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Breaking Input Stage of Spectrum Analyser with Radio TX
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xrunner:

--- Quote from: Bicurico on November 22, 2023, 08:36:00 am ---The reasoning of this thread (and my worries) is: what can happen if I sit in on my desk (with all devices at 1m-2m distance) and start transmitting on cheap chinese radios, which claim up to 8W power?

According to xrunners calculations, there is in fact reason to worry, but it is borderline: sure enough, the SA's don't have an antenna connected to the input port, so little power should go in. And I could of course just move away, when I want to hit the TX button.

But thinking about this, led me to imagine other scenarios:

Imagine you ordered a brand new SVA1032. Its input is specified at +30dBm = 1 Watt.

Now the DHL guy, who has your package in the back of his van, but somehow close to his CB antenna is broadcasting to his colleagues that he will be delivering this last package now. He is transmitting 4 Watts. Could he break the packaged SVA1032 by doing so?

What if a taxi is stopped next to the DHL van and is transmitting by radio that he just dropped of the passenger. His antenna is half a meter from the SVA1032 in the DHL van...

I know that the van has a metal shield, but for the sake of it, imagine it is sunny Australia and the van is like an open pickup truck.

Could the SVA1032 be "endangered"?

--- End quote ---

I'm thinking of an experiment that will (hopefully) put your mind at ease, but I need to check some things out today. I will post the results here ...


--- Quote ---When I mentioned "fragile", I was wondering that a switched off SA should be imune to RF radiation next to it. The input stage should be in a metallic shielded box and a relay should cut off whatever could come in through the open RF input.

I know, perhaps I am just paranoid...

--- End quote ---

I understand, but let's see what my experiment shows.


--- Quote ---Dummy load: I know what that is and it wasn't what I meant.

I meant this: https://pt.aliexpress.com/item/1005006031464052.html as opposed to this: https://pt.aliexpress.com/item/4000412423002.html

--- End quote ---

Mmmm ... one model is a 10W att. with SMA connectors "as opposed to this" the others are 2 W att. with N connectors. What's your point? Is it the fact that the 10 W has cooling fins and the others do not? It's a design choice made by the manufacturer. The design choice is "what cooling system is required to dissipate X amount of power in a certain size package. What enclosure do we need to put the attenuation components in?"

In other words, the one with cooling fins could have been made larger without cooling fins if they wanted to put it in a bigger package that would dissipate the heat by virtue of it's larger thermal mass.

And by the same token, the 2 W designs with no cooling fins could have been made smaller if they wanted to put the attenuation element in a package with cooling fins. But then the N connectors might be bigger than the attenuation package so what would be the point?

The point is "cooling fins" do not tell you anything about what power something can dissipate. It just tells you, for the package the manufacturer chose, they needed cooling fins to dissipate the Max power rating of the device without a fan. You see, it all comes down to design choices - lowest cost, good performance, package size, etc.

Anyway, let me try some experiments and I'll be back ...
xrunner:
For you Bicurico

The test: What we want to know is, using the N female connector as the "antenna", how much power is received at a measurement device behind the connector. It could be a spectrum analyzer, a power meter, or a receiver. It doesn't matter what is receiving the power, because it all depends on how good of an antenna the bare N connectors is.

I have a Baofeng UV-S9X3, a 3 band (146, 220, 440 MHz) HT. It puts out on high power ~4.7W on 146 MHz, 3.2 W on 220 MHz, and 4 W on 445 MHz. This is my test transmitter.

I will use a Boonton 4220 power meter to measure the power at the "output" of the N connector. The sensor can measure down to -60 dBm. As I said above, it doesn't matter what measures it as long as it is accurate.

First, is the power meter working correctly? The output of the Baofeng should be ~5 W or +37 dBm (depending on the charge state of the battery). I put 40 dB of attenuation in front of the power sensor to prevent damage. I compensated the power meter reading for this in the dB reference offset. I read +37.59 dBm or 5.7 W which is acceptable. But this is with a system connected with coax. What happens if we just use the antenna of the HT?

I removed the attenuators from the system and set the reference back to 0 dB, and screwed on a female N connector. This is our input - our "antenna", just like the spectrum analyzer would have with nothing connected to it. Now I want to transmit from the HT using it's antenna and see how close I can get to the N connector before reaching a danger level for damage to the sensor.

I started across the room transmitting and walked slowly to the sensor. It quickly became obvious that there was nothing to worry about, certainly at any distance greater than a hand's length. Then I continued slowly until I had the end of the HT antenna jammed up into the end of the N connector. This is as much coupling as I could achieve in any orientation around the N connector.

The Max coupling I could get gave -2.27 dBm (631 uW) for the 145 MHz band and -2.22 (600 uW) for the 450 MHz band. This is nowhere near the damage level of my power sensor (or spectrum analyzer of +20 dBm (100 mW)). So you see, the output of the HT is +37 dBm and the best coupling with the antenna stuck in the end on the N connector gives -2.2 dBm. One might think that this would be dangerous before this experiment, but even as it stands, the loss is +37 down to -2.2; a loss of 39.2 dB.

So we were checking whether a spectrum analyzer with an open N connector can accidentally be damaged by rf. What if the N connector has an antenna on it then? I put one on and then transmitted at 5 W and of course the antenna works like a charm. I could have easily exceeded the damage level of the power sensor of +24 dBm by moving just a bit closer than you see in the picture.

OK Bicurio, are you going to sleep better? Perhaps you're still not sure? Just don't keep an antenna on your spectrum analyzer.  ;)

cncjerry:
How many people remove all their cables all the time?  Brings up a good point, don't go chasing DX with your 2kw amp while cables are attached to sensitive inputs.
Bicurico:
Thank you very much.
That really helps ease my mind.
I realized that I lack sensibility to evaluate signal loss over air and distance.
Sorry for the short answer, I am away. I will try to replicate some of this and have much more confidence now

Kind regards,
Vitor
xrunner:

--- Quote from: Bicurico on November 25, 2023, 08:24:53 am ---Thank you very much.

--- End quote ---

No problem I found it interesting as well. Never had a reason to anything like that.


--- Quote ---That really helps ease my mind.
I realized that I lack sensibility to evaluate signal loss over air and distance.
Sorry for the short answer, I am away. I will try to replicate some of this and have much more confidence now

Kind regards,
Vitor

--- End quote ---

I'd be interested in your results if you duplicate them, or if they are different.
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