full duplex SSB with the two parties on opposite sidebands? - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: February 16, 2016, 05:30:25 am
content: Does anybody think that would be possible?You would need to use a directional coupler of some kind to isolate the receiving and transmitting signals, also they would need to be really clean! Also you would need very low SWR.

author: ivaylo
author_rank: Frequent Contributor
timestamp: February 16, 2016, 05:39:18 am
content: Sure, this beast used to be able to do it

author: Richard Head
author_rank: Frequent Contributor
timestamp: February 16, 2016, 05:51:37 am
content: Been done years ago. For some reason never caught on.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: February 16, 2016, 11:55:47 am
content: It's a bit of a novelty and both stations would need to be set up correctly with very clean transmitters and high dynamic range receivers.Another approach to full duplex I've heard work here in VK3 involves a remote receiver, one station transmitting AM and a VHF link.Station A (the crossband controller) transmits an AM signal on 3.6 MHz. He has a remote AM receiver about 50km away. Its audio is transmitted via a UHF link to Station A's home location. Audio from this link goes into a mixing box and fed to the 3.6 MHz AM transmitter (along with audio from the microphone of Station A). This audio is also transmitted on a local VHF frequency.Station B transmits LSB on 3.6 MHz. His signal is picked up by the remote AM receiver and transmitted to Station A via his UHF link. Even though it's only an AM receiver, the carrier from Station A forms a BFO so that the signal as heard by Station A is resolved correctly even though the remote AM receiver does not receive SSB by itself (without the carrier from Station A). The transmission from Station B is retransmitted by Station A on Station B's frequency as well as on the local VHF frequency. Station B doesn't have to listen to that local VHF frequency but if he does he can enjoy full duplex communication with Station A and also hears his signal come back.Potentially Station C could be transmitting USB on 3.6 MHz and be retransmitted by Station A so it could be a 3-way full duplex (triplex?) crossband. I haven't heard this but can't see why it wouldn't work.

author: voltz
author_rank: Frequent Contributor
timestamp: February 16, 2016, 12:32:19 pm
content: If you're talking true full duplex TX RX on either sideband, then my main concern would be front end swamping or even component break down. The more you can attenuate the TX at the RX section during transmit the better. You'd also need a very selective receiver.1) using some kind of super narrow notch filter tuned to the TX frequency in the RX path.2) using a 3 port circulator arrangement allowing both TX and RX to be connected simultaneously to one antenna.3) internet connected remote TX or RX station in another location.If its not full duplex but more like a break in system, (listen only during the TX gaps) then RF pin diodes could be used to block the RX during TX speech. Better than VOX, where is actives during the speech envelope and has no delay.Well there's some ideas, but yes, pretty sure its possible one way or another...

author: cdev
author_rank: Super Contributor
timestamp: February 16, 2016, 08:59:39 pm
content: Well, the context I read about it was a thread about diversity reception on the softrock mailing list. Softrock is a QRP transceiver and if you don't use amplification the signal is limited to around a watt.https://groups.yahoo.com/neo/groups/softrock40/conversations/topics/84222A number of interesting things in terms of signal intelligibility can be done with multiple receivers operating alongside one another and then processing the output.

author: AF6LJ
author_rank: Unknown
timestamp: February 16, 2016, 09:11:32 pm
content: All of the methods I have seen that successfully worked involved split site TX/TX and some degree of frequency separation. One of the problems encountered is broadband noise from the transmitter which cannot be filtered out if the RX and TX are at the same site.A Rat Race (aka circulators ) could be used to isolate the transmitter from the receiver if a common antenna is used. This only provides some isolation, and inhibits frequency agility, circulators are also sensitive to changing antenna conditions.Above 25MHZ cavities could be configured as a douplexer and used with reasonably good success but they are expensive and subject to detuning caused by temperature changes. Size also becomes a problem as a cavity for 25MHZ is over six feet tall for optimum Q.

author: TheSteve
author_rank: Unknown
timestamp: February 16, 2016, 09:18:09 pm
content: Possible - surePractical - not even closeIf you did want full duplex on a single frequency it would be easiest to go digital - digitize and switch between tx and rx really fast, then assemble full audio again at either end(its been done).

author: AF6LJ
author_rank: Unknown
timestamp: February 16, 2016, 09:20:22 pm
content: Maybe....Just, maybe.You could do this with Codec-2 and some custom hardware.

author: retrolefty
author_rank: Super Contributor
timestamp: February 16, 2016, 10:08:26 pm
content: There have been lots of radio sets (military for sure) that have ISB, Independent SideBand, transmission or reception but not in the full duplex mode. As others have said much too difficult and expensive to obtain full duplex on a single frequency.

author: voltz
author_rank: Frequent Contributor
timestamp: February 16, 2016, 10:55:36 pm
content: Switching fast between RX and TX presents its own problems. Too slow and the sample rate becomes lousy = bad quality audio. Too fast and the RX will start to hear its own TX reflections from nearby objects. like Radar. Maybe there's a optimum point but might take some hard sums to figure that out.I favor the remote site option. Its very doable and stations with existing internet remote rigs can do this right now. Of course both stations talking would need remote their own internet remote station. Bit complicated but the result would be duplex LSB/USB. Or any split frequency for that matter. But is it all worth it?

author: TheSteve
author_rank: Unknown
timestamp: February 16, 2016, 11:27:19 pm
content: Quote from: voltz on February 16, 2016, 10:55:36 pmThe fast switching was tough to get right. The product I worked on back in the late 90's had a range limitation before the delay was just too extreme to compensate for. The product was a wireless line extender for getting phone lines to remote locations using on single frequency. The advantages were huge of course in that it needed only a single licensed frequency, required no duplexer(was UHF) and it was frequency agile without any retuning.To the OP - if you want to look into full duplex communications further I'd look into VHF/UHF amateur radio repeaters which is the easiest place to find something that transmits and receives on the same band using the same antenna at the same time. If you were more interested in simply knowing if a LSB and a USB signal can co-exist on the same frequency at the same time then the answer is sort of yes as they wouldn't technically be using the exact same frequency but the transceiver might display it as the same frequency. If you shifted the frequencies so the USB and LSB signals overlapped then it would not work very well. Either would also require a level of extreme front end filtering which I don't believe exists or that is able to handle strong and weak signals.

author: cdev
author_rank: Super Contributor
timestamp: February 16, 2016, 11:56:29 pm
content: QSK seems to me to be pretty interesting, with CW. Sometimes QSK is done using PIN diodes..Definitely a nice thing if you are using CW is to be able to hear if somebody wants to break in.Its weird little things like that that make all the difference with something, whether its fun or not, sometimes.Sort of unrelated but useful to know.. Its my understanding that full duplex communications is dangerous in a mobile setting but PTT isnt. And the reason is pretty obvious once you think about it. PTT is more forgiving of people stopping and it lets people be distracted, they just unkey the mike and do whatever traffic adjustment they need to do, but cell phones, not so easy..Especially if you are talking to somebody important in your life. So, the lesson is, don't talk on your cell phone while driving! But a ham radio mobile is okay.Cell phones end up being orders of magnitude more dangerous to use in cars. Even hands free.

author: voltz
author_rank: Frequent Contributor
timestamp: February 17, 2016, 12:10:40 am
content: Its kinda off topic but yes, QSK (full break in) is exactly that. Hearing between the dits and dahs as you send. Semi break in being the same as SSB VOX, with its delay setting as desired. I know full QSK is not always liked by some hams, its a bit distracting hearing stuff while you send.. But everyone to their own.One point i made earlier comes back, the idea of using pin diodes to block the frontend activated from the SSB speech envelope. Not to be confused with VOX operation, as it would be instant - like QSK full break in for voice. Never seen that on a radio (yet). Quite like the idea of that.

author: TheSteve
author_rank: Unknown
timestamp: February 17, 2016, 12:12:49 am
content: Quote from: cdev on February 16, 2016, 11:56:29 pmYeah, for such an old form of communication CW having QSK is pretty darn advanced! I swapped the input and output TX relays in my Icom 2KL 500 watt linear amp to vacuum type (2ms switching) which gives it QSK ability quite nicely.

author: cdev
author_rank: Super Contributor
timestamp: February 17, 2016, 12:18:34 am
content: Look at youtube user SM5BSZ's videos, the video titled "ensemble 5" (or maybe its ensemble 4)he uses a pilot tone some KHz off to allow the switching to be considerably more precise in terms of timing - to prevent receiving and transmitting at the same time you need a signal that has a very clear beginning and ending, which is in the signal path, not in the USB control path which is USB 1.1 on the SR, even slower than USB 2. the indeterminancy of timing on the radio creates a non-optimal situation Its adequate he says for most QSK but not for radar, certainly.. and it seems to me that even for QSK it would be preferable to use the pilot tone.I had always read that the softrock needed more filters but according to Leif the signal is very clean, if you do it this way. Its an interesting discussion and visual explanation by somebody who clearly knows their subject extremely well.YouTube really is an amazing resource for learning.

author: cdev
author_rank: Super Contributor
timestamp: February 17, 2016, 12:26:56 am
content: Its the efficiency of being able to use a very weak signal and it having such a narrow bandwidth.Quote from: TheSteve onTodayat 19:12:49

author: voltz
author_rank: Frequent Contributor
timestamp: February 17, 2016, 12:37:52 am
content: Are you sure that was CW activity around 7.040? Not a data mode or RTTY contest?

author: vk6zgo
author_rank: Super Contributor
timestamp: February 17, 2016, 03:30:48 am
content: Quote from: retrolefty on February 16, 2016, 10:08:26 pmThe PMG's Dept (later Telecom Aust),used ISB extensively for long haul HF comms,in the '50s,'60s & into the '70s.They had separate transmit & receive Carrier frequencies,as well as separate Tx & Rx sites.They thus had both frequency & geographical separation,.That said,some of the systems were really Simplex,using VODAS/VOGAD devices,which operated on the users voice,so as to not need PTT.I think the Perth-Melbourne system was full Duplex,though,as the distance between attended Rx & Tx sites could be made much larger in the cities,than the semi-unattended sites in remote towns.If you are a Communications carrier,it makes more sense to be able to use the two sidebands for different conversations.If you use the other sideband for the receive side,you can only have one conversation,per system.To get back to where you were,you then need another Tx,Rx,& a second frequency.As there was no advantage in to using one frequency allocation,it was easier to use multiple frequencies to begin with.

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: February 17, 2016, 04:10:40 am
content: This concept has me thinking about repurposing some grain silos as a duplexer.

author: cdev
author_rank: Super Contributor
timestamp: February 19, 2016, 01:57:54 am
content: In the US way way back in the day they used some kind of multiplexing device to turn POTS phone lines into many? Maybe slick something? Slick 76, slick 96?

author: retrolefty
author_rank: Super Contributor
timestamp: February 19, 2016, 02:13:29 am
content: Quote from: cdev on February 19, 2016, 01:57:54 amI worked maintaining such systems in the 70s. We had both frequency division multiplexers and time division multiplexer. They generally required higher cost dedicated 4 wire 'conditioned' telephone lines for the 'high speed' side. Worked great but most of the data streams we were multiplexing were 300 baud or lower. Times have changed for sure, now I talk on the internet with a 14Mbit link using my home POTS line.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 19, 2016, 02:14:19 am
content: Quote from: cdev on February 19, 2016, 01:57:54 amPhone lines between Towns & Cities used Carrier telephone systems,where baseband audio from different sources was modulated onto a number of carriers,thus increasing the capabilities of that line pair.Early systems used DSB,but later ones were SSB.As an example of everyday use in the 1960s,the "aerial" line into Wyndham,(a tiny outpost in the far North of Western Australia),carried 12 phone channels & 24 VFT telegraph channels.In "The Wet",that line was often washed out,so they had to go back to 4 channels on a HF ISB Radio system.You should have heard them complain!In fairness,though,"The Wet" was a lousy time for HF Radio,too!.

author: cdev
author_rank: Super Contributor
timestamp: February 19, 2016, 04:57:41 am
content: To be honest, I was preoccupied with something else. It seemed like really fast CW.If I see it again, I will document it better.Quote from: voltz on 2016-02-16, 19:37:52