Can a radio frequency be lowered by a passive circuit?

[timelessmidgen]

Is it possible to design an unpowered antenna, (possibly some kind of RLC circuit) which will receive a radio wave at a particular frequency and then re-emit it at a lower frequency?

 

[NFuller]

I don't believe so. Passive circuits generally do not alter the input signal frequency.

 

[tech99]

Is it possible to design an unpowered antenna, (possibly some kind of RLC circuit) which will receive a radio wave at a particular frequency and then re-emit it at a lower frequency?

I do not know a method of doing it passively, but it is possible to re-radiate at twice the frequency, and this is used for vehicle collision avoidance radars for instance.

 

[russ_watters]

Is it possible to design an unpowered antenna, (possibly some kind of RLC circuit) which will receive a radio wave at a particular frequency and then re-emit it at a lower frequency?

Even if there was, please note that the only power available is what was received, so that isn't much power to re-emit.

 

[berkeman]

Is it possible to design an unpowered antenna, (possibly some kind of RLC circuit) which will receive a radio wave at a particular frequency and then re-emit it at a lower frequency?

Since a diode is usually included in the list of "passive" (unpowered) devices, I believe the answer to your question is yes. Can you think of how you would use one or more diodes and two antennas to do what you are asking?

 

[timelessmidgen]

Many thanks for the responses everyone!
@berkeman: hmm, okay, it will take me a while to wrap my head around the circuit diagrams, but googling "diode frequency divider/multiplier" does indeed reveal the sort of thing I was wondering about!

 

[tech99]

Many thanks for the responses everyone!
@berkeman: hmm, okay, it will take me a while to wrap my head around the circuit diagrams, but googling "diode frequency divider/multiplier" does indeed reveal the sort of thing I was wondering about!

I have never come across away of dividing a frequency with diodes and I wait with baited breath.

 

[berkeman]

Many thanks for the responses everyone!
@berkeman: hmm, okay, it will take me a while to wrap my head around the circuit diagrams, but googling "diode frequency divider/multiplier" does indeed reveal the sort of thing I was wondering about!

A lot of the links I get with your Google search are for digital dividers and synthesizers, so keep an eye out for that. Many of the multipliers they show are for analog multiplication and mixing.

 

[berkeman]

I have never come across away of dividing a frequency with diodes and I wait with baited breath.

I haven't tried this, but it seems like it would work in principle:

Spoiler

AM modulate the carrier at a high enough frequency so that the envelope can be re-radiated at the lower frequency with a larger antenna. For example, modulate a carrier at 100MHz with an AM envelope around 10MHz. Pick up the 100MHz signal with an antenna, detect the envelope, and re-transmit the detected envelope with a 10MHz resonant antenna.

But as @russ_watters alluded to, you would need a pretty strong signal into the first antenna to get much transmitted out of the lower-frequency antenna.

 

[Tom.G]

But as @russ_watters alluded to, you would need a pretty strong signal into the first antenna to get much transmitted out of the lower-frequency antenna.

Since a diode is usually included in the list of "passive" (unpowered) devices, I believe the answer to your question is yes. Can you think of how you would use one or more diodes and two antennas to do what you are asking?

Following thru on that theme, use energy harvesting of the received signal to obtain a DC voltage. Use that voltage to operate a Tunnel Diode oscillator at the desired frequency. Just don't expect much conversion efficiency.

(took a while for that one to percolate up into consciousness)

 

[tech99]

But as @russ_watters alluded to, you would need a pretty strong signal into the first antenna to get much transmitted out of the lower-frequency antenna.

It is possible to frequency double with a diode and re-radiate a signal sufficient for some purposes. An idea of the range obtainable can be found as follows:-
Assume a typical mobile transmitter and receiver at 100MHz. TX power 10 watts (10 dBW), and minimum receive power 10^-14 Watts (-140 dBW). So it can work over path of 150dB. If we split this system gain between two paths, go and return, each can be 75dB.
Now if we allow 2.5dBi antenna gain at each end and a loss of 10dB for the frequency doubler, we can work over a path of 60dB between isotropics.
We can find the free space distance this will allow by using a path loss formula such as the following,
Loss (dB) = 126 + 20 log (D/lambda)
where L is the loss between isotropic antennas, D is in miles and lambda is in cm.
60 = 126 + 20 log (D/300)
D = 0.15 miles

 

[tech99]

Apologies for an error in my calculation. The antenna gains increase the system gain to 70dB so that a range of 0.5 mile is possible.