Analog crossbander repeating C4FM?

It should be possible to use a conventional analog crossband repeater to relay a digitally modulated signal, such as C4FM.

I want a crossband repeater to use at home because I have a hard time reaching the repeater with my handheld. Now my group has decided to upgrade our repeater and buy one capable of using System Fusion. Therefore I am holding a bit back. At the time of writing this the C4FM capable crossband repeaters are fairly expensive. I feel it would be somewhat of a waste to buy an analog crossbander if I cannot utilize it fully when the new repeater is in place. But I firmly believe that an analog crossband repeater should be able to forward C4FM without problem.

I raised this question on Eham`s forum, but got turned down quite quickly. It`s easy to understand why: An analog radio is not able to generate or decode digital signals like C4FM. That`s obvious, but I`m still not convinced. If the signal is already generated, I cannot with the best of my intentions understand why it shouldn`t be relayed by a mobile radio in crossband repeat mode. Here`s why:

C4FM is a kind of FSK

C4FM is a kind of frequency shift keying, FSK. By altering between four different offset frequencies the radio transmits two binary digits at a time: +/-900 kHz and +/-2700 kHz. These offsets are overlayed on the carrier frequency. The deviation of C4FM is the same as conventional FM.

These frequencies are all within the passband of an analog FM transceiver. The digital signal would be heard as “noise” through a conventional FM transceiver, much like other kinds of FSK. In other words, an analog repeater would treat the signal as ordinary sound. Relaying sound in the form of FM is exactly what an analog repeater is designed to do.

The type of repeater probably matters

Because there are several different kinds of crossband repeaters there have to be several different ways they handle the signal. The idea of passing a digital signal through an analog repeater can only be accomplished as long as the repeater does nothing else to the signal than transverting it to the new frequency. All information must be retained and forwarded the exact same way it arrives at the antenna. That is, if the repeater does some kind of processing to the audio, we cannot be sure that the signal is intact after it leaves the repeater.

Similar things have been done before

When searching to find the answer I ran across VE6CPK`s site. He claims to have done something like this with D-star and C4FM. But the mobile radio he used was the Icom 5100 and the Icom 2820. Both of these are already D-star enabled, and they may handle the signal differently compared to “your random analog repeater”.

On the contrary, NC5P reports that he has tried doing this with the Yaesu FTM-400 without success.

Testing the hypothesis

I would really like to try this myself, but for now I am unable to because I don`t possess the required equipment.

In a few days I receive my new Yaesu FT-70DE. I will try to use one of my analog transceivers to record a digital transmission sent from the FT-70DE, and then send the playback in return from the analog transceiver. If the FT-70DE is able to demodulate the signal, there is absolutely no reason why an analog crossband repeater shouldn`t be able to forward the same signal.

This method has its weaknesses. C4FM uses identifiers to tell the receiver who the transmission is from. With the ID being identical to the receiving transceivers ID, we could run into trouble, but not necessarily. This experiment may be able to confirm my hypothesis, but a failed experiment doesn`t mean the hypothesis isn`t true. Also, if the experiment is a success, I still won`t know which crossbanders that will successfully forward C4FM and which won`t. This may work with some radios, and not with others.

I also plan to team up with a fellow radio amateur or two, and test out this hypothesis some day, when we have the appropriate equipment at hand. Maybe we`ll be able to test this with a few different repeaters.

Summary

There should be no reason why a crossband repeater in general shouldn`t be able to forward a digital signal, unaltered. For this to happen the repeater cannot process the audio in any way. It can only transvert to a higher or lower frequency, and then pass on the signal.

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