How To Set Up A Cobra 25 Ltd Classic
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Help getting a little Cobra 25 LTD set up
- Thread starter JSMachine
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- #1
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 past 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 information technology as low key as possible. I have a 4' fiberglass RAMI antenna. I've heard some RAMIs are multiband, but I don't know well-nigh this one. Just for kicks, allow's say it is a regular fiberglass antenna. I've mounted it to a bracket on the commuter's side front fender that comes upwardly in between the hood and fender (in the gap). This was just a test fit. I later took it dorsum off and painted it black, and so proceeded with mounting the antenna.
Information technology is contrary the radio antenna which is on the passenger side. I've heard RAMI antennas are non tunable. I'thousand not certain if this is the case or not. I ran this same antenna on my little truck in high school, and information technology seemed to work okay. I don't know if I always had the SWR set though. I can't rememeber. Here is the antenna.
I'thou 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 middle virtually the trans tunnel. It only has about 2.v' of coax to reach the antenna. I accept heard that I need 18' of coax..is there any truth to this?
And so came trying to effigy out how the two conductors in the coax were wired. Recollect, it's been since loftier 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 metallic base which acts every bit a nut and has threads). It is my understanding that the copper wire that encircles the antenna rod is the "Hot" wire. Hither is the copper wire which I have broken the solder connection from the base
[/QUOTE]
Then, I was trying to effigy out how the base of operations of the antenna would connect to the mounting bracket without grounding itself out (considering the mount is obviously grounded). I then discovered that insulating washers and sleeves need to exist used.
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I'yard simply trying to effigy out if the organisation will work. I decided to just mount the antenna without insulating sleeves. I asunder the copper wire from the base of the antenna, and soldered the centre wire of the coax directly to it, and and then took the shielding (braided wire) and grounded it to the mount. The mountain is installed to the fender with self tapping screws. I figured this would be a good enough ground until I can run 1 to the frame. I will practice that soon when I get a risk.
The radio comes on and receives fine. I spent nearly some of last night listening on several channels. My radio is a Cobra 25 LTD, and I was thinking that information technology had the built in SWR scale. I looked a piddling closer and discovered that only the 29 models (and peradventure college finish ones) have information technology. Mine does not take information technology. All I have is the meter that shows wattage and dB.
In any example, 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 cardinal somewhere abroad from zero - like somewhere in the middle, then when I talk into the mic the needle should jump over to the red.
Apparently 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 earlier I tin can go to that 1 though, so I may just purchase one. Any hints on where to buy 1 locally? Maybe radio shack? I could guild, merely that would surely be a calendar week or more wait.
If information technology turns out that I tin can't become my SWR right with the antenna setup I take, I will get another. I merely want to come across if this 1 works commencement. I don't like to spend coin on things If I don't have to.
- #two
Exercise yous have a dummy load?
A nice 10w 50-52 ohm resistor volition piece of work, put it across the coax in identify of the antenna.
If the coax is practiced you should get a very low SWR.
Supplant that antenna! I take an old peterbilt antenna that the solder came loose, I did manage to solder
it back onto the stud, all the same I would not use it as a vehicle mounted antenna. I do not trust the vibration
and flexing from the vehicle in movement to allow the previously dammaged and re soldered connection to
last. I apply my repaired antenna as a portable Dipole I can fix while camping etc.
Yep those antennas can be tuned. It is washed by slowly hacksawing little $.25 at a time off the end to
shorten information technology, recheck SWR and repeat.
You lot can become a four' fiberglass firestik antenna with a tunable spiral tip. Easy to tune and you lot dont have to
worry about accidentally shortenning the antenna too much. Mine ran me $35. Just make certain
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 costless? How most the And then-239 on the stud, is it make clean and corosion free?
Practice yous have a expert DC ground to the chassis on the mount?
no your method of testing wont work. Believe it or not, that fiddling 1 1/2" of steel with the thread on information technology to fit the mount adds length to the antenna
and past eliminating it, it is now electrically short, and will throw out the SWR.
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- #3
I took a closer look at your mount.
Your entirely missing the antenna stud. Its a ii piece part with an insulator to keep the center
usher from shorting to the chassis. Information technology has an SO-239 connector on one end to take 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 lot need something like this to become in that mount.
http://lghttp.14077.nexcesscdn.net/...a0ffb21846ac53c0/e/c/economy_antenna_stud.jpg
- #4
I accept no mode of checking SWR at the moment. That'south why I asked where I could get 1 locally. Bank check out the end of my postal service. I realize it'due south long, but everything is in at that place.
Check out my original post to see what I did most this. Again, information technology'due south in at that place.
The coax is RG8X. It is in proficient condition. The PL259 (back of cb) is in great shape, clean and tight. Check out the original post. I am not using a Then-239. Read my post to see what I did.
Information technology's in the original mail. For now information technology is grounded to the mountain, which is cocky tapping screw mounted to the fender. I will run a good chassis ground when I get a chance. I presume where I take it is good enough for something though, for the fourth dimension being.
Non sure I'thousand following y'all here.
- #5
I took a closer look at your mount.
Your entirely missing the antenna stud. Its a two piece part with an insulator to keep the center
conductor from shorting to the chassis. It has an SO-239 connector on one cease to accept the PL-259 on the coax, and a femal thread to take the antenna. Without this, you volition short your radios finals and blow em upwardly.You demand something like this to go in that mount.
http://lghttp.14077.nexcesscdn.net/...a0ffb21846ac53c0/due east/c/economy_antenna_stud.jpg
Delight go back and read my post. The issue of insulation and all of that is covered. I promise. The picture y'all are looking at is but a mock up. Information technology wasn;t even permanently mounted at that point. That pic was simply to testify the location of the mount.
- #6
Not sure what you have local to buy an SWR meter, maybe bank check a truck stop.
A couple we have hither sell antennas, SWR meters, Mounts and radios.
I must have missinterperated the movie, I idea that was your install, didnt realise information technology hadnt been mounted yet.
As well you mentioned you painted the mount. If you painted the whole thing, you may have issues getting a
good ground when you claw everything up if the pain insulates the stud, or your screw heads from the vehicles fender. Use split up lock washers on the bolts/screws used to mountain information technology to the fender. These should bite through any pigment that is there emmet make a ground. The studs usually have a lock washer with them
This should bite through the pigment to make a decent basis.
RG8X is good cable, its what I utilise for my mobile installs.
Your setup will work, but you need to put information technology together properly. Mockups will have completely unlike SWR
than a propor setup, and you may not even be able to tune information technology at all if the environmental factors dont remain the same.
Mount your mount, Put the stud on, and utilise a DMM to cheque a proficient 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 merely real mode to exam 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 performance, The antenna is a loaded type, it i basically a coil of wire. i/iv wavelength at 27Mhz would be around 9ft (104") Your antenna is four'
and loaded to get in await like either a 1/iv wavelength or 5/8 wavelength (which would be around 22 anxiety) to the transmitter, matching information technology 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 concluding 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 non physically equally long, (the height of the vertical radiator or the length of the scroll if it were straigtended out completely volition be shorter) The gyre on the antenna relies on inductance and reactance at a given frequency to brand the antenna "look" electrically as long as a concrete 1/4 or v/8 wave antenna. By eliminating both the small bits of metallic at the base of the antenna (in your example the stud mount, and the threaded stud on the antenna) you lot are finer trimming off near 2" of length from
the antenna. Everything that is on the outside of the shield of the coax finer becomes part of the antenna and effects its resonance.
Mount everything solid, with a practiced footing, catch an SWR meter and a short jumper, hook it up and your
set for tuning. But I would strongly advise replacing that antenna. If the radio is valuable to you, you lot dont want information technology to fail without you knowing while your transmitting and kill it.
RF is funny, tiny tiny changes can make a huge deviation.
- #seven
Crazy how the length changes things. I have the shielded part of the coax split of from the main eye usher under the hood, so that would mean that the antenna is effectively going all the manner under the hood. Hither is a movie.
I know this probably looks like the worst rigged thing You have ever seen, but I hope, I am very wearisome and picky virtually things being right. If you lot look closesly, the coax is visible in the bottom of the moving-picture show with its gray jacket. The shielded office of the wire that is grounded is the dark-green tape coming off at xc degrees. Y'all can see some of the bare wire nether the screw caput. The wire is in straight contact with the screw head, and the screw is in directly contact with the hole it created in the fender. I know this might not be perfect, just I can bet you that information technology is at least 95% grounded. I volition run a defended ground wire to the frame, I just haven't had a take a chance to do it. I merely put Everything in like two days ago.
The blackness electric tape starts where the green stops. It is wrapped around the center core conductor (which is still in its potent 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 ane that wraps around the mast. So I guess technically, my antenna starts downwards the where the green wire wire splits off, since you say that whatever isn't shielded becomes part of the antenna.
Not sure, simply this is a pain. If I had known that it was going to be this much trouble to hook it upward, information technology would probably be still laying in the attic. lol
- #8
Y'all can become 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.
Y'all should replace the antenna...a Firestick (fiberglass) isn't ideal, only it would cost most $15, is real simple, and doesn't require whatever tuning (and they tin can be had in a variety of dissimilar lengths). By having your coax divide and so far under the hood now, you accept essentially made the antenna longer...and then 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 considering 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 at that place, information technology isn't optimum; only information technology should get yous working. I would estimate yous would become the functioning yous are looking for if the setup is put together correctly.
As far equally your coax length, you but need the minimum length required to get you lot from the radio to the antenna...nothing more than, nothing less.
As far equally your radio burial the needle in the red...could be annihilation, but I'k guessing the guy who "tuned" it for you set the needle up to show "full power" to make you lot experience good most his piece of work. Its hard to find good cb technicians...
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- #ix
Yep the antenna begins where the shield and the centre conductor separate. Every bit someone said above become a new antenna. What y'all have might work but I doubt it. You have added several inches to the antenna which volition affect its resonant frequency and impedance.
BB
- #10
Starting at 2:20 in this vid, this guy is explaining what I'm talking about. What he is describing every bit "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 subsequently he adjusts the radio, it yet doesn't do too much more than.
CB Radio - How to adjust modulation within a CB Radio past CBradiomagazine.com - YouTube
Mine dead keys in the carmine. Information technology also swings all the fashion to the cherry-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 cheque information technology, because I'one thousand afraid I may mess up the transmission circuitry..Not sure what the SWR is doing at this indicate, then I am merely going to wait.
- #11
Like I stated previously, the needle can be made to evidence anything...its a simple internal adjustment.
- #12
Ok, well all this existence said, is there any manner to tell whether or not I have messed something upward 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 take an SWR meter. I want to be sure the radio is OK before i spend coin on the correct antenna setup.
- #xiii
Its quite doubtful you hurt the radio...if it was onetime, 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 however moving which indicates its putting something out notwithstanding. Most times when the last pops, the meter stops moving too.
- #14
The radio is dated '02 on the rear.
- #xv
A DMM won't be able to help yous much. You need an SWR meter or a Watt meter to decide if y'all have any RF output.
BB
- #16
So here is another question. I'yard looking at correctly mounting the antenna, and I realize that I volition need a lug that mounts to my piddling subclass, but is insulated. Something like this:
Firestik K-four Lug CB Antenna Stud Mount | Correct 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 tin ever attach the hot wire (heart core) to the lug like the one pictured in the link. Does the shielded core need to cease right there as well, and so footing to the bracket? This is an example of what kind of cable end Im talking virtually.
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"grand not paying for that. -Only more than what I want to spend to become this working.
I'yard trying to avoid buying a cable terminate of some sort. I recall in that location is probably some sort of termination for the end of the coax which would merely spiral correct on to the lesser of the antenna or the lug, but where and how does it basis?
- #17
So here is another question. I'thousand looking at correctly mounting the antenna, and I realize that I will need a lug that mounts to my little subclass, but is insulated. Something like this:
Firestik 1000-4 Lug CB Antenna Stud Mountain | 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 adhere the hot wire (center core) to the lug similar the one pictured in the link. Does the shielded core demand to terminate right there every bit well, and and so ground to the subclass? This is an example of what kind of cable terminate Im talking nearly.
Firestik K8 CB Coax Cable | Right Channel Radios
I would assume the footling grounding strap just grounds to the mount screws.
I do see the Firestik Firering, but I"yard not paying for that. -Just more than than what I want to spend to get this working.
I'm trying to avoid buying a cablevision finish of some sort. I think at that place is probably some sort of termination for the end of the coax which would only screw correct on to the bottom of the antenna or the lug, but where and how does it footing?
I read your posts, so I sympathize you are only interested in very short-range communications, you're keeping this project on a tight budget, and you lot just want to get on the air quickly and get on with your life.
I think you lot're disruptive things when you lot say "center core" and "shielded core." I'yard going to phone call it the center conductor and the shielding. The eye 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 information technology 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 fourth dimension and money to your project, on the bright side is that pickup trucks are rife with extra holes, nuts and bolts. Giving that "other one-half" of your antenna more metal can brand a huge departure in performance.
In improver to the 1000-four, there'southward also a G-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 (non stainless steel), R-four and R-4A.
Here is a parts diagram of each:
http://world wide web.firestik.com/Instl-Art/te-k-4.gif
http://world wide web.firestik.com/Instl-Fine art/te-m-4a.gif
On the K-four (or R-4), the "insulated ring terminal" in that K-four diagram is what you'd attach only to the center usher of your coax. The "shoulder washers" insulate the center usher and antenna from the mount. The shielding from the coax should exist attached to your mount; probably at one of the screws attaching it to the trunk panel.
There'south also a trick to neatly terminating your own cable, probably better than the ane in your link. (Parts: coax cablevision, solder, ring terminals, oestrus-shrink tubing; Tools: awl [or like tool], soldering atomic number 26, crimpers, lighter; Time: 5 to 15 minutes)
- #18
I read your posts, then I sympathize you are only interested in very short-range communications, you're keeping this project on a tight budget, and y'all just desire to get on the air quickly and go on with your life.
I retrieve yous're confusing things when you say "center core" and "shielded core." I'yard going to call information technology the center usher and the shielding. The middle conductor goes to the antenna element and the shielding goes to the mount/chassis. The center usher, 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 information technology 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 than metal tin can make a huge difference in performance.
In addition to the K-four, there's also a K-4A, which has an And then-239 socket on it so y'all can use off-the-shelf coax with PL-259 plugs. There are besides cheaper versions (not stainless steel), R-4 and R-4A.
Hither is a parts diagram of each:
http://www.firestik.com/Instl-Art/te-m-4.gif
http://www.firestik.com/Instl-Art/te-thou-4a.gifOn the K-4 (or R-4), the "insulated ring terminal" in that Thousand-4 diagram is what yous'd attach just to the eye conductor of your coax. The "shoulder washers" insulate the center usher and antenna from the mount. The shielding from the coax should be attached to your mount; probably at 1 of the screws attaching it to the trunk panel.
At that place'due south likewise a trick to neatly terminating your own cable, probably better than the i in your link. (Parts: coax cable, solder, ring terminals, oestrus-shrink tubing; Tools: awl [or similar tool], soldering iron, crimpers, lighter; Time: 5 to fifteen minutes)
The way yous described everything is exactly the way I have information technology. The but matter I oasis't washed is "bond" as you lot say - the mount to the frame. It is bonded to the fender, but I do want to practise the extra step and bond that to the frame via a grounding strap or piece of wire.
- #19
You lot may non have to bail the metal parts.They SHOULD be bonded already.
Look for ground wires connecting the trunk panels to the frame.
There should be a multitude of them. While the additional bonding may help improve information technology,
It is not saying it wont work wothout it. I haven't added aditional bonding to any of the vehicles I have washed
HF installs on, and all work exceptionaly well and resulted in the expected range.
If you lot are planning to conect your antenna with your cale in the same fashion as those firestik cables,
while at that place is nothing wrong with doing that, just make sure the connections are skillful. Solder on some
ring terminals. A self tapping screw into the mount bracked for the coax ground should exist enough. Endeavor and make the connections as curt as humanly possible.
Seal the coax. Get some outdoor rated silicon, and seal up the finish of the coax in one case everything is connected. Moisture getting into the cablevision will poison it and get in garbage.
- #xx
this is what you need to do
I took a closer look at your mount.
Your entirely missing the antenna stud. Its a 2 slice role with an insulator to keep the centre
usher 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 brusque your radios finals and blow em upwardly.You lot demand something like this to get in that mountain.
http://lghttp.14077.nexcesscdn.net/...a0ffb21846ac53c0/e/c/economy_antenna_stud.jpg
I suggest that you follow these instructions. All you lot demand is a stud, like the one in the motion picture, and some other antenna that's non damaged. I adopt a four foot Francis that needs no tuning. Brand certain the insulator ring is on elevation of the mount. You will demand coax with screw on connectors on both ends to fit the stud and radio, unless you lot are forcing it through a tiny pigsty in the burn wall. That's my estimate, if you lot're avoiding the pl259 at the antenna mountain.
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How To Set Up A Cobra 25 Ltd Classic,
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