Stowing the surplus coax

[Papa]

We're installing a hood mounted antenna on the EB and about 14 ft of the coax will be stowed - but where? Would there be problems if the coiled coax tied off in the middle was left under the hood rather than pulling it all through the firewall and trying to find a space under the dash?

 

[KA5IVR]

There is no reason not to cut off the excess, if you are able to install a PL-259 connector. If not, just stuff it up under the dash with some wire-ties.

 

[dblosch]

Firestick brand of antennas seems to think excess coax should be looped a certain way. They specifically mention not making a small, convenient coil.

Frequently Asked Questions

Hope that helps
Dan

 

[john101477]

I was always told never to coil tightly if at all and that coax should be ran in increments of 3ft. so your coax should be 3,6,9,12 etc. Mind you this is what I was always told.

 

[DaveInDenver]

For a resonant 50Ω antenna, the formula for coax length is:

 

[xtatik]

Firestick brand of antennas seems to think excess coax should be looped a certain way. They specifically mention not making a small, convenient coil.

Frequently Asked Questions

Hope that helps
Dan

Unfortunately, when they say this they're also telling their more astute customers that their antennas are not very efficient.
They use the term "RF choke" which is a misnomer. Coiling the feedline would actually be forming a "line balun", "line isolator" or as most hams call them, an "ugly balun". If you notice in one of their other answers, they are recommending a 5.5 meter minimum length feed line. There's a good reason for this.....it's because they are relying on a 1/2 wave section of the feedline to serve as either a radiating portion or counterpoise portion of the actual antenna....2 x 5.5m = 11m = CB. Not a good sign.
If you coil this feedline portion it will stop the common mode current from fully electrifying and enveloping that side of the antenna/feedline. Their antenna needs this (it shouldn't if well designed) in order to work.
In an installation that involves a properly resonant antenna (unlike a FireStik), having a line isolator would be desirable in keeping reflected power out of the feedline.
As for the extra coax in this case...that's a lot to spare and store for a mobile installation. I'd source or make a more appropriate length. Having a few add'l feet is not a problem and you can roll it up in any fashion that doesn't severely kink or pinch it. With some of the cheaper foam dielectric "kwax" the stiffer center conductor can migrate through the foam and come in contact with the shield. This is bad.

 

[suntinez]

There is no reason not to cut off the excess, if you are able to install a PL-259 connector. If not, just stuff it up under the dash with some wire-ties.

Agree with this – or under the hood ok too, if well secured and not subject to heat issues.

I don't know about the Firestick brand per se (other than it's popular) but I disagree with their FAQ on this – however, may be accurate for their brand. Sometimes you want an RF choke, depends on the installation.

Main thing is you want to eliminate as much as possible is common mode currents, where signal is propagating on the outside of the cable.

Every single piece of a radiation receiver affects the overall performance, mobile installations are particularly quirky. Baluns or toroids are often used in base stations to offset CMCs and tune, seldom in mobile.

Just did a quick google on “common mode current coax” and got this site, which seems pretty accurate IMO.

http://www.signalengineering.com/ultimate/coax_basics.html

edit: just saw xtatik's post, agree with all that!

 

[DaveInDenver]

To expand on Randy's post, a coiled length of coax is an air core 1:1 common mode choke balun. You use this when your antenna is somehow not resonant and your feedline becomes part of the antenna. This is generally a bad thing. Now there are times when a length of coax IS critical, but that would be a 1/4λ long and is serving an impedance transformer (that is not the case of 18' on CB, which is 1/2&#955. I don't know why they tell you not to cut it, there's no reason it should matter.

 

[DaveInDenver]

Just did a quick google on “common mode current coax” and got this site, which seems pretty accurate IMO.

http://www.signalengineering.com/ultimate/coax_basics.html

This website is good and mentioned an important point, velocity factor. Firestik's magical 18 feet is 1/2λ of CB frequencies in free space. That length has no meaning in the coax, which with a 0.66 vf (typical of RG-58) would mean 1/2λ is about 12 feet.

 

[xtatik]

To expand on Randy's post, a coiled length of coax is an air core 1:1 common mode choke balun. You use this when your antenna is somehow not resonant and your feedline becomes part of the antenna. This is generally a bad thing. Now there are times when a length of coax IS critical, but that would be a 1/4λ long and is serving an impedance transformer (that is not the case of 18' on CB, which is 1/2&#955. I don't know why they tell you not to cut it, there's no reason it should matter.

This is true. I've had to run sections of 75 ohm (I believe 1/4w long) when feeding delta loops in the past to serve this purpose.

 

[DaveInDenver]

This is true. I've had to run sections of 75 ohm (I believe 1/4w long) when feeding delta loops in the past to serve this purpose.

Very common technique. You can also feed non-resonant doublets with critical lengths of coax to make it work (e.g the G5RV). But this only works at one small range of frequencies and it's even probable that efficiency won't take a major hit.