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Don Kemp's avatar

A friend of mine ( now a silent key) put a simplex repeater up on 145.510. the antenna was atop of a 450 ft. tower in Columbiana county, Ohio. The coverage was phenomenal. Stations out of local simplex direct range had no problem. Stations who could hear the primary signal could not get a grasp that you had to wait till you heard the second repeated primary signal and screwed everything up. There will be a steep learning curve until the local repeater oriented guys finally figure it out. 73, Don, NN8B

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Steve Stroh N8GNJ's avatar

Don: The best thing about TIME DOMAIN DUPLEX Single Frequency Repeaters is that they're (at least to the ear of us mere humans) Full Duplex, so that irritating and confusion delay waiting for the other person to finish their transmission isn't a factor.

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Paul Elliott's avatar

- QRP Labs SSB using Polar Modulation -

Here is a great youtube presentation by Hans about this. He details the trials and tribulations along the way to finally making it all work:

https://www.youtube.com/watch?v=bsDBcFXgHbY

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Steve Stroh N8GNJ's avatar

Paul - Thanks for that. Will mention that next issue.

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Uncle Milburn's avatar

I've been using https://www.wm7d.net/perl/ulsquery.pl as my lookup for decades.

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Steve Stroh N8GNJ's avatar

Uncle - That's a nice one too. I briefly got sucked into WM7D's links - that's quite the rabbit hole.

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Justin AB3E's avatar

Man the SFR stuff is even more tempting now that I know there is actual repeater hardware that presumably can handle longer-term high duty cycles...

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Steve Stroh N8GNJ's avatar

Justin - Agreed! My mind is still reeling about the potential of TDD SFRs.

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K4HCK Cale's avatar

Kudos to WB2OSZ for trying to implement some organization to the APRS spec. And who else is better positioned to do so? If you've ever watched the output of Direwolf scroll through a terminal window, the lack of clarity around the spec becomes clear. Lots of comments along the lines of "this is probably a message from station X"

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Steve Stroh N8GNJ's avatar

Cale - Agreed! WB2OSZ's work on collecting all the ephemera into a modern APRS specification is, in my opinion, nothing short of heroic.

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Werner OE1WRS's avatar

Tetra uses single frequency repeater in DMO mode, we do have a number of Tetra repeaters in OE land (and other countries) which work on one frequency without duplexer etc. those are even networked so when you talk on one in one city you are also heard on the other repeaters in other cities (or on mountains). Most Tetra terminals can act as repeater….

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Steve Stroh N8GNJ's avatar

Werner - Thanks for that info. Will correct my statement saying that Tetra can't do TDD SFR.

For those, like me who aren't very familiar with TETRA, see https://www.teltronic.es/en/what-is-dmo/

In radio communications, unlike Trunked Mode Operation, TMO, in which the transmissions are made through a TETRA network infrastructure, the DMO, Direct Mode Operation, describes the ability of TETRA radio terminals to communicate among them operating independently from the network, as if they were simple walkie-talkies.

However, the most frequent use of the DMO is to create an extension of the network allowing communications in areas where there isn’t coverage. Through gateways, DMO users can maintain contact with TMO users. In this way, the radios operate like mini base station that can provide service to others that are outside the network.

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Jerome's avatar

The Anytone AT-D578UV series of mobile DMR has supported SFR for years and, about two years ago, Radioddity introduced the GD-88 handheld with that capability. Radioddity advertised SFR as a huge thing, yet failed to be noticed by the ham crowd.

The main problem with SFR is that it needs to be supported by the code plug of the handhelds connecting to the repeater. It also needs the handhelds to support DCDM (Dual Capacity Direct Mode), not all do.

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Jerome's avatar

Something else about SFR: they don't really increase the spectrum available. A classic DMR repeater will allow two communications on two 12.5 KHz channels, a SFR will allow one communication on one 12.5 KHz channel. They may even jam the bands a tiny bit more, as they produce a bit more spurious because they use simple filters while classic repeaters must use cavity filters for their duplexer.

The real interest is ease of deployment. Find a free frequency and a simple device can make an ad-hoc repeater. The use cases given by Retevis in the link you get for the HD1 correspond to that ease of deployment: search and rescue in mountains, concert venues, etc... For a dual frequency repeater, you need to tune the duplexer on the frequencies and that takes time and somebody with a VNA. A SFR just needs one to enter the frequency on the VFO.

Last but not least, the bulk of amateur operators are not interested. The majority sees DMR as a way to use a hotspot and doesn't even realize that it can work simplex or that a repeater may be available nearby. Search and rescue or Emcomm people in general refuse to use anything but FM. For Emcomm that is even more unfortunate, as DMR capacity to send text messages is invaluable.

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Steve Stroh N8GNJ's avatar

Jerome - Thanks for your substantive comments.

I agree that SFRs don't, technically, increase the "spectrum" available. But because they are single frequency, they don't have to be coordinated by "the repeater pair guardians", and thus enable the use of (SFR) repeaters in cases where a conventional repeater simply wasn't practical.

I posit that SFRs are still a net gain (beyond the tactical use cases you describe). A use case for SFR that I can easily imagine is a localized SFR on, for example, a college campus for the curious Hams attending that college. A SFR can easily be moved in frequency, and can be put on single channels

I agree that "The bulk of amateur operators are not interested. The majority sees DMR as a way to use a hotspot and doesn't even realize that it can work simplex or that a repeater may be available nearby."

But such folks aren't the intended audience of Zero Retries, nor what I imagine the primary use case to be for this capability. Writ large... for MOST of what I discuss in Zero Retries, "The bulk of amateur operators are not interested". While I write Zero Retries for those of us Amateur Radio Operators that are technically curious, the primary intended audience (this has evolved over the course of Zero Retries four years of publication) to NewTech Amateur Radio Operators. I'll write about this last part extensive in Zero Retries 0210 which commences the fifth year of publication.

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Jerome's avatar

Oh yes. There is a reason why I come to read what you write and this is it.

BUT, when we talk about DMR, we talk about electronics. The manufacturers need some volume for their handhelds or they go bankrupt. Therefore enough people need to see the light or we won't have nice things.

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Steve Stroh N8GNJ's avatar

Jerome - We're in violent agreement about the need for a mass market for vendors to make "nice things". But I don't see that arising from the current "market" of typical Amateur Radio Operators. They just don't see the need for data over DMR, or IP400, or New Packet Radio, or dozens of other things I discuss in Zero Retries for the 3000 or so readers.

It's just not going to happen. "We 3000" aren't going to convince a mass of Amateur Radio Operators to adopt these new technologies. Thus... all is lost, right?

If I felt that way, that what I write about in ZR is... and will remain... a tiny niche of Amateur Radio (which is also a tiny niche), then there would be little point in trying.

But, what I'm seeing, is the quiet rise of NewTech Hams that are coming into Amateur Radio and not being noticed by most, because they're not spending big $ for $ HF rigs, they're not joining the ARRL or other traditional national radio societies, etc. Instead they're working on new technologies like SDR, like microwave networking, like IP400, etc.

I don't know what will "flash over" to create a mass market for "nice things" that we want. It could be a new national / international organization that promotes such things. It could be an "out of the blue" capable manufacturer of "nice things" like TAPR was with the TNC2. It could be a particular product from an existing manufacturer that finally has the right combination of "nice things" features that create a mass movement like Meshtastic has done. I don't know HOW this is going to happen... but I'm convinced it IS going to happen. Again, this is a story to be told in Zero Retries 0210 next week.

I'm a student of Amateur Radio history in the early and mid packet radio era and that era birthed a number of companies making "nice things" that were successful and profitable for some time. In THIS era, and we've never had better technology and better tools to evangelize new technology.

My sense is that there is some event in the near future that will cause some kind of "flash over" into wide adoption of "nice things". I just don't know what it is, looking ahead, but we'll definitely recognize it when it begins. And when that "whatever it is" happens, it's going to be so cool, so interesting, that techies will get their Amateur Radio license just to participate in "nice things".

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Jerome's avatar

You are preaching to the choir here. I certainly know that the new generation of licensed and unlicensed operators is developing nice things. And, in some countries like Germany, the radio societies make some space for them: DARC had an interesting SDR presentation last Saturday at Friedrichshafen.

So: yes, it will work in the end.

But if you want mass-produced DMR handhelds with the SFR function, the importers will need to sell a truckload. That is the deal, sorry.

(And since you are in the USA, the easiest way to attain that volume would be to convince the FCC to allow DMR on GMRS...)

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Steve Stroh N8GNJ's avatar

Jerome - I think we'll have to agree to disagree here. I just did a bit of back and forth research on GMRS with perhaps DMR being used, and there be dragons. But what I've seen is that the cheap Chinese radio manufacturers are trend followers. If one manufacturer starts seeing some reasonable viral success on a feature like DMR SFR, or data over DMR, the rest quickly start adapting to that trend because they don't want to be left behind and all the others want to sell as many radios as they can too. So... we need to find that pebble in the right place to start the avalanche of manufacturers adopting such a feature. But for that to happen, we need to have some well defined use cases and successful examples to point to, and as we've collectively discovered... all of this capability actually exists to some degree, but it's scattered and inconsistent. So what CAN be done, and WHO is doing it, and what we collectively NEED must be brought into a coherent narrative. I think that (what we need from DMR) is now the primary subject of ZR 0211 in a couple of weeks. Thanks very much for this exchange!

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Steve Stroh N8GNJ's avatar

Jerome - Thanks for that info. Now I know what to look for - DCDM in DMR radios.

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Jerome's avatar

DCDM is only half what is needed. DCDM allows standard participants (that is: non repeaters) to define the time slot timing. TDMA needs some kind of synchronisation for everyone to agree when is time slot 1 and when is time slot 2. Either a real repeater does it or a clever system is needed. That system is call DCDM.

But that is not enough. If you want your handheld to use a SFReapeater, you need to be able to program it to listen on one time slot and send on the other when pressing PTT. That should be trivial and maybe some open source "code plug" programming software will add the capability to handheld which do not have it, but using the manufacturer's software or the device buttons, that capability is often not foreseen. Anytone allows it (AFAIK) but, for example, Radioddity which has a device able to be used as a SFR (the GD88), does not have the programming capability in the other devices they sell (at least not the ones I could try). I contacted them, they are nice people and understood my question, but apparently they cannot get the change from their Chinese manufacturers...

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Steve Stroh N8GNJ's avatar

Duplicate post removed.

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