Antenna Scope measurements and questions

[sonnytiger]

I have a rigol DS1052E, I was messing around with a length of wire strung up that would be appropriate as a monopole CB radio antenna. I had it hooked to my scope with 1M input impedance and 15pF input capacitance. I was having trouble seeing anything other than the 60Hz signal so I attached a large value inductor to pass the signal to ground. On the scope I saw a 1MHz signal, but not really any kind of repeating 27MHz signal. I didn't really expect to see anything as a) how am i to know if there is any CB radios operating anywhere near me?, b) the input impedance of the scope is not the usual 50 Ohms used in radio stuff and c) I didn't know what kind of voltage swing to expect on a relatively inefficient antenna. I also thought about using a capacitor in series with the antenna to only pass the high frequencies I was looking for, but calculations revealed that it would require a capacitor of like 25pF, wouldn't the input capacitance of the scope be doing something? or is it in parallel with the probe causing higher frequencies to pass to ground? Any kind of answers to my a, b and c statements or my other questions would be nice, as well as any other applicable information. Also how is the antenna signal filtered out in a amplifier?

[KJDS]

To answer your last question, look up superhet receivers.

[sonnytiger]

i will read about that thank you!

[vk3yedotcom]

On the scope I saw a 1MHz signal, but not really any kind of repeating 27MHz signal. I didn't really expect to see anything as a) how am i to know if there is any CB radios operating anywhere near me?,

At least in Australia 27 MHz CB radio activity is a shadow of its former self.  So it's probably not a good choice to be testing equipment.

Instead I'd recommend first trying a local signal source, eg a 1 transistor crystal oscillator or oscillator module.

It might also be educational to try probing around the 455 kHz IF stage of an old AM transistor radio that's tuned to a station. Look at what comes up on the screen. Then tune between stations and watch any change.  Then probe later on in the receiver around the detector and audio stages and note different frequencies and signal characteristics.

Note that radio signals picked out of the air are extremely weak and require a considerable level of amplification.  Whereas a signal that's gone through the front end of a radio and to the IF strip would have had a significant amount of amplification.

[sonnytiger]

On the scope I saw a 1MHz signal, but not really any kind of repeating 27MHz signal. I didn't really expect to see anything as a) how am i to know if there is any CB radios operating anywhere near me?,

At least in Australia 27 MHz CB radio activity is a shadow of its former self.  So it's probably not a good choice to be testing equipment.

Instead I'd recommend first trying a local signal source, eg a 1 transistor crystal oscillator or oscillator module.

It might also be educational to try probing around the 455 kHz IF stage of an old AM transistor radio that's tuned to a station. Look at what comes up on the screen. Then tune between stations and watch any change.  Then probe later on in the receiver around the detector and audio stages and note different frequencies and signal characteristics.

Note that radio signals picked out of the air are extremely weak and require a considerable level of amplification.  Whereas a signal that's gone through the front end of a radio and to the IF strip would have had a significant amount of amplification.

OK I will do this! Thanks for the suggestion! I think I have an AM radio lying around haha

[sonnytiger]

From what I have read and what I have heard here, I seem to be better off messing around in something a bit easier like AM, which I will do.

Thanks for the info guys!

[xrunner]

I have a rigol DS1052E, I was messing around with a length of wire strung up that would be appropriate as a monopole CB radio antenna. I had it hooked to my scope with 1M input impedance and 15pF input capacitance. I was having trouble seeing anything other than the 60Hz signal  ...

If the CB radio was in your shop transmitting you might be able to see the RF, but other than that the signal will be way too weak to see on your scope. I used to have a Heathkit field strength meter that was entirely passive, you could see the meter move as you walked around your yard and transmitted with a 100W transmitter, but it depended on the RF being extremely strong in the near-field. If you got too far away from a strong transmitter it would simply not register anything, that's because it was simply a wide-band detecting circuit with no amplification.