NXDN Repeaters Using 6.25

KC3ECJ · Jan 7, 2016

Can one repeater do two 6.25khz channels at once?

Or are seperate repeaters needed?

Would an all in one unit that contains two transceivers be considered 2 seprate units for licensing?

Can somebody(preferably somebody that owns or maintains such a system) clarify?

mmckenna · Jan 8, 2016

Kenwood's = no
Icom's = maybe, if they have both RF modules installed.

I'm running all Kenwood NXR-900's, they are single frequency at a time.
Many years ago when I was looking at the options, I looked at Icom. At that time (3+ years ago) the IDAS repeaters were single channel, but you could add a second RF module.

NXDN is an FDMA format, not TDMA like DMR.

I've heard of people running two 6.25 channels in a 12.5KHz envelope, but I've never done it myself. Likely there is an Icom LMR wonk on here that could clarify how they do things.

My 800MHz NexEdge trunked system is running 5 channels, all spaced 500KHz apart through a transmit combiner to a single TX antenna. RX antenna is connected to a tower top amp and fed down to a RX multicoupler.

W2GLD · Jan 8, 2016

With ICOM, specifically the IC-FR5000 and IC-FR6000 repeaters, each 6.25 kHz channel requires it's own repeater module; however, with this series of repeater from ICOM, both modules can fit inside a single 2U 19" rackmount chassis. Also, keep in mind that these 6.25 kHz channels should not be adjacent channels, unless you are going to add a ton of filtering.

A better solution for running two channels in a single repeater would be to utilize DMR (a.k.a. MOTOTRBO). What happens in this platform is the repeater is centered on a 12.5 kHz channel; then it splits two "timeslots" using TDMA architecture to accomplish the same goal with less infrastructure. With this platform, you need a single repeater, a single duplexer, and a single antenna.

domes · Jun 8, 2016

W2GLD said:
With ICOM, specifically the IC-FR5000 and IC-FR6000 repeaters, each 6.25 kHz channel requires it's own repeater module; however, with this series of repeater from ICOM, both modules can fit inside a single 2U 19" rackmount chassis. Also, keep in mind that these 6.25 kHz channels should not be adjacent channels, unless you are going to add a ton of filtering.

A better solution for running two channels in a single repeater would be to utilize DMR (a.k.a. MOTOTRBO). What happens in this platform is the repeater is centered on a 12.5 kHz channel; then it splits two "timeslots" using TDMA architecture to accomplish the same goal with less infrastructure. With this platform, you need a single repeater, a single duplexer, and a single antenna.

Just a ruff estimate on my part but I think you may be off by about 1,959 lbs & 14 oz. You can use a 2-ch hybrid combiner to combine the 2 close spaced transmitters. These are just isolators and a coupler and is about the size and cost of a good duplexer. They only weigh about 20lbs. You can plug the antenna output of the hybrid into your typical duplexer, also about 20 lbs. On the receive side of the duplexer you only need a 2 oz. T connector to connect the 2 RX's to the duplexer. The only concern on the receive side is the T will produce about half dB loss and each receiver will add another dB loss. A tolerable loss. I've done this myself. With good filter engineering you could eliminate the TX half of the duplexer and save 10 lbs and a few $. I have never looked inside ICOM's magic 2 for 1 box but I suspect that this is their configuration.

The myths of combining NXDN channels are highly over-exaggerated.

domes · Jun 8, 2016

2 NXDN repeaters on one antenna trunked together will give you real 2 channel trunking efficientcy on 12.5. Not quasi-kinda-sorta-maybe trunking. What you do on one channel won't block use of the other.

fwradio · Jun 9, 2016

You can use two 6.25 kHz channels inside the 12.5 kHz envelope. The FCC allows it if properly licensed and it does work (speaking from experience). You have to license the 12.5 kHz center and the 6.25 kHz channels, so each pair of channels is actually 3 on the license. Icom put a white paper out about it.

petnrdx · Jun 16, 2016

I keep seeing people post that they have combined two 6.25 transmitters.
Sounds interesting.
I have experience with combining. LOTS of VHF, a little UHF. Less on other bands.
Never that close.
Always around 150 khz or greater at VHF, more at higher freqs.
Knowing that freqs are getting rarer in a lot of places, you have to utilize the pairs you have.
So, those of you that are doing this, what kind of loss are you experiencing?
I would expect a total of 6 dB ish minimum on a two channel at VHF.
Which is not that bad, compared to not having a frequency to use.
At that spacing, has to be hybrid.
Someone want to describe what they have done that is working?

domes · Jun 18, 2016

If you don't have much experience with 6.25, there is something you should know about duplexers that is often misunderstood. Narrowing a receiver improves the sensitivity. If you look at specs of various radios & repeaters, the typical WB 25KHz receiver is spec'd at .35 uv sensitivity. .30 at 12.5 and .20 at 6.25. That is quite an improvement and is one of the reasons that 6.25 NXDN out preforms NB FM or any other digital radio. If you are trying to reuse a duplexer, you may find that a duplexer with 70-80 db TX/RX isolation that worked well with 50 watts and .35 Rcvr and just good enough with .30, may not provide enough isolation with a .20 rcvr. This is especially true with those close VHF pairs. Receiver desense is hard to recognize with digital because you can't hear the telltale signs, you just have poor performance. You need to use a good hi spec duplexer with 6.25.

petnrdx · Jun 20, 2016

Good point.

Karl-NVW · Jul 19, 2016

Time to get the RIGHT loss numbers out here: the loss per side of the two-port transmit hybrid is 3.4 dB to the output port, and the loss from using the ultra-lightweight BNC T connector paralleling the two receivers is 3 dB. Both are added to whatever insertion loss you already have for the existing duplexer.

NXDN Repeaters Using 6.25

KC3ECJ

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mmckenna

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W2GLD

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domes

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domes

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fwradio

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petnrdx

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Karl-NVW

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