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Repeater Channel Spacing with Digital & Analog Repeaters?

Discussion in 'Ham Equipment' started by Moleculo, Feb 23, 2012.

  1. Moleculo

    Moleculo Administrator Staff Member

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    I've been trying in vain to coordinate a frequency pair for my D-Star repeater. (I will have a lengthy rant about the effectiveness of the local coordination body in another upcoming thread.) Since I'm not getting any help, I'm going to have to pick my own frequencies, put the repeater up uncoordinated, and do my best not to interfere with the coordinated repeaters.

    I want to make sure I understand the bandwidth characteristics of D-STAR before I finalize my pair. For reference, I've studied this page, which has some very good information on real tests that they've performed on the bandwidth usage of D-STAR as compared to a typical analog signal: The Utah VHF Society - D-Star channel spacing recommendations

    Here's an interesting spec analyzer plot showing D-STAR on the left and analog on the right:



    [​IMG]

    In the summary table on that page, they show that the typical -30db bandwidth for D-STAR is about 11khz and -55db bandwidth at about 14khz. They go on to recommend a minimum channel spacing of 12.5khz for D-STAR segments of a coordinated repeater plan, although they said that you could get away with 10khz if it wasn't for random frequency stability issues present in most transceivers.

    In Southern CA, the 70cm band is channelized into 20khz segments for repeaters like this: 445.020, 445.040, 445.060, etc. If I understand the spec plot above as well as the analysis on the referenced page, I think I should be able to squeeze a D-STAR repeater between two distant analog repeaters on a frequency like 445.030, right? If my TX -30db bandwidth is about 11khz, my D-STAR repeater would take up roughly from 445.0255 to 445.0355, right? If I'm reading their chart right, the -30db bandwidth of a typical analog signal is 17-18khz. So, if I squeeze my station between them, it looks to me like I'm more likely to receive interference from other repeaters than cause interference.

    Am I looking at this correctly? The machines I have in mind of squeezing between are so far in the distance, I don't think it will be a problem even if I was right on top of their frequency, however I'm obviously trying to be careful if I have to do this uncoordinated.
    #1

  2. W5LZ

    W5LZ Crotchety Old Bastard

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    Mole',
    I think the problem you have is that there are a limited number of usable 'channels' for a repeater (on any band), and the availability of on empty channel just isn't there.
    A 'digital' signal is typically narrower than an 'analog' signal sure, but the difference isn't that great. That depends on just how well the transmitter is adjusted, which isn't going to be the best in all circumstances. So, that 2 Khz space between signal edges is probably the best compromise.
    Squeezing between two allocated channels could be possible if propagation never changed (deals with the distance to that other two repeaters), but it does. You run into that with 'properly' coordinated repeaters quite frequently. I'm sure you can imagine what it would be like if you throw an uncoordinated repeater into that mix.
    I don't know what the best solution would be. Maybe acquiring an already allocated freq.pair? And I can guess how likely that would be.
    Good luck.
    - 'Doc
    #2
    1 person likes this.
  3. hookedon6

    hookedon6 W9WDX Amateur Radio Club Member

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    well, all I can say is: there are 2 good days in a repeater owners life, the day you buy it and,......:whistle:.


    good luck dealing with the Kilocycle Kops
    #3
  4. Moleculo

    Moleculo Administrator Staff Member

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    In SoCal, 2m and 70cm are coordinated by two different coordinating bodies. Here's an interesting piece of information that I just discovered: The 2m coordinating body has carved out a piece of the band for digital repeaters (D-STAR) and they've gone to 10khz spacing. Some of the frequencies are within 10khz of frequencies allocated to analog repeaters. I would have to study the list to see if there are any with 10khz of each other in close proximity, but the fact that they've gone to 10khz spacing tells me that it probably is possible to do what I've described above.

    An interesting side note is that it seems that the 2m coordinating body (TASMA) has embraced and being more proactive about finding ways for these two technologies to coexist. The 70cm coordinating body (SCRRBA) seems to be a lot less cooperative.
    #4
  5. FatHam

    FatHam Administrator Staff Member

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    And I gleaned some info from a TASMA guy this morning that SCRRBA isn't pro-actively dealing with the new narrow band formats AT ALL. They've got all kinds of band width that they're protecting so "Floyd" can have his three member "Sunday Night Net" once a week, even though the frequency isn't in use six days and twenty three hours out of the week.

    He also pointed out that every Mormon Temple in So. Cal has an "emergency pair" just sitting there on a "closed" repeater, waiting for the call to go out that it's time to break out the rations in the garage.

    He went on to say that it took going over TASMA's head to the FCC, complaining that new technologies involving narrower bandwidth are the future, yet the coordinators that they've assigned to ride heard on it in So. Cal were not recognizing the newer technologies. Perhaps it's time to start a letter writing campaign. Obviously it worked on TASMA.
    #5


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