Welcome to the FlexRadio Community! Please review the new Community Rules and other important new Community information on the Message Board.
The latest SmartSDR Software:
SmartSDR v4.1.5 | SmartSDR v4.1.5 Release Notes
SmartSDR v3.10.15 | SmartSDR v3.10.15 Release Notes

The latest 4O3A Genius Product Software:
The latest 4O3A Genius Product Software and Firmware
How to Receive Technical Support::
If you are needing assistance with FlexRadio products, please refer to the product documentation or check the Help Center for known solutions. Need technical support from FlexRadio? It's as simple as creating a HelpDesk ticket.

Pre amp on 2nd receiver slice causing signals to appear with no antenna connected?

Gary Ashdown
Gary Ashdown Member ✭✭
in SmartSDR for Windows

After testing my 3 radios, 6600, 6700 and 8600 I have noted the following on all of them.

A. With no antenna connected to either #1 or #2 antenna ports and turning the pre amp on the receiver slice with no antenna connected, signals appear.

b. These signals are coming from the opposite antenna port which has an antenna connected.

c. This is particularly evident on 50mhz.

Has anyone else noted this as it appears to be an isolation problem between receivers when the pre amps are used.

regards

VK8AW / VK4ABW

Answers

  • KD0RC
    KD0RC Member, Super Elmer Moderator

    Hi Gary, I just did a quick check of 6 meters, and cannot find any spurs in the ham band when no antenna is connected, even with the RF Gain set to +32dB. It does not make any difference if I have an antenna on any other ports. I am running a Flex 8600.

    Do you have a GPSDO connected to the 10 MHz port? When I first got a 6400, I bought a Leo Bodnar GPSDO and it introduced a ridiculous amount of noise into the rig. I got rid of the wall-wart power supply and now power it directly from the main station supply (Astron RS-70M). That completely cured the problem.

    Another thing to try is disconnecting things connected directly to the radio one by one to see if that is what is conducting noise into the rig. I suppose another potential source could be common-mode interference coming in on your coax.

  • Gary Ashdown
    Gary Ashdown Member ✭✭
    edited March 20

    Hi Len,

    It's not spurs or noise, it's actual signals. I first noted this on 50mhz when i switched my 2nd slice receiver to a disconnected antenna port. The 2nd slice was still decoding strong JA's clearly but at a much weaker level.

    I then tested this on 14mhz with the same result. Disconnecting the only antenna stops all signals.

    I also noted the internal birdies and these are present on both receivers with no antennas connected.

    The way to test this issue is - connect an antenna to port 1 and put the slice receiver on an amateur band with strong signals. Then create a 2nd slice receiver, allocate it to the antenna port with no antenna and turn the pre amp on and put it on the same frequency as the 1st slice receiver.

    Let me know what happens.

    Gary VK8AW / VK4ABW

  • KD0RC
    KD0RC Member, Super Elmer Moderator

    Hi Gary, OK, I just did that on 20 meters (7:30 PM local time here). When I set the preamp up to +32 dB, I can indeed see some signals on the open antenna port. Removing the antenna on ANT1, all signals drop to zero. An S-9 plus signal on the main antenna shows up as an S-1 signal on the port with no antenna. With the +32 dB pre-amp engaged, the noise floor (as measured by the panadapter) is around -152dBm. The main antenna gain is set to +8 dB and the noise floor is around -106 dBm. So the isolation between antenna ports seems pretty high, but I am no engineer…

    Just did another quick test, I left the main antenna connected, but put one RX on one open port and the other on a different open port and still get the same signals. Does this imply common mode reception by my coax? Or does this indicate antenna port isolation issues?

    Any RF engineers who can duplicate this and give an opinion?

  • Gary Ashdown
    Gary Ashdown Member ✭✭
    edited March 20

    Hi Len

    The antenna isolation on the 6000 series radios seems to be weaker than my 8000 series radio.

    I've put my radios on 28.074 now and there is plenty of signals on the 2nd receiver with no antenna. This starts to happen once the pre amp gets to 10db.

    It could be isolation at the antenna port or the way the pre amps work? I think they are software pre amps?

    I find it amusing to be decoding JA's on 50mhz with no antenna on that port :)

    Gary

    VK8AW / VK4ABW

  • KD0RC
    KD0RC Member, Super Elmer Moderator

    Hi Gary, they are analog preamps and attenuator (two 16 dB amps and an 8 dB attenuator in the 8600). They are controlled digitally, but the amplification is analog at RF.

    I don't know if it is antenna port isolation or common-mode signals from the feedline. Hopefully, someone in the know will answer that question.

Leave a Comment

Rich Text Editor. To edit a paragraph's style, hit tab to get to the paragraph menu. From there you will be able to pick one style. Nothing defaults to paragraph. An inline formatting menu will show up when you select text. Hit tab to get into that menu. Some elements, such as rich link embeds, images, loading indicators, and error messages may get inserted into the editor. You may navigate to these using the arrow keys inside of the editor and delete them with the delete or backspace key.

Comment As ...

Categories