Monday, March 14, 2011

75Ohms, why not?!

I have been experimenting a bit with more or less cheap (inexpensive that is) materials lately. Came across something called RG-6, or, in other words, very inexpensive 75Ohms coax-cable for TV-satellite reception.
TV-sat-RX means, that losses at frequencies between 1GHz and 2GHz are acceptable. The cable itself proofs on that point, foam dielectric....
We amateurs are using stuff called RG-58, RG-213 etc. for ages.... This relatively expensive stuff is rated 50 (or 52) Ohms, as most of our transceiver's aerial connectors. Most of the modern times transceivers however still employ so called UHF-connectors... something sooo outdated, you would not believe it! And further, look at the inside of 'm transceivers, there is not 50Ohms match found anywhere.... So, what is that hype about 50Ohms anyway?! I don't know!
And hence, I don't care!
However, nowadays UHF-connector have a thread for the the shield. The soul of the sat-coax is copper and hence can be soldered easily. There is hence an easy way to adapt 50Ohms PL-connectors to 75Ohms F-connectors.

F-connector 75Ohms coax and a 50Ohms UHF-connector

My local hardware store provides me with satellite-grade 75Ohms cable for a price of about €5.- per 10m including two of the so called F-connectors... a price, RG-58 cannot match, not to speak about the (two) UHF-connectors aka PL259.
Let's further search for justification of using more expensive 50Ohms-stuff rather than 75Ohms-mass-ware. A dipole is mentioned to have an impedance of 60Ohms.... OK 50Ohms is marginally closer to that than 75Ohms would be.... but... does that matter? I don't think so!
Let's turn to the connectors for a change. I mentioned UHF-connector (aka PL-connectors) before. To the time these connectors have been invented, names like UHF were justified, I believe. However, in modern times, this merely reflects a relict from the glorious past... UHF connectors are considered good for shortwave and lower frequencies. Connectors considered suitable for UHF as of today would be BNC and N-connectors (both 50Ohms nevertheless).
Still 50Ohms, but why? I believe, we stick to 50Ohms (or 52Ohms) for reasons of tradition. Industry seems to like that, since there still is a reason to produce (and sell) 50Ohms coax like RG-58 and RG-213.
As radio-amateur, or HAM for good measures, you always want to watch your options.
I watched mine, and I came to the conclusion that, where ever I would need coax-cable the cheapest commercial (i.e. mass market) solution available was the best for the purpose. Hence I will use RG-6 or whatever (low loss) will be available for cheap at my local hardware store. Who needs specialized retailers, if a tiny change in the setup will do?
Up to now I was writing about low power and low-noise reception. Let's face the other option for a change... QRO! When facing high power, I would anyway not consider using coax-cables. Those just get hot on losses. For QRO, I personally would/will/do go for open wire feed-lines. If that appears too difficult to do, I still would propose using high ohmic window-line for the purpose.
Conclusion: use cheap satellite-TV-coax and forget about expensive 50Ohms stuff!