Question about Cobra 19 DX 40-Channels Base CB Radio

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Swr damage What transistors are typically damaged with SWR mismatching

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The transistors you are referring to are the rf power output transistors commonly called "finals". swr is the amount of your outgoing radio signal that is reflected back into your antenna ,through your coax(antenna Wire)and backs up in the radio circuitry. this backup will cause excessive heat to build up in the "Finals" therefore damaging the finals. this damage is serious but not major, just about any decent cb shop can resolve this problem. In a healthy antenna system the less swr you have the more efficent your cb radio is, this translates into more effective radiated signal therefore increasing the effective range of your radio. This information courtesy of Outlaw's CB Shop,Clinton SC.

Posted on Aug 15, 2007

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Posted on Jan 02, 2017

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How do you calibrate the SWR meter in a 706 mkiig? The service manual shows how but does not expand on what dummy loads to use.


A standing wave ratio meter is rarely required to be accurate and isn't usually used with a dummy load.

Mostly if the correct antenna is used the SWR meter is placed in series with it so the antenna can be adjusted or tuned to produce the lowest reading, which is the smallest amount of energy being reflected back into the transmitter or in other words the combination of transmitter and antenna are working as efficiently as it can be.

Clearly the readings are merely relative but a degree of accuracy is needed when an inefficient or mismatched antenna is used and the SWR cannot be lowered to a safe limit. When the SWR is too high the transmitter can be damaged and so it is a good idea to know roughly the reading where the transmitter stops being safe.

A dummy load would typically be used for testing and setting up a transmitter without radiating a signal. Nonsense and spurious signals are considered antisocial and even ******* so tweaking the power, adjusting the modulation and checking for stray harmonics would be carried out using a dummy load.

Apr 01, 2017 | Radio Communications

2 Answers

SWR displays briefly then back 1.1


Your problem is not very well described, but let me guess: while you transmit, SWR peaks shortly to a high value and then drops back sharply.
There are two possible reasons:
1) Strong radio transmission pulse somewhere very close to the antenna;
2) Bad contact between connector and coaxial cable somewhere between SWR meter and antenna. Points to check:
- Coaxial connector on the cable from SWR. Is it soldered properly?
- The coaxial cable: can you see mechanical damage, stress? If you find scratches, broken isolation, sharp bends, etc, try to replace the cable.
- Connection between coaxial cable and antenna (or connector): is it soldered properly, is it not corroded or mechanically damaged?
- Connections inside the antenna (if there are any such joints): look for corrosion and mechanical damage, also loose screws.

If you find multiple places of potential problem, go step by step from antenna towards the SWR meter: if the most distant problematic location is fixed and it still shows this strange behavior, go closer.

Rarely this could be a problem in the SWR meter itself. That can be identified if you replace antenna by a dummy load. Dummy load should never cause bad SWR "flashes" (unless it is mechanically broken, too).

Mar 30, 2014 | Cobra 29 LX

1 Answer

My swr shoots all the way up on my base station


Generally, high SWR relates to an antenna problem. It could be as simple as the coax connector isn't tight enough. You could have damage to the coax cable, or the antenna may be damaged due to wind, or water in it. If you have access to an antenna analyzer, it will help you troubleshoot your antenna and coax, if not, start with a thorough visual inspection, looking for damage, cracking, or water infiltration.

Dec 27, 2013 | Ranger RCI-2995DX Base Scanner

1 Answer

My radio is not broadcasting reports.


Maybe its not transmitting because a Final Transistor has blown. Check to see if it receives well, change antennas,microphone...get a meter to measure SWR/POWER out. If it don't transmit or receive it may be seriously damaged. A final only cost several dollars if you know how to solder one in place. Its always like 2SC2078(Radio Shack carries them) it has 3 prongs to be soldered on the board with a heatsink and screwed to back with a ceramic insulator and heat glue. Swr over 2.1 will cause transmitter damage. Bad antennas,lightning may cause these problems. Get an antenna factory tuned at near 1.5 for 27mhz.Wilson 1000's,K40,Antron A99(18 ft.base) are good antennas.Look at www.copperelectronics.com for radio supplies,antennas and new radios.

Apr 20, 2011 | Radio Communications

2 Answers

Everything works but can only send and receive within 50 yards of other radio


check swr's and the antenna installation. if all is good. have the radio checked out. ITS should be FREE!!!! to see if it has any rf power out put could have a bad rf final transistor.....

Feb 04, 2011 | Cobra 29 WX NW ST 40-Channels Base CB...

1 Answer

C.b. is transmitting weak.


If it is low audio, it could be a problem with your mic or his speaker. If your signal is low, I suspect it is likely to be an issue with your antenna system.

The antenna systen consists of the feedline or coax cable and the antenna. The feedline should be inspected for evidence of damage from being pinched, cut or water entry over its entire length., Remove, replace or reposition to prevent the cable from becoming damaged in any way.

The antenna itself should ideally be located at the highest point on the vehicle. Further, it should be secured to firmly into the metal parts of vehicle - preferable to a large, flat surface. The surface should be grounded.

Matching the antenna system for lowest SWR to the transmitter is required. SWR is a ratio of "power out" of the transmitter to "power that is reflected back" from the antenna. Ideal theoretical ratio or "match" is 1:1 (or one to one). Real-life is more like 1.1:1 (one point one to one). Most transmitters are perfectly happy with a match (or mismatch as the case may be) of 1.5:1 and some even up to 2.0:1. More than this is asking for trouble. A mismatched antenna system can cause RFI and failure of the output final transistors.

Matching or lowering the SWR of a radio to an antenna system requires that an SWR meter be installed between the antenna coax or cable and the transmitter's antenna connection point. There are two types of meters - dual needle and single needle. The dual type is more expensive, but easier to work. The meter must be capable of working on the frequency it is to used on. This means if it is marked as a 400 - 512 Mhz or UHF type, it will probably not be accurate on 27 Mhz or HF.

Once the meter has been installed, transmit the highest frequency or channel and take readings. Do the same at the lowest and a middle frequencies and record findings. Physically adjust the length of the antenna whip up or down in the antenna base - by no more than 1/4" inch (for 27MHz CB frequencies) and check again. After a few tries in one direction, you should begin to see how shortening or lengthening the whip changes the SWR of the system. Once set correctly, the middle frequency will have the lowest SWR value while the lowest and highest frequencies will have pretty close to equal, but higher SWR values. It would look something like this: Ch1 = 1.6:1 Ch 20 = 1.2:1 Ch40= 1.6:1 Of course, if Ch1 = 1.7:1 and Ch 40 = 1.6:1 that would be fine, too. Once completed, the SWR meter can be removed from the antenna system.

A poorly matched antenna system can cause permanent damage to the transmitter, if not corrected.


Sep 21, 2009 | Cobra Radio Communications

1 Answer

When I key the mic the radio blinks out and then comes back on


This can be due to one of several problems. Insufficient sized wires to the power source or a high SWR (standing wave ratio) between transmitter and antenna. The first is probably not a problem unless connected to an RF amplifier to boost output wattage. A high SWR however, is a more likely culprit. SWR is a ratio of power out of the transmitter to power that is reflected back from the antenna. Ideal theoretical ratio or "match" is 1:1. Real life is more like 1.1:1 (one point one to one). Most transmitters are perfectly happy with a match (or mismatch as the case may be) of 1.5:1 or even 2:1. Beyond this is asking for trouble. A mismatched antenna system can cause the failure of the output final transistors.

Matching or lowering the SWR of a radio to an antenna system requires that an SWR meter be installed between the antenna coax or cable and the transmitter's antenna connection point. There are two types of meters - dual needle and single needle. The dual type is more expensive, but easier to work. The meter must be capable of working on the frequency it is to used on. This means if it is marked as a 400 - 512 Mhz or UHF type, it will not be accurate on 27 Mhz or HF.

Once the meter has been installed, transmit at half power (or at least the amount needed by the meter) on the highest frequency and take readings. Do the same at the lowest and a middle frequency and record findings. Physically adjust the length of the antenna whip - by no more than 1/4" inch (for 27MHz CB frequencies) and check again. After a few tries, you should begin to see how shortening or lengthening the whip changes the SWR of the system. Once set correctly, the middle frequency will have the lowest SWR value while the lowest and highest frequencies will have equal higher SWR values. It would go something like this: Ch1 = 1.6:1 Ch 20 = 1.2:1 Ch40= 1.6:1 Of course, if Ch1 = 1.7:1 and Ch 40 = 1.6:1 that would be fine, too.

Double check everything once more at full power, so that you know the transmitter won't be damaged under full power conditions. Make any last adjustments and lock everything in place.

Aug 21, 2009 | Radio Communications

2 Answers

High swr ch. 1- 5 and ch. 40 - 5


This is a common problem in CB's. You can either get the radio tuned properly or tune the antenna. Sometimes the CB will put out more power on a channel than is needed and this could end up in a damaged radio. You can get the radio tuned to prevent the RADIO from high SWR, or you can tune the ANTENNA so tha antenna will better suit your CB's power.

Hope this helps!

Jun 06, 2008 | Cobra 29 LTD CB Radio

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