Strange Transmit Spectrum Display with Pixel Loop antenna on XVTR port

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  • Updated 4 years ago
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With a Flex 6300, with a Pixel Loop antenna on the XVTR port, the Panadapter transmit spectrum looks like this:



With the Pixel Loop on ANT2 port I get the normal display:



The Pixel Loop antenna is wired KEY to TX, which turns off power to the amplifier.

  Has anyone seen this?  Is it affecting my actual transmit signal?

73, Javier

KC2QII
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Javier, KC2QII

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Posted 4 years ago

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Stan - VA7NF

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The modulation appears to be 120Hz (Counted 8 in a 1Khz span).  My guess your strong signal is mixing with local noise inside the turned off pixel amp.  We know the very strong local signal; do you have power line noise present, or near any strong 60Hz signal?

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Javier, KC2QII

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Stan,

 No strong power noise or 60 Hz source near by.  Per the Pixel Loop manual, during PTT, the internal relay disconnects power to the amplifier and ground the RX feed connection to the receiver.

Javier
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Steve - N5AC, VP Engineering / CTO

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Official Response
This is not an indication of transmit purity. I'd have to go research the exact relay positions in this configuration to say what the cause is for sure, but the display you get in transmit is based on where the RX ends up. If it receives a lot of signal, you can end up in a mild overload condition which is what the top looks like to me. As a general rule, don't rely on the spectrum during transmit as a lot of factors, many external to the radio, can make this display show conditions more reflective of a receiver effect than a transmitter effect. You're better off to understand what is typical for your installation and investigate if you see large differences.
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KD4HSO

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I have the exact same issue with my Pixelsat loop hooked to the RX port on the 6500.  If you unplug the AC adapter from the loop supply box you'll see those levels drop.  The loop is powered from an AC (not DC) adapter, so it should have it's own internal rectifier.  My guess is the rectifier circuitry stays powered during transmit, so the RF is coupling around the disabled amplifier and mixed by the rectifier, thus the 120 Hz sidebands.  Set the TUNE power to 1 watt, and unplug the PTT connection from the loop supply so it stays powered.  The signal received by the Flex will be much cleaner.

All that said, this issue led me to re-working the ground system on my antenna yesterday, adding radials and Epsom salt to the rods.  I have a simple inverted L running up the side of my house, and coupled at the base with an SGC-235.  Monitoring my TX signal with a 8566B spectrum analyzer fed by a passive high-Q loop, I still have 120 Hz side bands at -40 dBc on 40 meters (less at other bands), with just 1 watt tune power.

Pretty sure I'm getting strong coupling into the AC wiring in my house (my garage door opener goes off-line if I key up full power on 40m).  It's not my DC power supply as I can run station of battery.  I hope it's just a near-field issue, and I'm not radiating that.  May try to find another Flex owner to monitor my signal on a narrow pan-adapter setting.  Or get a portable spectrum analyzer out a few 100 ft from the antenna.  I could also run the SSB audio from my KX3 into a laptop for FFT.

TX audio from 6500 monitored by KX3 sounds good, as does +/- 1 KHz around the tune tone with 100 Hz CW filter.  Of course -40 dBc is hard to discern from just listening.
(Edited)
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Ross - K9COX

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The actual voltage sent to the preamp is 20 VDC from the "power inserter". No AC is present at the antenna preamp.
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KD4HSO

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Yes, but the input to the power inserter is 24 VAC.  Since the amp inhibit input is active low, something still has to be powered up in power inserter.  I don't know if that relay is simply killing DC power to the remote section, or also disconnecting the RF.  Maybe just the former.
(Edited)
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Javier, KC2QII

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I just updated to SmartSDR 1.5.1 this morning.  With the new firmware, the display issue during transmit on Ant 1 with the Pixel loop on Ant 2 disappeared.

Javier
(Edited)