Sudden and persistent 'flutter' on VHF only

Discussion in 'On the Road' started by W4EAE, May 27, 2021.

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  1. W4EAE

    W4EAE XML Subscriber QRZ Page

    This problem began three days ago. While moving, I have substantial 'flutter' when in motion to three VHF repeaters which are all mounted on the same tower about 12 miles to the NE from the farthest point in my commute. It is to the extent that the signal from the repeater is often unreadable. I will stress that this is a completely new problem, and also stress that it is only when I am in motion. Here is the outline of the problem:

    --It only occurs on 2m. I can use 70cm repeaters on the same tower without issue.
    --It occurs on both of the VHF capable radios installed in the car (with two separate antennas). If I monitor a repeater with both at the same time, the flutter is simultaneous.
    --It only occurs on receive. Report of my TX audio from others on the repeater indicate that it is pristine.

    I have taken several steps to eliminate various things as the cause, and I am stumped. Here is what I have done so far:

    --Connected an antenna analyzer and determined that SWR was <1.5:1 across the 2m band.
    --Inspected the connectors and coax from the radios to the antennas.
    --Inspected the antennas themselves.
    --Swapped out one of the antennas for another dual band mobile antenna I had sitting around.

    Any ideas that take in to account what I have stated above?
     
  2. K7JEM

    K7JEM Ham Member QRZ Page

    Have you changed or added anything in the vehicle? I had an instance where a voltage converter plugged into a cigarette lighter outlet was causing extreme noise on 2M, made all incoming signals seem weak, but didn't affect uhf reception at all. I would look at something like that since it is happening on different radios with different antennas.
     
    AG6QR likes this.
  3. AG6QR

    AG6QR Premium Subscriber QRZ Page

    I had a similar experience with a USB phone charger that fit into a lighter outlet.

    If you have a VHF radio that supports AM mode, listen to it as you turn the vehicle on and off. Interference producers are a lot easier to hear on AM.
     
    K7JEM likes this.
  4. K4VLF

    K4VLF Ham Member QRZ Page

    I was once told that "pickett fencing" is common on mobile VHF RX, due to hitting a peak of the waveform, vs hitting a zero.

    I have not been convinced that is true.
     
  5. W4EAE

    W4EAE XML Subscriber QRZ Page

    I have not made any changes to the car, or added any new devices to the mix.

    The problem with turning the car off is that the flutter only happens when the car is in motion.

    I have scanned the band with an AM receiver. One of the two radios in the car is an Icom IC-7100. I had my wife slowly spin the VFO back and forth across the band while we were in motion. (I would have had her drive so I could operate the radio, but my car is a manual). We could not find any noise that rose above an S3 (which is normal downtown).

    FYI, you can see my commute if you search for W4EAE-9 on aprs.fi.
     
  6. K7JEM

    K7JEM Ham Member QRZ Page

    I assume you tuned it over the 2M band, while moving. Does the S meter reading change while on AM or SSB? That is where you need to test, and in the AM or SSB mode, not FM. On FM, the S meter reading is pretty much useless, since it is tied to a different point in the demodulator. Check the SSB noise floor before moving, and while rolling. Noise floor on 2M should be pretty much "zero".

    This problem definitely sounds like a local noise source "desensing" your 2M FM RX, since the antennas are both good, and it happens on two different radios. Unless all three of the repeaters are having an issue, but others would have noticed that.
     
    AG6QR likes this.
  7. K7JEM

    K7JEM Ham Member QRZ Page

    The wavelength is so short on VHF+ that you move through a lot of "nulls" as you drive. Typically, if you are in good range to the repeater, it won't be noticed, since the signal will still be near or better than "full quieting" all of the time. It is not unusual for a signal to vary by 10dB as a mobile moves around. I have been parked in a parking lot where I could barely hear the repeater, then move a foot or two and have the signal come right up. Same thing on TX to the repeater, except the null is not always in the same spot. IOW, sometimes you hear the repeater well, it doesn't hear you good, you move 2 feet and the opposite will happen.
     
    AG6QR likes this.
  8. W4EAE

    W4EAE XML Subscriber QRZ Page

    The 2m band scan was done in AM mode, and the 2m band is not that quiet where I live.

    My radios already run off of an auxiliary battery, so my next test will be to basically disconnect and/or disable everything the car that isn't necessary for locomotion. Because I have a manual, it isn't very feature dense to begin with. If I turn off the compressor, lights, and AM/FM radio, pretty much the only electronics running will be the ignition and the radiator fan.

    I total get what you are saying about variance in 2m repeater reception over small distances, but what flummoxes me about about this is that it is totally new. I went for years without this problem, and then it was suddenly there.
     
  9. K7JEM

    K7JEM Ham Member QRZ Page

    There are lots of DC converters used in modern cars, and lots of computers and electronics embedded in every system. It is possible that something has gone bad in one of those things. How is your aux battery connected and isolated? Can you bypass that and connect right to the car battery. even temporarily for an RX test?
     
  10. AG6QR

    AG6QR Premium Subscriber QRZ Page

    Sorry, I didn't mean to suggest turning the car off while in motion. I meant to suggest turning the car on and off while stationary, with an AM receiver tuned to a relatively quiet spot in the 2m band.

    Does the AM noise level increase when the car is turned on?

    The flutter while in motion is likely to be multipath. Radio signals will naturally have pronounced areas of higher and lower signal strength, on the order of a half wavelength apart. As you drive through these, the signal strength will vary. But if you're listening on FM, you won't often hear the variations unless your signal to noise ratio is on the margin of receivability.

    When I had a very similar experience, it was because a new local noise source inside the car pushed the signal to noise ratio to the point where nearby repeaters were only full quieting at the peaks.

    It's also possible you have a problem that you're not capturing as much signal as you used to. A problem with the antenna or coax could be the issue, but that would likely (but not absolutely certainly) show up with an SWR check. Perhaps the repeater you listen to has had a recent drop in signal strength?
     

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