what to do with excess coax in a mobile ?

[BOOTY MONSTER]

i was surfing and found myself at firestick's web site...
http://www.firestik.com/FAQ.htm

and i saw this.....
Q I only needed 9 feet of coax to go from my radio to my antenna. How should I handle the excess?
A What ever you do, do not roll it into a small convenient coil. It will become an RF choke. If you cannot let it lie loose under a seat or in a headliner, wrap it into a yarn-like skein of about 12 to 16 inches, put a wire tie in the center and tuck it under your dash, seat, etc.

i didnt do that , i spent a lil while weaving it through my truck and tucking my coax so it wouldn't be coiled or doubled across itself or have any sharp bends in it as well as being pretty much out of the way .im not having any problems .

but.......for future reference and those on here that may wonder in the future .....is what the firestick site wrote correct ? i know to never coil coax but ("If you cannot let it lie loose under a seat or in a headliner, wrap it into a yarn-like skein of about 12 to 16 inches, put a wire tie in the center and tuck it under your dash, seat, etc.") that dosent seem right to me either .
i'm the first to admit that i dont know it all so what do y'all think ?

 

[Marconi]

BM, the site tells us to never use coiled up coax in a mobile setup, but does not explain why or what happens.

What do you think happens if you used an RF choke as described here?

 

[BOOTY MONSTER]

i really dont know . i was told it would reduce my output and really mess up my swr , so i didnt do it that way .

 

[Marty]

I found out by trial and error that coiling your coax reduces your output and receive. I usually zig zag it back and forth behind the seat, there was no Internet then so i like to know the best way to run coax my self.

 

[Marconi]

Within reason, this choke is nothing more than a coil having a ratio affect of 1:1 on thru-put, meaning amp's in equal amp's out. The choking effect, if any, will be that this choke, by induction, can develop a high impedance characteristic in the area on the outside of the coil and thus minimize RF from flowing back to the station. This can be helpful when you have an unbalanced feed point with common mode currents (CMC) flowing back to the transmitter, which is not unusual in mobile type installations.

If the choke works as it should in this regard, it does so to choke off only theses RF currents (CMC) flowing on the outside of the coax shield and nothing else.

Now, if this is true then how does this jive with what you have read?

 

[Marconi]

Marty, could you describe for me how you determined your test for the transmit side of your argument.

 

[Marty]

Yea you sound like a mudduck now Rabbitman uncoil your coax. That was all I needed to know and he was my uncle.

 

[W5LZ]

After looking at the web site, I wonder who they got to do those 'FAQ' thingys, they certainly are not all correct. More misconceptions than not. It would certainly help if they explained the 'why', and not just give a 'flat' answer. Those 'flat' answers are wrong as much as they are right, and in this case, not very 'right' at all.

There's nothing wrong with having a 'choke' in your feed line. Having it near the antenna would make it work 'better' though. If putting a choke in line makes for worse reception or transmitted signal, then you got other problems, not the choke. (Uh, unless you coiled up half a mile of the stuff.) Depending on how much 'extra' you have, a choke is one option, cutting the extra off would be good too.
- 'Doc

 

[BOOTY MONSTER]

i was of the impression that unless you really know what youre doing to keep the mobile coax at 18 feet long . something to do with keeping a 50 ohm impedance or something like that but i could be wrong .

Marconi thanks for your input also . i didnt coil mine so its not a problem for me .

 

[loosecannon]

if we are speaking of FIRESTIK brand antennas, and other brands with similar designs; there is a BIG reason not to coil, or cut that 18 feet of coax that comes with the antenna.

the reason?
the coax is part of the radiating system.
yep, thats how firestik antennas become "resonant".
that is why they use 18 feet of coax, and tell you not to coil it, or cut it.

i figure their FAQ section applies ONLY to their antenna design.

anyone ever bought a used firestik antenna without the factory coax, and installed it on their vehicle only to find that it would not tune up no matter where you put it?

good ol' firestik; taking advantage of new CB'ers since 1962. LOL
loosecannon

 

[Marconi]

I don't know if it is true or not, but loosecannon makes a very good point in his last post.

I could maybe argue that the feed line would not be laying vertical and thus would not be producing a good vertical and omni signal, but like I said I just don't know for sure.

 

[4600turbo]

The only reason to use 18' of coax is to get an accurate reading on a SWR meter near your radio.

As long as the antenna is tuned. Then you could use any length of coax.

And like Marconi said sometimes a choke is a good thing. It may even clean your TX and take out Harmonics in your system.

I have read that it may reduce your RX though. I haven't seen much problems with my RX when using RF chokes like the one described.

 

[Captain Kilowatt]

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.

 

[BOOTY MONSTER]

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.

 

[ken white]

If you coil the coax and the coil is tightly wound the coax will be damaged and will not have a 50 ohm impedance anymore. IIRC the radius of the coil of coax must be greater than at least 10 times the coax's diameter, but I learned this so many years ago I am not 100% sure of the values in this fact.

Coiling coax into a choke only stops common mode currents, i.e. non-radiated currents, and will not affect radiated currents. So coiling the coax will not make the common mode currents radiate, but rather will force the non-radiating currents to find a new path back to the source. The original problem with the antenna is still there...

The only way to reduce common mode currents in a non-dipole antenna is to tune the antenna correctly which may mean relocation to a better spot, no nearby reflective objects and a nice big flat ground plane.

I have used FireStik antennas and have not had any problems with them. They are a good cheap antenna that will take a beating on the trail and still provide adequate trail communications. There are better antennas out there, but they will cost more and they don't like trees.

:shock: