Very interesting CB problem

[Guest Tom E]

SWR meters are harder to find than hens teeth around here. I finally had to pay premium price from Radioshack. When I set it up with my scoot, I got >3, so I took the rubber cap off and shaved 1/4th inch off the end of the antenna whip. Funny, it just looked like nothing but fiberglass, but when I checked it, it measured 3 again. It seemed so krazy that I did it over and over again, so I started cutting off the rubber cap and the measurement came down - not by much but at least

[BOO]

Tom,

Not sure what you are cutting there. Must be one of those antennas that is considered the "marshal mod". If that is the case I'm not sure they are trimable?

Usually on that type of antenna there is a stinger in the top end of the antenna that is adjustable for SWR but I don't remember seeing one in those fiberglass antennas.

Maybe if you give me some more information I might be able to help you.

Jerry

[Marcarl]

Maybe a dumb question but also maybe safe than sorry: how did you use the meter?

[Guest Tom E]

Well, I was trying for the Marshall mod, but, if SWR's are hard to find, 3' firesticks are impossible to find. I have a wound fiberglass antenna. The package indicated that it could be trimed, but being ignorant, the instructions were too ambiguous for me. I didn't see any metal in the part I trimmed.

 

As far as the meter, I calibrated it per instructions, and re-checked the process over and over.

[gibvel]

I take it you don't have the splitter on the CB anymore (silver rectangular box covered by a clearish rubber boot that's attached by a bracket to the CB). That has a screw to adjust the SWR.

[Guest Tom E]

I take it you don't have the splitter on the CB anymore (silver rectangular box covered by a clearish rubber boot that's attached by a bracket to the CB). That has a screw to adjust the SWR.

 

 

????? Yes - I still have it. ???? What???

 

Ok I have my spair unit at the computer. I see a "CB Matching" adjustment screw. Is that what you are talking about?

[Guest Tom E]

and, by the way, still, what does that little rubber cap have to do with the SWR? Why would the SWR change when that little rubber cap is put back on. There has got to be a physics lesson in this.

[Freebird]

ANYTHING that you add to the top of the antenna will affect the SWR. You adjust, then put the cap one, check and then remove and adjust again.

[Guest Tom E]

ANYTHING that you add to the top of the antenna will affect the SWR. You adjust, then put the cap one, check and then remove and adjust again.

 

It's near perfect without the cap - do I need it?

[gibvel]

????? Yes - I still have it. ???? What???

 

Ok I have my spair unit at the computer. I see a "CB Matching" adjustment screw. Is that what you are talking about?

 

Yes. When I adjusted mine I had a T connector on the back of the meter. then one end screwed into the CB unit and the antenna screwed into the other. I took that splitter unit out of the bracket and took the rubber cover off but, If I ever do it again I'm going to try do it with the splitter on and cut an X or something over the adjustment hole. It was kind of tricky to put it all together but it can be done.

 

Then you just key the mic and adjust the screw for the best signal. Do this on channel 20. If you do it on a low or high channel the SWR will change. I did mine on 1 and found that 40 was a little higher.....did it on 20 and all the channels were the same.

[bisquit]

FYI..If there are any truckstops near where you live, especially the big ones.ie Petro,Flying J, Pilot, Loves etc. They will have anything you could possibly want for a CB. They typically carry all kinds of antennas,microphones,cables,connectors,and even SWR meters. They cater to truckers and just about evry trucker in the country has a CB in his truck. Many of the larger truckstops will also have a CB shop either in their business or at least real close by...Don

[Bummer]

I didn't have much luck at the truck stops. I did best at Walcott CB.

 

Fiberglass antennas consist of a fiberglass rod with wire wrapped around it. The wire is the real antenna. The fiberglass rod is just there to hold the wire up. Some fiberglass antennas must be cut to adjust. Some have to be hacksawed, others are shrink wrapped and you can pull out a bit of the wire and snip it off. Others have a screw that serves as the adjuster. If the screw is all the way in but the SWR is not quite there yet you can trim a little metal off the screw (threaded end) and keep trying.

 

Generally speaking, a high SWR means too much radiating mass.

 

The tip has to be there when you test SWR if you are going to have it on there when you're transmitting. I don't know if it HAS to be there, but it does serve to seal the end of the antenna.

 

Remember that the trunk lid has to be down or it'll affect the SWR reading. You can't have anything within one wavelength or it'll affect the SWR reading. You have to test it setup the way you intend to run it.

 

If you run a splitter you should adjust the antenna SWR with the splitter out of the circuit. After antenna SWR is set put the splitter back in the circuit and readjust SWR using the screw on the splitter.

 

Some parts:

 

3' Firefly

 

AR1A splitter

 

203EZ folding mount

 

300-340 stud mount

 

PL-259 right angle adapter

 

I found the stud mount and the 90 at a truck stop. The rest I had to order. I got PL-259 and motorola solderons at a local electronics shop.

[GeorgeS]

If you make the Antenna " Longer" the Resonant Frequency, of the antenna, Goes Lower.

 

If you make the Antenna " Shorter" the Resonant Frequency of the anetnna Goes Higher.

 

To get a Low SWR, basicly what you are doing is Adjusting the Length of the antenna, so it is Resonant at the Frequency you are useing. Which is About 27.2 Megahertz, about the Center of the CB Frequency Band.

 

 

The Cap, being discussed Used by Fire Stik, on their Antenna's Does affect the " Resonant " length of their Antenna's. I noticed this when tuneing my new firestik.

 

I Re Read their Instructions, and it does say, that the Cap Affects the SWR.

So you have to make the adjustment, then reinstall the Cap before takeing a reading on the SWR Meter with the new Adjustment.

[bisquit]

Bummer if you are anywhere near the Iowa 80 truckstop in Walcot then you have at your disposal the biggest truck stop in the world. Their cb shop should have absolutely anything you could possibly want for a radio. If ya ever get a chance to walk through the chrome shop you'll be amazed. They have a Peterbuilt all decked out with lights and chrome on one side and a Kenworth on the other, both on rotating turntables. Along the front wall they have a Freightliner with a 53' trailer and both are decked out with all the lights and chrome. All this is INSIDE the chrome shop. They are supposed to open a museum of antique cars and trucks somewhere there on the property, it might already be open I havent been out there in a year or more......Don

[rod]

While on the subject of CB's is the fold down mount listed in this thread a direct replacment for 2nd Gen fold down? Mine is all rusted. Thanks Rod

[BOO]

By the way if you add a fold over to the antenna that will also change the swr.----it becomes longer.

Jerry

 

 

 

 

 

While on the subject of CB's is the fold down mount listed in this thread a direct replacment for 2nd Gen fold down? Mine is all rusted. Thanks Rod

[Ladyrider]

This was a great thread. Over my head, but Tom was learning a lot. I am saddened he did not get to make it work. He surely would have if he had more time...you guys are all an awesome source of knowledge and information. God bless you each one.