How to know there is a receiver or not when I am FM transmitter?

"Boki"

• Easy - just announce, over and over, that you want listener reports.

Say your phone number and wait for any calls.

........ Phil ;-)

What a garbage reply.

Phil Allison =E5=AF=AB=E9=81=93=EF=BC=9A

A common technique -- used by the RFID guys, and also pretty much the same technique used in some inventory control tags -- is to monitor "the load" on the transmitter. That is, you run your RF power amplifier off a constant voltage or current source and then monitor the resultant current or voltage; the receiver purposely places a changing load on its antenna (e.g., by shorting it!), and this causes the load seen at the transmitter to change. Probably the easiest way to think of this is as if your transmitter:receiver system is a loosely coupled transformer (and in the case of low frequency RFID tags, where the separation of the tag and transmitter is far less than a wavelength, this is a physically legitimate model as well), and back on the primary (transmitter) side you're just trying to determine what the load (receiver) side is doing.

The above is the also the way that so-called "grid dip meters" work.

Another approach for detecting receivers is to build a receiver tuned to the IF (intermediate frequency) of the receiver you're trying to detect. This is how -- historically -- vans in countries that had taxes on TV (e.g., England) would drive around and attempt to discover tax evaders. Here in the U.S., some states outlaw radar speed gun detectors, and these "radar detector detectors" operate on similar principles.

If you think about it, the ability for a transmitter to detect a receiver relies on the fact that the receiver is generally a form of parasitic transmitter itself (at the most basic level, the act of an antenna receiving a signal perturbs the electromagnetic field that it's in). This can be through explicit design (the case for RFID tags) or just as a side-effect of common design (the IFs in TVs). It's certainly a lot easier to detect a receiver if you have its explicit cooperation, however!

The job of making a receiver nearly impossible to detect (e.g., as "radar detector detector proof radar detectors" claim to do) is pretty trivial compared to somehow trying to build a a near-perfect receiver detector.

What are you trying to do?

---Joel

"Boki" = Another FUCKWIT

What a garbage reply.

Phil Allison ??:

You cannot know if there is a receiver there when you are an FM transmitter: FM transmitters don't know anything. They have no brains.

........ Phil

What do you mean, "so-called" grid dip meters?

I'm looking at the schematic for my Measurements Corp. Model 59 and I'll be damned if there isn't a microammeter connected in the grid return.

Hi Wes,

Haha... nice. :-) By the time I knew V=IR -- and probably by the time I was born -- I don't think they made grid dippers that had Real Live Toobs in them anymore...! :-)

---Joel (who was never, ever made to learn what the small signal model of a tube is, although it was mentioned at some point that "they're pretty much like FETs," so stuck on an island with a few tubes but no transistors, I *might* still have a shot at building a transmitter to call for help... at least if I can get The Professor to build me that nuclear reactor first...)

Tim Wescott =E5=AF=AB=E9=81=93=EF=BC=9A

Ya, of course, I think so.

A well-known customer write FM scanning function in specification...

Not to be rude, Boki, but did this customer write that specification in a language you read fluently? If not there's a good chance that something is being mis-interpreted, such what you describe is a rather odd request without some additional specification as to *how* it's supposed to be accomplished.

I spent some time on the phone yesterday with a guy who kept claiming he "couldn't share his drive" (this was on a Windows PC) when what he actually

On 30/03/2006 the venerable Boki etched in runes:

It is a very sensible reply to a garbage question. Also, garbage top posting corrected

Joel Kolstad =E5=AF=AB=E9=81=93=EF=BC=9A

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Thanks, but not my case...

Hi Joel,

I have two model 59's, which BTW are the best of the bunch.

Even when I was still employed and "my" lab had $250K worth of HP network analyzers available, I had a model 59 rat holed away. I had to hide it because Metrology couldn't "calibrate" it so to them it was worthless and had to be surplused.

When I started in the business as an electronics tech, the first job I worked was assembling Phoenix Missile i-f amplifiers from blueprints and feeding back to the guy writing production assembly planning. At the time (and a long time after) this was the most sophisticated air-to-air missile ever built. It had a pair of cascode vacuum tubes (Nuvistors) in each channel of the i-f amp and also used three klystrons.

Fifteen years later I was the engineer responsible for introducing a high power IMPATT diode klystron replacement into production and the Nuvistors had long been replaced with FETs and Gunn effect oscillators had replaced the two klystron oscillators. Time marches on.

I must admit with today's surface mount and hybrid stuff, it's pretty hard to stuff a GDO coil into a circuit and measure resonances :-)

Wes

This is the service used by the professional FM transmitters for measuring listenership:

Tim.

Right. If you go to any big radio station, there's a big round plate-current meter on a black wrinkle-finish panel. Every time a listener turns on his radio, you can see the current jump up. Some meters are even calibrated in kL, kilo-listeners.

Right.

John

Well, certainly. Or at least you can do it for any _legal_ receiver. Just get your government to mandate that all receivers must have a function which reports if it is powered on, and when it is powered on, which station it is tuned to.

Of course, that goes completely counter to the whole philosophy of _broadcasting_ in which there is no way to know how many receivers may be tuned to that station. With newspapers, you can know how many are printed, if not how many are actually being read, but with radio, television, and any other broadcasting, if the receiver is not also a transponder in some sense, there is no way of knowing.

Does a lighthouse keeper turn of the light if he doesn't know if there are any captains out there looking at his light?

(Is this really so difficult to figure out yourself without asking??? This really isn't rocket science or brain surgery, is it??)

I don't know whether it's still done today, but I understand the Brits used to drive trucks up and down the streets sniffing for LO leakage to find "unlicensed" TV sets ;-)

...Jim Thompson

;-)

Bwahahahahaha ;-)

...Jim Thompson