Best Mobile Coax

[W5LZ]

This a little bit of 'nit-picking' but it's still going to hold true...

All conductors of RF/AC have a velocity factor, which includes coax cable, inside and outside. The VF for the inside/outside aren't going to be the same, but it's there. That 1/3 wave length isn't going to be very close to a 1/3 wave length after working in that outer VF. It probably would be close enough to -help- in reducing those CMCs but I don't think I'd count on it much. Anything close to that feed line's outer side is going to affect it, unlike the inside of that braid.
There's all kinds of justifications for that 18 feet feed line length, most of them are a lot of 'over thinking' when the simple reason is that it's a good average length for most mobile applications, it'll reach from here to there just dandy.
- 'Doc

 

[CDX-007]

I use LMR-240UF for mobile installs, but that's just my preference. It's easy to route, low loss, double shield and the outer casing holds up well for years of trouble free service.



I'm curious as well. If you have CMC wouldn't that indicate a tuning problem with the antenna? Wouldn't you see better results by adjusting the antenna rather than altering the coax length? I thought that CMC only traveled on the outside (shield) of the coax. How do you get CMC on the center conductor?

Good morning, Welcome to Tag-Team USA!

I never stated that we didn't tune the antenna. A car can be both Blue AND a Ferrari.

Maybe I forgot how easy it is to get a nice perfect 50Ω, X=0 tuning on a mobile antenna

Remember, most of these guys were running a 350 or 667 amp with a mag mount antenna less than a ¼λ from the radio & amplifier, and on a roof smaller than a ¼λ.

The CMC I was referring to eliminating by the non-resonant 1/3λ or 2/3λ was on the outside, you may want to re-read my post.

This a little bit of 'nit-picking' but it's still going to hold true...

All conductors of RF/AC have a velocity factor, which includes coax cable, inside and outside. The VF for the inside/outside aren't going to be the same, but it's there. That 1/3 wave length isn't going to be very close to a 1/3 wave length after working in that outer VF. It probably would be close enough to -help- in reducing those CMCs but I don't think I'd count on it much. Anything close to that feed line's outer side is going to affect it, unlike the inside of that braid.
There's all kinds of justifications for that 18 feet feed line length, most of them are a lot of 'over thinking' when the simple reason is that it's a good average length for most mobile applications, it'll reach from here to there just dandy.
- 'Doc

Quick, inform Sirio that their testing facility is all screwed up and their Gain Master really can't work with it's 5/16λ of above-CMC choke coax offering an expected 5/16λ of CMC current for the bottom ½ of that 5/8λ because it really "isn't gong to be very close to a [5/16λ] after working in that outer VF".

 

[Marconi]

Good morning, Welcome to Tag-Team USA!

I never stated that we didn't tune the antenna. A car can be both Blue AND a Ferrari.

Maybe I forgot how easy it is to get a nice perfect 50Ω, X=0 tuning on a mobile antenna

Remember, most of these guys were running a 350 or 667 amp with a mag mount antenna less than a ¼λ from the radio & amplifier, and on a roof smaller than a ¼λ.

The CMC I was referring to eliminating by the non-resonant 1/3λ or 2/3λ was on the outside, you may want to re-read my post.


Quick, inform Sirio that their testing facility is all screwed up and their Gain Master really can't work with it's 5/16λ of above-CMC choke coax offering an expected 5/16λ of CMC current for the bottom ½ of that 5/8λ because it really "isn't gong to be very close to a [5/16λ] after working in that outer VF".

007, I agree completely in keeping the outside length at some non-resonant length and the closer to perfectly in the middle of a resonant line the better...just like you describe.

For a nice range of frequencies like CB that length will look to be a high impedance path for CMC and help some to minimize the bad responses. It has nothing to do with tuning anything but the feed line.

007, why would you figure VF important with regards to the CMC flowing on the outside of the shield in the bottom half of the GM radiator? Your previous comments explained well just how VF is relevant to the inside of the coax, but irrelevant to the outside. At least that is the way I see it.

I think your best answer is do the real world thing and experiment a little, and you explained you did it because it seemed to work before you knew anything about VF probably. Guys use to talk a lot about "the load," but we don't hear the term used much anymore. For those CB professionals out there: it's about the same as loading a big truck nice and even, with good weight distribution over the entire trailer...so everthing thing travels nicely.

 

[W5LZ]

I don't play "tag-team" games, sorry. You did answer my question, though the answer may not be in your favor. Thank you.
- 'Doc

 

[CDX-007]

007, I agree completely in keeping the outside length at some non-resonant length and the closer to perfectly in the middle of a resonant line the better...just like you describe.

For a nice range of frequencies like CB that length will look to be a high impedance path for CMC and help some to minimize the bad responses. It has nothing to do with tuning anything but the feed line.

007, why would you figure VF important with regards to the CMC flowing on the outside of the shield in the bottom half of the GM radiator? Your previous comments explained well just how VF is relevant to the inside of the coax, but irrelevant to the outside. At least that is the way I see it.

I don't believe the VF of the outside of the coax to be significantly less than 100%, I was being facetious with that last smart-assed remark I posted... just feeling a little irritated, I shouldn't though, short night's sleep, short fuse I guess.
- But it does seem like if I say "The sky is blue" I get replies asking why I don't think the sky is blue... :blink: - Maybe I need to word things differently.

I think your best answer is do the real world thing and experiment a little, and you explained you did it because it seemed to work before you knew anything about VF probably. Guys use to talk a lot about "the load," but we don't hear the term used much anymore. For those CB professionals out there: it's about the same as loading a big truck nice and even, with good weight distribution over the entire trailer...so everthing thing travels nicely.

Yep, but you need a little swrzzzz so the transmitter will be happy and see the load...

 

[CDX-007]

I don't play "tag-team" games, sorry. You did answer my question, though the answer may not be in your favor. Thank you.
- 'Doc

No, I know, I was just feeling a little ganged-up-on, but I still don't agree with your summation of the coax outside VF or else the Sirio Gain Master couldn't work as they claim.
Sirio count on, (and have tested it to have) half of the 5/8λ on the braid / shield as radiating CMC and that's exactly what it measures.

 

[WR0220]

You are correct - I misread what you were saying in regards to the location of CMC, and for that I apoligize. My post wasn't meant as any part of a "tag team" effort, I was just curious why one would alter coax length to combat CMC. I'll be the first to admit I don't know everything when it comes to antennas - and I'm definitely not an expert - but that practice has never been discussed in anything I have read/learned/applied to date other than as a "band-aid" solution so the radio or amp will see a lower SWR. My understanding is that CMC is caused by a tuning mismatch (imbalance) between the antenna (including the ground plane) and the feedline. I have had very good luck reducing CMC by minimizing ground plane loss (bonding vehicle panels), insuring good coupling between the mount and vehicle body (metal to metal contact) and adjusting the length of the antenna. I have only assisted with 1 installation that required a choke under the headliner at the feedpoint of the antenna to reduce CMC. A mag-mount would offer a greater challenge for sure, but I would think it is still possible to minimize CMC without having to factor in coax length - other than to increase the length to compensate for a choke if it couldn't be minimized by other means. Can you point me to any info that discusses this in further detail? Like I said, I'm curious and would like to learn more about this practice.

 

[CDX-007]

I should've placed a after that 'Tag-team' comment!

It always seemed to do the trick and years later when I became conscious of Velocity Factor it snapped into focus for me. I wish I would've thought about DC bonding panels, that might have saved hours of chasing RFI!

 

[Marconi]

I don't believe the VF of the outside of the coax to be significantly less than 100%, I was being facetious with that last smart-assed remark I posted... just feeling a little irritated, I shouldn't though, short night's sleep, short fuse I guess.
- But it does seem like if I say "The sky is blue" I get replies asking why I don't think the sky is blue... :blink: - Maybe I need to word things differently.



Yep, but you need a little swrzzzz so the transmitter will be happy and see the load...

Get a little rest and you'll be alright. I figure as much, "...so never mind."

007, if you measure the bottom half of the GM I think you'll find it is a bit shorter than the top half. That likely as to do with a possible difference is shield and top wire diameters. So, IMO Sirio has designed this one under very tight technical specs. Else, I don't think we would see the little things in its construction that seem a bit unique and well thought out. This is likely what one has to do to make something different that really works. In other words and for those CB professionals out there, that choke wasn't just put where it is, because it looked nice.

I don't think we need to worry too much about needing a little swrzzzz, because were it not for the fact that SWR is a characteristic of antenna function, we would still be sending out smoke signals in order to communicate. It's just like smoke signals, it don't happen without some fire.

For me the proof of how well the Gain Master works regarding no CMC flowing below the bottom can be verified by anyone with a simple Field Strength Meter. Just set FSM up at the base of the antenna, or anywhere near the feed line, or wrap a FSM wire antenna 10' feet long or longer around the feed line itself...and you'll be hard pressed to see the meter deflect due to RF detection.

I have some antennas that respond wildly with feedline radiation, and others with just a little, but the Gain Master shows me about a 1 on my digital FSM that goes well over 100 on the scale. That is no radiation around an antenna bud.

 

[CDX-007]

Get a little rest and you'll be alright. I figure as much, "...so never mind."

007, if you measure the bottom half of the GM I think you'll find it is a bit shorter than the top half. That likely as to do with a possible difference is shield and top wire diameters. So, IMO Sirio has designed this one under very tight technical specs. Else, I don't think we would see the little things in its construction that seem a bit unique and well thought out. This is likely what one has to do to make something different that really works. In other words and for those CB professionals out there, that choke wasn't just put where it is, because it looked nice.

I don't think we need to worry too much about needing a little swrzzzz, because were it not for the fact that SWR is a characteristic of antenna function, we would still be sending out smoke signals in order to communicate. It's just like smoke signals, it don't happen without some fire.

For me the proof of how well the Gain Master works regarding no CMC flowing below the bottom can be verified by anyone with a simple Field Strength Meter. Just set FSM up at the base of the antenna, or anywhere near the feed line, or wrap a FSM wire antenna 10' feet long or longer around the feed line itself...and you'll be hard pressed to see the meter deflect due to RF detection.

I have some antennas that respond wildly with feedline radiation, and others with just a little, but the Gain Master shows me about a 1 on my digital FSM that goes well over 100 on the scale. That is no radiation around an antenna bud.

Marconi, did you measure the coax coming out from the top of the coil and find it shorter than the top wire? I only gave it a cursory going over but after looking at their diagram and considering the overall length minus the coil & mounting area I recall when I assembled and installed it 5 weeks ago, that it was centered.
Now I'm really going to have to take one apart and measure it in infinite detail, perhaps I'll be eating crow!

 

[DJboutit]

RG 213 214
LMR 400
RG 12 for a cophase setup is what I am using now

 

[Kamikaze]

007 - 1

Tag team - 0

Nice explaination and drawing.

Tnx.

 

[GnG8d]

I know this has been asked before, but it's been a while since I've been on here, so here goes one more time.

Which is the best coax, the RG-58A/U or the RG8X (Mini 8)?

Here is what I use .........