coax cable question ....
#1
Registered User
Thread Starter
Join Date: Jan 2003
Location: lancaster,pa
Posts: 344
Likes: 0
Received 0 Likes
on
0 Posts
coax cable question ....
i stopped in a local cb shop and was talking about installing a radio in my truck. he asked if id be using the 18ft coax cable.... i replied i guess so although i dont need near that much to get from my antenna to the radio...... his reply, yes you should use the 18ft cable and dont ball it up under the seat.... lay it out back and forth on the floor under the carpet.. thats better for your swr's............ so anyway my question is........is this correct ?? i hate to question his theory but i just want opinions from guys who know about this cb stuff... thanks guys, scott
#2
Administrator / Scooter Bum
It's my understanding that the length of the coax has something to do with the length of a radio wave. Longer or shorter and it could be impossible to get the SWRs down low enough.
Also, don't just coil the excess and tuck it under the seat. You will have created a "Choke".
Make a figure 8 with the excess.
Also, don't just coil the excess and tuck it under the seat. You will have created a "Choke".
Make a figure 8 with the excess.
#3
Registered User
Join Date: Nov 2002
Location: outside of Duluth MN
Posts: 1,234
Likes: 0
Received 0 Likes
on
0 Posts
Yep, the guy was right. You need to use 18' in order to get the lowest SWR (standing wave ratio). If you do coil up your coax you will make what we used to call "an FCC choke". It will limit your range.
Tom
Tom
#5
Registered User
Join Date: Feb 2002
Location: Sturbridge, Taxachusetts
Posts: 1,424
Likes: 0
Received 0 Likes
on
0 Posts
18 feet is one half wavelength for the general CB band, but this is not the corrrect length to cut your coax. The "experts" often forget the following. Electrons flow slower thru the wire cable than they do thru a vacuum; about 66% to 80% slower. This called the velocity factor of the cable. THis figure is available for each type of coax used. RG-59/U (foam) coax has a fixed velocity factor of .79
Here is how to figure out your true 1/2-wave:
492 x (Velocity Factor) / Frequency (MHz)
So, the true 1/2 wavelength for the CB band is about 14 feet or so depending on the channel. You can add multiples of 14 feet for longer lengths. Trust me on this, cut your coax using this method and your CB will get out.
Here is how to figure out your true 1/2-wave:
492 x (Velocity Factor) / Frequency (MHz)
So, the true 1/2 wavelength for the CB band is about 14 feet or so depending on the channel. You can add multiples of 14 feet for longer lengths. Trust me on this, cut your coax using this method and your CB will get out.
#6
Administrator
Uhoh.
I didn't cut mine, I think it was 18ft as Firestik recommended, but I do have the excess kinda coiled up next to the fuse box under the hood.
Maybe that's why I can't get my SWR down below 1.8:1.
I did what Firestik recommends, and serpentined it like a skein of yarn, then cable tied it in the middle though.
phox
I didn't cut mine, I think it was 18ft as Firestik recommended, but I do have the excess kinda coiled up next to the fuse box under the hood.
Maybe that's why I can't get my SWR down below 1.8:1.
I did what Firestik recommends, and serpentined it like a skein of yarn, then cable tied it in the middle though.
phox
#7
Registered User
Unless the firestick has a coil to match the impedance, you will be running a 2.1 to 1 mismatch, which is normal for a stick antenna. Differences in the coax length will make a difference to show your current swr.
Trending Topics
#8
I don't know much about CB's. All I know is I have dual firestick 3' antennas mounted on opposite sides of the bed with an 18' coil just kind of snaked under my carpet. I had it professionally tuned and they guy had no problems with it. My Radio Shack CB picks up great reception. Now, if I could only figure out what the RF gain is supposed to do, I'd be in great shape.
#9
Registered User
Originally posted by BigBlue
I don't know much about CB's. All I know is I have dual firestick 3' antennas mounted on opposite sides of the bed with an 18' coil just kind of snaked under my carpet. I had it professionally tuned and they guy had no problems with it. My Radio Shack CB picks up great reception. Now, if I could only figure out what the RF gain is supposed to do, I'd be in great shape.
I don't know much about CB's. All I know is I have dual firestick 3' antennas mounted on opposite sides of the bed with an 18' coil just kind of snaked under my carpet. I had it professionally tuned and they guy had no problems with it. My Radio Shack CB picks up great reception. Now, if I could only figure out what the RF gain is supposed to do, I'd be in great shape.
If firestick packs the coax with the antenna, thay have matched it. The coil is bad though, creats a choke that stops rf from coming through. Put it any way except a coil. Sorta does what the rf gain does
Thread
Thread Starter
Forum
Replies
Last Post
SILVER GUS
3rd Gen High Performance and Accessories (5.9L Only)
13
02-21-2009 08:41 AM
Dave88LX
Performance and Accessories 2nd gen only
6
12-10-2006 06:13 AM
Ridiculous
Performance and Accessories 2nd gen only
22
07-08-2006 08:11 PM
surfram
3rd Generation Ram - Non Drivetrain - All Years
10
02-12-2004 07:20 AM