75 Ohm radio

[NiCdeth]

Why don't the radio manufacturers build everything to match a 75 ohm load? Decent quality 75 ohm coax can be quite cheap, but for some reason even bargain-basement 50 ohm coax is very expensive in comparison.

[GeeFull]

Why don't the radio manufacturers build everything to match a 75 ohm load? Decent quality 75 ohm coax can be quite cheap, but for some reason even bargain-basement 50 ohm coax is very expensive in comparison.

It has not always been the "norm" for 50 ohm coax, many years ago in the days of more common valve PA's and PI tank tuning on their outputs, many Hams pressed 75 ohm co-ax into service, when coupling up to half wave dipoles etc., which exhibit closer to 75 Ohms than 50 Ohms, unless pruned to accomodating length!
Some early valve TX equipment even had 75 Ohms stated as their termination impedance, and some like old Codars, even had Belling Lee TV type connectors for the antenna output!
The problem with a lot of modern low loss 75 Ohm co-ax, despite it being very low loss with high frequency handling characteristics, is its dielectric strength when high levels of RF are shoved up it.
Sure its low loss, and excellent for RX low loss purposes, but it simply isn't designed to handle high RF TX power levels.
You can still get RF TX grade 75 Ohm co-ax, if you want it, but be careful with a lot of stuff currently sold as "low loss 75 Ohm co-ax", as whilst it has excellent low loss RX characteristics at very high frequencies, it simply is not designed for TX purposes, although it will handle relatively low TX RF without any problems.
As to why 50 Ohms became the standardised "norm", ask manufacturers, I am sure it was they that spearheaded it, rather than amateurs themselves!

[kr0ne]

Don't know if it's true but I've read a few times over the years that 50 ohms became the norm because early co-ax in the US was constructed from stabdard sized copper pipe.

I think it was 0.75" stuff inside 2" stuff, which works out to something like 49.97 ohms.

[Tigersaw]

We changed to 50 ohm during the war, as it requires less copper than 75 ohm. Once the war ended there was shed loads available surplus, and thus became the new standard.

[Dragonfly]

I read somewhere that 50 Ohm coax is a compromise as 75 ohm is best for recieving and 30 Ohm is best for transmitting .

[kr0ne]

30 ohms is best for power handling and 77 ohms has the lowest attenuation.

75 ohms is a compromise between the lowest attenuation and the best match for a dipole (73 ohms) while also being easily matched to a folded dipole (300 ohms) with a 4:1 balun.

50 ohms is a good compromise between 30 and 77 ohms but I guess there are many stories about how it came to be.

[GeeFull]

The last two post above seem to have the reason nailed, well Amphenol cables on demand agree with them anyway!
http://cablesondemandblog.com/wordpress ... ial-cable/

[Transwarp]

Here endeth the lesson.

[kr0ne]

Good article! Funnily enough, I was thinking about 93 ohm co-ax after posting earlier. Some old network gear used to use this... Arcnet being one, if I remember rightly.

I had some Arcnet NICs in the '90s (they were old then) and could never get them to work. Knowing what I know now, I've often wondered if this was because of cable impedance. Back then, one thick cable with BNC connectors was the same as any other thick cable with BNC connectors to me!

[Admiral]

As I used modern wideband 'DC to daylight' sets then an ATU is a must, therefore it generally matters not a hoot what the impedance of the coax is.

[paulears]

Why don;t the manufacturers build radios to use 75 Ohm cable? They could, but what would be the point? RF systems use 50 Ohms cabling and connectors and video systems use 75 Ohm. Just a standard, as people have said. They don't design radios so just a few people can save a pound or two on the proper cable. I work with lots of digital audio and lighting. The cable impedance for these should be 120 Ohm, but most people use the wrong cable, to save a bit of money. For the majority of the time, it works - until it doesn't! Fault finding then becomes a real pain in the bum.