Help getting a little Cobra 25 LTD set up

[JSMachine]

I have a Cobra 25 LTD Classic that I bought when I was in high school (about 15 years ago). It was peaked and tuned by a local radio guy when I bought it from him. I found it in the attic the other night and I decided to put it in my daily driver full size chevy pickup just for kicks. The radio is not amplified - just pretty much factory other than the few things he modified on the inside.

I don't want a conspicuos install. I'd like to keep it as low key as possible. I have a 4' fiberglass RAMI antenna. I've heard some RAMIs are multiband, but I don't know about this one. Just for kicks, let's say it is a regular fiberglass antenna. I've mounted it to a bracket on the driver's side front fender that comes up in between the hood and fender (in the gap). This was just a test fit. I later took it back off and painted it black, and then proceeded with mounting the antenna.

It is opposite the radio antenna which is on the passenger side. I've heard RAMI antennas are not tunable. I'm not sure if this is the case or not. I ran this same antenna on my little truck in high school, and it seemed to work okay. I don't know if I ever had the SWR set though. I can't rememeber. Here is the antenna.

I'm concerned about the ground plane and all that jazz. Would this be a suitable place to mount it?

I'm not looking for something that will reach out to some ridiculous distance. Just something that will work reliably for a couple miles.

The CB is mounted under the dash in the center near the trans tunnel. It only has about 2.5' of coax to reach the antenna. I have heard that I need 18' of coax..is there any truth to this?

Then came trying to figure out how the two conductors in the coax were wired. Remember, it's been since high school since I worked with this stuff. I removed the plastic from the base of the antenna and found that the copper wire which encircles the antenna was soldered to the base (the metal base which acts as a nut and has threads). It is my understanding that the copper wire that encircles the antenna rod is the "Hot" wire. Here is the copper wire which I have broken the solder connection from the base

[/QUOTE]

Then, I was trying to figure out how the base of the antenna would connect to the mounting bracket without grounding itself out (because the mount is obviously grounded). I then discovered that insulating washers and sleeves need to be used.

-----------------

I'm simply trying to figure out if the system will work. I decided to just mount the antenna without insulating sleeves. I disconnected the copper wire from the base of the antenna, and soldered the center wire of the coax directly to it, and then took the shielding (braided wire) and grounded it to the mount. The mount is installed to the fender with self tapping screws. I figured this would be a good enough ground until I can run one to the frame. I will do that soon when I get a chance.

The radio comes on and receives fine. I spent most some of last night listening on several channels. My radio is a Cobra 25 LTD, and I was thinking that it had the built in SWR scale. I looked a little closer and discovered that only the 29 models (and maybe higher end ones) have it. Mine does not have it. All I have is the meter that shows wattage and dB.

In any case, When I dead key my mic, My needle goes into the red. There is really no difference when talking and dead keying. From the vids I've watched on youtube, it should dead key somewhere away from zero - like somewhere in the middle, and then when I talk into the mic the needle should jump over to the red.

Obviously the SWR is off, so I am not going to key the mic anymore until I get it checked. I have a friend with a guage. It may be a while before I can get to that one though, so I may just buy one. Any hints on where to buy one locally? Maybe radio shack? I could order, but that would surely be a week or more wait.

If it turns out that I can't get my SWR right with the antenna setup I have, I will get another. I just want to see if this one works first. I don't like to spend money on things If I don't have to.

 

[RC286]

Do you have a dummy load?
A nice 10w 50-52 ohm resistor will work, put it across the coax in place of the antenna.
If the coax is good you should get a very low SWR.

Replace that antenna! I have an old peterbilt antenna that the solder came loose, I did manage to solder
it back onto the stud, however I would not use it as a vehicle mounted antenna. I do not trust the vibration
and flexing from the vehicle in motion to allow the previously dammaged and re soldered connection to
last. I use my repaired antenna as a portable Dipole I can set up while camping etc.

Yes those antennas can be tuned. It is done by slowly hacksawing little bits at a time off the end to
shorten it, recheck SWR and repeat.

You can get a 4' fiberglass firestik antenna with a tunable screw tip. Easy to tune and you dont have to
worry about accidentally shortenning the antenna too much. Mine ran me $35. Just make sure
to replace the plastic tip weather cover before checking the SWR. Believe it or not, the stupid plastic
cap changes the impedance of the antenna, affecting the SWR.

What kind of coax are you using? Is it in good shape? How about the PL-259 connectors? are they clean
and corosion free? How about the SO-239 on the stud, is it clean and corosion free?

Do you have a good DC ground to the chassis on the mount?

no your method of testing wont work. Believe it or not, that little 1 1/2" of steel with the thread on it to fit the mount adds length to the antenna
and by eliminating it, it is now electrically short, and will throw out the SWR.

 

[RC286]

I took a closer look at your mount.
Your entirely missing the antenna stud. Its a 2 piece part with an insulator to keep the center
conductor from shorting to the chassis. It has an SO-239 connector on one end to accept the PL-259 on the coax, and a femal thread to accept the antenna. Without this, you will short your radios finals and blow em up.

You need something like this to go in that mount.

http://lghttp.14077.nexcesscdn.net/...a0ffb21846ac53c0/e/c/economy_antenna_stud.jpg

 

[JSMachine]

Do you have a dummy load?
A nice 10w 50-52 ohm resistor will work, put it across the coax in place of the antenna.
If the coax is good you should get a very low SWR..

I have no way of checking SWR at the moment. That's why I asked where I could get one locally. Check out the end of my post. I realize it's long, but everything is in there.

Replace that antenna! I have an old peterbilt antenna that the solder came loose, I did manage to solder it back onto the stud, however I would not use it as a vehicle mounted antenna. I do not trust the vibration and flexing from the vehicle in motion to allow the previously dammaged and re soldered connection to last. I use my repaired antenna as a portable Dipole I can set up while camping etc.

Check out my original post to see what I did about this. Again, it's in there.

What kind of coax are you using? Is it in good shape? How about the PL-259 connectors? are they clean and corosion free? How about the SO-239 on the stud, is it clean and corosion free?

The coax is RG8X. It is in good condition. The PL259 (back of cb) is in great shape, clean and tight. Check out the original post. I am not using a SO-239. Read my post to see what I did.

Do you have a good DC ground to the chassis on the mount?

It's in the original post. For now it is grounded to the mount, which is self tapping screw mounted to the fender. I will run a good chassis ground when I get a chance. I assume where I have it is good enough for something though, for the time being.

no your method of testing wont work. Believe it or not, that little 1 1/2" of steel with the thread on it to fit the mount adds length to the antenna and by eliminating it, it is now electrically short, and will throw out the SWR.

Not sure I'm following you here.

 

[JSMachine]

I took a closer look at your mount.
Your entirely missing the antenna stud. Its a 2 piece part with an insulator to keep the center
conductor from shorting to the chassis. It has an SO-239 connector on one end to accept the PL-259 on the coax, and a femal thread to accept the antenna. Without this, you will short your radios finals and blow em up.

You need something like this to go in that mount.

http://lghttp.14077.nexcesscdn.net/...a0ffb21846ac53c0/e/c/economy_antenna_stud.jpg

Please go back and read my post. The issue of insulation and all of that is covered. I promise. The picture you are looking at is only a mock up. It wasn;t even permanently mounted at that point. That picture was simply to show the location of the mount.

 

[RC286]

Not sure what you have local to buy an SWR meter, maybe check a truck stop.
A couple we have here sell antennas, SWR meters, Mounts and radios.

I must have missinterperated the picture, I thought that was your install, didnt realise it hadnt been mounted yet.

Also you mentioned you painted the mount. If you painted the whole thing, you may have issues getting a
good ground when you hook everything up if the pain insulates the stud, or your screw heads from the vehicles fender. Use split lock washers on the bolts/screws used to mount it to the fender. These should bite through any paint that is there ant make a ground. The studs usually have a lock washer with them
This should bite through the paint to make a decent ground.

RG8X is good cable, its what I use for my mobile installs.

Your setup will work, but you need to put it together properly. Mockups will have completely different SWR
than a propor setup, and you may not even be able to tune it at all if the environmental factors dont remain the same.

Mount your mount, Put the stud on, and use a DMM to check a good DC ground. Hook the coax to the
stud. Resolder the wire to the threaded stud on the antenna, and thread the antenna into the stud mount.
This will be the only real way to test the setup.

Now the reason that hooking your coax directly to the wire wont work is an antenna is a resonant electrical
device that must resonate on or very near the frequency of intended operation, The antenna is a loaded type, it i basically a coil of wire. 1/4 wavelength at 27Mhz would be around 9ft (104") Your antenna is 4'
and loaded to make it look like either a 1/4 wavelength or 5/8 wavelength (which would be around 22 feet) to the transmitter, matching it to the 50ohm impedance of the amp. Even being a few inches out in length can drastically affect the resonance of the antenna, and RF will be reflected back into the radio instead of radiated out of the antenna, possibly dammaging the radios final output transistors.

A loaded antenna is more touchy to tiny changes in length, fractions of an inch can make a difference, this is because the antenna is not physically as long, (the height of the vertical radiator or the length of the coil if it were straigtended out completely will be shorter) The coil on the antenna relies on inductance and reactance at a given frequency to make the antenna "look" electrically as long as a physical 1/4 or 5/8 wave antenna. By eliminating both the small bits of metal at the base of the antenna (in your case the stud mount, and the threaded stud on the antenna) you are effectively trimming off nearly 2" of length from
the antenna. Everything that is on the outside of the shield of the coax effectively becomes part of the antenna and effects its resonance.

Mount everything solid, with a good ground, grab an SWR meter and a short jumper, hook it up and your
ready for tuning. But I would strongly suggest replacing that antenna. If the radio is valuable to you, you dont want it to fail without you knowing while your transmitting and kill it.

RF is funny, tiny tiny changes can make a huge difference.

 

[JSMachine]

Crazy how the length changes things. I have the shielded part of the coax split of from the main center conductor under the hood, so that would mean that the antenna is effectively going all the way under the hood. Here is a picture.

I know this probably looks like the worst rigged thing You have ever seen, but I promise, I am very tedious and picky about things being right. If you look closesly, the coax is visible in the bottom of the picture with its gray jacket. The shielded part of the wire that is grounded is the green tape coming off at 90 degrees. You can see some of the bare wire under the screw head. The wire is in direct contact with the screw head, and the screw is in direct contact with the hole it created in the fender. I know this might not be perfect, but I can bet you that it is at least 95% grounded. I will run a dedicated ground wire to the frame, I just haven't had a chance to do it. I just put Everything in like two days ago.

The black electrical tape starts where the green stops. It is wrapped around the center core conductor (which is still in its stiff plastic jacket) all the way up to the base of the antenna, past the mounting base. Right there, it is soldered directly to the copper wire seen in the pics above, the one that wraps around the mast. So I guess technically, my antenna starts down the where the green wire wire splits off, since you say that whatever isn't shielded becomes part of the antenna.

Not sure, but this is a pain. If I had known that it was going to be this much trouble to hook it up, it would probably be still laying in the attic. lol

 

[sdeeter19555]

You can get an SWR meter at a truckstop or a RadioShack. The RadioShack one will have a power meter and SWR capability, I've had several of these and they are basic and pretty durable. Probably should think about finding a truckstop, RadioShack, or a CB shop nearby to get your supplies.

You should replace the antenna...a Firestick (fiberglass) isn't ideal, but it would cost about $15, is real simple, and doesn't require any tuning (and they can be had in a variety of different lengths). By having your coax split so far under the hood now, you have essentially made the antenna longer...so your SWR will be off.

Mounting the antenna where you have it will make you stronger to the front and side the antenna is on and not so much to the rear and opposite side because the truck's body is in the way (creates a "shadow" so to speak). Your mount is similar to a lot of the commercially available deals out there, it isn't optimum; but it should get you working. I would guess you would get the performance you are looking for if the setup is put together correctly.

As far as your coax length, you only need the minimum length required to get you from the radio to the antenna...nothing more, nothing less.

As far as your radio burying the needle in the red...could be anything, but I'm guessing the guy who "tuned" it for you set the needle up to show "full power" to make you feel good about his work. Its hard to find good cb technicians...

 

[WA0CBW]

Yes the antenna begins where the shield and the center conductor separate. As someone said above get a new antenna. What you have might work but I doubt it. You have added several inches to the antenna which will affect its resonant frequency and impedance.
BB

 

[JSMachine]

Starting at 2:20 in this vid, this guy is explaining what I'm talking about. What he is describing as "3" and "baseline", notice how his needle is in the middle. It doesn't swing much from there when he talks into the mic. Even after he adjusts the radio, it still doesn't do too much more.

CB Radio - How to adjust modulation inside a CB Radio by CBradiomagazine.com - YouTube

Mine dead keys in the red. It also swings all the way to the red (really the same place as dead key) when I am talking into the mic.

Like I stated though, I'm not gonna do anything though until I redo the setup or check it, because I'm afraid I may mess up the transmission circuitry..Not sure what the SWR is doing at this point, so I am just going to wait.

 

[sdeeter19555]

Like I stated previously, the needle can be made to show anything...its a simple internal adjustment.

 

[JSMachine]

Ok, well all this being said, is there any way to tell whether or not I have messed something up in my transmission circuitry with the setup I currently have? I only did a couple of radio checks, and got no response. I did this close to an interstate hwy and using channel 19. I assume that If I were working ok I would have gotten a response.

Is there any way I can test it with a DMM?

I do have that, I just don't have an SWR meter. I want to be sure the radio is OK before i spend money on the correct antenna setup.

 

[sdeeter19555]

Its quite doubtful you hurt the radio...if it was old, I'd say maybe; but Cobra 25s are pretty tough little radios. Its not like you didn't have an antenna hooked up, and your meter is still moving which indicates its putting something out yet. Most times when the final pops, the meter stops moving too.

 

[JSMachine]

The radio is dated '02 on the rear.

 

[WA0CBW]

A DMM won't be able to help you much. You need an SWR meter or a Watt meter to determine if you have any RF output.
BB

 

[JSMachine]

So here is another question. I'm looking at correctly mounting the antenna, and I realize that I will need a lug that mounts to my little bracket, but is insulated. Something like this:

Firestik K-4 Lug CB Antenna Stud Mount | Right Channel Radios

My question is where does the ground need to be? If I run the coax up to the bottom of the antenna, I can always attach the hot wire (center core) to the lug like the one pictured in the link. Does the shielded core need to terminate right there as well, and then ground to the bracket? This is an example of what kind of cable end Im talking about.

Firestik K8 CB Coax Cable | Right Channel Radios

I would assume the little grounding strap just grounds to the mount screws.

I do see the Firestik Firering, but I"m not paying for that. -Just more than what I want to spend to get this working.

I'm trying to avoid buying a cable end of some sort. I think there is probably some sort of termination for the end of the coax which would just screw right on to the bottom of the antenna or the lug, but where and how does it ground?

 

[KK4HBW]

So here is another question. I'm looking at correctly mounting the antenna, and I realize that I will need a lug that mounts to my little bracket, but is insulated. Something like this:

Firestik K-4 Lug CB Antenna Stud Mount | Right Channel Radios

My question is where does the ground need to be? If I run the coax up to the bottom of the antenna, I can always attach the hot wire (center core) to the lug like the one pictured in the link. Does the shielded core need to terminate right there as well, and then ground to the bracket? This is an example of what kind of cable end Im talking about.

Firestik K8 CB Coax Cable | Right Channel Radios

I would assume the little grounding strap just grounds to the mount screws.

I do see the Firestik Firering, but I"m not paying for that. -Just more than what I want to spend to get this working.

I'm trying to avoid buying a cable end of some sort. I think there is probably some sort of termination for the end of the coax which would just screw right on to the bottom of the antenna or the lug, but where and how does it ground?

I read your posts, so I understand you are only interested in very short-range communications, you're keeping this project on a tight budget, and you just want to get on the air quickly and get on with your life.

I think you're confusing things when you say "center core" and "shielded core." I'm going to call it the center conductor and the shielding. The center conductor goes to the antenna element and the shielding goes to the mount/chassis. The center conductor, and the antenna element, must be insulated from the mount. The mount must be bonded to your vehicle's chassis. Typically that will be at the point where you bolt it to the body.

You're probably going to need to bond your fender to the frame (and bond other parts together, too). While that would add time and money to your project, on the bright side is that pickup trucks are rife with extra holes, nuts and bolts. Giving that "other half" of your antenna more metal can make a huge difference in performance.

In addition to the K-4, there's also a K-4A, which has an SO-239 socket on it so you can use off-the-shelf coax with PL-259 plugs. There are also cheaper versions (not stainless steel), R-4 and R-4A.

Here is a parts diagram of each:
http://www.firestik.com/Instl-Art/te-k-4.gif
http://www.firestik.com/Instl-Art/te-k-4a.gif

On the K-4 (or R-4), the "insulated ring terminal" in that K-4 diagram is what you'd attach only to the center conductor of your coax. The "shoulder washers" insulate the center conductor and antenna from the mount. The shielding from the coax should be attached to your mount; probably at one of the screws attaching it to the body panel.

There's also a trick to neatly terminating your own cable, probably better than the one in your link. (Parts: coax cable, solder, ring terminals, heat-shrink tubing; Tools: awl [or like tool], soldering iron, crimpers, lighter; Time: 5 to 15 minutes)

 

[JSMachine]

I read your posts, so I understand you are only interested in very short-range communications, you're keeping this project on a tight budget, and you just want to get on the air quickly and get on with your life.

I think you're confusing things when you say "center core" and "shielded core." I'm going to call it the center conductor and the shielding. The center conductor goes to the antenna element and the shielding goes to the mount/chassis. The center conductor, and the antenna element, must be insulated from the mount. The mount must be bonded to your vehicle's chassis. Typically that will be at the point where you bolt it to the body.

You're probably going to need to bond your fender to the frame (and bond other parts together, too). While that would add time and money to your project, on the bright side is that pickup trucks are rife with extra holes, nuts and bolts. Giving that "other half" of your antenna more metal can make a huge difference in performance.

In addition to the K-4, there's also a K-4A, which has an SO-239 socket on it so you can use off-the-shelf coax with PL-259 plugs. There are also cheaper versions (not stainless steel), R-4 and R-4A.

Here is a parts diagram of each:
http://www.firestik.com/Instl-Art/te-k-4.gif
http://www.firestik.com/Instl-Art/te-k-4a.gif

On the K-4 (or R-4), the "insulated ring terminal" in that K-4 diagram is what you'd attach only to the center conductor of your coax. The "shoulder washers" insulate the center conductor and antenna from the mount. The shielding from the coax should be attached to your mount; probably at one of the screws attaching it to the body panel.

There's also a trick to neatly terminating your own cable, probably better than the one in your link. (Parts: coax cable, solder, ring terminals, heat-shrink tubing; Tools: awl [or like tool], soldering iron, crimpers, lighter; Time: 5 to 15 minutes)

The way you described everything is exactly the way I have it. The only thing I haven't done is "bond" as you say - the mount to the frame. It is bonded to the fender, but I do want to do the extra step and bond that to the frame via a grounding strap or piece of wire.

 

[RC286]

You may not have to bond the metal parts.They SHOULD be bonded already.
Look for ground wires connecting the body panels to the frame.
There should be a multitude of them. While the additional bonding may help improve it,
It is not saying it wont work wothout it. I haven't added aditional bonding to any of the vehicles I have done
HF installs on, and all work exceptionaly well and resulted in the expected range.

If you are planning to conect your antenna with your cale in the same fashion as those firestik cables,
while there is nothing wrong with doing that, just make sure the connections are good. Solder on some
ring terminals. A self tapping screw into the mount bracked for the coax ground should be enough. Try and make the connections as short as humanly possible.
Seal the coax. Get some outdoor rated silicon, and seal up the end of the coax once everything is connected. Moisture getting into the cable will poison it and make it garbage.

 

[MeddleMan]

this is what you need to do

I took a closer look at your mount.
Your entirely missing the antenna stud. Its a 2 piece part with an insulator to keep the center
conductor from shorting to the chassis. It has an SO-239 connector on one end to accept the PL-259 on the coax, and a femal thread to accept the antenna. Without this, you will short your radios finals and blow em up.

You need something like this to go in that mount.

http://lghttp.14077.nexcesscdn.net/...a0ffb21846ac53c0/e/c/economy_antenna_stud.jpg

I suggest that you follow these instructions. All you need is a stud, like the one in the picture, and another antenna that's not damaged. I prefer a four foot Francis that needs no tuning. Make sure the insulator ring is on top of the mount. You will need coax with screw on connectors on both ends to fit the stud and radio, unless you are forcing it through a tiny hole in the fire wall. That's my guess, if you're avoiding the pl259 at the antenna mount.