Is there a way to preamp the received signal? I‘m Guessing it would require a synchronized pin diode circuit or equivalent to keep outgoing slot (Tx) from blowing up the Rx preamp. Or, maybe power sensing. Obviously the radios that do SFR have a way internally to protect Rx from Tx. Not urgent but some of the Co comms guys are going to test some SFR solutions in place of regular DMR repeater and that repeater has a preamp to give it balanced performance. Im guessing the SFR will need the boost as well. TDMA is still a second language at best for me and I have no instinct for dealing with it’s subtleties.
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Single Frequency Repeat
- Thread starter otobmark
- Start date
Josh
Member
I can't answer your question, but owning and having used SFRs as well as traditional repeaters, I'm responding.
A traditional repeater uses a duplexer and has a fair amount of loss for the frequencies it is meant to pass through. This is where a pre-amp might be handy, however... a pre-amp also lessens the effectiveness of the duplexer to some degree depending on the setup.
A Single frequency repeater doesn't need any of that, so much like simplex, whatever the radio receives unobstructed by other interference sources to the receiver, it will transmit.
Of course, there are less losses to the transmitter output vs a traditional repeater as well, but 3db isn't a whole lot.
I would try it out as-is. My experience is that a SFR is viable, but not a long-range solution... but I've never had a good high profile site to test mine out properly.
-Josh
A traditional repeater uses a duplexer and has a fair amount of loss for the frequencies it is meant to pass through. This is where a pre-amp might be handy, however... a pre-amp also lessens the effectiveness of the duplexer to some degree depending on the setup.
A Single frequency repeater doesn't need any of that, so much like simplex, whatever the radio receives unobstructed by other interference sources to the receiver, it will transmit.
Of course, there are less losses to the transmitter output vs a traditional repeater as well, but 3db isn't a whole lot.
I would try it out as-is. My experience is that a SFR is viable, but not a long-range solution... but I've never had a good high profile site to test mine out properly.
-Josh
Which Kenwood radio are you talking about?
Completely logical observation. Duplexer is 4 can with preselector and preamp is 9-12db range so is a bit more than just making up duplexer loss. As is repeater is balanced even with handhelds. The site is a bit of a unicorn, outperforming splat maps considerably. As I understand this is a test not a permanent changeover. I’m speculating but I believe they are looking for field expedient solutions for disaster comms. I know they have tried drone born repeaters for about 20mi range on 460ish MHz.
Subscribers are 1300, 5000, Viking and maybe Hytera. The SFR I don’t know what they are using at present but if concept makes them happy I’m guessing they will get Hytera commercial units (nice looking and not cheap).
Surprise for me was that the agnostic solution worked best when subscriber units were NOT in DCDS mode. Of course if Rx subscriber is close enough to Tx subscriber it’s a toss up as to rather you copy direct or SFR. If in the overlap there is destructive interference it is not obvious from my brief unqualified testing so will need to be further investigated. Logically I expected collisions from untimed signals. My before coffee theory as to how this might happen is that since the SFR I used Tx’s on opposite slot it Rx on (in 1 out 2 or reverse on the fly) it may assume the untimed subscriber is one or the other TS’s and Tx’s on the opposite- creating a crude form of timing. Pure speculation for sure.
Josh
Member
I've been toying with SFR for a while and my research has shown that there isn't a set standard on how to do it. My initial venture into it was with a Radiodity DB40D (or whatever) CCR. The method it advertised was a "mesh" network type system, which you are describing. The problem I had was that a large portion of the time it didn't work right... plus it was "double vocoded" -or at least that's how it sounded. I can't recall if user IDs passed properly or not. It worked great in close range due to actually being simplex communication, but at a distant it was too spotty to be reliable.
I'm not sure how Motorola does theres, but a Retevis SFR I've tested works very well, but in order for it to work right the subscriber radios need to be set to DCDS mode, and transmit on one slot while the receiving radio has to be on the other slot. The same goes for a portable radio (inexpensive compared to Hytera) The Retevis P1, works well too... it does SFR.... but so far nobody makes a radio that can transmit on one slot and receive on the other... when this happens that this type of SFR operation will be viable.
But to answer the initial question about preamplifiers, my bringing up their typical use to overcome filter loss, causes me to wonder if one is even necessary (but if needed, it would have to go between the switching diode and receiver inside the radio itself). If the frequency slot received is clear, then it should work like a simplex radio since the transmitter is only active between slots.
The whole concept, while been around for several years now, is intriguing. I have wondered why it isn't employed more.
I'm not sure how Motorola does theres, but a Retevis SFR I've tested works very well, but in order for it to work right the subscriber radios need to be set to DCDS mode, and transmit on one slot while the receiving radio has to be on the other slot. The same goes for a portable radio (inexpensive compared to Hytera) The Retevis P1, works well too... it does SFR.... but so far nobody makes a radio that can transmit on one slot and receive on the other... when this happens that this type of SFR operation will be viable.
But to answer the initial question about preamplifiers, my bringing up their typical use to overcome filter loss, causes me to wonder if one is even necessary (but if needed, it would have to go between the switching diode and receiver inside the radio itself). If the frequency slot received is clear, then it should work like a simplex radio since the transmitter is only active between slots.
The whole concept, while been around for several years now, is intriguing. I have wondered why it isn't employed more.
Josh, my initial work around for DCDS was to create a 2 channel scan list in the subscriber radios with the 2 channels being identical except for TS--one was 1 and the other was 2-- all in DCDS mode. When the SFR output on TS1 the the scanning subscribers would stop on the channel with TS1. Just like my scenario mentioned above if the receiving subscriber was in range of transmitting subscriber the radio might stop on that direct signal (TS2 in this example). As you mentioned what we want is a single memory channel with split TS for Tx & Rx.
Some of my subscriber radios don't do DCDS (Viking- at least not yet. Come on EFJ, give us DCDS, split TS, and DMR roaming) and Moto does a different scheme with split CC for repeater extenders. I think the Hytera system does use split TS single memory position method. Anytone ccp probably similar.
Some of my subscriber radios don't do DCDS (Viking- at least not yet. Come on EFJ, give us DCDS, split TS, and DMR roaming) and Moto does a different scheme with split CC for repeater extenders. I think the Hytera system does use split TS single memory position method. Anytone ccp probably similar.