VHF frequencies, repeaters etc





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  1. #1
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    Default VHF frequencies, repeaters etc

    OK since I hav'nt started "studying" yet, and cannot get my hands on one of those dummies book I have some questions, hopefully the hammers here can be of assistance.

    Now please excuse the ignorance, terminology etc, I am still learning there!

    You have a VHF frequency allocated to your by ICASA.

    When using repeaters I understand that the TX and RX is different, must they be on different frequencies, or can it be on a single frequency split with tones?

    What I am trying to achieve is to have 4 seperate communication channels, where we have 3 areas that will each use their own channel, and the 4th channel will be open for shared use by all three areas.

    I am busy with a motivational letter to ICASA and need to know that before I make my name completely bollie.

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    I would say you would be spot on there. But thats my opinion .It would then be nice to have all 4 channels programmed into the radio.
    ORA
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    Ian

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    Dirk, all repeaters have two frequencies, separated by either a large band of frequencies in the case of Commercial frequencies, but narrower, in the case of Amateur radio (Hams).

    You will have, for example, a Commercial Repeater TX frequency of, say, 148,500MHz matched with a RX frequency of, say, 153,500. The splits are not all the same, some are 4.5MHz, others 5MHz, or 6.25MHz, 7.0MHz, etc. But Amateur Radio Repeater frequencies vary by as little as 600kHz.

    There are Simplex (same frequency) Repeaters, known as "Parrot" Repeaters: These are usually used for radio alarm systems, Paging systems, etc, and ICASA has been known to approve them for Voice frequencies for certain applications. What happens with this type of repeater is that it receives a signal, then re-broadcasts it following a short delay. Sounds kind of weird if you transmit a spoken message and then hear your own voice repeat the message a few seconds later . A Parrot Repeater is not recomended for general usage however.

    ICASA grants Repeater frequencies on a per application & motivation process. The larger Metropolitan areas (JHB, CT etc) quite often have a waiting list for Repeaters because the available repeater channels are all taken. The smaller areas generally get Repeater Licences easier, but with a minimum of 40 radios on the Application.


    -F_D
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  4. #4
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    I understand, but can use use a single frequency with splits for the tx and rx on a repeater, or must it be on different frequencies?

  5. #5
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    No, a Repeater "pair" is always a dedicated pair of frequencies, neither of which is ever used as a single (Simplex) frequency. It is, of course, possible (but not legal!) to use two different Simplex frequencies for a Repeater, but they have to be separated by around 5MHz or so. Using the same Simplex frequency with two different tones will not work, then you are back with a Parrot Repeater again.

    You can apply for a VHF Repeater frequency as well as a Simplex one, all depends on the motivation letter, the number of radios, and the availability of frequencies in your area. A Repeater frequency is generally not used more than once in a 200Km radius of the Repeater, to ensure non-interference with other users.


    -F_D
    Eric Skeen is the Family Dog
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    Dirk, you cannot run a conventional repeater on just a single frequency.

    "Split" = two different frequencies.

    Tones will not solve the problem either.

    A lot of radio manufacturers make it sound as though tones somehow provide privacy or give you extra radio channels. Nothing could be further from the truth. Tones give you neither privacy nor extra channels.

    The only way you can have any kind of repeater working on just a single frequency is the parrot method mentioned above.

    Parrot repeaters are all but useless for the kind of application most people, including 4x4 folks, require.

    The only way it's going to work is with two separate frequencies.

    A repeater has an input (receiver) and an output (transmitter).

    When a mobile station transmits, it does so using the input frequency of the repeater so that the repeater "hears" the mobile station.

    While receiving the signal on the input, the repeater simultaneously transmits (rebroadcasts) the audio (the mobile user's voice) from that signal using a transmitter set to the output frequency.

    The other mobile stations would, at that time, be receiving the signal being transmitted on the repeater output frequency.

    If you attempted to make the input and output frequency of the repeater the same you'd have a huge mess because repeater would be "hearing" itself the whole time. The output of the repeater would be received by the input of the receiver and the whole thing would fall over.

    Additionally, if a mobile station were to transmit you would then have two transmitters (the mobile and the repeater output) operating simultaneously on a single frequency, which would result in the two signals interfering with each other and no one would be able to hear anything at all.

    It would just be one groot gemors.

    The parrot repeater gets around all of this be working according to the following process:

    Parrot repeater sits there "listening" for a signal.

    Mobile station transmits and sends its message.

    Parrot repeater makes a recording of the message while the mobile user is talking and transmitting.

    As soon as the mobile station stops transmitting, the parrot repeater plays the recording back, transmitting it on the same frequency for all the mobiles to hear.

    In this way, only one transmitter is active at any time and the parrort repeater is only ever receiving or transmitting (not both) at any given moment, unlike a traditional repeater which simultaneously receives and transmits.

    Tim

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    Dirk,

    I see you are in good hands. I agree with FD and Tim. It is far more complicated but is correct as explained.

    FD, since it seems as if you are on the commercial radio side, perhaps the forum and the 4x4 in general might think of going the Trunk Radio option. Maybe you have more info. It could work for the 4x4 people county wide. As far as I know there are 2 service providers and it has some limitations regarding coverage in remote areas and short range comms. I'm just not sure what the financial implications are.

    Regards.
    Andre ZR6AGA 8)
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    Hi Andre,

    Nice one! We deal with Fleetcall.

    A "Trunked Radio System" is basically "Cellular Radio", as opposed to Cell phones. It is a good option to HF/SSB, but the downside is that it is only available in the major Metropolitan areas and along National Highways. It is a "Wide-Area" means of radio communication, rather than Country-wide. But it certainly has its advantages! One can talk from, say, Cape Town to Polokwane (sp?) quite comfortably and reliably at all times.

    The downside is the price of the radios, running at around the R5000 mark for a Kenwood TK785. Excellent radio, though! The monthly airtime costs range from around R100 to R350 per radio, depending on the area coverage required.

    I'm not sure what the coverage would be in the areas that people like playing, and for off-road use it will have its limitations if there are no Trunked Radio Base stations nearby. But, if enough people use a certain area in the country often enough, Fleetcall will usually install a Base Station nearby, as long as there is a guarantee of sufficient radio traffic.

    An advantage to the user is that he is not required to apply for an ICASA licence as the RF Communications licence has already been issued to Fleetcall (and also the second Trunk Radio Operator, Q-Trunk). Of the two, Fleetcall seems to have the larger network.


    -F_D
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  9. #9
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    Ok thanks chaps you have answered my question.

    Btw this question is not forum related, but rather neighbourhood watch.

    So if I want to establish communications via repeater then we would have to apply to two different frequencies.

    We are currently using simplex, but over a short distance it's really nice and clear, but rapidly deteriorates to being totally useless, thus the need for the repeater. We are currently using a sponsored repeater, but it's very busy at night when we need it most, between 10pm and 4am. Pointless having to wait for 3 minutes just to talk.

    FD, how would I then go about to apply for a license for use on a repeater without any simplex on it? To apply for 40/50 radios is definitely not a problem, we already have close on 20 radios on the network in our area.

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    FD, how would I then go about to apply for a license for use on a repeater without any simplex on it? To apply for 40/50 radios is definitely not a problem, we already have close on 20 radios on the network in our area.

    Dirk, You would use the same application form as is loaded in the Radio Licence section of the forum. Give me a shout at work tomorrow and we can follow it through, as the info is slightly different to what I have added here.


    -F_D
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  11. #11
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    Will do, but it might not be tomorrow.

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    FD,
    That is tooooooooo much. As far as I know, Q-Trunk had some Security channels that were seperate from the normal trunk channels that was in normal voice use. I attended a presentation some 15 years ago and it was the case then. I was then involved with radio and technical security as jack of all trades, master of none and at the same time combining both fields with computer technology and running a electronic maintenance centre.

    Dirk,
    Good luck. Seems like you want to do it right. With the radio spectrum as is today, don't take risks. Do your radios the proper way and not like some security moegoes did in Pta. They caused interference into other users spectrum and they were really thrashed by ICASA.
    Andre ZR6AGA 8)
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    Dirk what is it that you want to do with the four area's and how far away are these area'a from each other. Do you realy need a repeater or will simplex operation work between the area's. ?

  14. #14
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    Quote Originally Posted by Knersus View Post
    Dirk what is it that you want to do with the four area's and how far away are these area'a from each other. Do you realy need a repeater or will simplex operation work between the area's. ?
    Simplex works, using mobile unit. Handhelds are not good enough. Comms are there, but scratchy and unclear from boundary to boundary. So the only way to get clear comms is via a repeater. We are busy with our application, we have already identified and secured permission for the installation site.

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