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-   -   CB cable questions on trimming to length. (http://www.risingsun4x4club.org/forum2/showthread.php?t=16663)

Rezarf 10-11-2011 08:17 PM

CB cable questions on trimming to length.
 
I remember in the Ham class I took at RS a few years ago there was something about the length of the cable coming into discussion... the particulars escape me. I think it was keeping the cable in increments of 8' (or some number).

I am installing a replacement CB this week (Cobra WX ST II) in the 40 and bought a coaxial cable from Johnson's corner folks last time I was through FoCo. My question is two fold...

First, should I keep the cable length at 18' or can I trim it without effect? I have an adjustable tip 4' Wilson Firestick if it matters. I'd also like to add an antenna splitter for the radio/cb if it doesn't effect anything negatively? Thoughts?

Second, I bought a mount that looks like the one below and I am wondering where the best place to put it might be? I was hoping to mount it to the rear as to keep it clear from trail obsticals, but would a side rear quarter mounting location make more sense?

I don't want to mess with punching holes where I don't have to! :hill:

Thanks in advance-

http://www.rightchannelradios.com/images/CBs/ptsbm.jpg

Rezarf 10-12-2011 08:45 AM

Does anyone care about a lowly CB operator anymore?:o

DaveInDenver 10-12-2011 08:59 AM

There is nothing special about CB feedlines. Cable length makes no difference if the antenna is tuned and presents a 50 ohm feedpoint impedance.

The 9' foot thing would be if the feedline is acting like an impedance transformer, which is a special case if the antenna is 1/2w.l. and does not have a matching network. A 1/4w.l. of 50 ohm coax would transform a very high impedance as seen on a 1/2w.l. antenna.

However, a good antenna would be built with one (e.g. the Larsen antennas) and the cable length should be the length between your radio and the antenna with enough slack to lay out of the way.

What antenna are you using?

MDH33 10-12-2011 10:25 AM

I think Dave answered all of your questions regarding the cable length. :thumb:

I can give you some feedback on your antenna and mount as I'm running that same setup on my 40. I put the mount in the location of the OEM antenna on the drivers side of the cowl. I had to add a support (regular i-bolt) above it to keep the antenna from moving too much because the mount was threatening to crack. It works really well there and looks good I think.

You can sort of see it in this picture:

http://mdhuber.smugmug.com/photos/95...4_cQqTe-XL.jpg

Rezarf 10-12-2011 11:21 AM

Wow, Martin that blue 40 right next to yours is amazing! :D

Thanks man, where did you get the I-bolt? Just a normal I bolt from the hardware store that fits the antenna?

thanks-

Drew

Rezarf 10-12-2011 11:21 AM

Quote:

Originally Posted by DaveInDenver (Post 194050)

What antenna are you using?

Thanks Dave-

I am running a 4' Wilson Firestick with adjustable tip.

DaveInDenver 10-12-2011 12:10 PM

Quote:

Originally Posted by Rezarf (Post 194064)
Thanks Dave-

I am running a 4' Wilson Firestick with adjustable tip.

So one of those fiberglass jobs, probably electrically 5/8λ. Does it have a matching coil at the bottom? My guess would be if it's like the Firestik it does not and will probably be sensitive to coax length.

Nathaniel did a 2m ham antenna build that would demonstrate the principle, there would be a tapped coil at the feed point and the antenna would look DC grounded. For 2m a 5/8λ would look something like this.

http://i34.tinypic.com/ftf1ug.gif

The Firestik is a 5/8λ but doesn't have a matching coil, so they have to use the coax to match the impedance. Problem is they (IIRC) say to use 18' of RG-58. Assuming it's an ideal transmission line that's going to just mirror the feed point impedance at the other end. But in the real world coax has a lower-than-lightspeed propagation (called velocity factor, which is roughly 0.66 for RG-58), which means the coax is going to be some electrical length around 3/4 wavelength, which makes it act like a transformer. In that case the length is critical. You could use around 6 feet if you want to experiment.

Ham antenna manufacturers build the coil into the base so that the length of coax is not critical.

Rezarf 10-12-2011 12:45 PM

It does have a tighter "coil" (a wire under the plastic sheath) at the bottom and is much less tight towards the top.

DaveInDenver 10-12-2011 01:00 PM

Quote:

Originally Posted by Rezarf (Post 194088)
It does have a tighter "coil" (a wire under the plastic sheath) at the bottom and is much less tight towards the top.

A photo would help, but does the coil have a tail sticking out?

rover67 10-12-2011 01:55 PM

I run a splitter for my radio and CB so that they can share a CB antenna and the only thing I have noticed that suffers is the radio reception. It kinda sucks now...


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