Power Output Drop and Calibration Issues on Flex 8400M

Riccardo, are you testing on a dummy load or live antenna (regardless of actual SWR readings). The support team will want to know as you don't mention what your test setup is.

But, it does sound like your radio may need to come home for a spa day.

Actual RF power out can be affected in many ways, now just the finals. It can be related to components in the LPF or the ATU.

You still need to get on a dummy load.

Before Submitting a Power Output Concern

To avoid unnecessary bench fees, please complete the following tests before sending your radio in for service. If we cannot reproduce the issue, a bench charge may apply.

Test Setup Requirements

• Set TUNE to 100%
• Connect to a quality dummy load rated for your power level using a good quality patch cable. New 100W dummy loads are available from DX Engineering for around $50.
• Use a quality wattmeter that covers all HF bands
• Set the ATU to OFF or BYPASS
• DC supply voltage must be 13VDC or higher while transmitting in TUNE mode. You can verify this in real time using FRStack (mkcmsoftware.com/Flex/Index). Supply voltage during transmit directly affects RF power output. The radio is designed to operate up to 15VDC (13.8VDC +/- 15%).

Measurement Procedure

Sweep through all bands from 160m to 6m using the TUNE function and record your power readings for each band. If using a smaller dummy load, monitor it for heat and allow adequate cool-down time between bands.

What to Expect

A variance of +/- 10% from the rated 100W output is within normal operating tolerance and is not considered a defect.

Opening a Support Ticket

If you still believe there is an issue after completing these steps, open a support ticket and include all of your recorded measurements along with your test setup details. This will help us assist you as efficiently as possible.

Why the Difference Between 90W and 100W Is Insignificant (sorry for the rant) :)

The relationship between transmit power and perceived signal strength on the other end is logarithmic, not linear. This is measured in decibels (dB), and the math tells a very clear story.

Going from 100W down to 90W is a loss of just 0.46 dB.

To put that in perspective:

• The human ear (and the S-meter on the other station's radio) cannot detect a change of less than 3 dB under any practical operating condition
• 3 dB requires halving your power, dropping from 100W to 50W
• 6 dB requires dropping to 25W before most operators would describe it as a noticeable change in signal
• Even a full S-unit (6 dB) is considered only a marginal difference in actual copy on a real HF band

So a 10W difference at the 100W level is, in practical terms, completely inaudible. The noise floor of a typical HF band, QSB fading, and atmospheric variation will all mask that difference many times over.

QSB Fading Dwarfs Any Power Difference

Here is the part that puts it all in context. On a typical HF path, QSB (selective fading) routinely causes signal level swings of 10 to 20 dB or more over just a few seconds. That is the equivalent of your signal fluctuating between 100W and 1W repeatedly, completely beyond your control. Against that backdrop, a 0.46 dB difference from a 10W power variance is not just small and it is entirely invisible. The band is doing far more to your signal than your power output ever could.

A helpful way to think about it:

You would need to reduce power from 100W to about 79W just to lose a single dB. Nobody is going to hear that either.

The takeaway is simple: if your radio is putting out 90W instead of 100W, your signal on the other end is identical for all practical purposes. Focus your energy on antenna efficiency, feedline loss, and operating technique — those factors move the needle far more than a 10% power variance ever will.