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Cb Woes


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I know there is another CB thread floating around here, but I didn't want to hijack it. I'm having a transmit issue with my CB. It's a recent problem, that started after awhile of inactivity. I receive transmissions just fine, from trucks on a highway that's 15 miles from here. However, my SWR for transmitting is non existent except on higher channels. I know this is a tuning problem, but my antenna is not tunable as far as I can see. I re-ran grounds from both my radio and antenna to a common point. I tested continuity in my cable, the cable is good. That fact my radio worked flawlessly before and is all of a sudden unable to transmit a strong signal is worrying. I've got a video and a few pictures:

 

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Here's a couple things to check and a couple general comments.  Borrow or buy a dummy load like in the link below.  Take your coax off the radio and put the dummy load on the radio then check your output at top middle and bottom of the channels.  If the output is more normal then re-connect your coax to the radio, get a barrel connector. remove the coax form the antenna mount put on barrel connector and dummy load.  Re-check your output.  If the output is normal there then the antenna is the cause. If the antenna was originally tuned for the whole band the output should be best on 19 and less on 1 and 40 when transmitting into the antenna.   Get rid of the spring.  All it is going to do is cause you head aches because it adds 3" plus or minus to the length of the antenna and the antenna is cut to be  resonant as is.  It appears that you have a 102" whip.  The tip on there is basically cosmetic so if the antenna is too long you can cut some off.  There is usually a set screw in the tapered part that the whip fits into.  If there is that is your adjustment.  You will have a reasonable amount of travel up and down for adjustment.  If you don't reach a resonant point/low SWR when lowering the antenna into the spring then you could cut some off the top as mentioned before.  Also, your RF gain should always be set to max. Use the squelch to quiet the speaker.  The RF gain setting CAN affect what the meter shows even on your output.  I'll toss this next out for clarity and just in case.  If you check your SWR on 40,19,1 in that order and it is better/best on 40.  Then the antenna is more resonant at a higher frequency so it is too short for the lower frequencies.  If it is better or best on 1 then the antenna is resonant at a lower frequency and is too long for the higher frequencies.  If the SWR is lower on 19 than 40 or 1, then it is closer to a good match for the whole band.  Check a channel about half way between 1 and 19 and see if it's lower or higher than 19. If it's lower than 19 then the antenna is still too long overall and if it is higher the antenna is still too short. Also check a channel half way between 19 and 40 and see if its lower or higher than 19 and adjust accordingly.  If adjustments have little to no affect on the SWR that is typically a grounding issue.  It may be necessary to run your antenna ground all the way to the battery and the radio ground the same way.  Seems like overkill but it usually helps at least.

 

  My basis for the above is my experience as a Ham operator since 1978 and a CB'er before that.   Hope this helps and nothing mentioned is meant to infer you not already knowing it.  Just a refresher etc.  If you have any other questions holler and I will do my best to answer as best as I can or point you to somewhere that will give you the answer. Good luck!!

 

http://www.ebay.com/itm/like/150565947364?lpid=82

 

Later,

 

Larry

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Wow, I appreciate the time you took to write this out. I did forget to mention it is a 102" whip, unsure of the brand. It doesn't have a set screw, I literally took apart the mount, spring and antenna assembly to try and tune it, but it isn't a tunable antenna. I've never heard or seen that before. I never checked before either because it transmitted and received just fine. Does a 102" even need a spring?

 

I'll have to find a place that sells those dummy loads, radio shack maybe? My RF gain is almost always at max, and when it is it produces the same readings I had in that video. I forgot to set it before the test. I'm getting some 12.6 volts at the electrical connector, but could it be the radio isn't receiving enough power?

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The spring isn't necessary or really wanted on any antenna.  The 102" whip is more than flexible enough to not get damaged especially where you have it mounted.  Rat shack may still carry dummy loads.  Most any CB shop will have them I'm sure.  12.6Vdc is a bit low but no enough to make that much difference in power out.  You might think about running the power right to the battery as a precaution.  I've always run fused lines right to the battery for my CB's or HF/VHF/UHF radios. Or set up a terminal strip under the dash that is wired straight to the battery to power stuff I want hot all the time.

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I have to wait a couple days before I can try anything, but could it be possible something inside the radio blew? The fact that it does transmit and receive tells me no, but I'm no expert with radio systems outside of aviation. I cracked it open a few weeks back to see if anything seemed out of place, but all the resistors I checked, connections, etc were all in great shape. There wasn't even any dust inside of it. 

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Ok.....So don't shoot me for asking the silly question.... But it doesn't look like you actually checked the SWR..... I say that because I have the same EXACT radio, and the left switch is set to transmit vs. SWR mode. You need to actually go to cal, calibrate the SWR, then get a reading. That should make it VERY apparent if you have an SWR issue, or a transmitter issue (inside radio).

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Actually, according to the manual (I double checked my initial assumptions), if the switch is in S/RF mode, it shows the broadcast power. This is different than the SWR, so while the low reading still shows we have an issue, you still need to flip over to cal, deadkey the radio, turn the knob until the needle is on the calibrate mark, and then go to the SWR setting and take a reading. 

 

A SWR reading of 0 means that no signal is generated. A 0 reading is basically impossible, as this would mean that 100% of your radio signal is being reflected away, which just doesnt happen. 

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Cobra has all the manuals online :) but could be 1 of 2 things. Either your internal SWR meter is junked, and you should get an external SWR meter to check (google is your friend), OR your radio's transmitting circuits are fried from either age or out of spec SWR (and didn't notice) and you've junked the radio. 

 

 

Those are the 2 causes I can think of for not being able to calibrate the SWR meter.

 

Besides that, just check all connection in the antenna setup, especially the bracket. Take apart the stud that attaches between the bracket and the antenna/coax cable. Maybe if the plastic grommets were out of place or rust was present inside there? That's just an off chance, but free to check :)

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Skel130... you have the SWR backwards.  Zero or as close as you can get it is what you want.  A high number means that more power is being reflected back.  SWR means Standing Wave Ratio. The ratio between power out and power reflected back.  The lower the better.  The transmit seems to be OK cuz it does show output on some channels.  The issue with the cal not working right could be the match between the radio and the antenna is so far off that the cal can not compensate for it. 

 

   The advice to check the mount for washer alignment and rust is good to check!!  Most CB today have protection circuits in them in case of high SWR.  They cut back the power output so it doesn't fry the finals.  If you can find a dummy load and check things like I had mentioned earlier that will tell you if the radio is working OK and help track down where the problem is.  An outboard SWR meter would be a good idea too as skel130 mentioned.  The built in SWR meters are notorious for not being too accurate.  If you have an volt ohm meter you can use that to get a rough idea if there is rust and it will tell if there is a short some where in the antenna circuit too. 

 

Here's a link to an SWR table.  As an example... with 4 watts output and 1 watt reflected back... that is equal to a 3 To 1 SWR.

 

http://radio.feld.cvut.cz/personal/matejka/download/VSWR%20table.pdf

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You misunderstood me, I do understand lowest SWR is best, but in CB radios, getting your SWR under 1.0 on a CB radio is basically impossible on a good day. When I said a 0 reading would mean 100% is being reflected away, I meant 'sent out' from the antenna. Looking back, I can see how that might read wrong though. Same page  :thumbsup:

 

I'm curious as to if it is posible to burn parts of the transmit circuit out, so you were getting no out put on low channels, but capable of it on high end channels? Not sure on that. I know more about radio theory than radio circuitry. 

 

Now I hadn't thought about the antenna being SO far out of tune as to be out of range for the SWR. Such as rather than being in tune for CB bandwidths (26.9-27.4) that the low end of the antenna was at something around 27.4, and the top end was 28 something. Though, that seems unlikely as that would be something like a 2 inch shift in antenna length... And should still show up on the SWR meter, even if only briefly. 

 

But I think the best option is going to be the external SWR meter/dummy load. 

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You misunderstood me, I do understand lowest SWR is best, but in CB radios, getting your SWR under 1.0 on a CB radio is basically impossible on a good day. When I said a 0 reading would mean 100% is being reflected away, I meant 'sent out' from the antenna. Looking back, I can see how that might read wrong though. Same page  :thumbsup:

 

I'm curious as to if it is posible to burn parts of the transmit circuit out, so you were getting no out put on low channels, but capable of it on high end channels? Not sure on that. I know more about radio theory than radio circuitry. 

 

Now I hadn't thought about the antenna being SO far out of tune as to be out of range for the SWR. Such as rather than being in tune for CB bandwidths (26.9-27.4) that the low end of the antenna was at something around 27.4, and the top end was 28 something. Though, that seems unlikely as that would be something like a 2 inch shift in antenna length... And should still show up on the SWR meter, even if only briefly. 

 

But I think the best option is going to be the external SWR meter/dummy load. 

Actually.......I'm thinking wrong. SWR under 1:1 is impossible, As 1:1 would be 100% output, zero loss. So if you were to have 0.9:1 as a theoretical SWR reading, you'd be getting more output than the radio generates. So under 1:1 shows there is an issue with the meter or output. 

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You misunderstood me, I do understand lowest SWR is best, but in CB radios, getting your SWR under 1.0 on a CB radio is basically impossible on a good day. When I said a 0 reading would mean 100% is being reflected away, I meant 'sent out' from the antenna. Looking back, I can see how that might read wrong though. Same page  :thumbsup:

 

I'm curious as to if it is posible to burn parts of the transmit circuit out, so you were getting no out put on low channels, but capable of it on high end channels? Not sure on that. I know more about radio theory than radio circuitry. 

 

Now I hadn't thought about the antenna being SO far out of tune as to be out of range for the SWR. Such as rather than being in tune for CB bandwidths (26.9-27.4) that the low end of the antenna was at something around 27.4, and the top end was 28 something. Though, that seems unlikely as that would be something like a 2 inch shift in antenna length... And should still show up on the SWR meter, even if only briefly. 

 

But I think the best option is going to be the external SWR meter/dummy load. 

Actually.......I'm thinking wrong. SWR under 1:1 is impossible, As 1:1 would be 100% output, zero loss. So if you were to have 0.9:1 as a theoretical SWR reading, you'd be getting more output than the radio generates. So under 1:1 shows there is an issue with the meter or output. 

The RF output section is common to all channels so with apparent output on the upper channels the finals are at least working to some extent. 

 

I can see how I misunderstood how were talking about SWR.  I was looking at it like we usually do in that you are looking at the antenna in front of you with the radio behind you and all the reflecting is being done by the antenna back to the radio.  Definitely on the same page!!

 

The spring in the install really shouldn't be there.  It adds considerable length to the antenna which is already cut to be resonant.  The 102" whip is quite flexible so the spring is really just overkill and additional places for rust and corrosion to grow.    If a person insists on running one with that antenna, the length of the antenna needs to be shortened by the length of the spring and then tune the antenna.

 

As far as an SWR below 1:1... I would definitely say a problem of some sort but....it could be that you have created a possible first step on a perpetual motion machine...  that is getting something (power greater than the radio output) for nothing.  And in the process become a billionaire!!  And quite famous too!!    Sorry.... just an attempt at a little humor!!

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As far as an SWR below 1:1... I would definitely say a problem of some sort but....it could be that you have created a possible first step on a perpetual motion machine...  that is getting something (power greater than the radio output) for nothing.  And in the process become a billionaire!!  And quite famous too!!    Sorry.... just an attempt at a little humor!!

 

"Billionaire"? Dayum - you'd be like the man who invented everything ever invented, from the taming of fire and the wheel to everything else ever conceived... combined!   :D Such a thing would change everything we thought we knew about the universe.

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As far as an SWR below 1:1... I would definitely say a problem of some sort but....it could be that you have created a possible first step on a perpetual motion machine...  that is getting something (power greater than the radio output) for nothing.  And in the process become a billionaire!!  And quite famous too!!    Sorry.... just an attempt at a little humor!!

 

"Billionaire"? Dayum - you'd be like the man who invented everything ever invented, from the taming of fire and the wheel to everything else ever conceived... combined!   :D Such a thing would change everything we thought we knew about the universe.

You'd have all the monies. 

 

 

But I agree with the spring statement earlier. Take that out! Could actually be the root of the problem, especially if you're getting output only on high channels. Longer whip = higher frequency, meaning the antenna would be too long for the low end channels. 

 

And thank for the info on the transmit circuits, just a bit more I can file in my brain under "I doubt I'll ever actually need to know this" aka, 90% of my brain.

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Ummmmmm... longer antenna means longer wavelength which means lower frequency, not higher:

 

file_51526.jpg

 

Makes me think the spring is not the problem, unless it is an issue of the quality of the connection. The added length contributed by the spring would be a bonus for the lower channels, not the upper, but that is the opposite of what he is reporting.

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  • 2 weeks later...

I've got a power supply in the cab direct from the battery now, plugged the CB up and vastly improved the ground. Turned it on and transmitted, no transmit on channels lower than 39, and very weak above. I did buy a new coax cable, and an external swr meter. I'll get another video up with my tests, and maybe you guys can tell me for sure my radio is somehow fried.

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No video and only one picture, but I'll explain whats going on.

 

Weather drastically affects a cb, apparently. Bright sunshine means high SWR, clouds mean normal and rain means no transmit at all. This is my experience, if it seems weird, tell both my SWR meters. 

 

It's overcast today, went out and tested on channel 19 (my most used channel). Calibrated both SWR meters, one in my radio and the other external. I'm using 2-20ft coax cables. I transmit, and get a reading on both meters of about 1.5. Ideal, I'd imagine. I ask for a radio check, but this would be the one day noone is around. I check channel 40 and I get about the same reading, and channel 1 and I get nothing at all. I unplug the meter and plug the antenna back up to the radio, key the mic on 19 and get nothing at all on the radio swr meter. Now, I'd imagine the 40 feet of coax has something to do with that, but how do I fix it once I eliminate that extra coax? Does the antenna need to be raised or lowered? I have the radio powered and grounded directly to the battery, so that eliminates a power problem. Ideas?

 

63NzJpD.jpg

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