Building an FM Trap in LTSpice & Antenna input Power..

[Mikejo]

I have an "RTL-SDR".

They are notably sensitive receivers, I have some FM stations nearby, and what I would like to do is model an FM trap (Chebyshev or Butterworth) in LTspice.

What I'm interested in knowing is, how much power will enter the circuit at the receiver antenna.

I'm guessing it's somewhere around 25-100 mV AC?

 

[slicerwizard]

Does this help?

Power vs. Voltage in 50 Ohm System Equations Formulas - RF Cafe

 

[Mikejo]

That's just way too much information, I just need a ballpark idea so I can set the voltage parameter for the input of the filter that I'm designing. I'm guessing it's going to range depending on the distance from the transmitter I'm just looking for a ballpark figure. Like I said possibly between 25 mV and 100 mV or something like that? I would just like to present to the filter something realistic.

 

[krokus]

I have an "RTL-SDR".

They are notably sensitive receivers, I have some FM stations nearby, and what I would like to do is model an FM trap (Chebyshev or Butterworth) in LTspice.

What I'm interested in knowing is, how much power will enter the circuit at the receiver antenna.

I'm guessing it's somewhere around 25-100 mV AC?

Are you doing this aa an engineering exercise? If not, it would be less hassle to purchase one.


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[prcguy]

For a generic worst case starting point I would use 0dBm. I've measured as much as -10dBm aggregate power off a Discone antenna in a suburban environment and that could go higher with close proximity to some high power transmitters.

 

[Mikejo]

No, it is not an engineering exercise or anything I'm just trying to build an FM trap I don't want to buy one, I want to enjoy the experience of building it. All I'm looking for is a simple answer as to how does the energy that's broadcast reach my radio and in what form does it reach. Is it in the form of alternating current, DC current, and at what level millivolt, nanovolt, whatever? It's not a hard question to answer I would guess, unless it is the goal is to make it so by some people.

(No snark intended).

 

[krokus]

That's just way too much information, I just need a ballpark idea so I can set the voltage parameter for the input of the filter that I'm designing. I'm guessing it's going to range depending on the distance from the transmitter I'm just looking for a ballpark figure. Like I said possibly between 25 mV and 100 mV or something like that? I would just like to present to the filter something realistic.

That will depend on a lot of factors, including:
Transmitter power
Transmitting antenna
Distance from transmitter
Terrain between sites
Flora between sites
Receive antenna


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[Mikejo]

I guess it's not a question of "what is in the way of getting to my antenna" so much as it is a case of what IS getting to my antenna? I know it's coming in a wave form, but I don't know what it is? Is it AC, DC, gummy bears? And when it when it hits my antenna, in the form of electrical energy (In whatever form), what are the possible ranges in strength? is it nanovolts millivolts, kilovolts… Lightning bolts?

 

[Ubbe]

I don't know what filter calculation programs that require knowing a voltage. All components will handle all the voltage that ever can come from your antenna. It is attenuation level and bandwidth that are used as variables when you design a filter.

FM broadcast transmitters are in the 88-108Mhz range and I guess you have nothing below 50Mhz to monitor but if you would like airplane frequencies at 118Mhz to be untouched you'll have to make a sharp filter at the high frequency end or sacrifice some attenuation in the FM broadcast band.

How much you'll need to attenuate depends of how high power level from the strongest transmitter you have from the antenna and the most important parameter are how well your receiver can handle strong signals without loosing sensitivity and/or have overload issues and intermodulation problems.

I would say that look at 40dB attenuation (10,000 times) of the 88-108Mhz range if you don't bother with aviation monitoring.

/Ubbe

 

[Mikejo]

.........All components will handle all the voltage that ever can come from your antenna.........

Thanks Ubbe, I think that's the answer I was looking for!

P.S. i'm using LTSpice, and all the examples I see are using AC voltage at 1 V input.

 

[majoco]

I measured across the VHF FM band with my Ailtech NM 37/57A field intensity meter from a discone up on the roof and the strongest stations gave me just over -70dBm - say 100uV in 50 ohms. I am line-of-sight to the transmitters.

 

[prcguy]

Aah, but what is the aggregate power of all frequencies hitting the front end of your receiver? Stick a power meter or millivolt meter on your antenna and see what happens.

I measured across the VHF FM band with my Ailtech NM 37/57A field intensity meter from a discone up on the roof and the strongest stations gave me just over -70dBm - say 100uV in 50 ohms. I am line-of-sight to the transmitters.

 

[majoco]

Aaah - but they won't all be in phase.....