Ohm impedance mismatch

[Bonkk083]

I have a antenna that can use 50 or 75 ohms but I'm using 75 ohm low loss rg-11 and my scanner is 50 ohms are they any converter or transformer to connect to the scanner to fix the mismatch, the way I'm using it currently am I losing a lot from receiving farther away signals

 

[merlin]

This is not needed especially if you are just receiving. What little mismatch there will be
has little significance.

 

[Ubbe]

I've measured several scanners over their frequency range and they are way off 50 ohm on most of their frequencies. If a scanner happens to be 50 ohm and you use 75 coax then it's a 1,5:1 SWR that equals a 0,18dB loss. If it is a 50 ohm to 100 ohm mismatch then the SWR are 2:1 and the loss will be 0,6dB.

SWR Table

/Ubbe

 

[wtp]

yes, i know you want the best signal.
but unless you want to hear a guy on a handheld 10 miles away, don't worry too much about it.
there are some on here that use a paperclip to listen.
and i am sure there are some that spent hundreds of dollars on an antenna.
i have used RG6 on some things, and back of the set on other radios, and even a TV antenna.
for reception only, there are no rules.

 

[G7RUX]

Mismatch will affect both receive and transmit in the same way so it isn’t true to say that it doesn’t matter much for receive as opposed to transmit, although there won’t be the problems with heating that you might see with a mismatched transmitter systems.

All that said, the mismatch losses you will see with this sort of system will be small and not worth worrying about in this case. As has been pointed out, the loss for perfect 50/75 ohm mismatch is around 0.2 dB, quite hard to measure accurately. Additionally, a wide and receiver input impedance will likely vary quite a bit from the 50 ohm you might expect, with typically significant reactive components which vary wildly across the operating frequency range. Antennas are rarely exactly what you expect either so you might see the same sort of mismatch losses at the opposite end of the feeder too.

In short, don’t worry about it as it will be fine.