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Voltage at the Radio vs Voltage in the Radio (as reported via API)

W9ATW9AT Member ✭✭

I don't get 100W out on most bands into several different power meters into a few different dummy loads--all tried separately of course <grin>.  I have read a few explanations of why that would not be the case due to a few variables including actual power available to the radio, accuracy of the SWR meters, the real impedance of the dummy loads, etc.

Of course 10-30W less may not make as big a difference in db at a distant station, that is not my point here.

I'm guessing that FRStack reports my 6700's voltage via the SmartSDR API. 

My first question is:  Does anyone know generally speaking where that voltage, as measured by FRStack, is measured internally in the radio?

Since I've tried several power meters and several dummy loads, i thought I would take a closer look at the power supplied.

My second question is:  When being advised to supply 13.8V + or - 15%, where is that voltage meant to be measured--at the PowerPoles of the radio or internally somewhere such as where SmartSDR reports its voltage reading?

Of course, I have found some voltage variations at different points in the power circuit (at the power supply, at a 12v power distribution hub, at the radio, etc. 

So, for simplicity, I conducted the following measurements (rounded) with a 10ga, 10" power cable directly to the radio (temporarily not even including a fuse in the circuit).

50Amp Astron power supply with 10ga, 10" power cable to radio:  14.1V at output

- Off:  14.1V -- at radio PowerPole connectors
- RX:  13.9V -- at the radio PowerPole connectors
- RX (internal):  13.1V -- inside the 6700  (as reported by FRStack 3rd party app.--presumably via the SmartSDR API)
- TX (100%):  13.7V -- at the radio PowerPole connectors
- TX (100%):  12.6V  --  inside the 6700 (as reported by the FRStack 3rd party app.)

If I increase the voltage output of my power supply, I am trying to be careful by asking:

My third question:  Where is the prescribed 13.8V +- 15% meant to measured (e.g. at the PowerPoles?  Internally?) 

My fourth question:  For optimum power output, is that prescribed level meant to be with the radio off, radio on RX, or the radio on TX under full load (100%)?


Mark W9AT


  • N8SDRN8SDR Member
    edited December 2018
    Are your power meters PEP (peak reading) or average power?

  • Bill -VA3WTBBill -VA3WTB Member ✭✭✭
    edited July 2018
    With the radio under full load, you don't want the voltage to drop below 13.8v
    Many people including some Flex staff run there voltage as high as 15v.

    In FRstacks the voltage is read at the fuse in the radio, that is what you want.
    On the Flex meter you can down load and install, tells you the voltage before the fuse and after the fuse, there is a little difference between them but that's splitting hairs.
  • W9ATW9AT Member ✭✭
    edited July 2018
  • W9ATW9AT Member ✭✭
    edited July 2018

    Your answers:

    - "With the radio under full load, you don't want the voltage to drop below 13.8v"

    - "In FRstacks the voltage is read at the fuse in the radio, that is what you want"

    ...are exactly what I needed to know.  All my questions answered!  [even though I asked one answer twice!]

    As I appear to be "losing" (term used loosely here) nearly 1V during TX between the Powerpole connection and or after the fuse inside the radio, relying on one test "instrument" instead of two different ones when comparing power in and power after the fuse might be interesting.  I'll try to find that "Flex meter" software online.

    I can always check the difference just "outside" the internal radio fuse and "outside" the chassis Powerpole connector, but that would require opening the case (and taxing my laziness <grin>).  After connection cleaning, and any minor power supply adjustment, any such revelation may not be that important to getting optimum power set anyway.

    Thanks much Bill.


    Mark W9AT

  • Bill -VA3WTBBill -VA3WTB Member ✭✭✭
    edited July 2018
    Really, the voltage meter in FRstacks is all you really need. I am more concerned with people having 13.8v at the power supply then under load it drops to 12v or less at the radio.
    If FRstacks says your getting at least 13.8v on transmit I think your ok. I run my radio so I have 14.5 v at the radio fuse.

    If you want to send me off an email I can send you the Flex meter...
  • W9ATW9AT Member ✭✭
    edited July 2018
    Thanks for the tip, Bill, about Flex Meter by Denley Barnette W3XY.  It's a nice, simple tool to have in my Flex Radio user toolkit.

  • Bill -VA3WTBBill -VA3WTB Member ✭✭✭
    edited July 2018
    Dig around in it, you can do lots of things with it,,under source select RAD then it will read before fuse or after fuse
  • WA2SQQWA2SQQ Member ✭✭
    edited July 2018
    As part of my semiannual maintenance when I disassemble my 6500 I usually remove the fuse and clean the contacts using some Deoxit on a lint free cloth. The first time I checked, I was able to measure a .15v voltage drop. Since cleaning it, nothing large enough to measure. More important, absolutely no change in performance.
  • Bill -VA3WTBBill -VA3WTB Member ✭✭✭
    edited July 2018
    There seems to be something to that, I have heard that before. I wonder how many operating hours it takes before the contact begins to go south?
  • WA2SQQWA2SQQ Member ✭✭
    edited July 2018
    After cleaning it, the first time, about 18 months ago I have not seen any reoccurrence.  Honestly, it's "splitting hairs".
  • Andrew ThallAndrew Thall Member ✭✭
    edited January 5
    With my 6300, I'm using a 35 amp Astron supply (recently upgraded from a 20 amp supply).  FRStacks shows the idle V+ at 12.8 and key down 10.5.  My external meter shows 13.7 v at the terminals of the power supply.  Should I have any concerns?
  • edited August 2018
    Sounds like your D.C. power cables are to small a gague. I get no voltage drop with 10 gauge wire. Also keep the length as short as possible.
  • Bill -VA3WTBBill -VA3WTB Member ✭✭✭
    edited August 2018
    You need at least 13.8v on transmit. Your are getting a lose in the power cable,,did you make the cable longer?  Try and shorten it if you can.
    And try to adjust the Voltage on the power supply to make up for the cable lose.
  • Andrew ThallAndrew Thall Member ✭✭
    edited August 2018
    Thanks.  I shortened the power cable and all is well in Flex land. I was really amazed at how much power dropped in such a relatively short power cable.  I have a 10 gauge power cable on order.
  • DonSDonS Member ✭✭
    edited December 2018
    I am using the cable supplied by flex with the Anderson power pole connectors on one end and ring terminals on the other end which connect to the back of the Astron 50-M.  I am showing 13.66 volts on the back of the Astron and 13.3 volts with FRStack and 13.34 volts with K9DUR's SDRMonitor in the radio so a 0.3 volt drop from ps to radio side of fuse in the radio.  In transmit the voltage drops to 12.3 volts or a one volt drop with about 85 watts out into a dummy load on 20m.    

  • Neal Pollack, N6YFMNeal Pollack, N6YFM Member ✭✭
    edited December 2018
    The original poster did not specify which power meters were used.
    Clearly, there are two points of interest here;

    A.   As indicated above,  voltage drop across wire size and length is very real and important,
           even more so at lower voltages like 12 volts. (Which is one reason we never parallel
           our 12 volt solar panels, but run them in series for higher voltage, and lower overall power
           loss due to conductor size/length.  It's also the reason that long electric utility distribution
           lines use very very high voltages.)

    B.  Very few Ham watt meters are accurate.  (and.... it just don't matter)
           Another engineer and myself spent some time in 2017 collecting various ham analog
           and digital watt meters, and charting them with a lab calibrated HP-438A instrument
           using rather expensive endpoint sensors.   We found wild variation of between 5 and 22%,
           that varied with frequency transmitted (in industry terms;  Tilt).  We even found  >$500 digital
           meters with external sensors that performed far worse than $139 analog models.  Even the old
           Bird favorite was at best 5% if not old, roughed up, and abused.   So consider most of the Ham
           digital and analog watt meters an approximation, not a lab grade measurement.    And since it
           takes roughly doubling the power  twice on your transmit to move the S meter  1 unit on the far
           receiving end,  it really does not matter.
           REF: Raise 100 watts to 400 watts = 6db increase or 1 s-unit on a properly calibrated s-meter.
           [I was going to publish an article called "What's a Watt?",  but other priorities  and projects have
            gotten in the way  (which is probably just as well, since the article would put me on the "hit list"
            of several name brand companies and Ham manufacturers). ]



  • DonSDonS Member ✭✭
    edited December 2018
    I am using the  LP-100A

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