This all makes sense.? ?So what I do not understand is how with a 5w transmitter - which (if my math is right) is 37dbm, a 40db attenuator gets you to the minimum -25dbm safety threshold for the Ultra.? It would seem to me that it is -10dbm and therefore (risking) causing damage, no?

Joe

On Thu, Jan 23, 2025 at 3:53?PM W0LEV via <davearea51a=[email protected]> wrote:

OK, I'll try:

Your confusion may come from not understanding the dB and logarithms related to actual power.

1)? The advantage in reasoning/thinking in dB space is that the dB, like logarithms, add and subtract directly.?

2)? So converting to dB in power, usually dBm (dBmilliwatts or 1E-3 watts), allows one to directly deal with powers in adding and subtracting attenuation directly.?

3)? Also realize that every 10 dB, be it power or attenuation, is a power of 10 greater or lesser.? So if you start with 1-watt and insert a 10 dB attenuator, you end up with 100 mW.? If you start out with 10-watts and insert a 20 dB attenuator, you end up with 100 milliwatts.? Remember this as it makes things easier without converting to dBm or dBw (dBwatts).

4)? How to convert from RF power to dBm, again, remember dBm is in dBmilliwatts or 1E-3 watts which is 0.001 watts.

5)? Convert power to dBW: ? dBW = 10 x log10 [power in watts].? That "10" after the log indicated log[base 10].? The Brits. use the Neper which is log[base e], the "natural logarithm".We're concerned only with log[base 10]. ?

Example:?? Assume you have 10-watts.? So:? dBW = 10 x log [10-watts] = 10 x [1] = +10 dBW

6)? Since we now know dBm is 0.001-watts or 30 dB BELOW 1-watt, we must add that 30 dB to our +10 dBW to express the power in dBm (remember, there are "more" dBm's in a watt than a single watt or dBw).? So, our 10 dBW beomces +40 dBm.?

Now you can add and/or subtract directly the value of any attenuator you install to get power in dBm.??

Dave - W?LEV? ? ? ?

Virus-free.

On Thu, Jan 23, 2025 at 8:34?PM Joe Tomasone via <joe=[email protected]> wrote:

I've been reading the responses to my own thread on here and watching several YouTube videos and still feel like I do not have a good handle on the attenuation required.??

In the videos, I have seen multiple hams use a 40db 10w attenuator with no internal attenuation to test?a 5w HT with apparently no ill effects.? ?I've also seen some use more attenuation for lower power outputs.? ?To my understanding from reading what has been written here and these videos:

1. I would need around 60-80db of attenuation to safely input a 1w signal into the Ultra.?

2. I should not use the internal attenuator - or, alternatively, I should.?

3. I should use a variable attenuator in conjunction with the 40db attenuator, or I should just use 2 40db attenuators, or I should use the 40db attenuator plus the internal on "auto".?

4. I should use a CleanRF.com RF-S2K & dummy load with a 40W attenuator or by itself (but the CleanRF is only specced?for 3-30Mhz, so I don't understand how it would work anyway...)


I could really use some authoritative answers on how to safely test VHF/UHF/HF rigs without letting the smoke out of the Ultra.? ?I have the 40db attenuator (10w, covers the spectrum) already. I'm not sure what else I need to obtain.

Thanks,?

Joe AB2M

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Dave - W?LEV