View Full Version : Conversion to Single Antenna

06-17-2007, 07:09 AM
Conversion to Single Antenna

Thanks to Steve Bumgardner (Bummer)

While removing the gas tank I managed to lean in too far and snap the center load on the CB antenna. Guess I should have put it up while I was working on the bike. Live and learn.

Problems: The stock antenna is $140 or so, and it's obviously not very stout. The twin antenna system reduces the efficiency of the CB antenna. Probably not enough to worry about, but it's there if you like to sweat that sort of thing.

Solution: One antenna serving both radios.

Note: Some splitters reduce output power. Testing indicates that the Firestik splitter does not reduce final output power to the antenna. 4 watts in, 4 watts out.

Many thanks to Marshall Gammon for his site http://bludolphintravel.com/gmg/marshallmod.htm It was a wonderful starting point.

Preparation: Read the manual with the splitter. Read and understand the manual that came with your SWR meter. Check out the help files on the Firestik site http://www.firestik.com/Tech_Docs.htm If you've never worked with coax you should probably read http://www.firestik.com/Tech_Docs/Coax_Procg.htm for tips.


1 Firestik Firefly 3 foot CB antenna $18.00 http://www.walcottcb.com/product_info.php?cPath=28_97&products_id=897

1 300-340 stud mount base $5.00 http://www.walcottcb.com/product_info.php?products_id=251

1 folding mount $15.00 This part is way too sloppy but it works if you require a folder. http://www.walcottcb.com/product_info.php?cPath=29_112&products_id=262

1 90 degree PL-259 adapter $5.00 http://www.walcottcb.com/360359-coax-right-angle-adapter-p-233.html

1 CB FM splitter $18.00 http://www.walcottcb.com/product_info.php?products_id=221

1 PL-259 male plug $2.50

1 PL-259 reducer model UG176 $.50 http://www.radioshack.com/product/index.jsp?productId=2103401
1 Motorola female $1.50

2 Motorola male $2.00

Solder and shrink tubing


The CB lives in a plastic box under the trunk. The lid is held in place with two rubber straps. This mod assumes access to the CB by removal of the trunk as needed.

(Remove six screws in the bottom of the trunk, then close the trunk and carefully rotate the rear up and forward. There's wiring up front. I used the 'balance with forehead' technique to hold it up whenever I needed to get to the CB. The red spot's almost gone. Be careful. It's way too easy to put a nasty scratch in the trunk while you're doing this. Sigh.)

Remove and do as you will with the old antenna. Mine may eventually grow up to be a flag mast. Remove and retain the wire between the antenna and the CB.

Place the stud mount base on the antenna support with the nylon washer at the top. If you are using one place the folding mount in the stud mount, with the fold pointed front/back. Screw the antenna into this. Don't get carried away with the antenna itself. All else should be tight. Screw the 90 adapter into the bottom and aim it toward the radio.


Antenna base.

I had to use a file and cut two notches in the stud base. Most PL-259 females have notches all around so that the locking points on the male can keep it from rotating while maintaining good contact. This one had none. A half dozen swipes with an edge of a triangular file did the job.


The notch

Use the old wire to measure where to cut the CB Antenna Out wire on the splitter. Solder a PL-259 male plug on the wire you cut off. This is your new antenna wire. Put a male motorola plug on the CB Antenna Out wire of the splitter.


Wiring details.


Finished wiring I didn't shorten the FM antenna wire the first time. It doesn't have to be, but it makes the box a bit crowded.

Screw your new antenna wire into the 90 at the base of the antenna.

Test for continuity and shorts. The tip of the antenna should have continuity with the center of the CB antenna wire. It should not have continuity with the outside of the plug. The CB antenna wire shield (outside of plug) should have continuity with the antenna support that came with the bike.

Screw the end of the antenna wire into the splitter's Antenna In plug. Route this wire through the hole the old antenna wire went through. Plug the CB out wire into the CB box (gray plug).

Remove the FM antenna, base and all. Cut the end off of the FM wire and solder the female Motorola plug on. The core goes to the center of the plug. The shielding goes to the body of the plug. Use some shrink wrap to cover the whole thing. Cut a notch in the side of the plastic box and route the FM antenna wire with the new plug into the box. I made a notch in the lip of the top and a matching one in the slot of the box. I kept this a tight fit to keep water out.

Plug the splitter's FM Out lead into the FM antenna wire, fold it up into the box beside the CB. Place the splitter under the wires coming into the box on the left. Massage all the wiring to fit. Close it up.


In the box.

Now that everything fits, it's time to set the Standing Wave. THIS MUST BE DONE or you'll toast your CB output transistor.

Remove the plug from Antenna In on the splitter. Remove the plug between the splitter and the CB. Add a male PL-259 to male motorola patch cord to the antenna input wire (gray plug). Let the patch cord and the antenna wire dangle outside the box. Put the trunk back in place. Plug the antenna wire into Antenna on the SWR meter. Plug the wire from the radio into Radio on the SWR meter.

Set the standing wave as per meter instructions.

I got a meter at the Rat Shack. I got a http://www.walcottcb.com/product_info.php?products_id=934 and a http://www.walcottcb.com/12mfpl-pl259-to-motorola-female-12-long-p-198.html to connect the meter to the antenna system.

I had to screw the adjuster on the antenna most of the way in. After some tinkering with the tip I got a 1.3 on 20 and just under 1.7 on 1 and 40.

Once the antenna SWR is set you have to tune the splitter. Connect the antenna to Antenna In on the splitter. Connect a male motorola to male PL-259 patch cord to CB Out on the splitter. Connect these to your meter and check SWR. You'll have to turn the CB adjusting screw on the splitter to bring the SWR back down to where it was when you set the antenna. Mine is turned all the way down.

You can now set the AM as per splitter instructions.

When the antenna and splitter are both set disconnect the meter and the patch cord. Put the antenna wire back on Antenna In and the CB out wire into the gray socket. Pack everything back into the box, put the lid on, and bolt the trunk back in place.

Breaker, breaker, two nine.

End result: Single antenna system for under $75.