Flex Rolls Back Power At the Edge of the Passband

W4RXK

I am a new Flex owner, less than a week. Tonight on 20m, my output was very low and fluctuated wildly 5 to 20, 20 to 0, 0 to 10, etc. I took the covers off expecting to see something smoked. Nothing. Changed power supply. Nothing, Changed coax. Nothing. Changed power meter. Nothing. Changed antenna A > B. Nothing.

My thought was - blown finals. Despite excellent SWR, grounding, etc.

After an hour of searching I found a thread that mentions the below.

* Power output can be erratic if you try to transmit on the edge of the TX passband. The transmitter will try to reduce the power out.

^ this was the culprit. My TX was at the very edge of the band.

On 20m FTP, I made a contact w/Kuwait (from the US) and was not holding the TX, meaning, I responded on his frequency. His frequency was 14.076.970 - right at the edge of the 3000K on the WSJT-X wide graph.

Can someone explain why this happens?

John KB4DU

Not the expert at this situation. The flex filters (of any type) have a really sharp cutoff at the edge of the band pass. Sometimes referred to as “brick wall” filters. This is possible because the filters are defined mathematically by the software. An oversimplified example is: if frequency in > 3000, frequency out =zero. A traditional hardware filter has a bell shaped cutoff curve that gradually rolls off the filter response, not the sharp cutoff curve that software has.

Erika - KØDD

Just bump the tx bandwith to 3100 hz and youll have full power up there. K0DD

David Decoons, wo2x

instead of bumping bandwidth, go into WSJT-X setup and in radio tab on right choose Fake It. This shifts TX frequency to keep your TX audio between 1500 and 2000 Hz.

73 Dave wo2x

John KB4DU

Just my opinion, Erica’s suggestion is the most appropriate for this situation.

W9WLX_Chuck

I usually use "Fake It"—it keeps the audio in the sweet spot. The only exception is 60m due to the 2.8 kc limit. On 60m I turn "Fake It" off and my TX bandwidth gets set to 100/2900, otherwise the out-of-band safeguard will disable TX.

73, Chuck

Geoff AB6BT

I never use "Fake It". I just make sure I set my TX frequency within the passband I have set.

Erika - KØDD

It really is best to not use fake it unless you really need it. If you have an automated station as I do things can get really exciting when using fake it. It seems more often than not, where the FT8 windows are located are not the best for default windows on some tunable accessories. My ALPHA 9500 will jump between segment 1 and segment 2 when moving from receive to transmit very unnerving! Especially if using serious power for a rare one. And the SteppIR also will start a retune under some instances, 6meters is one band for sure… Fortunately the DB-36 is still in a preinstall testing phase here. But I can see it start flashing. So If I'm NOT in fox hound I'll leave Fake it off. When in phone or CW in split you could select using the B vfo. but again you're stopping and changing a setting mid stream. I stand by my comment ONE TIME in 1000 qsos where you'd need to give the bandwidth a slight bump to open her up… once in the log, you can move it right back.

Erika - KØDD

Say for Chuck W9WLX…. If you are on 60m I agree for most instances what you said. But more and more the DXpeditions are going F/H on 60m… If you set your TX bandpass LOW to 1500 and TX bandpass high to 2000 and select fake it. Leave your receive bandpass open the full 0 to 3K you will be able to transmit everywhere in the 3K window and the F/H using fake it will also work. This is a really cool work around for 60m and also works everywhere else also on FT8 but I would leave it for 60m. Just make sure before you try to operate fone you reset your bandpass or you will get a really bad audio report from da DX… 500 hz wide fone audio in digu sounds like ****. Erika DD

David Decoons, wo2x

good point about the SteppIR. My Hawaii remote is using the MonstIR but I will usually check Hold TX Frequency and set TX around 1200 Hz. If running TX low, I run between 500 and 600 Hz depending on band conditions. This way for stations who call on my TX frequency and using radios whose TX bandpass is 2.4k (300-2700) they will not be at edge of their TX passband.

For 99% of the users, Fake It eliminates the complaint of power dropping off on some calls.

73 Dave wo2x