VHF noise in '95 Toyota Tacoma

Discussion in 'Mobile Radio Systems' started by KM6LYW, Aug 23, 2018.

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  1. KM6LYW

    KM6LYW Ham Member QRZ Page

    Frying bacon noise is pretty bad when tuned to weak VHF stations. I confirmed noise is coming in through antenna (using portable battery) and can hear it on HT. Noise is in engine compartment, front of engine, near ignition/efi wiring harnesses.

    I put a dozen RF chokes on everything, except fuel injector wires (hard to get to). Haven't found ECM yet. Fuel pump doesn't seem to make noise (key turned to run, but engine not started).

    Does anyone have experience silencing older toyota tacomas? (1995 v6).

    I've spent a few hours reading all the pseudo-science on the web to no avail. Looking for anyone who has silenced a similar setup. Any empirical observations or success stories? Reading and implementing all the theory hasn't helped at all.

  2. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Doesn't make any difference what you do! The RFI is no doubt caused by the ignition system, but that isn't why you hear it.

    Most modern amateur VHF transceivers are not true FM. If they were, you wouldn't hear any amplitude modulated signals, although you would have defense caused by them. Nowadays, they are PM (phase modulation). These too shouldn't receive AM RFI, but they do because there cost-performance ratio isn't very good. This fact makes them affordable. And too, everyone, it seems, wants to receive all manor of VHF frequencies, including the AM-based FAA frequencies. The net result? AM Reception, especially at a low SNR. You either live with it, or get a better antenna setup, if possible (improved SNR).
    KM6LYW likes this.
  3. KM6LYW

    KM6LYW Ham Member QRZ Page

    Thanks for the info. so, "better antenna setup," what would you suggest? Antenna is currently a little nagoya magmount on the cab. A higher gain antenna would increase signal proportionally with noise wouldn't it? how would it change the ratio of signal to noise? From what I've gathered, a weak signal causes the RF gain to automatically go up, pulling in all kinds of noise with the weak signal. A strong signal (S9) probably turns down the RF gain, which squelches the noise. So increase overall signal strength with high(er) gain antenna so all VHF signals are strong?
  4. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    In most VHF cases, it is NOT and RF gain issue. Rather the AGC cuts back the first IF gain, leaving the RF stage wide open. The key here is, to increase the SNR (signal to noise ratio). Drilling a hole, and using a decent antenna is a step in the right direction.

    So there is no mistake here... Mag and glass mount antennas have an excess of common mode, which means the coax cable is part of the antenna—not just a feed to it. In fact, the coax radiates more RF that the element in most glass mounts. Mag mounts are a bit better, but not by much.

    By the way, if the engine in your Toyota is a V6 with COP units on one side, with plug wires to the other side, replacing the plugs and wires will help a little, depending on their age.
    KM6LYW likes this.
  5. KM6LYW

    KM6LYW Ham Member QRZ Page

    engine is COP... plugs replaced 30K ago, though, I'm sure they're cheap. Will confirm resistive-type plugs and wire. I put chokes on the primary and plug wires, no change.

    I almost put a choke on the feedline to kill any common mode current, but it sounds like that's actually desirable on magmount?

    I'd like to at least reduce the noise. I use APRS quite a bit, and a pops and crackles kill all but the strongest packets. I tried moving the magmount to the tailgate, but it surprisingly didn't help, at all.

    thanks for the hints!
  6. KF5FEI

    KF5FEI Ham Member QRZ Page

    I've noticed mag mount antennas pick up noise from both my vehicle and passing vehicles -- but the noise from my vehicle is reduced or goes away when I use an installed antenna. On the wife's car, it is an NMO mount in the middle of the roof, and on my car, it is a Comet lip mount.
    KM6LYW likes this.
  7. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Common mode current flow is a two-way street—signals can get out, and signals can get in. In other words, common mode causes the feed line to become part of the antenna, both receiving and transmitting. So think about this! If the common mode current flow it high enough, you're in danger of too much RF exposure from the coax! And this doesn't address the RFI issues, both ingress and egress. Just another reason to sans shortcut installs.
    KM6LYW likes this.
  8. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Phase modulation, PM, was also used on most of the tube type commercial two-way "FM" units. In fact, PM was used in a lot of solid-state units like the Motorola Micor. However, when used with "DPL" (digital private line), the microphone audio was PM but the DPL was generated using true FM applied to the channel element.

    Glen, K9STH
    N5RFX likes this.
  9. N5RFX

    N5RFX Ham Member QRZ Page

    Later synthesized radios like the Syntor, Spectra, Genesis (MT1000) etc. used 2 spot modulation. It is similar to what your described. The higher frequency modulation went to a varactor at the VCO, the low frequency component went to a varactor at the reference oscillator. The DMR radios (non-Motorola) that I have schematics for, also use 2 spot modulation.

    Back to the noise issue, I have a Motorola Spectra (110W) and a Connect systems CS800 in a 2010 Toyota Tundra and do not have any noise issues. I have not tried a PM radio in that vehicle.

    Mark N5RFX
    KM6LYW likes this.
  10. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    FM or PM is in the transmitter, both modes use exactly the same receiver. The old Foster-Seeley discriminator, with a couple of limiter stages, was, for all practical purposes, immune from noise which is primarily AM. However, more modern ways of demodulating FM / PM are not, usually, as immune from noise as were the discriminator types of receivers. But, the newer types of demodulation are definitely considerably smaller in physical size than the old discriminators.

    Glen, K9STH
    KM6LYW and N5RFX like this.

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