Panadapter display of xmited cw signal

WX6P

Hi, can you tell me what determines the size of the displayed cw signal that I am transmitting from a slice in panadapter? I am interested on why in 160 the displayed cw signal of 500Hz is a barely visible red line tha grows as you go higher through the bands to the point that in 10 or 15M is displayed with a thick red line and a wide yellow surround? I am sending you some screen grabs to illustrate my conundrum. If it was based on propagation/dBm it would change at night but it doesn't. It cannot be based on SWR because mine is low across the bands. So what is it?

Mike-VA3MW

Simply put, the width of your CW signal is not getting wider.

You didn't post your preamp settings, but as you go up in frequency the radio is actually able to visualize more RF.

I would have to get an engineering to explain it in detail and that would take time. Let me ask.

Steve-N5AC

When the FLEX-6000 series radios was designed we had several options for what to do during transmit. With the prior series of radios including the FLEX-5000, we displayed the generated frequency-domain representation of the transmit signal. In other words, the is the ideal signal provided to the PA, and not what is coming out of the PA. With prototypes of the FLEX-6000, we discovered coupling from the PA to the receiver during transmit that made the actual on-air signal visible in the panadapter. As we worked with the prototypes, we felt that this was a more representative display to show the customer rather than an idealized signal.

The coupling from the PA to the receiver varies with frequency: it is fairly low at the low end of the spectrum and increases with frequency. Every direct sampling receiver has multiple "overload points." There is the point at which the RF input to the ADC begins to clip. This is what we call "soft overload" because it begins as very infrequent overloads for a few nanoseconds. This has almost no material impact on the received signal in the narrow-band case. As the signal level grows, a more significant overload occurs which causes a limit in the ADC to be reached for a longer period of time. As the time increases, the impacts of this become visible in a broad frequency-domain spectrum like the panadapter. There are also situations where the digital part of a radio is not equipped to handle the soft overload case. In a FlexRadio, we provide sufficient bandwidth to never go into a hard digital overload where there is no way to represent the signal in the digital domain. Finally, with significant enough signal, the ADC could be on the brink of damage and the input is removed from the ADC to prevent damage. This is a hard overload and all signals disappear since the antenna is no longer connected to the ADC.

I bring the above up, because as you noticed, as you cycle through the bands the signal level increases in the receiver. For the top couple of bands, the radio may be in soft-overload from the coupling of the PA signal. There is no harm in this, but it can also produce artifacts in the panadapter that are not actually transmitted. In the end, we felt that the benefits of providing the display during transmit outweighed the drawbacks so we provided it.

TLDR: the coupling from the PA to the receiver while in transmit increases with frequency and the signal level in the panadapter only reflects this phenomena rather than any other physical reality.

WX6P

I see. I wish there was a way to create a combination between the overload points and the idealised signal.

In practical terms this means it’s almost impossible to visualise your transmitted cw signal on the panadapter when calling in a pile up on 160M.

In the higher bands, it also leads to a false visual impression that the cw signal being put out is actually stronger than it actually is.

Hmm…

Steve-N5AC

The overload discussion is simply to say that in the top couple of bands, you may see some "grass" in the panadapter that is not actually transmitted.

If this (accurate signal strength in the panadapter) is a particular concern/desire, you could purchase an off-the-shelf coupler with 60dB or so of coupling and then feed the coupled signal back into the radio for viewing on the panadapter. You will need to put the radio in full duplex mode if you decide to do this.

WX6P

I see. I will explore the coupler option although full duplex mode and probably an extra delay (I suppose) in signal display caused by the coupler doesn’t sound ideal.