Topics

FT8 and power levels


Mitchel Rought
 

A question of curiosity about FT8 and the level of power being used, I have a uBITX v6.2 with a usb dongle running approx.5-7 watts into either a  cobweb, 20m hexbeam, or a 20m delta loop, all antennas are run through a tuner and average < 1.1 swr at the digital freq.
I have made contacts and the average report I get back is anywhere from -13 to-26db
I am curious what everyone else is doing and seeing
thanks in advance


Rafael Diniz
 

What is the emission type / signal characteristics of this "FT8"?

Thanks,
Rafael

On 1/19/21 8:54 PM, Mitchel Rought wrote:
A question of curiosity about FT8 and the level of power being used, I
have a uBITX v6.2 with a usb dongle running approx.5-7 watts into
either a  cobweb, 20m hexbeam, or a 20m delta loop, all antennas are
run through a tuner and average < 1.1 swr at the digital freq.
I have made contacts and the average report I get back is anywhere
from -13 to-26db
I am curious what everyone else is doing and seeing
thanks in advance


jeanrenier2004
 

Hi Mitchel,

My experience using the uBIT-X in FT8 mode is quite similar.

The uBIT-X has a remarkably sensitive receiver, on a good day I can "hear" FT8 signals from New Zealand (about as far as one can get from Belgium) at -24dB S/N.  Its transmitter is of course limited to QRP levels, I am typically running at 5...6W (continuous, not PEP) into a simple (not so good) 20m dipole. 

FT8 is a great mode inasmuch it gives a more or less objective measurement of the S/N on both sides.  Of the 100+ QSO I made during the past months the average difference between received S/N at my end and the received S/N at the other end is as much as 15dB (in favor of the other guy).  Some are as low as 0dB, some as much as 24dB.

Since the same transmission path, including both antennas, is used for both directions I come to believe that a lot of OMs are transmitting FT8 at 100...200W, levels, some even over 1000W.  Not entirely the intent of Joe Taylor with WSJT-X, but I am afraid it is one of these facts of life. 
It means that I don't even try anymore to make a QSO with a station that come in with a S/N of less than -10dB.  On good days I can still reach out to NE USA and Canada. 

73,  ON4JRT/Jean.


Op 20/01/2021 om 00:54 schreef Mitchel Rought:

A question of curiosity about FT8 and the level of power being used, I have a uBITX v6.2 with a usb dongle running approx.5-7 watts into either a  cobweb, 20m hexbeam, or a 20m delta loop, all antennas are run through a tuner and average < 1.1 swr at the digital freq.
I have made contacts and the average report I get back is anywhere from -13 to-26db
I am curious what everyone else is doing and seeing
thanks in advance


Evan Hand
 

On Tue, Jan 19, 2021 at 07:28 PM, Rafael Diniz wrote:
What is the emission type / signal characteristics of this "FT8"?
FT8 messages are transmitted using 8-tone continuous-phase frequency shift keying (CPFSK). Each transmitted tone or channel symbol conveys three bits.

from 
https://physics.princeton.edu/pulsar/k1jt/FT4_FT8_QEX.pdf

73
Evan
AC9TU


Rafael Diniz
 

Thanks Evan!

On 1/20/21 7:41 AM, Evan Hand wrote:
On Tue, Jan 19, 2021 at 07:28 PM, Rafael Diniz wrote:

What is the emission type / signal characteristics of this "FT8"?

*FT8* messages are transmitted using 8-tone continuous-phase frequency
shift keying (CPFSK). Each transmitted tone or channel symbol conveys
three bits.

from 
https://physics.princeton.edu/pulsar/k1jt/FT4_FT8_QEX.pdf
<https://physics.princeton.edu/pulsar/k1jt/FT4_FT8_QEX.pdf>

73
Evan
AC9TU


Rubens Kamimura
 

Olá Evan, Curt e grupo.

O meu uBitx v4, voltou a oscilar no 40m, 7074kHz em FT8, mesmo com "fake It" selecionado no JTDX. E observo a falta de potência de RF de saída na faixa dos 10 a 15m.

Em qua, 20 de jan de 2021 13:11, Rafael Diniz <rafael@...> escreveu:

Thanks Evan!

On 1/20/21 7:41 AM, Evan Hand wrote:
> On Tue, Jan 19, 2021 at 07:28 PM, Rafael Diniz wrote:
>
>     What is the emission type / signal characteristics of this "FT8"?
>
> *FT8* messages are transmitted using 8-tone continuous-phase frequency
> shift keying (CPFSK). Each transmitted tone or channel symbol conveys
> three bits.
>
> from 
> https://physics.princeton.edu/pulsar/k1jt/FT4_FT8_QEX.pdf
> <https://physics.princeton.edu/pulsar/k1jt/FT4_FT8_QEX.pdf>
>
> 73
> Evan
> AC9TU
>







Evan Hand
 

On Wed, Jan 20, 2021 at 03:40 PM, Rubens Kamimura wrote:
O meu uBitx v4, voltou a oscilar no 40m, 7074kHz em FT8, mesmo com "fake It" selecionado no JTDX. E observo a falta de potência de RF de saída na faixa dos 10 a 15m.

Translation using Google:
"My uBitx v4, oscillated again at 40m, 7074kHz in FT8, even with "fake It" selected in JTDX. And I notice the lack of RF output power in the range of 10 to 15m."

I believe that you are saying that you are getting feedback on 40m and low power on 10 and 15 meters.

I have found that FT8 transmissions tend to add issues with RF in the shack that leads to oscillations and distortion in the transmition.  This is because the power output is maximum at 100% duty cycle.  The only way around this is to reduce the RF in the shack with common mode chokes and to keep it from the input to the uBITX and Computer interface.  0.01 uF capacitors on the inputs, along with ferrite rings or snap-on sleeves help.

The lower power at higher frequencies is a known characteristic of the uBITX design.  There are hundreds of messages both commenting on and addressing the issue.  It is a tradeoff for the full HF coverage with a minimum cost that is the design point for the uBITX.  You can search the board for messages on balancing power across the bands.  In particular look for messages from Allison/ajparent1/kb1gma.

Above are my opinions.  Yours can be different and that is OK.
73
Evan
AC9TU

Portuguese Translation:

Eu acredito que você está dizendo que está recebendo feedback em 40m e baixa potência em 10 e 15 metros.

 

Eu descobri que as transmissões FT8 tendem a adicionar problemas com RF no shack que leva a oscilações e distorção na transmissão. Isso ocorre porque a saída de energia é máxima em 100% do ciclo de trabalho. A única maneira de contornar isso é reduzir o RF no shack com choques de modo comum e mantê-lo longe da entrada do uBITX e da interface do computador. Capacitores de 0,01 uF nas entradas, juntamente com anéis de ferrite ou luvas de encaixe ajudam.

 

A potência mais baixa em frequências mais altas é uma característica conhecida do design uBITX. Existem centenas de mensagens comentando e abordando o assunto. É uma compensação para a cobertura HF completa com um custo mínimo que é o ponto de design para o uBITX. Você pode pesquisar o quadro em busca de mensagens sobre o equilíbrio da energia entre as bandas. Em particular, procure mensagens de Allison / ajparent1 / kb1gma.

 

 

Acima estão minhas opiniões. O seu pode ser diferente e está tudo bem.

73
Evan
AC9TU


Mitchel Rought
 

As the originator of this thread I would like to thank everyone who joined in, I knew when I bought my uBITX V6 it was low power and planned on that
Since August I have made almost 500 contacts using FT8 which include 18 countries
Running low power means you pay a lot more attention to antenna construction, cable runs and definitely paying attention to band conditions and timing
Will be looking at slightly more power in the future but right now 5-7 watts is still fun


Rubens Kamimura
 

Group,

Yes, uBITX has some problems, as it was developed to serve as economically as possible. That is why we are working and trying to improve. I am very grateful to the group for their help and attention. I have tried to work with a resting current of around 90mA and with injection (VR1 setting) for low power (QRP) in the range of 5 to 7 Watts (40m/FT8), to avoid RF feedback. But, there are inevitably unwanted feedbacks that we have to rework. Use JTDX with "fake it" selected.
73
Rubens
py2pvb


Em sex., 22 de jan. de 2021 às 10:55, Mitchel Rought <mitchelrought@...> escreveu:

As the originator of this thread I would like to thank everyone who joined in, I knew when I bought my uBITX V6 it was low power and planned on that
Since August I have made almost 500 contacts using FT8 which include 18 countries
Running low power means you pay a lot more attention to antenna construction, cable runs and definitely paying attention to band conditions and timing
Will be looking at slightly more power in the future but right now 5-7 watts is still fun


Rubens Kamimura
 

Hello everyone,

Grateful for the tips.
I changed the five relays of the uBITX v4, it improved a lot, for example it stopped oscillating the power, even with "fake it" unchecked for "split" operation. Now I need to resolve the issue of the absence of RF power in the range of 10 to 15m, I believe I will have to replace the Q90 transistor.
73
Rubens
py2pvb
--
Saudações fraternais, 
73's PY2PVB

Rubens Kamimura
Técnico em Eletrônica e Eletrotécnica - Aposentado
CFT/CRT-SP: 80254934820
Cel. +55 (18) 9-9819-2225 WhatsApp


Em sex., 22 de jan. de 2021 às 15:30, Rubens Kamimura <rubens.kamimura@...> escreveu:

Group,

Yes, uBITX has some problems, as it was developed to serve as economically as possible. That is why we are working and trying to improve. I am very grateful to the group for their help and attention. I have tried to work with a resting current of around 90mA and with injection (VR1 setting) for low power (QRP) in the range of 5 to 7 Watts (40m/FT8), to avoid RF feedback. But, there are inevitably unwanted feedbacks that we have to rework. Use JTDX with "fake it" selected.
73
Rubens
py2pvb


Em sex., 22 de jan. de 2021 às 10:55, Mitchel Rought <mitchelrought@...> escreveu:
As the originator of this thread I would like to thank everyone who joined in, I knew when I bought my uBITX V6 it was low power and planned on that
Since August I have made almost 500 contacts using FT8 which include 18 countries
Running low power means you pay a lot more attention to antenna construction, cable runs and definitely paying attention to band conditions and timing
Will be looking at slightly more power in the future but right now 5-7 watts is still fun