Is it possible to kill your transmitter with high SWR and a lossy RG58 cable?

[zikada]

I have a Kenwood TK880H that seems to not want to transmit on high or low power for more than a second before it dropped to 0 watts. I was using a RG58 cable for testing and a copper antenna. Could it be possible that I killed my radio in the process? I didn't test SWR but someone told me that it could have been really high and put RF back into the radio.

 

[mmckenna]

I have a Kenwood TK880H that seems to not want to transmit on high or low power for more than a second before it dropped to 0 watts. I was using a RG58 cable for testing and a copper antenna. Could it be possible that I killed my radio in the process? I didn't test SWR but someone told me that it could have been really high and put RF back into the radio.

Sure, it's possible.
High SWR can reflect enough power back into the radio to create enough heat to cook the final RF transistor.
It isn't usually an immediate failure, it takes some time for that heat to build up and do damage.

Likely what is happening is that the radio is sensing the high reflected power and throttling back the power to prevent damage. I think you might find that if you hooked up the radio to a known good antenna or a 50 ohm load that the RF output would go back to normal.

 

[zikada]

Sure, it's possible.
High SWR can reflect enough power back into the radio to create enough heat to cook the final RF transistor.
It isn't usually an immediate failure, it takes some time for that heat to build up and do damage.

Likely what is happening is that the radio is sensing the high reflected power and throttling back the power to prevent damage. I think you might find that if you hooked up the radio to a known good antenna or a 50 ohm load that the RF output would go back to normal.

Tried it a tram mobile ground plane antenna with rg8x and a diamond huslter with lmr400 same results. I'm going to have my friend bench it and see if he can diagnose the problem.

Its weird because if the radio turned on after not being used for a while, it will output the proper power limit for about 2 or so seconds before dropping. Once the radio warms up after that initial tx, no power output at all.

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[gatekeep]

One of 2 problems with that radio possible: either high SWR has caused an intermittent fault in the final PA IC or supporting circuitry, or there is an issue with the 8v rail regulator.

 

[zikada]

One of 2 problems with that radio possible: either high SWR has caused an intermittent fault in the final PA IC or supporting circuitry, or there is an issue with the 8v rail regulator.

Yea, you did say that to me before. You still want to take a look at the radio? I can give it to you next weekend.

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[gatekeep]

Well I almost guarantee at this point the final PA is the likely culprit. Probably a blown amplification stage, the PA is easy to replace but the part is hard to get. On the other hand I suspect the 8v rail, because VCO uses that voltage during excitation, and if the 8v rail regulator is having issues that might cause the VCO to unlock or otherwise be unable to modulate a carrier.

In either case the part(s) for the specific fault would need to be replaced. Since if I recall based on our previous communication you already replaced the radio, its almost easier to just use it as a receiver.

 

[K7MEM]

I have a Kenwood TK880H that seems to not want to transmit on high or low power for more than a second before it dropped to 0 watts. I was using a RG58 cable for testing and a copper antenna. Could it be possible that I killed my radio in the process?

There are several possibilities. "gatekeep" touched on a few. Many new radios have a foldback capability that reduces the output power, if the SWR is above a certain ratio (2:1 or 3:1). But the foldback should be very quick and not allow transmitting at full power for a couple of seconds. It's more likely something other than high SWR is the problem. While the Power Module could certainly be the problem, there are other possibilities.

I looked at the manual for the TK880H and it doesn't mention having foldback capabilities. Reading the circuit description, and looking at the schematic, I see it does sample the output power for stabilization and does have a thermal sensor. Its possible that the thermal sensor has failed and is shutting things down or, the Automatic Power Control (APC) circuit is getting incorrect information from the power sensors, and shutting things down.

IMHO, your best bet is to put it in a box and send it back to Kenwood for repair. If it was me, I would open it up and poke around a bit. But I don't know what kind of abilities you have.

I didn't test SWR but someone told me that it could have been really high and put RF back into the radio.

Who ever told you that needs to educate himself on SWR and what it means. Here is a link to a very good group of articles on SWR and Reflections. http://www.k6mhe.com/files/Reflect_rev1.pdf

A mismatch between the antenna and the feedline can cause some of your power to be reflected back towards the radio. It travels back down the feedline until it reaches the antenna connector on the radio. But it doesn't flow back into the transmitter and cause excess dissipation and other damage. The reflected power is then reflected back up the coax and to the antenna. The antenna will radiate some of that reflected power and the mismatch, at the antenna, will then reflect some of it back down the coax.

In the end all of the power that you send to the antenna will be radiated, except for the power lost in the coax. The reflected power is also subject to coax losses. There are on-line calculators available that can show you how much power is actually reaching the antenna, based on SWR, coax length, coax type, etc.

You originally said that you are using RG-58. That really isn't a good coax to use at the 440 MHz. RG-58 has about 12 db of loss at 400 MHz. A better selection might be some RG-6 (better) or RG-11 (best). Either of them will have less than 6 db of loss. You can often get these coaxes free from the satalite and cable installers. Yes, both are 75 Ohm cables, and may cause a little bit of a mismatch (SWR). But power loss due to mismatch is very small. Much smaller than the 6 db you gain from using a cable with lower loss. Try the on-line power loss calculators with RG-58 and RG-6/RG-11, and see the difference.

Of course, there is better coax, but I am just sugesting a, possibly, cheap upgrade.

Martin - K7MEM

 

[zikada]

There are several possibilities. "gatekeep" touched on a few. Many new radios have a foldback capability that reduces the output power, if the SWR is above a certain ratio (2:1 or 3:1). But the foldback should be very quick and not allow transmitting at full power for a couple of seconds. It's more likely something other than high SWR is the problem. While the Power Module could certainly be the problem, there are other possibilities.

I looked at the manual for the TK880H and it doesn't mention having foldback capabilities. Reading the circuit description, and looking at the schematic, I see it does sample the output power for stabilization and does have a thermal sensor. Its possible that the thermal sensor has failed and is shutting things down or, the Automatic Power Control (APC) circuit is getting incorrect information from the power sensors, and shutting things down.

IMHO, your best bet is to put it in a box and send it back to Kenwood for repair. If it was me, I would open it up and poke around a bit. But I don't know what kind of abilities you have.



Who ever told you that needs to educate himself on SWR and what it means. Here is a link to a very good group of articles on SWR and Reflections. http://www.k6mhe.com/files/Reflect_rev1.pdf

A mismatch between the antenna and the feedline can cause some of your power to be reflected back towards the radio. It travels back down the feedline until it reaches the antenna connector on the radio. But it doesn't flow back into the transmitter and cause excess dissipation and other damage. The reflected power is then reflected back up the coax and to the antenna. The antenna will radiate some of that reflected power and the mismatch, at the antenna, will then reflect some of it back down the coax.

In the end all of the power that you send to the antenna will be radiated, except for the power lost in the coax. The reflected power is also subject to coax losses. There are on-line calculators available that can show you how much power is actually reaching the antenna, based on SWR, coax length, coax type, etc.

You originally said that you are using RG-58. That really isn't a good coax to use at the 440 MHz. RG-58 has about 12 db of loss at 400 MHz. A better selection might be some RG-6 (better) or RG-11 (best). Either of them will have less than 6 db of loss. You can often get these coaxes free from the satalite and cable installers. Yes, both are 75 Ohm cables, and may cause a little bit of a mismatch (SWR). But power loss due to mismatch is very small. Much smaller than the 6 db you gain from using a cable with lower loss. Try the on-line power loss calculators with RG-58 and RG-6/RG-11, and see the difference.

Of course, there is better coax, but I am just sugesting a, possibly, cheap upgrade.

Martin - K7MEM

Thanks a lot for the very detailed answer, Martin.

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[gatekeep]

Another article on SWR: The Real SWR Page - Explained by WC7I - Will high SWR burn up my radio?

@K7MEM - Thumbs up. -- having looked at the radio quickly prior, there is definitely a problem with the radio. It was also mentioned, that at some point there was a possibility of the center conductor shorting to the shield on the coax.

Currently, the radio puts out full power for a few seconds, drops to nil and stays that way till power cycle. Interestingly, I didn't think of the thermal protection. IIRC, it was mentioned through a third-party that this particular radio gets rather warm during transmit (when it does transmit), so its possible as you mentioned the thermal sensing is screwed up.

Does Kenwood still depot these radios given the age?

 

[wa1nic]

Before sending it back or tearing it apart, locating and checking it on a known good 50 ohm dummy load might be worth doing.

 

[mmckenna]

Another article on SWR: The Real SWR Page - Explained by WC7I - Will high SWR burn up my radio?

@K7MEM - Thumbs up. -- having looked at the radio quickly prior, there is definitely a problem with the radio. It was also mentioned, that at some point there was a possibility of the center conductor shorting to the shield on the coax.

Currently, the radio puts out full power for a few seconds, drops to nil and stays that way till power cycle. Interestingly, I didn't think of the thermal protection. IIRC, it was mentioned through a third-party that this particular radio gets rather warm during transmit (when it does transmit), so its possible as you mentioned the thermal sensing is screwed up.

Does Kenwood still depot these radios given the age?

Thanks for posting that. I read through it and it reminded me of when my grandfather explained it to me pretty much the same way. Over the years I've simplified it and tend to keep using the incorrect explanation/terms/etc. Good to have a reminder.