50 Ohm Coax

[pkveazey]

I see a lot of You Tube videos about antennas and coax and I laugh and laugh at some of the stupid ass things these idiots put out. Now, with that said, I saw one the other day that made me scratch my head and say, "Hmmmm... this guy might be right". He was answering the age old question of why radios want to see a 50 Ohm load. He said that the transmitted signal loses less energy when it does out at 30 Ohms impedance and a received signal loses less energy when it comes down a 75 Ohm Impedance. So, somebody decided to find a sweet spot in the middle and they settled on 50 Ohms. Actually it is 52 Ohms but who gives a crap about the 2 Ohms difference. That made sense and the guy was quite convincing. The only thing that bothers me about it is why would the exact same frequency going out or coming into a coax have a different loss factor? For the moment, I'm going to go with the notion that he is right and accept his findings. I've never been one to give a crap about someone using 52 Ohm Coax or 75 Ohm Coax. Now, back in the old days, I remember a lot of Hams used a separate Receiver and Transmitter. Each one had its own coax. Ok some guys had an RF sniffer circuit relay box that would automatically switch a single coax cable back and forth but that was for the rich guys or the one's with enough brain cells to build their own. Ideally, you could use 75 Ohm coax on the receiver and 52 Ohm coax on the transmitter.

[JohnyMac]

You got me thinking there PKv.

It might be interesting if you have two antennas up to send on one using 30 ohms and the other using 75 ohms. My main rig has two antenna connectors and a A/B slit option.

With all the filters and such on the rig I wonder if the time given to playing with it would give me that much more performance.

Interesting discussion you stated their Brother. 

[wynnp4]

Honestly i never really thought about it.... I just buy 50 ohm RG8X and rock on...

I have a tower I hope to put up.one day... I was going to contact a buddy that works in the cable industry.... get a couple hundred feet of .500 75 ohm cable... there is a way to splice in a combination of 50 ohm and 75 ohm to balance it out to 50...  The .500 cable is hardline and would carry signal very well....

One day...

WP

Sent from my SM-G955U using Tapatalk

[JohnyMac]

LOL Wynn. We all have a few of "some day" projects. 

[pkveazey]

You got me thinking there PKv.

It might be interesting if you have two antennas up to send on one using 30 ohms and the other using 75 ohms. My main rig has two antenna connectors and a A/B slit option.

With all the filters and such on the rig I wonder if the time given to playing with it would give me that much more performance.

Interesting discussion you stated their Brother. 

That's a novel idea but finding 30 Ohm coax might be impossible. I often wondered why "Receive Only" coax was usually 75 Ohms. Think TV and some scanners.

[pkveazey]

Honestly i never really thought about it.... I just buy 50 ohm RG8X and rock on...

I have a tower I hope to put up.one day... I was going to contact a buddy that works in the cable industry.... get a couple hundred feet of .500 75 ohm cable... there is a way to splice in a combination of 50 ohm and 75 ohm to balance it out to 50...  The .500 cable is hardline and would carry signal very well....

One day...

WP

Sent from my SM-G955U using Tapatalk

You've brought up a good point about mixing 75 and 50 Ohm coax. Yes, if you know exactly what the velocity factor is on both coaxes is and you measure exactly then you should be able to make the system think its 50 Ohms. However, the literature that I've read pretty much says that 50% 75 Ohm connected to 50% 50 Ohm will yield the average of the two lengths which would be 62.5 Ohms. Hell, that's plenty close enough match. The Engineer for WRVA and WRNL radio station told me that when trying to match impedances you need to be inside 25 %. Hmmmm..... 62.5 Ohms would be about 20%. Hmmm... That's less than 25 % mismatch. Go for it. I've mentioned this before and Balun freaks sometimes get their panties in wadd over it. I don't like Baluns. If you want to match a 75 Ohm dipole to a 50 Ohm coax, just put a 1:1 Balun in the line. OK, you get that 1:1 SWR that you want but you lost some power in the Balun. Running it with no Balun, you'll usually get a SWR of about 1.5:1 and the amount of power loss is about the same as the loss going through the Balun. Hey, I know.... I'll spend extra money and get a perfect match that does me no good. With massive impedance mismatches you might need a 3:1 or 4:1 Balun to get the impedance into the ballpark. Now, going back up to the top of this epistle, let's say that you have 100 feet of 50 Ohm Coax and you add 25 feet of 75 Ohm Coax to it. The Impedance would be very close to 50 Ohms due to averaging based on length.

[pkveazey]

Honestly i never really thought about it.... I just buy 50 ohm RG8X and rock on...

I have a tower I hope to put up.one day... I was going to contact a buddy that works in the cable industry.... get a couple hundred feet of .500 75 ohm cable... there is a way to splice in a combination of 50 ohm and 75 ohm to balance it out to 50...  The .500 cable is hardline and would carry signal very well....

One day...

WP

Sent from my SM-G955U using Tapatalk

I had a second thing to mention about this. I found a Cable TV spool with 75 Ohm Hardline left on it and asked the cable crew if I could have it. They gave it to me and I ran the 75 Ohm Hardline up to my 160 meter 75 Ohm Dipole and ran it for many years with great success. I have to admit that I did use an antenna tuner so I could resonate anywhere on any band. One day I decided to run some LMR400 equivalent coax instead. I installed it and it worked perfectly but it wasn't any better than what was there to start with. Yes, I still use an antenna tuner to resonate it on any band. That's what I'm running right now.