Can anyone give me a sound electrical reason why we design antenna systems around 50 Ohm impedance?
Dave @KJ8U
Long answer ?microwaves101.com/encyclopedias/…
The executive summary of the long answer on Microwaves101.Com is that for coaxial cable with an air dielectric, the lowest loss is at about 77 Ω. Air-dielectric coaxial cable can, however, handle the most power when its characteristic impedance is about 30 Ω. You’ll find all the math at the Microwaves101.Com link above.
Hams want coaxial cables to both handle a fair amount of power and have relatively low loss, so whoever chose 50 Ω as the amateur radio standard did so because it happens to be about halfway between 30Ω and 77 Ω.
Nowadays, we rarely use air-dielectric coax. Instead, we use coax with a polyethylene dielectric. This increases the power-handling capability of the coax. Not only that, a coaxial cable with polyethelene dielectric has the least loss when the characteristic impedance is 51.2Ω. How lucky is that!
By the way, this line of reasoning also explains why 75 Ω is the standard for video and cable TV. These cables don’t have to handle a lot of power, so minimizing loss is the most important consideration in these applications.
GB Hoyt says
Oh man, I’d completely forgotten about this. Good reminder!
DE KG4GVL