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C4FM Cross banding....hmmmmmmm Surely

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by W4EGE, Apr 15, 2018.

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  1. W4EGE

    W4EGE Ham Member QRZ Page

    I’m not sure if this has been talked to death yet or not, but at least with respect to the ftm400 is has not.

    I would like to pass C4FM DN through something (likely an ftm400) from an HT to a node. On paper this should work. Transmit VHF DN from an FT2D to VHF on a FTM400 and then out UHF (analog) to a node. Once that UHF RF is received by another C4FM radio/node it should be intact right? RF is I understand this is just a a matter of what the endpoints are using to decode the C4FM signal. It Should not matter what the machines are doing in the middle with that DN RF...on paper, it should pass through should it not?

    In support of that idea...I can hear C4FM encoding on my Ft60...but of course it can’t decode it. If that signal is heard by any other C4FM radio it would know what to do with that RF as long as it was in DN mode.

    So...what am I missing? Why wouldn’t a FTM400xdr in analog cross repeat mode pass a C4FM transmission?
  2. K7JEM

    K7JEM Ham Member QRZ Page

    It might work, but probably not. A C4FM signal will get distorted when passing through an analog repeater. Some of the low frequencies needed for proper operation will be cut off by the filtering in the repeater. If you could just take discriminator audio from the RX and apply it to the analog modulator, at the right level and with minimal filtering, it might work.

    Digital signals like this have to have a very flat response from nearly DC to around 3000 Hz. I think the analog connection in the middle will destroy the needed bandwidth.
    M0DPX and W4EGE like this.
  3. W4EGE

    W4EGE Ham Member QRZ Page

    Here is where the different levels of license are apparent. Thank you for your thoughts.
    I had not thought about the filtering aspect. That being said I had another thought.
    If regular fm analog modulation is 25khz would seem that C4FM at 12.5khz would sit nicely in the center of the transmitting/receiving frequencies. I would think NEGATING the need to filter anything since C4FM is more compressed.

    Shouldn’t a 12.khz wide carrier frequency pass through something intended to be 25khz wide (analog)?

    I guess I’m missing something still...I’ll call it technician ignorance!
  4. KV6O

    KV6O Ham Member QRZ Page

    The 25kHz wide is the conventional channel spacing used by hams for a 5kHz deviated signal. It's not 25kHz wide. It more like 16kHz for a 3kHz audio signal.

    An FM analog repeater is made to pass a limited bandwidth signal, and does so by demodulating the received signal, and re-modulating it for transmission, sometimes with different characteristics - flatter or altered frequency response, added or removed PL tones, etc. This arrangement will not pass the full 12.5kHz (or so) of a C4FM signal.

    A non-inverting linear transponder would "pass" the signal, but even then there will be degradation, which is why a proper digital repeater demodulates/decodes the received signal and regenerates it for transmission.
    W4EGE likes this.
  5. W4EGE

    W4EGE Ham Member QRZ Page

    Thank you for that!

    I talked to Yaesu USA Today and asked this same question to them. The response back was a bit of a surprise. It was simply a matter of the FTM400 only having a single digital vocoder on VFOA only. The technician stated that it’s possible that there could be a future FTM400 that will repeat C4FM...this was not the first time they had heard this question.
  6. W4EGE

    W4EGE Ham Member QRZ Page

    A brief update and a case study in tenacity.

    I finally got around to getting a FTM400 at a fair price. I was resigned with the thought that I would not be able to cross repeat C4FM. No issue. I was after the APRS side of the radio anyway.

    I got onto the idea of passing APRS packets through cross band mode on the ftm400. Turns out it works rather well. After a short stint on the sofa i thought about the nature of packet audio. It is data over RF. Then by extension pondered “is not C4FM just longer strings of data over RF?”

    I went back out to my truck and configured everything to route from my node to the FTM400 and then to the FT2D. In cross repeat mode the ftm400 defaults to fm on both VFOs.

    I configured the node and powered everything up. Well...simply stated: it works...without fault.
    It seems to work as if these systems were designed to do it.

    A shock to me considering the Yaesu guy I spoke with was certain that this would not work. The lesson learned is that you should always try it’s the back of the napkin logic seems sound. And also...try not to overthink things. I’m thrilled with this discovery and hope word spreads that it is possible to cross repeat C4FM through a ftm400. In the end RF is RF and my node now has an enormous range.
  7. KD4UPL

    KD4UPL Ham Member QRZ Page

    Given the fact that the Yaesu tech mention only a single vocoder he didn't understand your question. Passing the C4FM signal thru an analog cross-band repeater doesn't involve a vocoder. So, his answer was useless.
    I'm glad you tried it and I'm glad it worked. As I was reading thru the thread I was wondering why you didn't just try it instead of ask the question. I guess you didn't have the radio yet.
    I haven't tried this with my FTM-400 yet. I just got mine about 6 weeks ago. I ordered it when they had what I though was a good deal going, a $75 coupon. Now, they have this $170 coupon. I guess I should have waited. Oh well. I hope you got the "good" price of $394.
  8. W4EGE

    W4EGE Ham Member QRZ Page

    I did not have the 400 yet. I was asking at the beginning when I was searching for a new mobile.
    All that being said...I tried the cross repeat again today and COULD NOT replicate the connection. I could pass the APRS no problem. So now I’m not sure exactly how I got it to work last night. I can hear the C4FM on the HT...but DN cant decode anymore. Maybe there is a new noise messing up the packets...I’ll sort this out tomorrow I hope.

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