|
In the operating manual I see the following warning:
```
# 1. Connect the Antenna first!
The antenna jack on the back must always be connected to a properly matched antenna with an SWR of less than 2 on the frequencies you intend to transmit on. This is a recommendation for all amateur wireless radios but more specifically the sBitx. The sBitx can put out upto 50 watts of power and if the antenna does not match, it can damage the power amplifier transistors instantaneously.
```
Does the sBitx have the ability to tune or check the SWR without damaging itself, or does one need to use an antenna analyzer before using the radio?
I ask because most of my operation is portable, so my SWR changes every time I set up my station. SWR can be high even for an antenna of the right length if it's deployed in a compromised position or if I made an error during setup (like last month when I forgot to attach counterpoises to an end-fed antenna :-)
Thanks,
Trevor KF0FTJ
--
=-=-=-= Trevor Stone -=- [Flwyd] -=- <tstone @ trevorstone.org> =-=-=-=
Computer science, eclectic philosophy, games, wits, esoterics, odd hats
Thou brazen boil-brained malt-worm!
{embrace society} POLAROID does it in seconds.
Calvin: The world is a complicated place, Hobbes. Hobbes: Whenever it
seems that way, I take a nap in a tree and wait for dinner.
|
Unfortunatel, no. The sbitx has no way to check the swr (yet).? Even if it had, as we are dealing with very high power, fiddling with the tuner for 15-20 seconds with large mismatch will send all the 40 watts back into the PA, destroying it. What will work is a resistive bridge. This will dissipate most of the power in big resistors until you are tuned up.? You can paraellel up ten x 470 ohm resistors of 1 watt to make each of the three legs of the resistive divider of the swr bridge. A DPDT switch can bypass it once you are all tuned up.
On Wed, Aug 3, 2022, 12:21 PM Trevor Stone < tstone@...> wrote: In the operating manual I see the following warning:
```
# 1. Connect the Antenna first!
The antenna jack on the back must always be connected to a properly
matched antenna with an SWR of less than 2 on the frequencies you intend
to transmit on. This is a recommendation for all amateur wireless radios
but more specifically the sBitx. The sBitx can put out upto 50 watts of
power and if the antenna does not match, it can damage the power
amplifier transistors instantaneously.
```
Does the sBitx have the ability to tune or check the SWR without
damaging itself, or does one need to use an antenna analyzer before
using the radio?
I ask because most of my operation is portable, so my SWR changes every
time I set up my station.? SWR can be high even for an antenna of the
right length if it's deployed in a compromised position or if I made an
error during setup (like last month when I forgot to attach
counterpoises to an end-fed antenna :-)
Thanks,
Trevor KF0FTJ
--
=-=-=-= Trevor Stone -=- [Flwyd] -=- <tstone @ > =-=-=-=
Computer science, eclectic philosophy, games, wits, esoterics, odd hats
? ? ? ? ? ?Thou brazen boil-brained malt-worm!
{embrace society}? ? ? ? ? ? ? ? ? ? ? ? ? POLAROID does it in seconds.
Calvin: The world is a complicated place, Hobbes.? Hobbes: Whenever it
seems that way, I take a nap in a tree and wait for dinner.
|
Many recommend tuning at a lower power. This risky as well. That is because at lower power, the output impedance of the PA will be different. Once you are tuned at QRP level, increasing the power will result in a mismatch again!? - f
Unfortunatel, no. The sbitx has no way to check the swr (yet).? Even if it had, as we are dealing with very high power, fiddling with the tuner for 15-20 seconds with large mismatch will send all the 40 watts back into the PA, destroying it. What will work is a resistive bridge. This will dissipate most of the power in big resistors until you are tuned up.? You can paraellel up ten x 470 ohm resistors of 1 watt to make each of the three legs of the resistive divider of the swr bridge. A DPDT switch can bypass it once you are all tuned up.
On Wed, Aug 3, 2022, 12:21 PM Trevor Stone < tstone@...> wrote: In the operating manual I see the following warning:
```
# 1. Connect the Antenna first!
The antenna jack on the back must always be connected to a properly
matched antenna with an SWR of less than 2 on the frequencies you intend
to transmit on. This is a recommendation for all amateur wireless radios
but more specifically the sBitx. The sBitx can put out upto 50 watts of
power and if the antenna does not match, it can damage the power
amplifier transistors instantaneously.
```
Does the sBitx have the ability to tune or check the SWR without
damaging itself, or does one need to use an antenna analyzer before
using the radio?
I ask because most of my operation is portable, so my SWR changes every
time I set up my station.? SWR can be high even for an antenna of the
right length if it's deployed in a compromised position or if I made an
error during setup (like last month when I forgot to attach
counterpoises to an end-fed antenna :-)
Thanks,
Trevor KF0FTJ
--
=-=-=-= Trevor Stone -=- [Flwyd] -=- <tstone @ > =-=-=-=
Computer science, eclectic philosophy, games, wits, esoterics, odd hats
? ? ? ? ? ?Thou brazen boil-brained malt-worm!
{embrace society}? ? ? ? ? ? ? ? ? ? ? ? ? POLAROID does it in seconds.
Calvin: The world is a complicated place, Hobbes.? Hobbes: Whenever it
seems that way, I take a nap in a tree and wait for dinner.
|
Hi Ashhar,
I¡¯m a bit confused. ?I get paralleling ten 470 ohm resistors but I¡¯m am uncertain how to use it. ?How do you connect this arrangement (three legs) to protect the finals? ?Is it in series with the antenna? ?Can you sketch to wiring?
Thanks in advance, Scott
|
Scot, You have to build a resistive bridge like this :
Here, each of the 50 ohm resistors should be able to dissipate 10 watts. So, you make a 10 watt resistor by paralleling 10 resistors of 470 ohms each.? - f
Hi Ashhar,
I¡¯m a bit confused.? I get paralleling ten 470 ohm resistors but I¡¯m am uncertain how to use it.? How do you connect this arrangement (three legs) to protect the finals?? Is it in series with the antenna?? Can you sketch to wiring?
Thanks in advance, Scott
|
? Scott, where did you get three legs? ?If all the resistors are parallel there will be only two legs. ?Connect one to the ground of your transmitter and one to the antenna pin in your output connector. ?A dummy load connect just like an antenna. ?It replaces the antenna for testing purposes.
All my dummy loads have a BNC connector and I connect them to the BNC out connector of my transceivers. ?
Google dummy loads¡. ?You will find dozens of them and check YouTube for videos.
Dave K8WPE since 1960 David J. Wilcox¡¯s iPad
toggle quoted message
Show quoted text
On Aug 3, 2022, at 6:07 AM, Scott KE8KYP <scott_massey@...> wrote:
?Hi Ashhar,
I¡¯m a bit confused. ?I get paralleling ten 470 ohm resistors but I¡¯m am uncertain how to use it. ?How do you connect this arrangement (three legs) to protect the finals? ?Is it in series with the antenna? ?Can you sketch to wiring?
Thanks in advance, Scott
|
I'm wondering if we could add an SWR bridge and use the reverse sense voltage to automatically fold back the RF power?? Perhaps control the gain of Q11 to accomplish this?
(I have an sBitx on order...)
73 Gody K3NG
On Wed, Aug 3, 2022 at 6:15 AM Ashhar Farhan < farhanbox@...> wrote: Scot, You have to build a resistive bridge like this :
Here, each of the 50 ohm resistors should be able to dissipate 10 watts. So, you make a 10 watt resistor by paralleling 10 resistors of 470 ohms each.? - f
Hi Ashhar,
I¡¯m a bit confused.? I get paralleling ten 470 ohm resistors but I¡¯m am uncertain how to use it.? How do you connect this arrangement (three legs) to protect the finals?? Is it in series with the antenna?? Can you sketch to wiring?
Thanks in advance, Scott
|
You can control it in software, much simpler! The main reason for not putting in an RF sensor is that raspberry pi doesnt have an ADC
I'm wondering if we could add an SWR bridge and use the reverse sense voltage to automatically fold back the RF power?? Perhaps control the gain of Q11 to accomplish this?
(I have an sBitx on order...)
73 Gody K3NG
On Wed, Aug 3, 2022 at 6:15 AM Ashhar Farhan < farhanbox@...> wrote: Scot, You have to build a resistive bridge like this :
Here, each of the 50 ohm resistors should be able to dissipate 10 watts. So, you make a 10 watt resistor by paralleling 10 resistors of 470 ohms each.? - f
Hi Ashhar,
I¡¯m a bit confused.? I get paralleling ten 470 ohm resistors but I¡¯m am uncertain how to use it.? How do you connect this arrangement (three legs) to protect the finals?? Is it in series with the antenna?? Can you sketch to wiring?
Thanks in advance, Scott
|
You can also get some TO-220 non-inductive resistors of a suitable
wattage, and mount those to the heatsink. 10-watt devices should be
OK, and 50 ohms is close enough.
Those devices aren't cheap, but they're effective. I use a 100-watt
50-ohm device mounted to a heatsink with an SO-239, and it's flat to
well over 100 MHz.
73
Jim N6OTQ
|
"Many recommend tuning at a lower power. This risky as well. That is because at lower power, the output impedance of the PA will be different. Once you are tuned at QRP level, increasing the power will result in a mismatch again! "
Interesting. Every auto tuner manufacturer's instructions I have ever seen recommend that you tune at low power. I have a remote auto tuner at the base of a wire vertical that I tune on every band change. I have always tuned with low power using several different rigs over the decades and never noticed a change in SWR. I'll pay closer attention, but if I'm getting a change it has been to small to notice.
Haven't received my sBitx yet, but it looks like there's room for a small board with SWR sensing and an absorptive bridge that you could manually switch in and out. If you wanted to get fancy you could route the measure to the Pi and integrate into the display.
--
? 73
??? Bob? KD8CGH
|
On Wed, Aug 3, 2022 at 02:45 AM, Ashhar Farhan wrote:
at lower power, the output impedance of the PA will be different. Once you are tuned at QRP level, increasing the power will result in a mismatch again!?
Well that's not good!
|
Please forgive my ignorance but I've never understood statements like this.?
"That is because at lower power, the output impedance of the PA will be different."
Since SSB is all about audio (no audio, no power) and the average power is about 40% of peak, what happens to the output impedance every time you speak into the microphone? Power is going to follow the audio and would be all over the place, from zero to peak power. How is this any different?
Max KG4PID
On Wednesday, August 3, 2022, 02:45:06 AM CDT, Ashhar Farhan <farhanbox@...> wrote:
Many recommend tuning at a lower power. This risky as well. That is because at lower power, the output impedance of the PA will be different. Once you are tuned at QRP level, increasing the power will result in a mismatch again!? - f
|
This is not correct.? The output impedance of the PA does not change
the SWR and resistance/reactance of the tuner/antenna combination.
What one is really doing is tuning out the reactance of the antenna
and setting the load resistance to the design value of the
amplifier.? A reactive load increases the dissipation of the PA
finals.? Tuning at low power within the voltage and dissipation
limits is safe.
David N1HAC
On 8/3/22 3:44 AM, Ashhar Farhan via
groups.io wrote:
toggle quoted message
Show quoted text
Many recommend tuning at a lower power. This risky
as well. That is because at lower power, the output impedance of
the PA will be different. Once you are tuned at QRP level,
increasing the power will result in a mismatch again!?
- f
Unfortunatel, no. The sbitx has no way to
check the swr (yet).?
Even if it had, as we are dealing with very
high power, fiddling with the tuner for 15-20 seconds with
large mismatch will send all the 40 watts back into the
PA, destroying it.
What will work is a resistive bridge. This
will dissipate most of the power in big resistors until
you are tuned up.?
You can paraellel up ten x 470 ohm resistors
of 1 watt to make each of the three legs of the resistive
divider of the swr bridge. A DPDT switch can bypass it
once you are all tuned up.
On Wed, Aug 3, 2022, 12:21
PM Trevor Stone < tstone@...>
wrote:
In the
operating manual I see the following warning:
```
# 1. Connect the Antenna first!
The antenna jack on the back must always be connected to a
properly
matched antenna with an SWR of less than 2 on the
frequencies you intend
to transmit on. This is a recommendation for all amateur
wireless radios
but more specifically the sBitx. The sBitx can put out
upto 50 watts of
power and if the antenna does not match, it can damage the
power
amplifier transistors instantaneously.
```
Does the sBitx have the ability to tune or check the SWR
without
damaging itself, or does one need to use an antenna
analyzer before
using the radio?
I ask because most of my operation is portable, so my SWR
changes every
time I set up my station.? SWR can be high even for an
antenna of the
right length if it's deployed in a compromised position or
if I made an
error during setup (like last month when I forgot to
attach
counterpoises to an end-fed antenna :-)
Thanks,
Trevor KF0FTJ
--
=-=-=-= Trevor Stone -=- [Flwyd] -=- <tstone @ >
=-=-=-=
Computer science, eclectic philosophy, games, wits,
esoterics, odd hats
?
? ? ? ? ?Thou brazen boil-brained malt-worm!
{embrace society}? ? ? ? ? ? ? ? ? ? ? ? ? POLAROID does
it in seconds.
Calvin: The world is a complicated place, Hobbes.? Hobbes:
Whenever it
seems that way, I take a nap in a tree and wait for
dinner.
|
Correct David.
How come so many still believe the myth that a bunch of passive elements ( antenna system) actually changes with power level?
simple, they have a crappy SWR meter. It either has poor directivity, or the indication changes because of heating in the diode / changes in the detection circuit like core saturation at higher power levels.?
|
David,
Here is the explanation as I see it. First, the impedance. It is the ratio of voltage to current. Let's skip RF and only consider DC, then we will expand this to DC.? Ex 1: Consider a 12 supply?that is delivering 40 watts of power to a resistor across it's terminals. It's current should be 3.3A, so that 12v x 3.3A = 40 Watts. Now, which also means that the resistor is 12V/3.3A =? 3.6 ohms. This is also the case of the full power of sBitx.? Ex 2: Now, consider that we have decreased the output of the power supply's current?only such that we are producing? 12 watts of power. The current will now be 1 A (as 12v x 1A = 12 Watts) and the load resistance required will be 12 ohms (12V / 1A). Hence, the load impedance is changed. If you still put a 3.6 ohm resistor, the voltage from the power supply will drop down and become unregulated and unstable, the power supply is overloaded.
Now, let's switch to an RF 'power supply', that is, a transmitter.? The output impedance of a power amplifier, derived from the above explanation is given as (Vcc * Vcc)/ (2 * Po). That is, output impedance of the power amplifier is calculated as square of power supply voltage divided by double the output power.
Ex 3: Consider a QRP transmitter like the BITX20 or QRP labs QCX. The Vcc is 12v and the output power is 5 watts, hence, the PA output impedance is (12 * 12)/ ( 2 * 5) = 14.4 ohms.?
Ex4: In the above Ex 3, now consider that you have reduced the power to 1 watt. The Vcc is still 12V, hence, the output impedance is (12 * 12) / (2 * 1) = 72 ohms.
In case of regular SSB transmitters, they are tuned to match at the maximum output to protect the transistors from excessive power remaining in the device at the peak output, it also makes sense as for CW and other continuous carrier modes like FT8 and RTTY, the output power doesn't vary but remains the same as the peak ssb envelope power.
- f??
This is not correct.? The output impedance of the PA does not change
the SWR and resistance/reactance of the tuner/antenna combination.
What one is really doing is tuning out the reactance of the antenna
and setting the load resistance to the design value of the
amplifier.? A reactive load increases the dissipation of the PA
finals.? Tuning at low power within the voltage and dissipation
limits is safe.
David N1HAC
On 8/3/22 3:44 AM, Ashhar Farhan via
wrote:
Many recommend tuning at a lower power. This risky
as well. That is because at lower power, the output impedance of
the PA will be different. Once you are tuned at QRP level,
increasing the power will result in a mismatch again!?
- f
Unfortunatel, no. The sbitx has no way to
check the swr (yet).?
Even if it had, as we are dealing with very
high power, fiddling with the tuner for 15-20 seconds with
large mismatch will send all the 40 watts back into the
PA, destroying it.
What will work is a resistive bridge. This
will dissipate most of the power in big resistors until
you are tuned up.?
You can paraellel up ten x 470 ohm resistors
of 1 watt to make each of the three legs of the resistive
divider of the swr bridge. A DPDT switch can bypass it
once you are all tuned up.
On Wed, Aug 3, 2022, 12:21
PM Trevor Stone < tstone@...>
wrote:
In the
operating manual I see the following warning:
```
# 1. Connect the Antenna first!
The antenna jack on the back must always be connected to a
properly
matched antenna with an SWR of less than 2 on the
frequencies you intend
to transmit on. This is a recommendation for all amateur
wireless radios
but more specifically the sBitx. The sBitx can put out
upto 50 watts of
power and if the antenna does not match, it can damage the
power
amplifier transistors instantaneously.
```
Does the sBitx have the ability to tune or check the SWR
without
damaging itself, or does one need to use an antenna
analyzer before
using the radio?
I ask because most of my operation is portable, so my SWR
changes every
time I set up my station.? SWR can be high even for an
antenna of the
right length if it's deployed in a compromised position or
if I made an
error during setup (like last month when I forgot to
attach
counterpoises to an end-fed antenna :-)
Thanks,
Trevor KF0FTJ
--
=-=-=-= Trevor Stone -=- [Flwyd] -=- <tstone @ >
=-=-=-=
Computer science, eclectic philosophy, games, wits,
esoterics, odd hats
?
? ? ? ? ?Thou brazen boil-brained malt-worm!
{embrace society}? ? ? ? ? ? ? ? ? ? ? ? ? POLAROID does
it in seconds.
Calvin: The world is a complicated place, Hobbes.? Hobbes:
Whenever it
seems that way, I take a nap in a tree and wait for
dinner.
|
There is room for doubt here.
An SWR meter assumes the transmitter wants to see a 50 ohm resistive load.
A uBitx/sBitx might work a bit better into a load that is somewhat different than assumed.
Jerry,? KE7ER
toggle quoted message
Show quoted text
On Wed, Aug 3, 2022 at 06:20 AM, David McGaw wrote:
This is not correct.? The output impedance of the PA does not change the SWR and resistance/reactance of the tuner/antenna combination.
What one is really doing is tuning out the reactance of the antenna and setting the load resistance to the design value of the amplifier.? A reactive load increases the dissipation of the PA finals.? Tuning at low power within the voltage and dissipation limits is safe.
David N1HAC
|
If it works well, it's good.
This does not strike me as all that odd.
I wouldn't worry unless it somehow caused significant distortion in the transmitted signal.
Jerry, KE7ER
toggle quoted message
Show quoted text
On Wed, Aug 3, 2022 at 05:21 AM, John AE5X wrote:
On Wed, Aug 3, 2022 at 02:45 AM, Ashhar Farhan wrote:
at lower power, the output impedance of the PA will be different. Once you are tuned at QRP level, increasing the power will result in a mismatch again!?
Well that's not good!
|
I would tune up any antenna matching network (if you have one) at low power,
that should be close.? Then increase power and tune again.
Alternantely, use something like an Antuino or nanoVNA to do the initial tune up.
If you have an antenna you know to be somewhat tuned, a matching network is not needed.
But you don't want to start at 40 watts, only to find that a windstorm took it down
and you have a 10:1 SWR.
For those who actually read my comments, it should be clear now.
I started out this morning by reading the most recent post and am working my way back.
Jerry, KE7ER
toggle quoted message
Show quoted text
On Wed, Aug 3, 2022 at 04:50 AM, Bob Benedict, KD8CGH wrote:
"Many recommend tuning at a lower power. This risky as well. That is because at lower power, the output impedance of the PA will be different. Once you are tuned at QRP level, increasing the power will result in a mismatch again! "
Interesting. Every auto tuner manufacturer's instructions I have ever seen recommend that you tune at low power. I have a remote auto tuner at the base of a wire vertical that I tune on every band change. I have always tuned with low power using several different rigs over the decades and never noticed a change in SWR. I'll pay closer attention, but if I'm getting a change it has been to small to notice.
Haven't received my sBitx yet, but it looks like there's room for a small board with SWR sensing and an absorptive bridge that you could manually switch in and out. If you wanted to get fancy you could route the measure to the Pi and integrate into the display.
|
Jerry,
A resistive bridge SWR meter should not work if this statement is true.
Am I missing something here?
73
Evan
AC9TU
|
My first guess for having different values of SWR reported at different power levels would be that the SWR measurement device cannot accurately measure the two power levels.? This does make sense to me.? So If you use a meter that can measure down to the low power level, it should be good to tune the tuner.
NOTE that the SWR meter is not part of the tuned circuit.? It is measuring at the cable's connector to the tuner.? The "measurement plane" in S Parameter terms.
Am I missing something?
73
Evan
AC9TU
|
David Wilcox K8WPE
Bob Benedict, KD8CGH
David McGaw