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?
Just a cable guy. So much crap. Did you know some of it is copper coated aluminum.still fine to 2.4
Copper plated steel, not aluminium.
One potential issue is that per-spec RG6 has quite thick copper, and remains low loss on the low HF bands. However since RG6 is generally used for UHF tv signals, much thinner copper would be fine – until you use it in the low HF, where it will become steel and tres lossy. So check the loss at 3.5MHz before using.
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 ;)
Or
555 could 555
555 have 555
555 used 555
That’s an ugly hack. ;)
@Joseph Eoff said: “I give up. You can’t (or at least I can’t) make a numbered list in a comment.”
Yup, you are right, you cannot make a REAL ordered list in WordPress Jetpack. But as others have demonstrated earlier, you CAN make a cheezy hack to get around it!
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.
The misonformation is by design, most probably to try to discredit the real Dave Jones in a petty act of self righteous retribution due to a percieved slight.
Pay them no heed.
Coming soon…Monster sustainably sourced oxygen-free copper coax. :-)
(I thought we’d put this “oxygen free” crap behind us years ago…)
I (mistakenly) thought “oxygen free” was regular copper cable with insulation that prevented oxygen from accessing the wire and causing corrosion.
That would be tranny lines made of copper pipes filled with electrically insulating and non-reactive sulfur hexafluoinert gas. Useful mostly in microwave systems.
…and TV transmitters. I think they use dry nitrogen gas and a pressure sensor to detect leaks rather than fluoroinert. MUCH cheaper.
8 inch copper pipes, the inner conductor is like 1.5″ or whatever the equation for impedance says gives you 50 ohms.
It just changes the velocity factor. This used to be more of a thing before you could get a NanoVNA, but it’s real easy to measure it directly and build a stub matching network to get the characteristic impedance you want. You can make a transmission line out of speaker wire if you like, and it’s a favorite of mine for cheap HF antennas.
Okay, I’m not Dave Jones but I’m the real Peter Walsh and I did a deep dive into impedance matching for one of my experiments. If you’re interested, I made some tools available for people who want to make bespoke coils:
https://hackaday.io/project/193000-inductance-design-tools
The surface characteristics of a conductor affect the transmission properties due to the skin effect. Rule of thumb, anything over 100 KHz is forced to travel on the surface of the conductor, and as a result the characteristics of the surface can matter.
I say can matter. Keep that in mind.
This comes up when people want to make high quality (Q factor) coils, should they use copper or silver? Silver will give you a 7% increase in conductance, and due to the skin effect you only need silver plating.
Copper will oxidize and so will silver, but silver oxide is itself a fairly good conductor and won’t dissipate much power.
Copper oxide has a high resistance, and the skin effect will try to push as much power into the high resistance as possible. Depending on the frequency, the skin effect will put all of your power within one thou (.001 inch) of the actual surface.
This is one reason we use enamel wires. It’s not just corrosion and insulation, it prevents oxidation that will dissipate power.
Now, I was always under the impression that there is no oxygen inside a cable so different cables don’t matter (I’m looking at YOU, MonsterCables), so I agree with everyone who’s not Dave Jones here.
But the surface characteristics of a transmission line do matter.
The skin effect is exponential. If you look up the skin effect thickness online, that’s the distance where 1/e of your signal power travels (IIRC – someone fact check this), and the next thickness down holds 1/e^2, and so on. So it makes sense that there might be some sort of resonance with speed of light in the signal traveling on the resistive surface of the conductor with the signal traveling further in the more conductive parts of the conductor.
I say makes sense, but I’d like to read a paper describing the effect before I make any judgement.
(And for the record, switching to silver is, in almost all cases, not effective. Usually there’s other aspects of the system that have a bigger effect on transmitted power, and these should be addressed first.)
Skin effect is best done with tuned wave guide. Bit annoying to use for long runs out to the backyard.
To be clear, the only way an oxide can change the velocity factor is by screwing with the reactance, not the resistance. The velocity factor’s just determined by the field characteristics (permittivity/permeability). It’ll increase the loss, though.
But the loss won’t be that large because the skin effect doesn’t actually apply entirely: the skin depth of the oxide is huge, since skin depth is proportional to sqrt(conductivity): most of the current will still be in the conductor unless it’s a very thick oxide layer. (Which it can be if it’s corroding!)
sigh, sqrt(resistivity)
This is one of the silliest and most ignorant posts ever.
Thanks!
While it is not really a hack, one can get adapters to connect coax with F connectors to 50ohm N connectorized systems cheaply on aliexpress and elsehwere.
That’s my go to for using cheap 75ohm coax on HF and elsewhere.
Especially when the low loss foam dielectric makes soldering connectors very annoying.
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.
Dielectric, not insulation material. For non-magnetic materials, VF is 1/sqrt(relative permittivity).
Also depends a bit on the shielding quality since that determines the propagation mode a bit. But it’s almost all dielectric. Conductor characteristics are a distant third.
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
Need to be a bit careful connecting to the aluminum jacket though…I investigated it a bit and figured out a way to use plumbing fittings, but there’s a dissimilar metals issue that needs to be addressed with NoAlOx or something similar. What did you do for connectors?
RE: “with NoAlOx or something similar. What did you do for connectors?”
Silver plated Amphenol (not cheap) and Noalox.
Still lower cost than LMR400/600 or 9914.
I worked on public service radio systems(Motorola dealer) and could
get them “cheap” (vs Digi-key,,,etc)
I tried AL solder a few times, just never had good luck doing it.
Other folks said it worked great?!?
Would that cause the sort of “rusty bolt” junction I’ve read about that causes harmonics? Or is that something different?
Any time you have two different metals and moisture, you have a dissimilar metals corrosion issue. There’s more to this than I feel like typing, (check the galvanic tables), but you can beat the problem by keeping the moisture out. NoAlOx is a heavy grease made specifically to do that for aluminum/copper connections. Works for aluminum/”anything” connections, too.
Regarding connectors, I did some playing around with aluminum CATV hardline and found plumbing fittings (for copper tubing with a crushable copper ferrule) would work for the outer conductor, and that the copper plated center conductor could be soldered to the center pin of a PL-259. Never went any further, but it looked feasible for HF. Silver plated N connectors sound good, but the center pin might be tricky…or not.
Bingo! I use F-6 —the aluminum bonded braid shield version RG-6— nearly everywhere, especially my outdoor runs.
It has nearly the same power handling capability
AND frequency vs. loss curve as RG-213.
Hey, dipoles most have a 75Ω center impedance.
So much misinformation here.
funny, my house it’s been running the same way with a 2.5GB/s coax with no problems…
Seriously, I’ve used 75 ohm coax for HF (it can actually be a better match to certain antennas). You could probably improve things a bit more with a 50-75 ohm transformer at your antenna tuner.
ATU should be next to the antenna feedpoint, never in the shack – that’s bad practice. 🤓☝️
“that’s bad practice. 🤓☝️”
Or use a resonate antenna…No need for the tuner :)
Sigh … a resonant antenna has nothing to do with its actual impedance. A W8JK antenna, for instance, can be perfectly resonant and yet it has a feedpoint impedance around 6 (six) ohms … which is an SWR of about 9:1.
Amateur radio antennas are probably in the top ten of technical subjects with the most ridiculous and persistent misinformation.
“a resonant antenna has nothing to do with its actual impedance.”
It was a poke at Joshua dude. Have a beer. :)
“ridiculous and persistent misinformation.”
On that one I agree!!
That depends entirely upon three things … the length of the coax, its loss per foot, and its voltage breakdown. In many cases the difference between an ATU at the antenna and one in the shack is almost undetectable on the air.
Correct. Harder to adjust and much more expensive, though.
It’s tough to figure out if you really need a proper impedance match. A ton of modern amps transfer a poor output match to their input via poor active directivity, so that can magnify things. But if you’ve got RG6 it’s relatively lossy anyway, so it’ll help damp reflections regardless.
The worst part about starting with a “less than ideal” setup is not knowing where the problem is when you have them. Once you’ve got some experience, it’s a lot easier to cheap out on stuff.
Yeah, it just depends on what your signal margin is and what you’re targeting. Hams generally can use super cheap coax because you’re already living in a noisy world and low frequency anyway. You could probably get away with even worse.
It has to do with the length of the cable and the length of the wave being received and reflectivity which is how you use that really complicated looking chart that’s a circle with a bunch of lines on it…….
It’s Smith Chart and though you should never Wikipedia in any graduate level class as I’ve been chastised for on a few occasions here’s the Wikipedia entry. I could go and find the journals and all the research on Google scholar which is a much better source but this does the job.
https://en.m.wikipedia.org/wiki/Smith_chart
AK6BB- rg6 coax is all I’ve ever used and works great. If your smart get to know your local cable tech, they always have spools of cable that are to short for what they need to do. I’ve gotten some 100+ lengths for free.An if you go to their main depot you might be able to snag some nice lengths of rg11 for free.
“It’s cheap” is generally my mantra. I don’t think I have a piece of ham equipment that is in any way/shape/form optimized.
Good ‘ol RG6 Quad with BNC connectors. I’ve been using that on HF since I saw the loss figures and did a little math. Even with the mismatch it’s less lossy than RG58. Plus it’s cheap and available locally.
But this opinion usually infuriates appliance operators who then go digging around for “data” to prove that it’s not the “right” solution.
I also use XT60 connectors for power instead of powerpoles. That one really brings out the ire of the emergency communication hams.
I think Powerpoles are mainly an American thing. Here in Europe, XT60 is quite popular for powering all sorts of devices. I also use it on all the power cables for my radios and all battery packs.
Round here XT60 is quite popular for all sorts of devices …. that are 36V or 48V i.e. all the ebikes.
Hence I stick to powerpoles for 12V stuff lest there be a terrible wailing and gnashing of teeth.
Oh…and good ol’ Ethernet cable (the yellow “thicknet” stuff) is slightly better than RG-213 at HF IIRC. If you were lucky, you could get it with N connectors already attached. I have a length feeding my inverted vee. The story goes that Metcalfe and company used RG-213 for the prototype.
Sadly, it’s somewhat less available now, than when I grabbed my length…
I’ve used RG6 for HF and 6 meter antennas and it works fine. For a single band antenna, just cut the coax to a half wavelength multiple and you won’t need a tuner if the antenna is resonant.
I also picked up a 1000 foot spool of RG11 from a hamfest, but I haven’t gotten around to using it yet.
75Ω coax is very useful for making antenna phasing harnesses too.
There’s another deeply misinformed person equating resonance with matched. They are NOT the same thing. MANY commonly used antennas are resonant but fundamentally don’t provide a good match to 50 ohm or 75 ohm coax. Would you like a list of them??
“another deeply misinformed person equating resonance with matched”
Dave,
You are seeing something in your head that is not being said.
Cody did not say that and neither did I
I DO understand the difference and Cody may as well.
Indeed. I cut to 1/2 wave at 80m – but I measure at 20m (14.3MHz) to cut the cable. This makes it a pretty accurate 1/2 wave on 80,40,20,10
He seems all concerned about power measurement in Watts. That’s a common problem, focus on the wrong power. Two-way communication has less to do with output power and far more to do with power delivered to the receiver.
When you start to think about the microvolts of signal that hit the receiver, the dB of losses in the system become worthy of effort to reduce.
Worry less about the Watts. Focus on the milliWatts.
You can easily match a length of this stuff to 50 ohms with a few feet of 50 and 75 alternated. Somewhere online I found a calculator for making this sort of un-un, and how much loss it has.
I found those instructions also. I found my myself in possession of some 75 ohm cable about the size of RG 213 and larger. about 1/2″ and larger, some was about 3/4″. was originally used with some obsolete radar. and the big 75 ohm was intended to be direct burial TV cable.made un uns with 50 ohm you can find the formulas. freq will determine length of 50 ohm needed. had to modify connectors to fit the larger stuff. the antenna end can be matched through gama, delta matches etc. the fat weeny (weenie?) 75 ohm coax is very low loss.
Oh and those un uns are hard to make at 800 mhz and higher.