2 Meter Repeater 'Noise'

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by K7IHI, Sep 1, 2014.

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  1. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Also, unlike commercial two-way equipment, amateur radio equipment does NOT usually employ a "roll off" filter to eliminate the CTCSS tone. As such, the CTCSS tone can often be easily heard when amateur radio equipment is used.

    Glen, K9STH
     
  2. KJ4TX

    KJ4TX Ham Member QRZ Page

    Interesting, so you mean that the annoying hum is per design and not a defect? It's the first time I've ever heard a repeater do that but then I've not spent a lot of time around repeaters in the past. The only reason I do now is because my Icom 746 Pro has 2 meters on it and I've been doing a lot of SSB and simplex on 2 meters... or at least as much as there is activity in those areas. I do regular sweeps through the band to see what is going on in the repeater areas when I'm sitting at the radios and stop to listen when ever I hear anybody talking. You guys may be right then although it still sounds like annoying 60 cycle hum to me. And it's almost at voice level, which sounds pretty loud for CTCSS tone. I'll still ask someone if I get a chance or if I make it to one of the VHF club meetings one of these days, just to satisfy my curiosity.

    Mike
     
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    Do You have a Frequency counter ?

    Hook it to your audio Output and measure the frequency.


    Have Fun.
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page


    I'm not there, so can't hear it.

    But do you hear this loud "tone" or "buzz" or "hum" on the rig's internal speaker, or are you using an external speaker or headphones?

    I can hear "every repeater's" PL tone if I use headphones or a large speaker; however it's usually pretty soft if I use the rig's internal speaker, which doesn't reproduce much below 300 Hz.

    And it's certainly possible they have it cranked up too high. Usually CTCSS modulation is only about 500 Hz deviation or so, although there's no regulations about this; I've just found that's usually about all it takes to work. Some systems may have it cranked up higher.
     
  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    TX:

    There are at least 3-possibilities:

    1. The repeater CTCSS level may be set higher than the maximum of 1/10th the deviation of the system. That would mean +/- 500 Hz deviation for the CTCSS tone. However, with modern, solid-state, CTCSS decoders, the CTCSS deviation can be set even lower.

    2. Your speaker may have much better performance at lower audio frequencies which would emphasize the CTCSS tone.

    3. Your receiver may have much better performance at lower audio frequencies which would emphasize the CTCSS tone.

    Since you are having the same problem on several repeaters, I would definitely suspect #2 and/or #3.

    Commercial two-way FM receivers, that have CTCSS decoders, have a high pass filter in the lower audio stages which greatly attenuates audio frequencies below 300 Hz. Most "add on" CTCSS encoder/decoder boards have such a filter included to make sure that the CTCSS tone is severely attenuated in terms of the speaker audio. Amateur radio equipment very seldom has a high pass filter to eliminate the CTCSS tone from the speaker audio.

    Even with repeaters that do not regenerate the CTCSS tone on transmitting, many repeater receivers also do not have high pass filters in the receiver audio and, as such, the tone from the station being received on the input frequency is passed through and is definitely heard on the repeater output.

    There are CTCSS high pass filter circuits on the Internet that can be added to receivers that have a problem with the CTCSS tone being heard in the speaker audio. It is generally possible to add such a filter to most mobile amateur equipment.

    Glen, K9STH
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Easier answer: Check with a dozen other users and see if they hear the same thing.

    If everybody hears it, something's probably wrong.:p
     
  7. KJ4TX

    KJ4TX Ham Member QRZ Page

    I don't have a frequency counter, but I did download an app program that will let me use my sound card to read the frequency. I just need to install it on the laptop I have on my radio desk.

    For speakers, I'm using both headset and a large (sorta) external speaker. It's called a center frequency speaker and works better in the voice frequencies than the other external speakers I've used in the past. I almost never use the internal speakers unless I have to for some reason. You guys could be right that it's just me. Although I've heard it mentioned by others on the repeater, it's only been a very few compared to how many are using it... and it is really annoying to me. well, at least I learned something. :D

    Mike
     
  8. KB0MNM

    KB0MNM Ham Member QRZ Page

    Mike,
    For filtering audio, they used to build a stereo component called a 'graphic equalizer', back in the days before Doctor Dolby had his way with compression and expansion. Your PC may or may not have the ability to take the output of your radio and pass it through a similar system via the 'sound card' interface. If you are the only one with the issue, it is possible that the speaker amplifier is the reason why you hear the CTSS as noise, so this would be effective. You also could be hearing 'hum' caused internal to your radio in the power supply section. For many folks, a cleaner 'feed' means installing a 'brute force' power supply filter, basically a series choke with capacitive input and output in parallel. This could conceivably decrease the life of your radio's power supply if it does not also include a 'clipper' circuit. Folks like the Tandy / R.S. and Kenwood have been selling these 'brute force' filters for years- usually to overcome alternator noise in mobile vehicles. If the radio does not make the 'hum' when connected to a battery supply ( no alternator ), yet does when connected to your power supply- you might have reason to look at that part of the system. No need to buy a 'brute force' filter if the radio 'hums' the same on 'clean DC' as 'noisey DC input'. Hum can also be part of a bad ground for your antenna system, ie. ground loop. If you can only hear the problem on a repeater with the low 'sub-audible' CTSS tone, no need for looking at the filter or antenna. Best of luck. KB0MNM



     
    Last edited: Sep 6, 2014
  9. KB0MNM

    KB0MNM Ham Member QRZ Page

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