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Comet cha-250b better near ground vs up at 25' on house??

Discussion in 'General Ham Radio Discussion' started by archjeb, Sep 5, 2018.

  1. archjeb

    archjeb Member

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    Folks,

    I was doing some testing of a CHA-250B. I know some of you will call it a dummy load and not a good antenna; but lets put that aside and consider the performance difference I'm seeing in two locations.

    Initially I had it about 30' from the house near the ground attached to a metal fence post. I had it this way because I was testing it in comparison with a fan dipole with an A/B switch to compare the two.

    The fan dipole was mounted on a mast on the top of the house roof at around 25' high. At the top of the mast is a dual band 2m/70cm vertical. The fan dipole was mounted offset a few feet from the mast with a non-conductive mount.

    Interestingly, the CHA-250B vertical was getting better signal reports than the dipole. Weird, but this was on the receive also - I noticed several S units difference.

    I took down the fan dipole yesterday and placed the CHA-250B on the mast feeding it with LMR240. There is still the VHF/UHF antenna mounted but with an offset bracket to separate the two antennas.
    Now, I'm seeing a lot of noise. On 20m I've got S7 of noise and my signal does not seem as good as before, such as when listening for the beacon stations.

    Now, I don't have two CHA-250B that I can test side-by-side with the antenna switch - I wish I had that to compare to see if its a location issue or whatever. Band conditions could have changed...or could it be something specific with the house such as man made noise with wiring, attic fans, etc?

    Also, antenna mast is grounded with #8 solid to ground rod. The previous fence post mount was obviously in ground, but the coax feed still was attached to the ground block/lightening arrester that all the other antennas were attached to, which also was attached to the ground rod.

    At this point, I'm thinking of mounting this antenna back on the fence post...



    Any thoughts on this. I'm scratching my head a bit on this.

    -Jeremy
     

  2. Riverman71

    Riverman71 Part-time Member

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    Jeremy,
    I once had a noise level of S7 that I finally traced to one of those older spiral type energy-saving light bulbs in my garage.
    On:S7
    Off:S1

    I couldn't believe my good fortune when I finally discovered this. The hobby was fun again.

    Jim
     
    Shadetree Mechanic likes this.
  3. archjeb

    archjeb Member

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    That makes me think...maybe I should shut down the main circuit breaker and see if things improve measurably. If so then, process of elimination on a per circuit basis.

    -J
     
  4. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    Would this have been a Metal post on a chain link fence row?
    All the Best
    Gary
     
  5. archjeb

    archjeb Member

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    No, just a stand alone T post. No fencing.
     
  6. Mudfoot

    Mudfoot Sr. Member

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    Put it back to confirm the absence of noise. If things get quiet, then it sounds like the source emission is weak, or within Part 15 specs. Power your rig by battery, shut main breaker. If noise goes away, then try each breaker to narrow it down. Other distant sources can use power lines to radiate the noise back to you. Usually, this type of noise is detectable at your meter. If this was the case, then you'd hear that noise all around the immediate neighborhood. Sounds like your source is local, but doesn't radiate very far, like a few yards or so. Also, your dipole is horizontal. 60 cycle radiators usually favor vertical polarization. Maybe your dipole suppressed the signal, but your vertical favors it.

    Do you have a recording of the noise?
     
  7. archjeb

    archjeb Member

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    I'll have to move it again and test - wish I had two of the same antenna to test with an A/B switch...for a true comparison.

    The noise is more white noise, like the noise floor increased by by a couple of S units. Additionally, signals are more buried under the noise. It doesn't sound like motor noise that I noticed when the AC is on. It sounds like increased white noise.

    I wish I did a local TX report before and after. That would have at least verified performance as a radiator and limit atmospheric condition affects in my test. But oh well.

    -J
     
  8. Mudfoot

    Mudfoot Sr. Member

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    I'd do breaker test first with radio on battery power.
     
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  9. archjeb

    archjeb Member

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    Mudfoot,

    I got a copy of the sound of the noise. Any ideas to pin point the noise source? Its AM on 3.8Mhz.

    I haven't shut down the power yet as I need to buy an AGM battery that I want to get long term for backup power for the radio.

    Thanks,

    -J
     

    Attached Files:

    #9 archjeb, Sep 11, 2018
    Last edited: Sep 11, 2018
  10. Mudfoot

    Mudfoot Sr. Member

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    Definitely sounds like powerline noise. Eliminate sources within your house first. If still have the noise, then move outside and start tracking it down. Call the power company and see if they have an RFI specialist to come and track it down.
     
    BJ radionut likes this.
  11. archjeb

    archjeb Member

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    Thanks. I'll give them a shot.
    The coax run to the top of the house is a couple of feet from the meter and antenna is directly above this (under ground power, but meter/service entrance is near shack). I'm really starting to wonder if that is why the noise appeared when I took the antenna from the ground (about 30feet horizontally away from the house) and moved it to the roof.

    Here is another sound clip with the RF gain turned down. You can really hear the rhythmic noise.
     

    Attached Files:

  12. Mudfoot

    Mudfoot Sr. Member

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    Walk around your property and neighborhood with an AM radio. Listen to the noise peaking. Better yet, if you have an HT with AM receive, you can use it to track it down. Use the attenuator if it has one. Borrow or make a 2-3 element Yagi to pinpoint the source. It's usually an insulator or loose/corroded hardware on the pole.
     
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  13. archjeb

    archjeb Member

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    So, I did some testing and there are definitely some noise generators in my office. I've identified several that I was able to power down and my noise dropped.
    Unfortunately, I can't power the equipment down regularly because its for work. What is weird though, is when I had the antenna mounted away from the house about 30', I didn't have the noise.

    If I unplug the antenna coax from the back of the radio, i don't have any noise. So I know that the noise is not coming in from the power source (power supply), but coming from either the feedline or the antenna.

    It is traveling around the AC circuits/lines because with an AM radio I can pick up the noise near the outlets.

    So my next question would be, what is the best way to test if I'm getting the noise from the feedline vs the antenna? Or from the ground bound to the mast to the grounding plate/lightening arrestor and ultimately the ground rod?
    Can I assume that if I attach a dummy load to the end of the feedline (at antenna feed point) and I have no noise, that its all coming from the antenna? At that point the only way to reduce the noise would be to move the antenna farther away from the house?

    Kind of thinking out loud here...but any additional pointers are appreciated.

    Thanks,

    -J
     
  14. Mudfoot

    Mudfoot Sr. Member

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    What are the sources of your noise? How are they powered, cheap switching wall warts? Maybe replace them with regulated ones, or get the chokes out and see if that helps. If your like me, you'll quiet all your stuff, only to hear some bullshit fire up from the neighbors. It's normal for many devices to only radiate hash and interference a few yards or more. Probably accounts for the null when your antenna was moved further away.
     
  15. archjeb

    archjeb Member

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    Its a mix of PC and Ethernet switches I use for testing for work. Some of it is fan noise based on the RPM of the fans which is variable depending on temperature; when the fans are running full speed, the noise is much greater.

    there is also some switching power supplies/wall worts that are used for some additional appliances. They are fed from two large power strips.

    There are other noise makers in the house, but these are the biggest culprits.

    -J
     

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