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6600 - RX on ANTA, TX on ANTB. Do I need to worry about overdriving the ANTA input on TX?

This applies to a single slice, receiving and transmitting on the same frequency but with different antennas. Assume 500 Watts TX power using SSB Phone.

Best Answers

  • KD0RC
    KD0RC Member, Super Elmer Moderator
    Answer ✓

    If you are not running Full Duplex (FDX), then you will be fine. If you are running FDX, then fill out the worksheet that Flex has in the documents section. It will tell you if you need more antenna separation or less power.

  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin
    Answer ✓

    Let me update a bit on what @kd0rc said.

    Unless your antennas are grounding when not in use, you need to be aware of energy coming back in to the shack/radio. This is regardless of any HF radio. I don't know of any that ground antennas in the radio but if there are some that do, I would love to know.

    It actually doesn't matter if you are running FDX or not since the RF energy still flow back on other antennas/feedlines.

    The cool thing about a FLEX-6000 radio is that if you are overloading the radio, we actually do measure it and send you an alarm.

    The end result is that you do not want to exceed +15dBm on a receiver. If you do, the radio will try to protect itself. If you were measure that with a watt meter it would be about 3mW.

    Just to be sure, we have a work sheet here: https://www.flexradio.com/documentation/flex-6000-fdx-power-calculation-worksheet/

    38.4.2 Preamplifier / Attenuator Block

    The preamplifier / attenuator block can raise or reduce signals before they are sampled in the ADC block. The overload limit of the SCU is reduced by any gain that is added by the preamplifier. For example, if 10dB of gain is selected using the preamplifier selection under the antenna controls in the panadapter, this 10dB addition must be factored into the overload calculations. The same holds for other amplification levels: if +20dB or +30dB are selected, the respective value must be added to overload calculations. You should only use pre-amplification when necessary and only what is required to raise the noise level in the receiver by 8-10dB over the noise level when the antenna is disconnected.

    The preamplifier has protection circuitry to prevent damage above levels of +10dBm. This protection circuitry will cause distortion in received signals if the level exceeds +10dBm. For this reason, pre- amplification should not be used when the input signal level will meet or exceed +10dBm. When performing all testing of a full duplex configuration, it is highly recommended that the preamplifier be disengaged until it can be determined that the power level from the transmitter will not reach +10dBm in any operating scenario.

    38.4.3 High-Performance Analog to Digital Converter Block (ADC)

    The ADC block, or digitizer, converts the received signals into digital data. All ADCs have overload points and damage points, but there is some variability in overload symptoms. With the FLEX-6000 Signature Series radios, the ADC overload point varies from +7dBm (FLEX-6300) to +9dBm (FLEX-6500 and FLEX-6700). This overload point is a “soft overload” meaning that at this point the receiver will begin to show a performance drop. The ADC generally functions better with increasing signal levels up to this point. At the soft overload point, the receiver will begin to develop spurs that will appear in the panadapter and these spurs will grow as power is increased. A digital overload point will be reached around +12dBm (level varies by receiver) at which point the receiver will cease to function normally, producing substantial distortion in received signals and rendering reception difficult.

    At levels above +15dBm (level varies by receiver), the ADC can be damaged so the FLEX-6000 contains circuitry to disengage the ADC from the antenna. While circuitry should protect the radio from a damaging signal, it is highly recommended that station configuration be designed such that signal levels above the soft overload point are prevented from entering the antenna connector of the radio. FlexRadio Systems assumes no responsibility for damage incurred from high signal levels entering the receiver. 

  • Ted S
    Ted S Member ✭✭
    Answer ✓

    Your dBm calculations are off by a factor of 10, +15 dBm is about 30mW, not 3 mW. Three mW is closer to +5 dBm, or more exactly +5.2 dBm.

    Ted, WR4T


  • jlag
    jlag Member ✭✭
    Both great answers, thank you Len and Mike. My next step was to actually measure it. Using the radio to measure it a low power to get a dB calc makes great sense. I have about 60 ft of horizontal separation between the antennas, TX is a vertical 1/4 and the first RX test will be with a mag loop. This should be an interesting experiment.

  • jlag
    jlag Member ✭✭

    @Ted, good catch, close to 32 mW it seems. Thank you.

  • WX7Y
    WX7Y Member ✭✭✭

    I have seen this error only a few times and here are my results.

    I had a next door neighbor who unfortunaly is now a silent key that ran full legal using a AL-82 Amp with one leg of his Antenna tied to my tower running parrellel to my big 160 meter Loop with the wires at a MAX 100' down to about 15' seperation.

    I installed a https://kd2c.com/rf-limiter or https://www.dxengineering.com/parts/dxe-rg5000hd?seid=dxese1&gclid=Cj0KCQjw0JiXBhCFARIsAOSAKqCvN9b5W-mXtiA-WVCdAFXMqRfNLiahX6gD2zFVk1vAVYHDSfKRGi8aAhJiEALw_wcB on the RX loop on the great and powerful Flex 6700 and worked perfectly with him running full legal power right next door, to Bad in flex's Wisdom they eleminated the RX Antenna loop through on the newer radios most people can't benifite from devices like the rf-limiter.

    On the other Radio's like the F6600's you can use a device like the https://www.dxengineering.com/parts/svp-sv-fesss?rrec=true to protect the 2nd Receiver if your really worried about it.

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