If 75 ohm is wrong impedance, then why 75 ohm is almost exclusively used for TV, Satellite Dishes, and Cable TV?
Neither 75 ohm nor 50 ohm are wrong. They match the system for which they are used.
The voice communication systems were developed first and they adopted 50 ohm as their standard. The Amature Radio and Cell/Mobile phone being voice communication devices, they also adopted the already existing 50 ohm standard for themselves as it was easy and natural choice.
TV was developed independent of voice communication system. Initially the VHF chanels were used, and the VHF TV antennas had 300 ohm impedance. As a result the twin-lead ribbon used to connect TV antenna to TV Receiver was designed as 300 ohm line. The input impedance of TV sets was also designed to be 300 ohms.
Later with introduction of UHF TV channels, the antenna design was modified, and folded dipole with 75 ohm impedance and GHz suitable RG6 coax were developed with 75 ohm impedance. Newer TV receivers were also designed having 75 ohm impdance.
Since the DVB-T dongle is designed as a TV Receiver, its front end chip has an input impedance of 75 ohms . So connecting a 75 ohm RG6 coax to a Generic DVB-T provides a perfect impedance match.
When DVB-T dongles with integral LNA were introduced, the designers used a 50 okm input/output impedance RF Amplifier chip. This caused an internal impedance mismatch (50 ohm to 75 ohms) at interface of LNA chip and Tuner chip. However as LNA chipās input impedance was 50 ohms, now it became suitable to connect a 50 ohm antenna coax.
Also 50 and 75 ohms are nominal values used as naming conventions. The actual impedances are 52 and 72 ohms. Even if they were actually 50 and 75, in a transmitting antenna and radio designed for 50 ohm coax, using 75 ohm RG6 would represent a 1.5:1 SWR which is quite good, representing only 3% (not dB) of power loss with 97% of the transmitted power reaching the antenna and being radiated (assuming the antenna itself is perfectly resonant at the operating frequency.) In the real world your SWR would probably be somewhere closer to 1.4:1 due to their only being a 20 ohm mismatch which is a 38% mismatch not a 50% mismatch.
In a receive-only system the slight amount of loss of using 75 ohm coax is insignificant compared to the attenuation of the length of coax. RG58 is 50 ohm coax but itās only 33% efficient at 1100MHz so you would be better off using RG6 which is 50% efficient. Of course LMR400 is even better at 74% efficiency. The higher the efficiency the lower the signal strength attenuation over the coax.
Like antennas, transmission lines are compromises. You may be forced to use what you can get or what you already have, and none of them are perfect so you end up selecting the least bad. Also, like antennas, any coax is going to be better than no coax.
Then you should get it from an authorized Times Microwave distributor or dealer to make sure youāre getting genuine coax and not a clone. Ham radio supply stores (DX Engineering, GigaParts) have it in bulk or by the foot but youāll need to also buy crimp-on connectors and a crimper tool and coax stripper.
Although speaking of clones, Iāve had excellent results with pre-made cables Iāve bought from a store called ācoolelectronicsā in China. I donāt know if they have a presence on Ali or Bangood though, Iāve ordered mine from an online shopping site called Lazada thatās only in certain SE Asian countries. Iāve purchased several coax cables from them and theyāre of excellent quality.
If you get Times Microwave brand Iād spring a few more bucks for their Ultraflex version. It may not be as stiff as regular LMR400 and has a tighter minimum turn radius making it easier to route the cable.