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Author Topic: Transmit and receive on different frequencies  (Read 3034 times)
KB3FFH
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Posts: 159




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« on: December 19, 2012, 09:30:05 AM »

Why do some hams receive on one frequency and transmit on another? Thanks Bill
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WB2WIK
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Posts: 20547




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« Reply #1 on: December 19, 2012, 09:46:32 AM »

"Split" operation is used mainly for pile-up management (by DX stations), so all the calling stations don't "bury" the DX station everyone's trying to hear.

But it's also used on bands where DX has privileges that "we" don't in the U.S.; for example on 40m voice modes, where they can transmit below 7100 kHz, but we cannot go below 7125.  If they want to work "us," and we "them," it has to be split in many cases.

Those are just a couple of examples, there are others.


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AC5UP
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« Reply #2 on: December 19, 2012, 11:39:29 AM »

There are thousands of VHF and UHF repeaters that operate with a T/R offset because it's impossible to transmit and receive on the same frequency simultaneously. The typical 2 Meter repeater uses a 600 kHz split with the user split set to the mirror image of the repeater split. This can also be described to as a half-duplex link.

A few years back there was some experimentation with simplex repeaters that relied on a controller with a digital voice recorder. Basic concept is that whenever someone keyed the repeater it would record what it received then immediately re-transmit whatever was heard. This made for clumsy conversations that encouraged a concise exchange but worked well as a low cost instant repeater to cover a temporary event.
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G3RZP
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« Reply #3 on: December 19, 2012, 12:03:04 PM »

There was a 'on frequency, real time' FM repeater built in the early 1980s. It was called the 'Groundsat' from Plessey: it used a direct conversion receiver and repeated on the receive frequency what it heard. Dynamic Range problems limited it to about 1 watt on transmit. Covered 30 to 76 MHz - the military tactical band.
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WB6BYU
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« Reply #4 on: December 19, 2012, 12:54:33 PM »

Quote from: G3RZP
There was a 'on frequency, real time' FM repeater built in the early 1980s...


And not your ordinary simplex repeater, either.  Wasn't that the one that used multiple
transmit antennas and put a tone on the output, then adjusted the phase of the
signal from each of the antennas to minimize the amount of the tone that it picked
up in the receiver antenna?  So basically they had an active real-time frequency-agile
phasing system to keep the TX signal out of the RX.
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G3RZP
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« Reply #5 on: December 20, 2012, 02:44:11 AM »

No, it used one antenna. The dynamic range limited it to 1 watt: that was both the amplitude and the phase noise limited DR. For some reason, it never caught on with the military.

It was designed at Roke Manor  research centre, the same place that came up with a very succesful method of tracking stealth aircraft by the reflections from cellular base station and TV transmissions.
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G8JNJ
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« Reply #6 on: December 20, 2012, 08:21:57 AM »

More info on groundsat at

http://www.amalgamate2000.com/radio-hobbies/radio/plessey_avionics_ptr3411.htm

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G3RZP
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« Reply #7 on: December 20, 2012, 08:45:18 AM »

These days they have BOWMAN, which is an acronym for 'Better Off With Map And Nokia'
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AC5UP
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« Reply #8 on: December 20, 2012, 08:57:50 AM »

The dynamic range limited it to 1 watt: that was both the amplitude and the phase noise limited DR. For some reason, it never caught on with the military.

Interesting concept... And the reasons why it wasn't used more widely should be obvious.

If one antenna is used for simultaneous TX and RX thanks to phasing and filtering, it stands to reason the TX power will be modest (as you mentioned) but wouldn't the RX sensitivity be degraded as well? A low power repeater that's somewhat deaf might have a working range no better than a conventional simplex radio link. Which then begs the question: What's the point of the repeater?

Considering the possibilities offered by digital encoding I'd say there are now many ways of achieving a secure communication link that's resistant to eavesdropping and direction finding.......
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W1VT
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« Reply #9 on: December 20, 2012, 09:08:32 AM »

It can also be useful when operating with a cheap crystal controlled CW rig--you can use the same frequency oscillator for your transmitter and receiver if you listen up 1 (or down 1).
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N2EY
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Posts: 3856




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« Reply #10 on: December 20, 2012, 09:09:12 AM »

Two more reasons:

1) Some hams just don't know how to zero beat their tx and rx frequencies and/or their rigs are not set up properly. (The old National NCX-3 cannot be set up properly for CW operation!)

2) Some hams still use crystal controlled transmitters.

73 de Jim, N2EY
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G3RZP
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Posts: 4391




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« Reply #11 on: December 20, 2012, 12:43:20 PM »

groundsat has the limitation that with phase noise and  IMDR, there's a limit to how much on channel signal you can handle. They could keep the rx sensitivity with a 1 watt tx power limit: in the case of rolling countryside, limited capabilities of tx antenna height, 1 watt could be enough to enhance communications in dead spots, which was what they were after.

But it didn't sell....................
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NR4C
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Posts: 306




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« Reply #12 on: December 21, 2012, 07:20:03 AM »

Once upon a time, long, long ago, many hams did not have any form of variable tuning available for their transmitters.  They used a plug-in crystal to set the frequency.  They often did have receivers with variable tuning so they would plug in their crystal, and call "CQ" and begin tuning the receiver around their "calling" frequency to see if anyone heard them.  And they'd repeat this until either someone heard them, or mom (or spouse) called them to dinner or sent them to bed. It was rare that two ends of a QSO would be on the same frequency.

Today, the appliance operators that hams have become have a myriad of wonderful aids to operating ham gear.  We have transceivers with synthesised VFOs, DSP, RIT, XIT, NR, noise blankers and "SPOT" buttons to automatically "ZERO-BEAT" the other guy to get the frequency exactly right.  We use digital readouts to read to one Hertz (Cycle) and regularly calibrate to WWV.  We can then gripe about the 'other' guy who can't tune his radio correctly.

But in reality, 'SPLIT' operation is a fact of life.  As has been said, many parts of the world use this technique so as to work (or NOT work) US stations from DExpeditions or in contests, and others use it to make the pile-ups controllable.

I have found that setting my XIT (transmit offset) to about 10 Hz, I am a bit more likely to be heard when all the other guys are actually on frequency when trying to bust a pile-up.

We've come a long way, baby!

...bill  nr4c
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G3RZP
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Posts: 4391




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« Reply #13 on: December 21, 2012, 08:05:37 AM »

Up to the beginning of the 1970s, crystal control on VHF was common here. For multi-op contesting, you'd run two receivers, and announce that you were tuning 'middle to both ends' or 'low to middle, middle to top' so people with several crystals knew roughly where to call.
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W5LZ
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Posts: 477




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« Reply #14 on: December 21, 2012, 09:24:57 AM »

And the really sad part of operating 'split' is that you NEED to listen to both sending and receiving frequencies so that you don't interfere with other conversations.  There's a -lot- of that kind of interference...
 - Paul
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