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Off frequency drift?

  • 31 May 2017 10:29 AM
    Message # 4863122

    While operating at the Invermere Rally, I got a report from Net Control that my transmission seemed to be 'off-frequency'. This was late on the 2nd day with no mention of problems before. Wondering what could cause my radio to TX unclear. Could run down battery capacity affect frequency response? Got the report near end of the day when battery pak was showing reduced reserve.

    Is there anything I can do to clean up transmission 'off frequency' report? I was using Kenwood TM-D710.

    Thanks

    Wilson  VA6JWL

  • 31 May 2017 11:22 AM
    Reply # 4863238 on 4863122
    Anonymous

     It would depend on radio of course but my experience is that Li-ion BATTERY PACK's tend not to give a warning when they are going to shut down.  

    For my ICom 92AD I have 3 old batteries.  One is always on the charger and if I remember, another is in my pocket when I go out.   Unless some one tells me my audio is sounding low, I find out the one I am using is done when I un-key and screen is blank. Happen to me all the time.

  • 01 Jun 2017 11:13 AM
    Reply # 4874998 on 4863122
    Anonymous

    The control operator stated that you sounded "off frequency". So the first question is whether you were truly off frequency or not.... were you operating on a simplex frequency and accidentally bumped the dial slightly on your radio? Or were you operating on a repeater frequency? (Usually the repeater duplexer is filtering things and wouldn't allow signals that were off frequency very much; but anything is possible). Are you familiar with the sound of being off frequency? If not, take two radios and dial them both on a frequency and then bump one of them 0.005Mhz up or down.....transmit and listen to the sound.

    Is it possible that you were not off frequency but there was noise/distortion on your signal which was being confused for being off frequency?  Do you have any RFI/EMI problems in your vehicle?  Were you parked near powerlines? In the area that you were operating, is it mountainous terrain?.....you might have been dealing with some multipath distortion on your signal (I've witnessed this when operating west of Calgary near springbank area). Perhaps moving your vehicle a few feet from your operating position might have changed things. What was the weather/atmospherics like at the time. High heat? Maybe when your day started you were in communication range (barely), and then later when the weather/atmospherics changed, you were now starting to lose communication.

    Were the communications on 2m (144Mhz) or 70cm (440Mhz) band. 70cm is more susceptible to fade, multipath, and even (trees) foliage.

    Talking about the possibility of low battery voltage....it's possible (you would need to test it).....you would normally expect the radio to operate normally and if the voltage dropped too low, it would shutdown. But a good example of radio distortion due to low voltage is the old Radioshack HTX-202 HT radios that were popular when I was first licensed. It was not uncommon to be in the middle of a transmission and if your battery was too low, those stations that were listening to your transmission would hear a low hum and no audio which would persist until you unkeyed the PTT button (only to realize that your battery just died). Then everybody on-the-air would have a laugh because they would recognize this sound and say, "oh, I guess his  battery just died, eh?".

    You indicated that you were using a Kenwood TM-D710 radio....this is a mobile radio that doesn't operate from an attached battery. What power source were you using to operate the radio? Was it tied in to the vehicle's electrical?  Is this directly to the vehicle's battery/fuseblock?  Or were you trying to operate from the cigarette lighter/powerpoint? Those powerpoints are not usually rated for the full current draw of this radio (12-15A)...they are usually rated at no more than 10A output.

    Were you using a separate gell cell, lead acid, or marine AGM battery to power your radio? Remember that most mobile radios are designed for 13.8V power (+/- 10%). Even a fully charged battery (after settling down) is only going to be at 12.8V...this is already -7% low from 13.8V. Therefore if you used the battery for awhile....it could very easily drop voltage to the point that the radio is unusable...this is also a problem for some mobile HF radios (such as the ICOM IC-7000)....it will shutdown when the voltage is less than 12V. Bear in mind that a battery's voltage is going to drop even further (while under  current load) than it's resting voltage.

    Unfortunately, "hearing some noise" is hard to pinpoint without more information. I imagine auto-mechanics who receive this description probably roll-their-eyes..... :-)

    Hopefully, I've given you some things to consider. Good Luck, and if you figure it out, it would be nice to know what the solution was.

    Tino, VE6MB


    Last modified: 01 Jun 2017 11:26 AM | Anonymous
  • 02 Jun 2017 11:20 PM
    Reply # 4877913 on 4863122

    Thanks Robert & Tino for the great info.

    I was on the VE6RIN repeater & checked the frequency shown on the radio & it was on the Repeater frequency & in Memory mode so bumping a dial would have gone a long ways off frequency, not the small amount suggested to test.

    I don't think I have any RFI  problems in my vehicle - when had the trouble, vehicle was not running. I was well up on a hill with no power lines nearby. But might have been distortion from the hills/valleys. But I had been at that location for over 6 hours with no problem before. 

    It was a 2m repeater - same frequency as our RYC repeater - without the tones.

    I was using a SLA battery that had been powering the radio all day - it is connected to the vehicle powerpoint, but there was no power from vehicle as key was off. With your explanation, it looks like the weakened battery didn't supply enough output for the Medium radio TX. After getting the report, I started the vehicle to add vehicle power to the battery & didn't get any more off frequency reports.

    In normal operation, I cannot TX at High power - 50 watts - with only the vehicle powerpoint -  I have to support the draw with the SLA battery pak. Vehicle powerpoint will support TX at Med power. If I forget & try on High, radio cuts out/goes off & then restarts - so I need to remember to manage it.  My SLA pak battery is about 7 years old & doesn't seem to produce the capacity it had when new - looks like it's time to shop for a replacement.

    Anyone had experience with using the Li car jump starter paks as surge supports for radios? For convenience, I'd like to be able to keep it on charging connection, but have it supplement the vehicle powerpoint for radio operation.

    BTW, I have a couple HTX-202's that my sons used over 20 yrs ago. Need to do the internal battery replacements.

    Thanks again for suggestions.

    Wilson

    VA6JWL

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