Jammer's interference

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Hello. I have a question about jammers. Based on this picture I made quickly, from what I've read online, Transmitter 2 will definitely be blocked with interference by the jammer since it's in its range. My question is, does the jammer also block Transmitter 1 and Receiver 2? Assume that the connection between the two receivers/transmitters are continuous. Thanks a lot!
 

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  • #2
berkeman
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There are two main ways that I know of to "jam" reception. You can overload the receiver with a much larger signal, or you can corrupt the RX data by adding another signal in that interferes with the first signal. Which version are you asking about?
 
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There are two main ways that I know of to "jam" reception. You can overload the receiver with a much larger signal, or you can corrupt the RX data by adding another signal in that interferes with the first signal. Which version are you asking about?

Hmm, I'd like to know from both please? Thanks so much!
 
  • #4
berkeman
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Hmm, I'd like to know from both please? Thanks so much!
Me too. :smile:

Can you post some links from your Google searching so far? Maybe we can both learn from those links.
 
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Me too. :smile:

Can you post some links from your Google searching so far? Maybe we can both learn from those links.

I mainly looked at the wiki for Radio Jamming. After looking at it some more, I noticed the wiki said:

"A transmitter, tuned to the same frequency as the opponents' receiving equipment and with the same type of modulation, can, with enough power, override any signal at the receiver"​

So, I suppose Receiver 2 in my picture would be the one affected?
 
  • #6
Baluncore
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Based on this picture I made quickly, from what I've read online, Transmitter 2 will definitely be blocked with interference by the jammer since it's in its range.
A jammer does not stop a transmitter operating. It jams some receivers.
A jammer only stops a receiver working as expected, when the jammer signal is sufficient to stop the receiver working as expected.
 
  • #7
berkeman
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A jammer does not stop a transmitter operating.
Actually, there is an exception to that. For CSMA (carrier sense, multiple access) communication like WiFi (and many wired networks), a jamming signal that looks like a continuous transmission can hold off all transmitters from trying to start their queued transmissions.

https://en.wikipedia.org/wiki/Carrier-sense_multiple_access
 
  • #8
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Ignoring berkeman's example and another example called decentralized trunking which prevents a transmitter from transmitting if the channel is busy, whether or not receiver 2 is jammed depends on the relative signal strength between the jammer and transmitter 1 at receiver 2's antenna. Normally a SINAD ratio (It is the ratio of Signal+Noise+Distortion divided by Noise+Distortion, expressed in dB.) of 12 dB is considered the minimum for communication. 20 dB is more common for reliable communication. So generally if transmitter 1's signal is more than 12 or 20 dB greater than the jammer's signal, receiver 2 will not be blocked. Note that that ratio can be improved by the use of directional antennas or digital coding with error correction. There are also jammer resistant modulation schemes.
 
  • #9
Baluncore
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Actually, there is an exception to that.
I disagree. A receiver is desensitised by, or detects the hostile jamming signal. The system then voluntarily disables associated transmitters. That is a bad policy as it allows the operator of the hostile jammer to easily estimate their effectiveness by listening to a transmitter.
 
  • #10
berkeman
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I disagree. A receiver is desensitised by, or detects the hostile jamming signal. The system then voluntarily disables associated transmitters. That is a bad policy as it allows the operator of the hostile jammer to easily estimate their effectiveness by listening to a transmitter.
Are you familiar with CSMA networks?
 
  • #11
Baluncore
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Are you familiar with CSMA networks?
yes.
The jamming carrier is received by the receiver.
 
  • #12
berkeman
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yes.
The jamming carrier is received by the receiver.
Sorry, I don't understand the response or your point.

(See my next post below...)

I deal mainly with CSMA networks at my work, and I know from painful personal experience what happens when there is an intentional or unintentional continuous transmitter on the network. Nobody can get through.
 
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  • #13
berkeman
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I disagree. A receiver is desensitised by, or detects the hostile jamming signal. The system then voluntarily disables associated transmitters. That is a bad policy as it allows the operator of the hostile jammer to easily estimate their effectiveness by listening to a transmitter.
yes.
The jamming carrier is received by the receiver.
Wait, I get your point now. You are saying that technically the jammer signal is causing the receiver to hold off the transmitter, and that the jammer has no effect directly on the transmitter itself. Yes, I can agree with that.

In CSMA networks, we really don't have much choice. You are right that it does give an easy way to detect receiver hysteresis thresholds. In fact, I use the effect in some of my transceiver testing where I don't have access to the digital RX signal directly -- I put a tone onto the network and increase it until I can see nodes ceasing their transmissions because they detect that the network is busy...
 
  • #14
Baluncore
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In CSMA networks, we really don't have much choice.
Yes you have got it.
Reasoning about jamming and it's mitigation requires getting the mental concepts in order. That understanding is obstructed by common phrases like “jamming a transmitter”, which come from the days of international propaganda broadcasting on short wave radio. Then, a ground wave transmitter in each city would be used to jam the majority of receivers in and near that city, so they could not listen to the international broadcast sky wave on that frequency.

Unfortunately you are trapped by a specific CSMA system, where jamming one receiver predictably turns off a transmitter. As you know that is a particularly vulnerable system, and one that should be avoided if at all possible. Such a feature denies you the use of game theory, something that can significantly increase the cost to deliberate jammers.

When subjected to deliberately hostile jamming, it is most important to maintain the character of the transmitter traffic, so the jammer cannot assess their success. To teach that lesson I quote directly from R.V. Jones, “Most Secret War”, 1978, page 329-30.
Before proceeding with the Gibraltar narrative, we must also remember the defence of Malta, where three old Gladiators, Faith, Hope and Charity, for a time faced the Luftwaffe alone. I had a little to do with that episode, for the Germans had installed some powerful new jammers on Sicily so as to render our radar on Malta useless. This would, of course, deprive the Malta air defence of any early warning, and a signal arrived in Air Ministry from the Signals Organization in Malta telling us that they were now badly jammed and asking if we could provide any help. I knew that the Germans judged the success of their jamming by listening to our radar transmissions to see whether, for example, they ceased to scan, as they might well do if they could not be used. I therefore signalled Malta to go on scanning as though everything were normal and not to give any kind of clue that they were in difficulty. After a few days the Germans switched their jammers off.

At the end of the War, I spent several days talking to General Martini, the Director General of Signals of the Luftwaffe, when he was a prisoner-of-war. He had been in his post since 1933, and had a long and detailed memory of the many events in which he and I had been opponents. At one point he specifically asked me about the jamming of Malta, and he told me that he had installed the jammers fully expecting to paralyse the Malta radar, but they seemed to have had no effect. He wanted to know what kind of anti-jamming devices we had installed in our radars so as to render them immune. He laughed ruefully when I told him that he had in fact succeeded, but that I knew the clues on which he would judge his own success, and had therefore advised the Malta radars to pretend that they were still working.
 
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  • #15
CWatters
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Hello. I have a question about jammers. Based on this picture I made quickly, from what I've read online, Transmitter 2 will definitely be blocked with interference by the jammer since it's in its range. My question is, does the jammer also block Transmitter 1 and Receiver 2?

Everything is relative but you have this the wrong way around.

Consider Rx 1: Tx2 and the jammer are roughly the same distance away from Rx1. So if they have the same power output Rx1 will see two signals of similar amplitude.

Consider Rx 2: Tx1 is much further away from Rx 2 than the jammer. So at Rx2 the signal from jammer will be much stronger than the signal from Tx1.

So the signal from Tx1 is less likely to be received by Rx2 than the signal from Tx2 is by Rx1.

Example:

Model aircraft clubs usually require pilots to stand close together. Why...

Years ago they used transmitters in the 35MHz FM band. If they stand far apart then their transmitters are spaced as per your diagram. If pilot Tx1 flies his model plane overhead pilot Tx2 then his receiver Rx1 is closer to Tx2 than it is to Tx1. Again this is similar to your drawing. In that situation Tx2 can act like an unintentional jammer blocking the signal from Tx1. Technically there is a bit more to it but that's the basic idea.

If pilots stand close together then the signal received by the aircraft from both transmitters is similar. This makes it easier for filters in Rx1 to reject the unwanted signal from Tx2.
 
  • #16
sophiecentaur
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A couple more from memory lane. When the HF bands were used as the only way to broadcast truth / propaganda, it was the practice to use the highest frequencies available for a particular path from transmitter to target area. Signals at these frequencies were the highest that would bounce from ionospheric layers. Jamming transmitters in the target country would transmit jamming signals but the path was much steeper to the local area and those signals would be lost to space so the jamming was ineffective. Smart eh?

The effectiveness of jamming is very dependent on the modulation signal. For AM, the demodulated signal is more or less the sum of wanted and jamming signal so even a lower level of jamming would spoil the listening experience. However, if you use FM (particularly wide band FM) there is a 'capture effect' and the demodulator will produce the demodulated FM programme of a wanted signal and reject an interfering signal that's just a few dB lower level because much of the interfering signal energy appears as out of band (ultrasonic) components as the demodulator is effectively 'chasing' the wanted slightly higher level carrier. This capture effect is taken into consideration in planning the service for an FM broadcast network. Note - once the interfering signal gets high enough, all hell breaks loose and you get nothing at all.
 
  • #17
jim hardy
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Another of my "boring anecdotes" ...............
this one from the days of analog TV when i lived in Miami

A friend complained that his "shop radio" in the garage got one station all over the dial.
I went over for a listen and sure enough, in between stations WMIA at 93.9mhz boomed in.
I dont remember if the others were distorted but i think so.....
Anyhow he was curious what might be wrong with the set....


We figured it out. It was a fun mystery.

He lived within sight of Miami's Channel 6 TV transmitter, a powerful one on a 1400 ft tower.
Channel 6 uses frequency band 82-88 mhz which lies just below the FM band (88-108 mhz).
In analog days their audio carrier was at 87.75mhz and most digital tuned car radios would pick up channel 6's sound when set for lowest frequency, 87.9 .
That was handy , you could listen to their news while driving to work.
upload_2019-1-26_16-28-52.png

Hmmm. Look at that channel 6 video carrier....

We noticed that the intruding station at 93.9, and channel 6's video carrier at 83.25, differ by 10.65 mhz.

We both knew that FM receivers have IF frequency typically 10.7 to 10.8 mhz.
The plot thickened.
We asked ourselves: "If Superheterodyne works, why not also SUB-heterodyne?"
That led to the questions :
" Who said your local oscillator has to be inside the radio? "
"Is that channel 6 video carrier booming in here, overdriving the first stage, causing it and WMIA to make a difference frequency that's in the receiver's IF passband?"
"Standard FM modulation is ±75khz so it's plausible that a 10.65mhz difference signal would get through a 10.7 IF ... "
"Maybe this particular set's IF is tuned to 10.70 and 10.65 sneaks in, or maybe the set's IF got tuned just a teeny bit low at the factory...."

Sure enough the local TV shop had a couple dollar "channel 6 trap" for the antenna lead-in, and that fixed it.

We called it "The case of the not-so-local oscillator".


old jim
 

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