New CB - Evil?

[Jim Stoltz]

So when I posted a pic of me driving the Allegro, Philip asked what the box by my left leg was. He thought it might be a CB. Good guess, but it wasn't. I got to thinking, though. The old CB crapped out at the end of last season. I got a new one and installed it today. When I took the pic it was on scan mode, and stopped on channel 6 where some nut job is always on an anti-government rant. It was also 6:x6 o'clock. So the picture captured a 6-6-6. Weird and not intentional!

[Steven Rose]

I have not had a chance to mess much with mine ,but it did fire up . So It should not be long .I used to have a good bunch of retirees to chat with . They were a lot of fun . Blu Ace signing out .

[Jim Stoltz]

So I cut the bad end off the cable, grabbed some slack from under the dash and poked it through the door. It tests out better than it did before - needed to cut the SWF back and it came within the 1.5 range that the manual recommends (even after dialing it back before, I still couldn't get it under 1.5). What's it mean? I dunno - just following directions.

Reception is much better - the crazy guy on channel 6 is crystal clear now. I bolted the cable back down with some slack for the door to open. It's interesting, but predictable I guess, that the antenna loses ground when I open the door (it's attached to the door). That shuts the CB down until I close the door again. I'm going to live with it for now.

[Rick Faunce]

That is not good because if you lose that ground while transmitting you could burn out the finals. And that ground is apparently just the latch which could be in and out with the flexing of driving. I don't know what antenna you have but most have a ground wire attached. You need to extend a ground to a chassis ground on the body somewhere. If there is no ground wire on the antenna then you need to attach one to the mount.

[Jim Stoltz]

Yeah I'll have to do that. It shuts off when the antenna ground is lost, too.

[Steven Rose]

On my allegro the roof is aluminum and i'll try to us it for ground and reflectivity .Witch will also make it directional if antenna is on rear of  RV. You will hear and transmit forward better  it antenna is placed center it will be more omni directional .I will have to see . Good luck and enjoy the adventure .

[Ralph Javins]

Good morning, Jim, et al; 

OK, yes, I do not have an Allegro, but I do have a GMC P-37 motor home chassis, so that should be at least a part qualification.  And I do play with radios, so I can talk about them. 

Your CB radio antenna.  Yes, having the coaxial cable end at the antenna mount making a good connection to the antenna radiating element is needed.  Making a good connection to the surrounding metal surface through the antenna mount is also a very good idea, but, in the case where there is a fiberglass body, that will be much more difficult.  Putting an insulated copper wire of probably 22 or 20 AWG, but not over 14 AWG, that is 8 feet and 8 or 9 inches long or 104 or 105 inches long on the body or the shield side of the antenna mount will help that problem.  Run this 8 foot wire out as straight as you can, either across the body up in the roof cavity, or down the front or the side of the fiberglass body on the inside.  This will provide the radial that the vertical CB antenna wants to see on the shield side of the coaxial cable where it attaches to the antenna mount.  That 104 or 105 inch length is the nominal one-quarter wavelength of a wire for the CB radio band. 

I had to do this with the CB radio that was in the Winnebago Elandan when it arrived here.  It works much better now.  With a 60 inch long radiating element mounted up at almost the top of the roof, it performs rather well, while also not being over the 13.5 foot maximum vehicle height above the road surface. 

Getting the VSWR down to 1.5 to 1, or some other "Standing Wave Ratio goal."  Most radios will actually perform quite well with a VSWR of 2.0 to 1 or less.  At 2.0 to 1, there is a 10% reflected power factor to consider.  The radio will be fine with that.  However, I will admit that some of the really inexpensive radios from the late 1970s and the 1980s did not have very much in the way of self-protection circuitry against a high VSWR or possibly even a short or an open in the antenna circuit.  For those radios, having a good antenna system from the coaxial cable connector on the back of the radio all the way out through the antenna really was important for the safety of those radios.  Most of the modern radios available now are a little more conservatively designed and built to be more tolerant of these problems. That is a good thing. 

Then there is the goal of reaching that elusive 1.0 to 1 VSWR.  You will probably not be able to achieve that.  With a perfect full size 1/4 wavelength vertical antenna over a ground plane, the nominal radiation resistance will be 36.5 Ohms. Feeding that antenna with the usual 50 Ohm impedance coaxial cable will give you a VSWR of 1.4 to 1; the ratio of the two values of the coaxial cable impedance and the radiation resistance.  That is as low as you really are going to get with a simple straight connection, and your radio will be quite happy with that.  With the addition of an impedance transformation device, or an "antenna tuner" or "Antenna 'MatchBox,'" then you can get the VSWR that the radio sees to approach the magical and elusive "1.0 to 1 ratio." 

Please note that those "antenna tuning" devices do not actually "tune" the antenna.  They just electronically null out the reactance and the impedance mismatch to make the radio think that it is seeing a perfect antenna. 

OK, Jim; now for your radio itself.  You need to find a local witch doctor or someone else equally qualified to perform the required exorcism to drive out the evil spirits inhabiting your radio at this time and producing that "6-6-6" warning message. 

Enjoy; 

Ralph 

Latté Land, Washington 

[Jim Stoltz]

Thanks Ralph and welcome to the group!

I haven't messed around with it lately, but I did reconnect the coax and it seems fine now. It is grounded to the aluminum door - there's no fiberglass anywhere on the thing (except for the nose).

[Jim Stoltz]

I finally got around to installing a new Firestick antenna today. I cleaned up all of the connections and the mount. Made sure the ground was good. I was able to easily tune it to 1.3-1.4 SWF and it passed all of its diagnostics. One thing off the to-do list for next week's trip!