Technical question from a long time listener, first time caller. Can low power Tx cause damage, excess dissipation?

[MIMTBR]

Hi all.
I have often used these forums as a great source of information. It's a blessing to have so much experience and knowledge searchable and available in a matter of seconds. However, I happened upon a post that I just had to ask about.
I was reading through posts related to my current favorite 2/70 rig, the TM-V71A.
A member named "cmdrwill" posted:

...All of our Kenwood EOC radios are set at 50, 20, and 12 watts. 5 watts is too low for the final RF amp's spec used in the Kenwood V and D series. Too low of a power setting causes excess dissipation, AKA heat damage to the RF amp...

I've never heard of this. Too low of a power setting causing excess dissipation. This is very interesting and I'm hoping someone can explain this to me. I searched the web but couldn't find anything on the subject.
Is this a specific issue for these radios or can this be an issue with any transmitter?
Thanks in advance for any insight, and it's nice to finally be registered on the forum.

 

[nd5y]

I've never heard of that either. Sounds like BS.

I have heard that on some older commercial radios with adjustable power (with an internal pot, not programming software or front panel/menu settings) if you set the power too low the transmitter can become unstable and put out spurs.

 

[KevinC]

I've never heard of that either. Sounds like BS.

I have heard that on some older commercial radios with adjustable power (with an internal pot, not programming software or front panel/menu settings) if you set the power too low the transmitter can become unstable and put out spurs.

800 MHz GE MVP's would do that. They were rated for 15w but would burn up the PA at that power level so we turned them down to around 9-10w, any lower and they'd go spurious.

 

[MIMTBR]

Very interesting. He went on to say,

Out of spec power settings DOES affect transmitter spurs., or "spikes", and over dissipation, which can burn up the PA.

So can these spurs generate heat and potentially cause damage?
Specific to the V71, I'd like to think the engineers didn't overlook this when setting the low power to 5w.

 

[prcguy]

On some specific radios its not good to run power below a certain level but in general it should not be a problem. In some cases its not the RF components that fail but a sub regulator that will dissipate more power when voltage is turned down inside the transmitter. Transmitter instability is another topic and common to many FM land mobile radios where the PA might be running in class C and they like to run saturated. Turning down the power below a certain level can increase IMD and other vile transmitter products.

 

[nd5y]

So can these spurs generate heat and potentially cause damage?

I have never heard anybody mention what happens to current draw on the older radios when they put out spurs.

Specific to the V71, I'd like to think the engineers didn't overlook this when setting the low power to 5w.

Kenwood radios are well designed. Low power setting won't be a problem.
I always wondered why most ham VHF/UHF FM mobile radios have 5 W as the minimum setting. 1 W (like marine radios) or less would be useful in some cases.

 

[vagrant]

I ran my Kenwood D710’s at 5W 90% of the time for years. No issues related to that. Sold two and they’re still going strong with new owners.

 

[RFI-EMI-GUY]

On some specific radios its not good to run power below a certain level but in general it should not be a problem. In some cases its not the RF components that fail but a sub regulator that will dissipate more power when voltage is turned down inside the transmitter. Transmitter instability is another topic and common to many FM land mobile radios where the PA might be running in class C and they like to run saturated. Turning down the power below a certain level can increase IMD and other vile transmitter products.

Best explanation right there^^^^^ .

I have actually seen a Micor station used as a link and the TX turned way down and the power control pass regulator would repeatedly fail. The FCC certification for commercial radios stipulates the lowest power set point (generally stipulated in the service manual), or default power level. If you go too low "funny stuff" happens either on power control loop stability or increased spurs, IMD and noise. Ham radio equipment is not much different. You should stay within operating parameters or use an external power attenuator if you need lower. If you must do otherwise, monitor the supply current and the spectral purity. Remember all this can change if the temperature or voltage changes.

 

[BinaryMode]

So the analogy would be like a dam holding back more pressure?

What is IMD?

 

[AK9R]

IMD = Inter-Modulation Distortion

 

[MIMTBR]

Thanks to all for the input and information.
This is something I haven't heard of and is very interesting.

Hypothetically, would you see supply current rise as the wattage was decreased into the "danger zone"? Or would current still decrease but not at a corresponding rate?

Or is it along the lines of reactance that de-tunes the final amp? De-tuned amps draw more current and create heat as a result. As things become less stable as the power is decreased, spurs and IMD increase... is this de-tuning the final?

 

[prcguy]

Thanks to all for the input and information.
This is something I haven't heard of and is very interesting.

Hypothetically, would you see supply current rise as the wattage was decreased into the "danger zone"? Or would current still decrease but not at a corresponding rate?

Or is it along the lines of reactance that de-tunes the final amp? De-tuned amps draw more current and create heat as a result. As things become less stable as the power is decreased, spurs and IMD increase... is this de-tuning the final?

Generally when the power is turned down the current goes down proportionally. On a 100w HF radio it will go down to a point where PA transistors are biased on for linearity and the current can still be a couple of amps with no RF out due to the bias drawing current in the transistors plus whatever current is required for other stages of the radio. If you turn down the RF power and at some point the current goes up, that's not good and could indicate transmitter instability or an oscillation, etc. I don't think this is a problem with any modern HF radio and if a VHF/UHF radio is known to have problems below a certain power level its usually known or even mentioned in its manual, like minimum power is X and don't go below that.

Then there are Gunn and Impatt diodes used in microwave oscillators and amplifiers. Yes, a diode can be an amplifier in some cases and it works on a term called "negative resistance" where lowering the conducting voltage will reach a point where the device draws more current and becomes unstable and can oscillate. Or the instability range can be excited by an external low level RF signal and it will lock onto that and produce more power that you can extract making an amplifier. You won't find this in any amateur or even commercial radio but I've built dozens of Gunn oscillators and Impatt amplifiers.

Or maybe just ignore what I just posted and enjoy your radio and don't worry about turning the power down.