DER EE DE-5000 LCR Meter
- Thread starter George S.
- Start date
So I am not clear on 50 ohm vs 75 ohm vs 93 ohm but do understance the difference in induction in coax cables. Please comment
here is some comparison
RG6QS loss 4.77db/100ft @ 550MHz 75 ohm cable 20pf per foot
RG174 loss 17.3dB/100ft @ 400MHz 50 ohm cable. 31pf per foot
RG62u loss 4.9db/100ft @ 500mhz. 93 ohm cable. 13.5pf per foot
here is comparison table
https://www.google.ca/search?q=coax...IBAzQuOJgBAKABAcgBCsABAQ&sclient=gws-wiz-serp
here is some comparison
RG6QS loss 4.77db/100ft @ 550MHz 75 ohm cable 20pf per foot
RG174 loss 17.3dB/100ft @ 400MHz 50 ohm cable. 31pf per foot
RG62u loss 4.9db/100ft @ 500mhz. 93 ohm cable. 13.5pf per foot
here is comparison table
https://www.google.ca/search?q=coax...IBAzQuOJgBAKABAcgBCsABAQ&sclient=gws-wiz-serp
For high voltage, the perfect impedance is 60 ohms. For high power, the perfect impedance is 30 ohms. For low loss, the perfect impedance is 75 ohms.
This means, clearly, that there is NO perfect impedance to do everything. What we ended up with was a compromise number, and that number was 50 ohms.
(4 MHz graph shown, the effects are less at audio frequencies at or below 20KHz)
You will note that 50 ohms is closer to 60 than it is to 30, and that is because voltage is the factor that will kill your cable. Just ask any transmitter engineer. They talk about VSWR, voltage standing wave ratio, all the time. If their coax blows up, it is voltage that is the culprit.
So why not 60 ohms? Just look at the power handling at 60 ohms - below 50%. It is horrible! At the compromise value of 50 ohms, the power has improved a little. So 50 ohm cables are intended to be used to carry power and voltage, like the output of a transmitter. If you have a small signal, like video, or receive antenna signals, the graph above shows that the lowest loss or attenuation is 75 ohms.
(Source www.belden.com)
This means, clearly, that there is NO perfect impedance to do everything. What we ended up with was a compromise number, and that number was 50 ohms.
(4 MHz graph shown, the effects are less at audio frequencies at or below 20KHz)
You will note that 50 ohms is closer to 60 than it is to 30, and that is because voltage is the factor that will kill your cable. Just ask any transmitter engineer. They talk about VSWR, voltage standing wave ratio, all the time. If their coax blows up, it is voltage that is the culprit.
So why not 60 ohms? Just look at the power handling at 60 ohms - below 50%. It is horrible! At the compromise value of 50 ohms, the power has improved a little. So 50 ohm cables are intended to be used to carry power and voltage, like the output of a transmitter. If you have a small signal, like video, or receive antenna signals, the graph above shows that the lowest loss or attenuation is 75 ohms.
(Source www.belden.com)