Full Duplex

There is a recent thread on EHAM regarding the Glories of QSK. The discussion revolved around what has come to be known as QSK in the ham community. Real QSK has to do with full duplex. It was from the days of separate receivers and transmitters. In the days of coastal stations and ships at sea, the coastal station had a transmitter plant, and a separate receiver site. The operator had access to the key, which controlled the transmitter, and the receiver. Since the RX and TX were active all the time you could hear between "dits" if you will.

Transceivers today are not full duplex. They share circuits like filters and the receiver shares the antenna with the transmitter. Thus the transmitter is switched on when a signal in the transceiver usually something called T+ is signaled to turn on by some external event like clicking the PTT or hitting the paddle. This turns off the receiver or at least parts of the receiver, turns on the transmitter and switches the antenna relay over to the transmitter. The receiver can not be used for receiving stations off the air when in transmit. When T+ turns off things go back to receive. I used to pick up this voltage in every so called "QSK" radio I ever owned to use to switch my amp since this was the earliest voltage to go positive. Ten Tec radios were designed like this and they were really the first to hawk this "feature" of so called QSK. It is not real QSK but simulated QSK since transceivers are half duplex. Like roofing filters you can advertise a concept into the general ham psyche and today we call this fast relay half duplex QSK. It's not QSK its just advertised as QSK. Like roofing filters we have been sold a load. That load makes us look like the smartest guy in the room so we continue to propagate the myth.

The Flex 5000 is different. It really can be made to do full duplex QSK like days of yesteryear if you have the second receiver installed. For true QSK you need 2 antennas. The Flex has a seperate RX2 antenna input. It also has a separate RX1 input and the ability to break the signal path of RX1 and insert something like a band pass filter or an amplifier.

Here are some shots of the various choices you can make in choosing antennas for 2 receivers and the transmitter


RX1 tap will tap RX2 to what ever antenna is connected to RX1. You can see the check mark box to open a the loop in the RX 1 signal path. The reason you have all this flexibility is you have 3 simultaneous processes going on in the flex at once i.e, full triplex RX1 RX2 and TX. If you choose you can set up the radio for full duplex operation by hitting ALT-D on the keyboard.

Here is my radio set up for full duplex


If you look at the VFO area you see both receivers and the transmitter tuned to 3501. RX1 and TX are set to Ant 1, and RX2 is set to RX2 ant input. On Ant 1 I have a full size 66ft 80M vertical. RX2 has my shorty 55ft self supporting 80M vertical that is on another part of the property. If I switch on the MOX I am in full duplex mode.


RX 2 is still active on 3501 and My TX is active on 3501. If I hit the key I transmit. No relays will click RF will exit antenna 1 and I will hear the RF on RX2's antenna

What you see on the panadapter is my transmitted signal received on RX2 If you look at the upper right Wattmeter you can see I am transmitting 97 watts. My transmitted signal reads +1 dBm on the RX 2 meter and you can see my CW spurs are down about -60dBm. Bob N4HY is working on an algorithm now that will improve this spurious response because the algorithm will automatically tune the signal for best spurious response in an ongoing fashion based on the frequency you are operating. It takes about 50ms to bring the transmitter into best alignment no matter where you are in the spectrum. The way the algorithm works now is the radio is aligned at once frequency in the center of the band. Spurious response on transmit is in the -100 dB range at that center frequency.

If I let up on the key I hear whatever is in RX2 NO RELAYS CLICK. The signal is always present.

Here is a shot of CO8LY transmitting on 3506.975 he is the guy on the green line which represents my 100hz filter on the panadapter


and here is a shot of me transmitting on 3501 while listening to CO8LY call CQ 6 khz up the band from my transmitter. I can copy him just fine even though I am transmitting 100W from a vertical that is 100 ft from a resonant vertical on the same band. Why I feel just like a coastal radio station. Hey wait a minute I live about 5 miles from the Atlantic!!! I am a coastal radio station!

CO8LY is perfectly Q5 and I copy his CQ and his call and I even hear a K9 calling him, through my string of dahs.

If you look here you will see a pile up on VP8KF on 3503 between me and CO8LY, the MOX is still on and I am still in full duplex QSK mode.


If I switch on the amp, I have 1.5 KW full QSK available. With the MOX turned on I run idle current on my amp and when I hit the key there are NO RELAYS that switch. 1500W flys out antenna 1 and I can hear it on antenna 2. I let up the key and still no relays, I hear on antenna 2. This is true QSK.

Here is a shot of the rig running SO2R


In this case RX1 and TX are on 80M. RX2 is on 40M I can tune RX 2 (in the left ear) while I am doing a run on 80M (in the right ear). If I hear something I want on 40 I click the TX button at the bottom of VFO B


and I am ready to transmit on 40M for the multiplier. Then I click back to 80 and continue the run. On 40 there is absolutely no indication you are transmitting on 80. No spurs or anything. The same is true of you are listening on 80 while transmitting on 40 or 160. I will include a picture of me transmitting 1.5kW on 80, while listening to 40 on a dual band vertical cut for 80 and 40.

So there you have it. I usually don't run full duplex since it requires 2 antennas, and my big vertical is a multiband antenna so I can flit around with ease working DX. I have a couple of rapid band change amps (Acom 2000 and ALpha 78 and LK-500NTC) that allow either for automatic band switching or one click switching. I also have the antennas set up so they can follow the band switch automatically. Pretty much point and shoot. Personally I run about 50ms of delay in my radio so I hear between words insteand of between dits, and my amps don't sound like machine guns. The AGC in this radio does not pump.

This is real QSK, the way QSK was done in the old days, which are now once again the new days. It is not the "advertised" QSK simulation that you see in QST adds. Adds with crap like "OOOO so smooth", while the relays in your amp sounds like a machine gun, and the AGC is pumping like someone pumping a hand water pump. Move over boys there is a new sheriff in town, and he's software defined.

73