[UK-CONTEST] RTTY Bandwidth

[David G3YYD]

Rob

RTTY occupied bandwidth is like CW bandwidth depends how hard the keying 
is and how strong the signal is. With little to no filtering then the 
bandwidth will be measured in KHz for a strong signal. A strong signal 
with good TX filtering will occupy about 400Hz of bandwidth. Then it 
depends how good the RX filtering and performance is. In high activity 
RTTY contests I normally use RX bandwidth of 250Hz at -6dB with around 
350Hz at -60dB.

With a +/-200Hz TX signal bandwidth and a RX +/- 150Hz (about -30dB down 
side of filter then PC software filtering. I can copy the next signal 
QRM free at about 350Hz spacing between signal centre frequencies. 
Before you think I have got my maths wrong remember it is TX to RX that 
counts and not TX to TX adjacent TX clicks can overlap. Of course if you 
are using a FT897 with SSB filters you will have a problem, both in 
filtering and dynamic range of the RX.

Some TX signals are a lot wider and therefore 350Hz spacing is not 
possible for all TX/RX pairings. Penalty points for transmitting wide 
signals would be an excellent idea and with SDR technology easy to 
provide proof. With today's very stable rigs and software RTTY could 
amend rules to a narrower shift, 85Hz?, has to be used. Or could go to 
Minimum Shift Keying (MSK) at baud rate/2, i.e just under 23Hz. Reducing 
the shift to 85Hz would reduce occupied bandwidth and adjacent channel 
spacing comes down by 85Hz.

Incidentally the minimum occupied bandwidth for a RTTY FSK signal can be 
very small compared to current practice.  Minimum shift of baud rate/2 
Hz coupled with optimum TX filtering gives occupied bandwidth of shift 
plus baud rate in Hz. For 45.45 baud this is  22.73+45.45=68.2Hz say 
70Hz. Allow a bit extra for signal spacing could have 10 run frequencies 
per KHz instead of less than 3 for the current practice. 75 Baud would 
occupy 102.5Hz with a bit extra say 8 per 10KHz.

Of course it will never happen as it is always easier to preserve the 
status quo. In the "RadCom Letter" subject heading on the reflector much 
has been written about contesters pushing the technology envelope so may 
be that is what we should do. We want (need?) more run frequencies per 
KHz so why not change the RTTY shift to a much smaller one? Given the 
nature of contesting, it would have to be a rule rather than just a few 
changing to their contest disadvantage.

73 David G3YYD

On 25/07/2011 14:14, G4LMW wrote:
> It is worth mentioning that the main reason that PSK was introduced was to
> help alleviate the overcrowding that was experienced with the RTTY ONLY
> contests.
>
> Also, should there be a move to 75 baud, then each signal will occupy 370Hz
> instead of 270Hz (happy to be corrected on my exact numbers).
>
> So, RTTY ONLY with 75 baud would be worse that ever!
>
> Regards
> Rob, G4LMW
>
> ----- Original Message -----
> From: "john reynolds"<g3rsd at btinternet.com>
> To:<uk-contest at contesting.com>
> Sent: Monday, July 25, 2011 2:57 PM
> Subject: [UK-CONTEST] UK Contest data modes in CC
>
>
>> I believe CC was intended to encourage and train operators in all modes of
>> on air communications used in contesting.
>>
>> Before the CC's I had never used Ry or PSK.
>>
>> Probably because of my age,it took me a long time to configure and use
>> Data Modes and it was thanks to other local Amateurs that I am now
>> comfortable with all modes.
>>
>> I do not think DATA should be dropped from these Contests, but it could be
>> modified!!
>>
>> May I suggest that on alternate Months RTTY only is used and it be left to
>> the individual to use whatever speed they wish to achieve maximum points.
>>
>> The following Month PSK would be the mode and here again speed is left to
>> the individual.
>>
>> May I also point out that a close "Observer" who has suffered Contesting
>> for Years considers that the "SHOUTING" mode should be dropped ...........
>> it is most unsociable  around the house!!
>>
>> John (JR)  G3RSD
>> _______________________________________________
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>> UK-Contest at contesting.com
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>>
>
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