Rick Muething


Digital modes have evolved much over the past 20 years.  There is however a basic underlying limit (Initially described by AT&T engineer Claude Shannon  in a landmark 1948 paper that proposed (which has now been proven) there is a basic limit to how much information can be sent within a given channel with a given amount of noise... and this is limited to the Energy per bit (Watts x symbol length divided by bits/symbol) divided by the Noise (No).  This is true whether we are talking about Deep space probes or HF Ionospheric propagation.  What has changed is better DSP (digital signal processing) and error correcting codes that allow us to more closely approach that limit.  Excellent examples today on HF are VARA (2300 Hz or 500 Hz bw) and Pactor 4 (1900 Hz bw). Pactor 4 is one of the first amateur mode to use dynamic channel equalization that improves its throughput dramatically in multi-path propagation common on HF.  If interested in some comparisons  of popular digital modes on HF and VHF/UHF see the write-up at


Rick Muething, KN6KB

On 10/27/2020 6:33 PM, Graham wrote:
I dont think its a  mode  / use problem,  I used to  have  regular contacts with the  USA  east  on 7  MHz using  amtor , long back, 
running  round  240 watts  carrier power , one guy even  asked me 'which state liverpool was in'  , unexpected  dx ..

the new  generation  of  data modes , reduced the  power needs to something like  25  watts  , a good  + 10 db gain , to  do the  same thing 

That was when the  bands where in  good  condition , I do  know from  tests  on 476 KHz , the  latest vara HF modem is  as  sensitive 
if not  more so, than the slowest  olivia mode , but  many times  faster and provides a  amtor like  fast  turn  round  qso  , using the  chat  module , interesting  test with  station  200  miles  south  of me , on MF  

73 -Graham

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