RFI, MRI, db loss... Does it really matter?

[77monarch77]

I hate to start WWIII here as there are probably many different ways of answering this question. Let's say that there is X amount of feet of lead-in coax. Between where it enters a building and where it connects to a receiver is, let's say, 10 feet. Let's also say, for the sake of argument, that everything about the antenna install is absolutely perfect (yeah, right). The situation is that there is 10 to 20 feet or so of additional coax that's just sitting there. Provided that you don't want to cut this extra, for whatever reason, what do you do with it? Do you coil the additional? Would you figure-8 it? Does it matter?

I used to install cellular phones in vehicles. The rule-of-thumb, at least what I was TOLD to do, was to figure-8 the additional. The reason was due to MRI induced into the reception signal, if the coax was coiled, radiating from various electrical and/or electronic devices and/or systems already in the vehicle. The amount of MRI was supposed to be amplified (although I don't know to what degree) if the coax was coiled versus if it was not.

Does similar logic apply in the above instance?

 

[jim202]

I used to install cellular phones in vehicles. The rule-of-thumb, at least what I was TOLD to do, was to figure-8 the additional. The reason was due to MRI induced into the reception signal, if the coax was coiled, radiating from various electrical and/or electronic devices and/or systems already in the vehicle. The amount of MRI was supposed to be amplified (although I don't know to what degree) if the coax was coiled versus if it was not.

Does similar logic apply in the above instance?

I guess part of the answer here lies in your deffinition of what MRI is. A simple guess on my part was that someone was blowing smoke when they trained you on installing cell phones.

As for the 10 feet of cable, unless your using some real **** cable, the 10 feet extra isn't worth the math to calculate the loss. Bear in mind thast the higher the frequency your working with the higher the looses will be. It also depends on the type of cable your using. The larger the cable, the less the loss is the best way of looking at it.

If your talking about the RG-58 cable you get at crap shack, then all bets are off. That stuff for the most part may work at the CB frequencies, but goes down hill fast as the frequencies go up.

Jim

 

[LarrySC]

First rule of thumb. Never coil extra coax for any reason. Just lay it back and forth. Years ago fellow came by with CB that would not talk two blocks. Said took it to 3 CB shops in area with no luck. I ask him how long was the cable. He pulled the seat over and he had about 25' coiled up. Cut it off at 9'. SWR now 1.7 rather than 20.0

 

[77monarch77]

Well Jim, it seems to make sense about the MRI (Magnetic Resonance Interference) although I don't know about the coax coil issue in that cellular situation. I know that MRI in automotive applications occurs quite frequently with many different types of radio installations if the antenna and/or coax is routed incorrectly.

I think that Larry has answered the question though. A SWR of 1.7 down from 20.0 is quite a significant improvement.

I'm curious as to what the actual reasoning behind the "don't coil" rule is??? Is it the amount or length of the excess? Jim raises a good point too. Perhaps 10 feet or under would not make a significant amount of difference as he mentions. In Larry's example, 16 feet was cut off and the remainder of 9 feet (under 10) made all the difference in the world.

I'm also now wondering if heavy shielding would make a difference. What about quad-shielding or heliax... or shouldn't that even be considered in the equation?

 

[N_Jay]

1) coiling coax has NO EFFECT as long as the SWR is not too high (the shield acting as an antenna)

2) 10 feet or so of even crappy coax is irrelevent at CB frequencies.

The change in SWR points to a problem with the cable or the connector, not an issue with coiled coax.

 

[77monarch77]

So... it's the frequency that's the culprit? I'm assuming, then, that somewhere around the 27Mhz area the difference is negligible and as you climb up in frequency that some sort of rule should apply? If this is true, then getting up into the 800Mhz+ cellular range and beyond would warrant that the coax be given some sort of attention. Am I correct in my assumption?

Yeah... I know... I shouldn't assume! I do this, however, on the pretext of trying to learn something.

 

[N_Jay]

So... it's the frequency that's the culprit? I'm assuming, then, that somewhere around the 27Mhz area the difference is negligible and as you climb up in frequency that some sort of rule should apply? If this is true, then getting up into the 800Mhz+ cellular range and beyond would warrant that the coax be given some sort of attention. Am I correct in my assumption?

Yeah... I know... I shouldn't assume! I do this, however, on the pretext of trying to learn something.

Go to the web site of any cable manufacturer.
Belden, Andrews, Times, etc.
Download their loss charts.
If they are not their order a catalog, as the charts are in the back.
Read and become educated.

Yes, for every type of cable the loss increases with frequency.
There is also a maximum recommended frequency for each cable.

 

[77monarch77]

<snip>Yes, for every type of cable the loss increases with frequency.
There is also a maximum recommended frequency for each cable.<end snip>

See... I DID learned something today. I knew that length and loss went hand-in-hand but never knew about frequency ratings or loss increase with frequency increase.

I will take your advice and study the charts. I have no problem with reading or studying and I think that most people don't. It does help, however, [as you've done for me], to point someone in the right direction.

Thanks.