They can pick up your jammer, which should not be placed in the middle of your troop concentrations.
The more powerful the jammer, the bigger the effective bubble and the further away you can place it from friendly forces and assets.
If you want to move up a gear, you can have a mobile jammer that moves around randomly and avoid easy targeting. You can use long cables to route the jammer emitter far from the rest of the jammer so enemy artillery will only hit a cheap and expendable emitter.
Moreover, your jammer should be working in conjunction with your own recon forces, counter battery radars, friendly artillery and aviation such that if the enemy tries to hit your jammers with artillery, they would expose their artillery positions to rapid counter battery fires as well as rapid UAV ISR recon to do damage assessment, fine tune friendly artillery and/or seek out fleeing enemy artillery units for aviation to follow up and mop up.
But this is the Russia forces, so a lot of that is probably not something they can do consistently across their forces. Which is probably another reason China is withholding direct military supplies. There is no silver bullet solution to the poor shape the Russian forces are presently in. You will need a full spectrum modernisation for them to fight like they should be able to, and that is going to require arms supplies on par with what NATO has been sending Ukraine.
Ukraine is just not remotely important enough for China to be worth that kind of investment, not even counting the diplomatic and economic costs of taking sides so decisively for China.
So if sending individual systems isn’t really going to make much difference, but incur full costs for China, it makes most sense for China to not get involved overtly.
I think this is the kind of calculations the EU has collectively just not even thought of to do before getting involved in supporting Ukraine.
As soon as you take a side, you incur full economic costs in the form of sanctions and/or loss of access to markets and key supplies.
Drip feeding arms into the conflict will only serve to prolong it, and the economic pain to yourself. Thus the logical play is to either not get involved, or to intervene decisively and with overwhelming force to utterly and irrecoverably tip the balance in the favour of your supported side.
Unless of course, you are a special case like Iran that both benefits from the current global economic earthquakes, and are also so sanctioned there isn’t much additional cost to getting involved since there isn’t anything left to sanction anyways. So it makes perfect sense for them to get involved to make a quick buck both from direct arms sales and also from prolonging the current favourable economic conditions and opportunities. This, funnily enough, also applies to the USA.
Jammers will draw HARMs towards them, exactly the US sent Ukraine some. If it were not for Tors, Buks and Pantsyrs, the Russians would have lost more these jammers.
Jammers are supposed to work with your own ESM systems. They are not set on 24/7 daily to create a static bubble. Instead the ESM picks up the enemy signal, then processes it for two results. The jammer stays quiet and invisible until it is called into action.
The first is to identify the signal, to determine friend or foe, then what classification of signal is it. Is it a radar? What type of radar will it be? Is it communication? What type of communication will it be?
The second is that the ESM then determines the location of the signal. There are a few means how this is determined but we won't go through these techniques (AoA, ToA and FoA).
Both data is sent to the jammer, with the signal waveform being replicated by the jammer. Then via a phase array, a beam is formed towards the offending source with a duplicate of the signal that has been either manipulated to give errors, or rotated to cancel it out. Incidentally this is also how shipborne EW systems work.
The ESM is a passive system and it would be working somewhere near the jammer. Because it is passive, detection is impossible except through optical means, and these systems can get away being small and not easy to spot.
The media almost never talks about it but I believe Russia has embedded into their brigades, sophisticated EW systems both ECM and ESM, which along with ground based SAMs, are able to neutralize Ukraine's drone forces to some degree. The ESM and SIGINT also tap to the Ukrainian communication chatter, overhearing conversations, and directionally locate them for targeting. That's likely how they were able to destroy some of the Ukrainian air defense systems. Picking up their radar signals, analyze them to identify the target, then directionally locate the source, and then send drones after them. The drones I suspect might have a HARM capability, this being the enhanced Lancet drone. Using a sample signal obtained by the ESM, the drone sniffed and followed the signal to its source. This may also be used against counter battery radars, and the Russian MoD occasionally reports the destruction of such radars in their public briefings.