Anyone who works with radio transmitters will know all about matching and impedance, and also about the importance of selecting the best co-axial cable connecting transistor and antenna. But here’s [Steve, KD2WTU] with a different take, he’s suggesting that sometimes a not-so-good co-ax choice can make the grade. He’s passing up expensive 50 ohm cable in favour of the cheap and ubiquitous 75 ohm RG6 cable used in domestic TV and satellite receiver installations.
Fighting that received wisdom, he outlines the case for RG6. It’s cheap and it has a surprisingly low loss figure compared to some more conventional choices, something that shouldn’t be a surprise once we consider that it’s designed to carry GHz-plus signals. Where it loses is in having a lower maximum power rating. Power shouldn’t be a problem to a shoestring ham for whom 100W is QRO. Another issue is that 75 ohm coax necessitates a tuner for 50 ohm transmitters. It also has the effect of changing the resonance of some antennas, meaning a few mods may be in order.
So we’re convinced, and with the relatively QRP shack here we can’t see RG6 being a problem. Maybe it’s something to try in out next antenna experiment. Meanwhile if you’re interested in some of the background on co-ax impedance choices, we’ve been there before.
The main issue with cheap 75 ohm coax is that it uses heavily oxygenated copper, which causes some signals to travel slower than speed of light, leading to weird resonance effects. More expensive 50 ohm coax uses oxygen-free copper and pretty much guarantees that signal will be transmitted undisturbed. Same reason why modern scope probes use 50 ohm cable.
All information signals travel slower than light, do you mean that RG-6 has worse dispersion characteristics/frequency response or what?
This is not even wrong.
You may be a Dave Jones, but I do not think you are the Dave Jones.
All signals in a cable travel slower than light in a vacuum.
Oxygen-free copper has zip to do with the velocity factor. The velocity factor (propagation speed of the signal as a fraction of light speed) depends on the dielectric (the insulator) of the cable.
Modern scopes normally use a resistive cable for the high impedance scope probes and a high impedance input. More expensive models have a 50 ohm input mode. Both modes have a use. Both modes can cause distortion of the signal and disturbances in the circuit being probed.
Well, dang it.
The comment system mangled my numbered list.
All signals in a cable travel slower than light in a vacuum.
Oxygen-free copper has zip to do with the velocity factor. The velocity factor (propagation speed of the signal as a fraction of light speed) depends on the dielectric (the insulator) of the cable.
Modern scopes normally use a resistive cable for the high impedance scope probes and a high impedance input. More expensive models have a 50 ohm input mode. Both modes have a use. Both modes can cause distortion of the signal and disturbances in the circuit being probed.
I give up. You can’t (or at least I can’t) make a numbered list in a comment.
:(
you can
(1) use patenthesis or
[2] use braces or
!3! use anythinhg else.
It won’t be formatted as a list but will be understood like that. And yes, that’s a hack ;)
Yeah, you’re definitely a phony. The real Dave Jones knows about transmission line impedance and velocity factor, and wouldn’t be spouting garbage about “oxygenated copper”. Please go away.
I think velocity factor is mainly dependent on the insulation material.
Belden RG-6 has a VF of 0.83, while RG-8 is 0.86.
I have used RG-11 on HF/6M for 30 years.
Low cost/low loss. I would not use it at VHF and up, but
it could be I guess??!?
Standard crimp RG213 connectors fit it fine.
On long runs I have used CATV hardline in the past without issues(at 1500 watts)
YMMV
funny, my house it’s been running the same way with a 2.5GB/s coax with no problems…