on opposite hill from the perspective of QRM issue:
This path loss calculator of course give you only calculation focused to the electromagnetic waves propagation in the free space, it mean there, where the 1.Fresnel zone is unaffected, it mean in practise propagation hill to hill on sight line. However, if the concurent station creates for you „all the band QRM“, would be useful to check, if everything is related to reality, o rif somewhere could be some serious technical problém, which could stay behind QRM phenomena.
Let’s say, the concurent station is 10 km far away and has 1kW power into 10dB gain antenna (it counts cable attenuation as well) and this beam is fixed into your direction. In case of „multibeaming“ arrangement on opposite hill it could be real. Then the signal from such station (in case of similar antenna on your side) will be on the input of your RX aproximamety (use the calc. for check):
Opposite 144MHz contest station:
Power output: 1kW = + 60dBm
Antenna gain: + 10dB (compare to dipole)
Path loss on 10km and 144MHz: 95dB
Your contest station:
10dB antenna gain as well.
Received power: 60 + 10 - 95 + 10 = -15dBm However level S9 on VHF represent the input power -93dBm, so -15dBm will be the signal level S9 + 78dB!
Now tune away your receiver from the opposite station frequency to a 100kHz distance. You may await, that in case of ordinary commercial transceivers the sideban noise of local oscilátor spectrum will be around -130dBc/Hz (it mean 130dB below oscilátor carrier, measured on 1Hz wide filter – because such sideband signal has character of noise, the power level is relevant to the width of filter and 1Hz is used as reference for such measure). However, if you are using SSB and you have 2kHz wide IF filter, the noise power level passing through your filter will be by 33dB higher (2kHz filter is by 33dB wider, than 1Hz filter). It mean the noise level ratio (compare to the carrier of local oscillator) will be on your receiver stronger - not 130dB less, but 97dB less bellow opposite station signal level.
And because signal of concurent station will be on your receiver S9+78dB, than – even in case of theoreticaly unlimited attenuation of the sideband noise on transmitting site, his signal will interfere by noise spectrum on your receiver in 100kHz spacing still (due to the sideband noise of your local oscilátor in your receiver) on the level S9 + 78 – 97, it mean about S7 level. If you are using LNA (for example with gain 20dB) the QRM will be not S7, but on S9. And if you add real transmitter performance on the transmitting side (oscillator sideband noise + wideband noise of transmitting chain + some intermodulations), the result will be even about 1S (6dB) worse.
It is a physics. It mean in case of two QRO stations in distance below 50km - particularly if one of them will have „multibeaming setup“, some QRM level is normal. Only if the interference level is much stronger, than above, o rif the concurent station produce QRM and/or splatters all the time although not use the multibeaming arrangement, or if he/they use only low power category or even „barefoot“ equipment only, then has some sense to put the complaint or try to find a solution. If not, you should just accept some level of interference. 144MHz contest is not using of HF bands…