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Author Topic: Dual RX vs. Full Duplex  (Read 1701 times)
KC0WDZ
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« on: January 21, 2008, 01:01:53 AM »

I am an Army radio man and I'm trying to equate civilian terms with Army terms with which I am more familiar. I am looking at a FT-8800R and some sites says that is "Dual RX" while others list it as "Full Duplex" I know there is a difference. Can someone just confirm for me that it is not actually "Full Duplex"? i.e. TX and RX simultaneously.
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K0BG
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« Reply #1 on: January 21, 2008, 05:53:28 AM »

It is a dual band radio. It can receive simultaneously on both on 2 meters and 70 cm. It is NOT full duplex.

I've not owned one, but as I recall it can receive on one band and transmit on the other band; this is called cross-band repeat.

It is a decent radio, and Yaesu has sold a lot of them. Personally, I don't like the display, but you may.

Alan, KØBG
www.k0bg.com
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K5LXP
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« Reply #2 on: January 21, 2008, 06:15:12 AM »

> it can receive on one band and transmit on the other
> band; this is called cross-band repeat.

Wouldn't that then demonstrate it's full duplex, TX on one band while receiving in another?

Numerous radios can operate on more than one band, but only one at a time (Icom 208H, et al).  True dual band radios are ones where each band can operate  independently of each other.  Note that there isn't a lot of practical application for full duplex operations in the amateur service but it's certainly possible to do so.  As an example, on my 2M repeater I have a separate 440 link which can serve as a 2nd RX input for the phone patch, so any dual band radio can make full duplex phone calls.  Predominantly, having a dual bander is most useful for being able to scan or monitor one band while operating another.

Mark K5LXP
Albuquerque, NM

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NA0AA
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« Reply #3 on: January 21, 2008, 04:12:43 PM »

I have two FT-8800 - while they will do cross band repeat, they do NOT do full-duplex.  You can set it for satellite use but the manual states it's not full duplex operation.

For those who might be confused "full-duplex" operation is the equal to a telephone circuit where you can talk and hear at the same time.
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AA4PB
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« Reply #4 on: January 22, 2008, 08:40:59 AM »

Its true that cross band repeat means that the radio is receiving and transmitting at the same time. To me, full duplex implies that the operator can talk and hear at the same time - like on a telephone.
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AA4TX
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« Reply #5 on: January 23, 2008, 05:51:33 AM »

The FT-8800 is full duplex in the VHF/UHF mode; that is to say, you can transmit for example on 146.520 while listening on 446.00. If the other station is transmitting 446.00 and listening on 146.52, you can have a full duplex conversation. This is also true for a LEO QSO - you can listen on the downlink to your own transmission on the uplink (wear headphones, or you will get feedback!)

The FT-8800 will not do VHF/VHF full duplex or UHF/UHF full duplex.

So the answer is yes, it is full duplex, but only in the VHF/UHF mode.

KI4LTX
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K0BG
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« Reply #6 on: January 23, 2008, 04:50:23 PM »

Well, not so fast John. Full duplex means that the user can carry on a conversation as if the connection was via telephone. That is not possible within one band unless you have a duplexer like those used on repeaters.

Those duplexers are usually BR/BP/BR, and BP/BR/BP depending on the configuration, and whether we're talking about the receive path, or the transmit path.

With respect to cross band repeat, there are no duplexers in use. That means, the person transmitting cannot hear him/herself on the output frequency. So, crossband repeat is just that; you transmit on one band, and retransmit the signal on a different band. Since you cannot hear the opposite band simultaneously, it is by definition, not full duplex.

This is true even if you have two radios involved. Although the frequencies are far removed from one another, the fact they would be collocated would requires a duplexer (or some other bandpass methodology) to keep one transmit signal out of the front end of the opposite receiver.

The short of it is, repeaters are full duplex. But, the transmitters transmitting to the repeater, and the receivers listening to the repeater, are not full duplex. While mobile, full duplex is possible, due to the close proximity of the transmit and receive signals all but eliminates multiple frequency full-duplex operation. Unless of course, you could come up with a method of readjusting the duplex cavities to track the separation frequencies. If there is such a system, I'm numb to it.

Alan, KØBG
www.k0bg.com
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AA4TX
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« Reply #7 on: January 24, 2008, 06:21:11 AM »

Alan,

With respect, I mentioned that the FT-8800 will not do full duplex in the same band (VHF/VHF or UHF/UHF).

However, I can do full duplex with the FT-8800 using UHF/VHF or VHF/UHF. (I am not talking about cross band repeat here, since I am not retransmitting the received signal. Since there is sufficient filtering for cross band repeat, there is also sufficient filtering for cross band full duplex.

If I take two FT-8800s, and configure them in this fashion:

FT-8800 #1
Right RX side @ 446.00
Left  TX side @ 146.52

FT-8800 #2
Right RX side @ 146.52
Left  TX side @ 446.00

Each operator can hold his mike open, and carry on a full duplex conversation, just like a telephone. Each operator hears the other without releasing the push to talk button. Since the FT-8800 has the capability of continuing to receive on one side of the radio while transmitting on another, (as long as you are not listening to the same band that you are transmitting on) cross band full duplex is possible. It just will not do this in the same band. But unless I am mistaken, full duplex is a mode that does not identify how the circuit is made, just that you have simultaneous transmit and receive as in a telephone conversation.

Now, I do not recommend this type of operation for the FT-8800, since it is not rated for 100% duty cycle, but it can be done for short periods. This is very useful in working LEO birds so that you can here exactly what you are transmitting on the up link.

By the way, as an RF Engineer, I must say that I find your work in general a very interesting read, and quite refreshing!

John
KI4LTX

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