what to do with excess coax in a mobile ?

[Captain Kilowatt]

so did i waste my time weaving my coax around making sure the slack wasnt looped or coiled ? since my roof mount uses terminal rings to connect my coax could or shoud i unweave it and run the shortest (but still hidden and protected) route to my antenna ,cut the slack and install new rings , then check to make sure my swr is good ? am i correct in assuming (i know..i know ) that the less coax the less resistance and loss and more signal in and out ? im running a 10-k dual coil on top of a SUV with a well grounded stud mount with almost 200 watts going to it.

I do not think you did anything wrong,you just didn't do anything any more right. Leave it as is.You won't have that much extra to worry about extra losses anyway.

Ken is right about not making a coil too tight especially if the dielectric is foam.The centre conductor can migrate out of the middle and eventually short to the shield especially in a hot place like a car in the summer.

 

[4600turbo]

A few points in no special order:

First,18' of coax is NOT the proper length of cable to get an accurate reading on a SWR meter near your radio. That length is approximately a PHYSICAL 1/2 wavelength not an ELECTRICAL 1/2 wavelength which is what is required to reflect the true antenna impedance.

Second,any losses incured by rolling the cable into a choke would be present in RX as well as TX. As a side note there are no losses created by rolling the cable into a coil. I have tested a full 100 foot roll of RG-58 at 450 MHz and there was NO additional loss when rolled up versus laid out straight.I was testing for losses at the time as I needed a relativly high power RF attenuator between the TX and antenna to limit ERP.The cable is self shielding and will not act like a normal wire coil.It is not a good idea however to roll a large amount into a coil due to unnecessary feedline losses due to extra length as well as heating if running real high power.

Third, I suspect the reason some manufactures say not to coil the cable is as stated above,the cable is part of the antenna system and coiling it will create an RF choke that will affect the performance of SOME antennas. Those antennas that are affected are the type of antennas I do not want to use.

If 18' is not the physical length of coax what is? Since 36' is a full wave.

 

[mopar2ya]

18' is the approximate length of a half wave at 11m in free space. You would need to multiply the wavelength desired in feet by the velocity factor of the transmission line in feet to get the proper physical length.

You know 4600 Turbo, I tried to help you understand this about a month ago on the CBRT forum. You must have missed it. Here is a snippet from that post.

=========================

In order for any of the above to be true, you have to go there (velocity factor). For example to make 1/4 or 1/2 wave length lines at 27.185 MHz you would have the following lengths. We will round the length of the wave in free space to 36.2’, keeping in mind that velocity factors for different brands of coax may be different than what I posted.

RG58 .66 vf coax
1/4 = 5.973’
1/2 = 11.946’
Full = 23.982’

RG8X .78 vf coax
1/4 = 7.059’
1/2 = 14.118’
Full = 28.236’

There you go.....

Mopar

 

[Captain Kilowatt]

If 18' is not the physical length of coax what is? Since 36' is a full wave.

18' IS a PHYSICAL 1/2 wavelength but NOT an ELECTRICAL 1/2 wavelength.Electrical wavelength is found by taking the physical length and multiplying it by the velocity factor of the cable.RF travels slower in coax cable,or any other type of transmission line, and the velocity factor plays a HUGE role in length calculations because of it.

 

[4600turbo]

18' is the approximate length of a half wave at 11m in free space. You would need to multiply the wavelength desired in feet by the velocity factor of the transmission line in feet to get the proper physical length.

You know 4600 Turbo, I tried to help you understand this about a month ago on the CBRT forum. You must have missed it. Here is a snippet from that post.

=========================

In order for any of the above to be true, you have to go there (velocity factor). For example to make 1/4 or 1/2 wave length lines at 27.185 MHz you would have the following lengths. We will round the length of the wave in free space to 36.2’, keeping in mind that velocity factors for different brands of coax may be different than what I posted.

RG58 .66 vf coax
1/4 = 5.973’
1/2 = 11.946’
Full = 23.982’

RG8X .78 vf coax
1/4 = 7.059’
1/2 = 14.118’
Full = 28.236’

There you go.....

Mopar

Ya, your right I didn't read all the material associated with the velocity factor.

But that is why most CB shops tell you to use 18' of coax, and that is why most packages come in those lengths. They don't take in to consideration the velocity factor.

 

[mopar2ya]

Someone with more information on CB history can better answer that one. It could be that somewhere in our past there was a compelling reason to sell 18' of coax.
It is a convenient length at 11m if the VF is .66 because it includes a 1/4 wave transformer within its length. In some installations this may be helpful. A co-phase harness for example.
Unfortunately that doesn't explain why RG8X is that length. Did marketing gurus latch on to that length because they could sell more coax that way? Who knows? Maybe they hired the hot dog bun marketing guy to figure out how to sell more coax.


Mopar

 

[ken white]

Someone with more information on CB history can better answer that one. It could be that somewhere in our past there was a compelling reason to sell 18' of coax.
It is a convenient length at 11m if the VF is .66 because it includes a 1/4 wave transformer within its length. In some installations this may be helpful. A co-phase harness for example.
Unfortunately that doesn't explain why RG8X is that length. Did marketing gurus latch on to that length because they could sell more coax that way? Who knows? Maybe they hired the hot dog bun marketing guy to figure out how to sell more coax.


Mopar

The open end of 18' of coax that has a velocity factor of 0.66 will create a low impedance standing wave (approximately 50 Ohms) to be seen back at the CB.

If the 1/4 wave antenna is tuned correctly, it will also be at its low impedance point at the feedpoint.

So, theoretically the standing waves in both the coax and antenna will be synchronized/matched at this low impedance connection point.

Maybe the hot dog guy already knew this and that is why he picked this length...

 

[BOOTY MONSTER]

thanks for all the replys . i appreciate the info .