Flex-6700 vs. Flex-5000 on 6 Meters

W9DR

I have completed A/B testing of a Flex-6700 vs. a Flex-5000 on 6 Meters. (receive only) The test was done under conditions of no e's opening, just ground wave/tropo and meteor scatter. I used an AR-2 6 meter preamp on the 5000 receive loop and no preamp on the 6700. I did have the 6700 internal preamp on to #3. Findings: The 5000 was about 2 dB more sensitive on signals.The noise blankers on the 5000 were much more effective. The 6700 NB was on and set to highest setting. Also notice that the 6700 pan noise floor did not drop down when the NB was turned on but the NB did reduce some of the noise some. Dave, W9DR

Gerald-K5SDR

The measured MDS in a 500 Hz bandwidth on 6m for the FLEX-6700 is -141 dBm (6 dB NF). That is in the 30 dB gain setting. We could in software enable the 40 dB gain setting for a -143 dBm MDS (4 dB NF) but I am not sure that really makes sense on 6m. The FLEX-5000A has a 6m measured MDS of -128 dBm in 500 Hz. I am not sure how you did the sensitivity measurements. Remember that the AGC threshold setting on both radios will directly affect maximum sensitivity.

Eric-KE5DTO

Note that there was a third-party AR-2 6m preamp on the FLEX-5000 receive loop.

Gerald-K5SDR

Missed that. That would probably put it in the class of the 6700 with 40 dB of gain. That is typically a lower NF than can really be used on 6m with the band open. When the band opens the noise floor come up due to noise propagation from the opening. I would love to know from serious 6m operators if you can really use a 4 dB NF when the band is open. You probably will loose at least 1-2 dB in the coax as well.

k0eoo

Hello Dave, My 6500 is much more sensitive than my 5000A as long as I have the +20dB preselector selected on the 6500. And my 5000A has a +25dB preselector in the receive loop too.... See if your 6700 is better with its preselectors on. Dennis, k0eoo

W9DR

Thanks for the responses. This measurement was not scientific but an A/B comparison, as I said in the original post, using a CW bandpass of 500 Hz and the AGC was set on both radios to fastest available settings. The signal source was two beacons on 6 Meters. ( weak CW signals) The measurement was done by visual observation of the Panadapters calibrated in 2 dB steps and FPS set on 15 for each radio.The SDR-6700 was tested WITH the internal preamp set to the highest setting. Again, as KE5DTO observed in my original post, the SDR-5000 had a 6 Meter preamp in the loop and the SDR-6700 did NOT. The point being, the SDR-5000 very much needs a external 6 Meter preamp and SO DOES the SDR-6700. What I did not say in the original post was that I DID measure the SDR-6700 WITH an AR-2 6 meter preamp in its loop and the signals were equal to the SDR-5000 with a 6 Meter preamp in the loop. (with both radios, the internal preamps turned off) When the band is open the external preamp is a moot point and best left switched out of the loop. As said in my original post, no e's opening. i.e. band was not open. Perhaps some serious 6 Meter ops can weigh in and compare these two radios in the real world of weak signal work. Dave, W9DR

Mike W8MM

Dave, Here's an experiment you can do that will help characterize your local antenna system noise temperature. For every combination of radio a or radio b with and without your preamp, observe the noise floor fom the antenna and then switch to a 50 Ohm room temperature load. See how much the noise floor increases from "Cantenna to Antenna" for every situation. Report the results and we can all calculate how much a preamp "should" help SNR at your house.

W9DR

Hey Mike, good to see you here. I will get you those numbers. Steve, K8DZ and I have talked about using this method many times and have wondered what NF is the lowest useful number for the 6 Meter band. Your sky temp may very. Hi Hi When you receive your Flex-6700, please do a real world comparison as I have between your 6700 and your 5000 with a 6 Meter preamp. Please share the results with us. Dave, W9DR

Steve K8DZ

I have a Flex 3000 (stock) and measured the NF on 6M to be 7 dB. Much much better than the 5000 w/o ext preamp as we all know. I also have an Icom 7700 w AR2 (0.5 dB NF) preamp ahead the RX input jack resulting in a 1.0 dB NF. Normally, the Flex 3000 is perfectly fine because I usually have a lot of man made noise. Noise is about 12 dB - 15dB above the level of 50 Ohm load. But occasionally, in one direction, my noise floor will be down to only 6 dB above the 50 Ohm load on the 3000. In this quiet case, I can tell an improvement using the radio with the 1 dB NF compared to the 7 dB NF. As Mike said above, we could calculate the S/N difference knowing these facts. I have not done that, just listening test. What does this have to do with the new 6000 series radios - it just shows that a lower NF can indeed be useful on 6M for weak signal operators like for meteor scatter work. If one has a lot of feedline loss, the low NF is even more important. Fortunately, the solution is simple - put the preamp in the loop because it will likely have a lower NF even if the Flex 40 dB setting is made available. One would not need the max gain of the internal preamp in this case and it could be turned down to help preserve the dynamic range. So not really any problem. A ready solution appears to be simply using loop in/out. 73 Steve