SDR's and rf limiters

[LD723]

Does anyone have an rf limiter attached to their sdr in-line? Something like this: Mini-Circuits
And if so how does it perform?

 

[a417]

I have an FM trap on the big antenna, and the flightaware bandpass on the antenna for the ADSB reciever. They work in my situation due to my 32,000 watt neighbor.

 

[dlwtrunked]

I have an FM trap on the big antenna, and the flightaware bandpass on the antenna for the ADSB reciever. They work in my situation due to my 32,000 watt neighbor.

an RF limiter has a different purpose--to prevent electronic damage to the reciever.

 

[dlwtrunked]

Does anyone have an rf limiter attached to their sdr in-line? Something like this: Mini-Circuits
And if so how does it perform?

I use one on the SDR in the car in case I forget and transmit on 2m or 70 cm on the nearby other antenna. How well does it work? I would have to test with and without it to see if it saves the SDR (AirSpy) when without it the AirSpy getting killed. I do not want to do that ~$200 test.

 

[vagrant]

I have limiters on various lines and also the vehicle scanner. I use two on an SDRPlay and no problems. I use the HP limiter. I forget the model number, but if you search for limiter and HP it should come up in several posts.

 

[LD723]

I use one on the SDR in the car in case I forget and transmit on 2m or 70 cm on the nearby other antenna. How well does it work? I would have to test with and without it to see if it saves the SDR (AirSpy) when without it the AirSpy getting killed. I do not want to do that ~$200 test.

I also have a question so let's say in your case you are transmiting on either 2m or 70 cm and the airspy is listening to local public safety. Does the transmission from from either the 2m or 70cm wipe out the signals the airspy is receiving in this case the public safety? With the rf limiter in-line of course

 

[vagrant]

Yes, it will wipe it out. You could try a DCI VHF/UHF filter. Still, the proximity will be rough and those filters are not inexpensive. To give the filter a chance you would want to separate the antennas vertically and horizontally and turn down your power to 5 watts or less. Maybe use a handheld.

 

[LD723]

Yes, it will wipe it out. You could try a DCI VHF/UHF filter. Still, the proximity will be rough and those filters are not inexpensive. To give the filter a chance you would want to separate the antennas vertically and horizontally.

Understood I'm curious on what kind of filter I need to be able to not have signals get wiped out by nearby transmitters like even my 1 watt portable or the fd talking on on mobiles at 50 watts when passing by my house lol while still being able to listen to vhf/uhf/700 800 mhz like how my local pd does it with their scanners in their vehicle. Funny thing is is when they transmit in their vehicle mobiles it doesn't wipe out their uniden BCD996P2 they have in their vehicles or at least it doesn't seem to be wiped out as the receive comes out clear. I've asked if they had any filters connected to it but they don't know as they did not install it.

 

[dlwtrunked]

I also have a question so let's say in your case you are transmiting on either 2m or 70 cm and the airspy is listening to local public safety. Does the transmission from from either the 2m or 70cm wipe out the signals the airspy is receiving in this case the public safety? With the rf limiter in-line of course

I never do this myself. The Limiter is in case I forget to make an attempt to avoid damage.

 

[mmckenna]

Antenna separation is key.
The limiters will only limit to a certain point.

The issue you are referring to is "receiver desense". When faced with a strong nearby signal, the receiver can essentially get overloaded enough that it becomes deaf to what you are trying to hear. Keeping the antennas far enough apart is what works. The limiter will help reduce the chance of damage.

As for the police cars, it really depends on the frequencies in use and how strong the received signal is. In-band is going to suffer from more desense issues. Radio traffic in other band may fare better.

 

[LD723]

Antenna separation is key.
The limiters will only limit to a certain point.

The issue you are referring to is "receiver desense". When faced with a strong nearby signal, the receiver can essentially get overloaded enough that it becomes deaf to what you are trying to hear. Keeping the antennas far enough apart is what works. The limiter will help reduce the chance of damage.

As for the police cars, it really depends on the frequencies in use and how strong the received signal is. In-band is going to suffer from more desense issues. Radio traffic in other band may fare better.

Gotcha makes total sense and thank you for the explanation 👍I was thinking about a limiter because I also became paranoid about transmitting near my sdr as I did it far way too much (1-5 watts) but haven't noticed any damage on my sdr's as they still receive

 

[thewraith2008]

Your SDR will have a diode clamp (e.g. BAV99) in the input RF path for excessive RF and static.
While not indestructible, it will do the job protecting the device in most normal situations.

Preventing front-end overload of a (sdr) receiver designed to be fairly broadband it's tricky.
Radios for a single band use a bandpass filter (BPF) and better SDRs use tracking filters to help attenuate out of band signals.
External filters can be used for AM/FM and TV bands and other filters can be found for specific bands if needed. Using to many inline and you get insertion loss and your target signal gets attenuated.
None of this will help for in band or adjacent frequency rejection. The selectivity of the IF will only go so far and doesn't help with SDR since the IF is hugh compared to those found in real radios (3-15KHz).

 

[LD723]

Your SDR will have a diode clamp (e.g. BAV99) in the input RF path for excessive RF and static.
While not indestructible, it will do the job protecting the device in most normal situations.

Preventing front-end overload of a (sdr) receiver designed to be fairly broadband it's tricky.
Radios for a single band use a bandpass filter (BPF) and better SDRs use tracking filters to help attenuate out of band signals.
External filters can be used for AM/FM and TV bands and other filters can be found for specific bands if needed. Using to many inline and you get insertion loss and your target signal gets attenuated.
None of this will help for in band or adjacent frequency rejection. The selectivity of the IF will only go so far and doesn't help with SDR since the IF is hugh compared to those found in real radios (3-15KHz).

I didn't know about sdrs having those diodes sounds pretty interesting. If that diode you mentioned was damaged in anyway by a portable of mine, would I still be able to receive or would the dongle be dead by then? I just feel like I damaged mine at least a little bit by transmitting one too many times throughout a few years right next to the antenna but I still receive signals mostly fine lol. I might just be paranoid by my previous actions with my radio near the sdr

 

[dlwtrunked]

Your SDR will have a diode clamp (e.g. BAV99) in the input RF path for excessive RF and static.
While not indestructible, it will do the job protecting the device in most normal situations.
...

With an a higher-end SDR on one end of the hood and 2-m on the other, I have "fried" the front-end of the SDR by accidently keying. Added a limiter after that and so far so good but I am probably more careful about transmitting (try to disconnect the mic when I am only listening with both). I would label that configuration as perhaps the primary one in this group that would be an issue and consider it "normal" - at least for me. But I can also picture a vehicle in the lane next to me similarly blowing out the front-end of my SDR if it were not (hopefully) protected with the limiter.

 

[thewraith2008]

I'm not sure when the diodes fail if they appear as a short or not. (never seen it occur)
Testing the diode in circuit may not be simple because of inductors that may be used in the RF path and GND.
This would make it look DC shorted when it may not be.

If the diode is shorted, you would expect a completely deaf device.

• Desoldering the part and so long as no other damage has occurred further in you would expect a normal receiving device.

If the diode is not shorted but as failed, so long as no other damage has occurred further in you might expect a partially deaf or normal receiving device (and now unprotected).

NOTE: I don't recommend desoldering this (or other) SMD components for the sake of testing as well as if you are not skilled at it.

 

[thewraith2008]

Using an SDR around a transmitter that you use is definitely going to require some additional protection or at least some 'best practices'.

Increase antenna spacing from (SDR) RX and TXing source as suggested.
Don't expect to be able to use the SDR while transmitting a few (tens or) hundred watts in close proximity
Maybe un-plugging the SDR from both USB+Antenna while TXing.
Use a bandstop on the SDR at the frequency you TX on if you really must use both at same time.
...

 

[LD723]

Thank you all for the suggestions I don't transmit as much as I used to so it's rare for me to use my portable near the sdr but I was just worried that my sdr's may have been damaged but I am hopeful it isn't since it doesn't appear to be deaf (at least to me lol) I did notice as soon as I stopped transmitting while looking at sdrsharp, the noise floor would be lower than usual then jump back up to normal a second after transmitting I'm assuming those could be the diodes at work but I have no idea lol.

 

[dlwtrunked]

I'm not sure when the diodes fail if they appear as a short or not. (never seen it occur)
Testing the diode in circuit may not be simple because of inductors that may be used in the RF path and GND.
This would make it look DC shorted when it may not be.

If the diode is shorted, you would expect a completely deaf device.

• Desoldering the part and so long as no other damage has occurred further in you would expect a normal receiving device.

If the diode is not shorted but as failed, so long as no other damage has occurred further in you might expect a partially deaf or normal receiving device (and now unprotected).

NOTE: I don't recommend desoldering this (or other) SMD components for the sake of testing as well as if you are not skilled at it.

Minewas repaired by a friend with the right equipment who could do the SMD work (in the meantime, I more carefully used used one of my 5 others) . There appeared to be no diode protection. (I am aware of others having similar happen.) The effect of the accident was was very week reception until the amplifying SMD was replaced. This was a higher end SDR (which I do not want to name as I cannot say similar would not happen to others. Protection at frequencies in the GHz range comes apparently comes at a price.

 

[dlwtrunked]

My was repaired by a ham friend who could do the SMD work (in the meantime, I more carefully used used one of my 5 others . There appeared to be no diode protection. The effect of the accident was was very week reception until the amplifying SMD was replaced. This was a higher end SDR (which I do not want to name as I cannot say similar would not happen to others. Protection at frequencies in the GHz range comes apparently comes at a price.

 

[thewraith2008]

There appeared to be no diode protection. (I am aware of others having similar happen.) The effect of the accident was was very week reception until the amplifying SMD was replaced. This was a higher end SDR (which I do not want to name as I cannot say similar would not happen to others. Protection at frequencies in the GHz range comes apparently comes at a price.

Strange not to include it as it's cheap protection to add.
Even the cheap RTL dongle has it (D6). (as does RTL-SDR.com (D?), Airspy Mini (D2), Airspy R2 (D4), SDRPlay RSP1A (D? x2))

If it does not have it, it's simple enough to add if you are so inclined.