attenuator at the radio output. So the radio transmission is very weak.
Tell us if the Field Strength Meter has an output when the antenna of the weak transmitter is connected to the antenna of the FSM.

 

[darklordchris]

FM stereo transmitters made to broadcast an MP3 player in the car to the car radio have an attenuator at the radio output. So the radio transmission is very weak.
Tell us if the Field Strength Meter has an output when the antenna of the weak transmitter is connected to the antenna of the FSM.

no that didn't work. would a more powerfull field strength meter be able to detect them? i have a plan from a book that i was considering building. the book is called Pirate Radio and Video: Experimental Transmitter Projects (Electronic Circuit Investigator)
by Newton C. Braga

it uses 2 double A batteries and measures it on the micro amp settings on a multimeter.
what about a kit? i have the fm transmitter velleman electronics kits k1171 oscillator. https://www.electro-tech-online.com/custompdfs/2010/06/illustrated_assembly_manual_k1771_rev1.pdf

you guys think that might work?

 

[audioguru]

The Velleman FM transmitter kit is missing a voltage regulator so its frequency will change as the battery voltage runs down and is missing treble boost that all FM stations have. Its sound heard on an FM radio will not have treble audio frequencies because all FM radios cut treble frequencies to match the boost by stations.