Cutting to length

[Cody Dixson]

Watched an interesting video about coax jumper lengths ect. Now before everyone gets wild and starts picking sides my main question is what equation is used to figure coax length. I'm in the process of turning current system on my cb in the semi from a single to dual antenna and i want to figure out how to calculate the length for each 75 ohm peice of feed line to get a good match and then a way to calculate what I'm guessing is the "multiples" of a resonate length. Hoping to shorten my runs without using 18ft

Any info is always greatly appreciated thank you!

 

[Slowmover]

9’ or 18’ each.

My Peterbilt had 6’. Got stretched and popped apart.

More is less of a problem:

1). Routing (plus attachment to body)
2). Need for CMC Choke or Isolator at antenna feed point (you might decide to wrap a toroid or two).
3). Yet another filter of one type or another at the radio.

In some installations, a filter at every quarter wave-length can be a suggested noise solution. 9’ is a handy number. You just won’t be able to do much more than one toroid with it (depends on diameter of coax wrap of toroid).

With 18’ legs I was able to wrap two (2) toroids into each at ends. Maybe 6’ left over to install with no excess. 3.25” diameter coils thru FT240-31 and -61 toroids.

Decide what you want to do and run a piece of 3/8” stiff cotton rope as substitute.

.

 

[Cody Dixson]

@Slowmover thanks for clearing that up for me ive been confused on if the length helps or is just there after reading a previous comment on another post about tuning the harness. the length of 50 ohm coax can be any length? Or would cuting it to length like some people talk about or resonate length be better? I watched a video where a tec took an peice of wire with one end and an antenna Analyzers and tuned the jummer untill it has a flat match. So if I sound crazy this is where my questions have stirred from

 

[Slowmover]

Co-phase needs exact length match. Past that is (on my part) conjecture.

Reading around this site will be of help on the question of coax length.

My take is that wavelength multiples are easiest as Total Coax Length isn’t of much concern in mobile install (past becoming part of a noise-related problem).

My concern is that it’s easy to cut TOTAL length too short.

When I get home from this month-long trip there’ll be a new 18’-leg cophase harness awaiting me to install in the Peterbilt. Higher quality cable plus ends plus assembly.

I’ll be installing feedpoint antenna filters at outer ends and have a few devices at the other before it reaches the radio. I’ll wrap excess into a Figure-8 and place into the console at what seems the best locations.

IOW, the truck determines what matters (install-related problems) in what’s crucial. 9’ or 18’ legs aren’t hard to deal with as I see it.

If you read around far enough you’ll also see that 18’ legs “help”, overall (Firestik). I’m not going that far, but cut to length just isn’t important enough in mobile. Fitting it to the vehicle is the priority.

 

[Cody Dixson]

@Slowmover 10-4 10-4 !
I'm hoping to take 2 equal lengths of rg6 with f type connectors to a t connectors then a short jumper of rg8x to the radio

I'll honestly have to go with 18 ft legs after thinking about how it's going to enter the truck.
The jumper of 50 ohm coax from T connector to the radio is my concern. Not that it's a massive concern but from what I've seen it's better to tune jumpers. Because alot of people have told me whatever length works. But would tuning the jumper lower swrs and increase power output efficiency.

 

[Slowmover]

@Slowmover 10-4 10-4 !
I'm hoping to take 2 equal lengths of rg6 with f type connectors to a t connectors then a short jumper of rg8x to the radio

I'll honestly have to go with 18 ft legs after thinking about how it's going to enter the truck.
The jumper of 50 ohm coax from T connector to the radio is my concern. Not that it's a massive concern but from what I've seen it's better to tune jumpers. Because alot of people have told me whatever length works. But would tuning the jumper lower swrs and increase power output efficiency.

The details you note are of interest to me also, but it’s past my ability (and interest) at present.

My opinion about big truck installs is that inefficient antenna mounts (ground) is where the larger problem (the real problem) lays.

I assume use of a dual final radio (35-watts) or greater.

A guy could crank major wattage, but even as it seems to get him OUT, he’s also bathing in RF inside the cab.

IOW, expect to hit limits early with each aspect of a system using typical CB truck antennas.

I’d imagine tuning the coax is a help, but is it enough to change the outcome?

What test after what formula to use? (There've been some heated, albeit funny, “discussions” on the subject).

I don’t want to sound discouraging on a subject in which I can’t help (coax length tuning), my only concern was that the coax harness be long enough for actual routing and any accessory additions.

.

 

[M0GVZ]

Watched an interesting video about coax jumper lengths ect. Now before everyone gets wild and starts picking sides my main question is what equation is used to figure coax length.

None because it's all bullshit. There is only one correct length, the length required to get from A to B by the route you wish to take.

However if you're co-phasing then yes it's critical.

I'm in the process of turning current system on my cb in the semi from a single to dual antenna and i want to figure out how to calculate the length for each 75 ohm peice of feed line to get a good match and then a way to calculate what I'm guessing is the "multiples" of a resonate length. Hoping to shorten my runs without using 18ft

You need to know the velocity factor of the coax for a start as that alters the length. Coax typically comes with two velocity factors, either 66 or 85. If you're making a jumper that needs to be a specific wavelength out of 66 it'll end up shorter than one made out of 85.

 

[Groundhog KSS-2012]

I'm weird I guess but, I always cut my coax in 1/2 wave lengths and have great results. Never had a problem. Even my jumpers are 1/2 wave. All are cut according to their specific velocity factor at 27.185, channel 19. But, I know many of you use whatever length of coax is needed and never have any problems either. It's just each to his own, or so to speak. Lol...

 

[brandon7861]

the topic that never dies... lol

If you have two antennas, one on each truck mirror, hopefully as close to a quarter wavelength apart as possible (hard, i know), and you want to connect both to one radio (radiation pattern to the front and back of your truck and jack shit to the sides, great for straight freeways... gotta love the co-phased give and take), it's simple...

If you have two antennas, presumably both having a 50Ω feed point impedance on their own, what you want is the cable to transform those individual impedances to 100Ω each so that their parallel combination at the tee (that's the radio's RF connector or any 50Ω coax connected to it) once again makes 50Ω for the radio to drive. Ideally, that requires a quarter wavelength of 70.9Ω transmission line feeding each antenna, but 75Ω is close enough for our purposes...

If you were to take the above advice and use a half wavelength of 75Ω coax to each antenna, just ask anyone with a smith chart what a half-wave length of coax does... it transforms the mismatch right back to the feed point impedance and the impedance at the input stays the same, and in effect, doing so with a cophased system puts two 50Ω loads in parallel for a 25Ω load at the radio, which will guarantee you a 2:1 SWR minimum on your best day.

Phasing cables should be 70-75Ω and a quarter wavelength long. Do not use 1/2w cables for phasing.

 

[Rwb]

things that go HUMMMM in the night cut to fit n use. best make sure you got metal as groundplane as lots of newer trucks have plastic roofs and be DAMM SURE mounts are well grounded im my opinion

 

[Eldorado828]

I stopped running co-phase on the big truck years ago. The one on the drivers side is the one in use with a single 8x coax while the passenger is only for cosmetics.

As far as coax length, have always ran enough to get from A to B with some slack just in case and to have enough for replacing pl-259's.

Is that the correct way? The argument continues and always will. Does it work? Always has for me.

 

[Groundhog KSS-2012]

I found this, from DX-antennas.com

WHAT LENGTH?
As mentioned the best length is the shortest, in this way the loss due to the cable is the lowest.
There can be one advantage by extending the length of the cable:.
A HALF WAVE coax cable or ANY multiple (1, 1½, 2 wavelength) length of this will show you: "exactly the same input resistance at both ends of cable". In normal words:
You can measure the exact SWR from the antenna with these lengths of coax cable.

If your antenna is truly 1:1 it doesn't matter, each SWR measurement anywhere on the line and you will find 1:1.
Only when your SWR isn't 1:1 but higher your measurements will change according to length, this is when it can come in handy to keep the line a half wave long (or any multiple).

 

[MadNachos]

I am not an expert on this but I have been cutting coax to whatever length I have needed and when terminated with good connectors SWR is very good so send it. Do what you need for your installation then work backwards if SWR is out of range. It isn't rocket surgery when it comes to CB unless you are pushing crazy power.

 

[brandon7861]

I found this, from DX-antennas.com

WHAT LENGTH?
As mentioned the best length is the shortest, in this way the loss due to the cable is the lowest.
There can be one advantage by extending the length of the cable:.
A HALF WAVE coax cable or ANY multiple (1, 1½, 2 wavelength) length of this will show you: "exactly the same input resistance at both ends of cable". In normal words:
You can measure the exact SWR from the antenna with these lengths of coax cable.

If your antenna is truly 1:1 it doesn't matter, each SWR measurement anywhere on the line and you will find 1:1.
Only when your SWR isn't 1:1 but higher your measurements will change according to length, this is when it can come in handy to keep the line a half wave long (or any multiple).

Discounting losses, SWR doesn't change with coax length, just the complex impedance changes. When SWR changes with coax length, the coax is acting as part of the antenna (commom-mode currents). Using a half wave coax allows you to read what the R±jX values at the antenna are.

 

[Groundhog KSS-2012]

Discounting losses, SWR doesn't change with coax length, just the complex impedance changes. When SWR changes with coax length, the coax is acting as part of the antenna (commom-mode currents). Using a half wave coax allows you to read what the R±jX values at the antenna are.

I've read that here yes.