Author Topic: 3rd Harmonic of VHF APRS interfering with 433 MHz sensors?  (Read 1686 times)

0 Members and 1 Guest are viewing this topic.

Offline miraculon

  • Sunrise Side Weather
  • Forecaster
  • *****
  • Posts: 3471
  • KE8DAF
    • Sunrise Side Weather in Rogers City MI USA
3rd Harmonic of VHF APRS interfering with 433 MHz sensors?
« on: January 17, 2019, 03:10:08 PM »
I have an RF Explorer handheld spectrum analyzer. In my area there are few Ham radio repeaters in general, but almost nothing happening on the 70cm band.

I set up the RF Explorer to scan from 420 to 450 MHz. Every so often, I would see a large signal at 433.124 MHz (according to the peak detector). I was thinking that this might be close to the 3rd harmonic of the APRS frequency of 144.39 MHz (here in the US). I got out my Kenwood TH-D72 with APRS and kept it nearby when the Explorer was scanning the UHF band.

Sure enough every time I saw one of the peaks near 433 MHz, I could hear the APRS packets on the Kenwood. This was exactly coincident in time.

3 x 144.39 MHz is 433.17 MHz, which is close enough for me.

Recently I had re-purposed my Acurite 00381 wireless thermometer to keep track of the freezer temperature. (it works, amazingly)

The Acurite uses 433 MHz, as to many other consumer weather sensors as found on this forum. I also have a Meade thermometer that uses 433 MHz.

Musing on this a bit, I began to wonder if strong harmonic signals from APRS might "stomp" on these sensors data transmissions.

APRS.fi shows the location of APRS digipeaters, trackers and other devices.

I thought that I would share this tidbit. My transmitters are fairly close, with the antennas mounted on the roof of the house and garage. I do have the power set low at 5W and 1W respectively. One would need to be fairly close to a transmitter for this to be a problem, but in my case I believe that this is a potential issue. (my transmitters are standard commercial units that supposedly pass FCC regs.)

Greg H.


Blitzortung Stations #706 and #1682
CoCoRaHS: MI-PI-1
CWOP: CW4114 and KE8DAF-13
WU: KMIROGER7
Amateur Radio Callsign: KE8DAF

Offline vreihen

  • El Niņo chaser
  • Forecaster
  • *****
  • Posts: 1018
  • K2BIG
Re: 3rd Harmonic of VHF APRS interfering with 433 MHz sensors?
« Reply #1 on: January 17, 2019, 05:39:14 PM »
My APRS TNC, transceiver, and Raspberry Pi running weewx and using an SDR to snag Acu-Rite data shared the same weatherproof housing before the Pi took a hit!  Interesting discovery.....
WU Gold Stars for everyone! :lol:

Offline Aussie Susan

  • Senior Member
  • **
  • Posts: 58
Re: 3rd Harmonic of VHF APRS interfering with 433 MHz sensors?
« Reply #2 on: January 17, 2019, 08:58:24 PM »
If a licensed 2M transmitter is generating a large 3rd harmonic then the owners/operators need to be told about it as something is probably wrong. The owners/operators also have a responsibility to correct this.
HOWEVER, 433MHz is 'open slather' cowboy territory and if a device is suffering interference then the answer is generally 'Tough!'.
The real answer probably lies in between somewhere.
Susan VK3ANZ

Offline miraculon

  • Sunrise Side Weather
  • Forecaster
  • *****
  • Posts: 3471
  • KE8DAF
    • Sunrise Side Weather in Rogers City MI USA
Re: 3rd Harmonic of VHF APRS interfering with 433 MHz sensors?
« Reply #3 on: January 18, 2019, 08:48:41 AM »
If a licensed 2M transmitter is generating a large 3rd harmonic then the owners/operators need to be told about it as something is probably wrong. The owners/operators also have a responsibility to correct this.
HOWEVER, 433MHz is 'open slather' cowboy territory and if a device is suffering interference then the answer is generally 'Tough!'.
The real answer probably lies in between somewhere.
Susan VK3ANZ

I am not sure how 'large' it is in absolute terms. I meant in comparison to other noises being picked up by the handheld spectrum analyzer.

The transceiver's spec (Alinco DR135 Mk III) is -60 dB spurious suppression, measured by ARRL labs at -70dB. I don't have the equipment to make this measurement myself. If my calculation is correct, this is a handful of microwatts relative to 5W.

The RF Explorer full scale was set to -20dBm and the peaks at 433MHZ are at -42dBm.

73 de KE8DAF, Greg


Blitzortung Stations #706 and #1682
CoCoRaHS: MI-PI-1
CWOP: CW4114 and KE8DAF-13
WU: KMIROGER7
Amateur Radio Callsign: KE8DAF

 

anything