Topics

QRP SWR meter recommendation? #ubitx

Mike KK7ER
 

So far I have been setting the antenna tuner using my Kenwood and then swapping in the uBITX.  But now we want to take the uBITX on the road.  Problem is that the uBITX does not put out enough power for full deflection on our MFJ versa tuner II so we cannot get an accurate SWR reading to tune the antenna.  Does anyone have recommendations for a QRP SWR meter?  Thanks!

73, Mike KK7ER
(and son KI7ZZS)

Joe Milosch <zzmiloschxx@...>
 

On Mon, 20 Aug 2018 08:31:35 -0700
"Mike KK7ER" <groupio@...> wrote:

Hi, try the Emtech ZM-2
https://steadynet.com/emtech/

It only handles 15 watts, but is highly regarded.
Comes as a kit or pre-built. Youtube has a few
good reviews.

Joe



So far I have been setting the antenna tuner using my Kenwood and then swapping in the uBITX.  But now we want to take the uBITX on the road.  Problem is that the uBITX does not put out enough power for full deflection on our MFJ versa tuner II so we cannot get an accurate SWR reading to tune the antenna.  Does anyone have recommendations for a QRP SWR meter?  Thanks!

73, Mike KK7ER
(and son KI7ZZS)


Jerry Gaffke
 

If all you want is an SWR meter and are on a budget,
consider this guy from Diz at $12 for the kit:
    http://www.kitsandparts.com/bridge.php

The Emtech would be nice though, no need to haul out that MFJ.

Might be a way to hack a switch into the MFJ to give a low power option.
I'm not familiar with it.

Jerry


On Mon, Aug 20, 2018 at 08:40 AM, Joe Milosch wrote:
On Mon, 20 Aug 2018 08:31:35 -0700
"Mike KK7ER" <groupio@...> wrote:

Hi, try the Emtech ZM-2
https://steadynet.com/emtech/

It only handles 15 watts, but is highly regarded.
Comes as a kit or pre-built. Youtube has a few
good reviews.

V Zecchinelli
 

I use this one.

http://www.4sqrp.com/4stuner.php

ince, N1VIN



On 8/20/2018 11:31 AM, Mike KK7ER wrote:
So far I have been setting the antenna tuner using my Kenwood and then swapping in the uBITX.  But now we want to take the uBITX on the road.  Problem is that the uBITX does not put out enough power for full deflection on our MFJ versa tuner II so we cannot get an accurate SWR reading to tune the antenna.  Does anyone have recommendations for a QRP SWR meter?  Thanks!

73, Mike KK7ER
(and son KI7ZZS)

Ken
 

I'm able to tune on the same tuner with a non Bitx/uBitx with 6 watts.

73

Ken VA3ABN

On Mon, Aug 20, 2018 at 11:31 AM, Mike KK7ER <groupio@...> wrote:
So far I have been setting the antenna tuner using my Kenwood and then swapping in the uBITX.  But now we want to take the uBITX on the road.  Problem is that the uBITX does not put out enough power for full deflection on our MFJ versa tuner II so we cannot get an accurate SWR reading to tune the antenna.  Does anyone have recommendations for a QRP SWR meter?  Thanks!

73, Mike KK7ER
(and son KI7ZZS)


ajparent1/KB1GMX
 

On Mon, Aug 20, 2018 at 09:37 AM, Jerry Gaffke wrote:
If all you want is an SWR meter and are on a budget,
consider this guy from Diz at $12 for the kit:
    http://www.kitsandparts.com/bridge.php
I second that.  Cheap and easy.  I have a few around here, one standalone in a box.
Some of the others are built into a radio.  Does the job.

Allison

Bill Cromwell <wrcromwell@...>
 

Hi Mike,

The tuner itself does not care how much power you use to 'set' it as long as it isn't *more* than it can handle. It is not impressed if your TX can pin the needle or not. If your uBitx will only drive the meter to half scale in it's "Forward" configuration then set the "Cal" knob full right. Flip the switch to "Reverse" and adjust the tuner for minimum as per usual. If you can get it all the way to zero - well zero is still zero. And less is still less. So what if you can't read that it is 1.25 to 1 if it is 1.25 to 1. A zero in "Reverse" is 1:1 no matter how high the TX didn't drive the needle in "Forward".

I built an SWR bridge that could be pinned with less than two watts (maxed at about 30) and discovered that it doesn't make any difference as described above. What does make a difference is a tuner and SWR bridge built for QRP power levels can be made much smaller and lighter:) Put the tuner and the bridge in the same box together for even more convenience out in the field. Like a scaled down version of your MFJ. If you build it into your uBitx you will forfeit using it with other radios but will be even more convenient when taking the uBitx to the field.

Keep us posted with progress.

73,

Bill KU8H

On 08/20/2018 11:31 AM, Mike KK7ER wrote:
So far I have been setting the antenna tuner using my Kenwood and then
swapping in the uBITX. But now we want to take the uBITX on the road.
Problem is that the uBITX does not put out enough power for full
deflection on our MFJ versa tuner II so we cannot get an accurate SWR
reading to tune the antenna. Does anyone have recommendations for a QRP
SWR meter? Thanks!

73, Mike KK7ER
(and son KI7ZZS)
--
bark less - wag more

Zachary Ledbetter
 

I would also recommend the ZM-2 tuner if you do need a SWR meter reading.

It has a resistive bridge to protect your rig, very fast to tune, and the range is amazing.  You could likely tune to low SWR on a pool umbrella with it!  Now that I think about it...Maybe I will give it a try this week! 

Best regards,



Zachary Ledbetter
 

Sorry guys.  A Typo on my ZM-2 recommendation.  If you “do not” need a metered SWR display.  

The tune to a dim LED works great except in direct sunlight.  In this case I use a small shade.  





Michael Monteith
 

I have one I built and was thinking of giving it a go.  Glad to see others feel the same way.   Perfect for the band and power range.

73, KM4OLT
Michael

Mike KK7ER
 

Bill, I guess you are right.  I ought to be able to do some experiments before we leave to get a sense for how the MFJ performs at low power.  If I can understand what readings correspond to 2:1, I can keep it below 2:1.  And we can order one of the kits and take our time building it.  Thanks!

73 Mike KK7ER

Mike KK7ER
 

Allison, the instructions are pretty terse.  Looks like the meter/meters does/do not come with it.  How does it work?  Use a DMM to measure forward and reverse voltages and do the math in my head?  Thanks!

73 Mike KK7ER

Jerry Gaffke
 

Correct, no meter provided.
A DVM is sufficient.

Or could go into analog pin A7 (with an analog switch to select forward/reverse) of the Nano. 
Perhaps both A6 and A7, use D0,D1 for the keyer if you want CW.
Let the Nano take care of the math.


On Mon, Aug 20, 2018 at 09:06 PM, Mike KK7ER wrote:
Allison, the instructions are pretty terse.  Looks like the meter/meters does/do not come with it.  How does it work?  Use a DMM to measure forward and reverse voltages and do the math in my head?  Thanks!

73 Mike KK7ER

Tom, wb6b
 

Here are some code snippets from a project I did. It calculates SWR from the voltages read from analog pins A6 and A7.

It does a number of other things, so here are the basic pieces of code for SWR extracted out into small snippets.

This first code segments captures selected analog pin voltages and applies an exponential smoothing filter to them.

for (int pin = 0; pin < 8; pin++) {
    if (_analogActive[pin]) {
      float scaledReading = analogScalingFactor * (float)analogRead(pin);
      _analogValue[pin] +=
        (scaledReading - _analogValue[pin]) * _analogFilterValue[pin];
   }
}


This next slice of code captures the max voltage and applies a slow decay to the max value.

  // Capture the peak analog values with a slow decay time.
  // The decay time is a starting point. May want to adjust it to taste.
  float decayValue = 0.001;

  for (int pin = 0; pin < 8; pin++) {
    if (_analogActive[pin]) {
      float analogValue = _analogValue[pin];

      _analogValueMax[pin] = max(_analogValueMax[pin], analogValue);
      _analogValueMax[pin] += (analogValue - _analogValueMax[pin]) * decayValue;

    } else {
      _analogValueMax[pin] = 0.0;
    }
  }


And here the SWR is calculated from the Max values.

    // "SWR"

    float vf = _analogValueMax[6];
    float vr = _analogValueMax[7];
    float vd = (vf - vr);
    vd = max(vd, 0.002);  // Limit max SWR reading before divide by zero
    // Calculate SWR from directional coupler voltages.
    result = String((vf + vr) / vd);


You can see the full code here:
https://github.com/mountaintom/SmartLCDandIOexpander_I2C/blob/master/examples/SmartLCDandIOexpander_I2C_Backpack/SmartLCDandIOexpander_I2C_Backpack.ino

Tom, wb6b

Gordon Gibby
 

If you look at the schematic of your tuner, there is probably one or two resistors in a voltage divider chain that you could parallel  something with to increase the voltage that gets to the meter.  Either before or after the rectifier diode or diodes.  Just make sure that whatever gets done to the forward circuit is the same as what gets done to the reverse circuit

Secondly, you don’t have to have a full-scale indication to figure out your swr.  It’s a relative measurement.   If the reverse is 50% of the forward on the meter range, most of the time youre at 3 to 1.  You could look at how yours is calibrated to verify that.   

You can first do a rough tune just using the receiver, adjusting for loudest receive noise. Then get the transmitter going & null out the return as low as you can.

Some of the other people may already have pointed it out, but there are several excellent little qrp swr kits out there that implement a simple resistive bridge circuit going to an LED diode, and you adjust tuner for the dimmest diode or so.   They are basically comparing against a 50 Ohm load and when your Antenna has been transformed into 50 ohms there is no longer relative voltage between the Antenna and the reference 50 ohms, so the light emitting diode goes dark.

Cheers
Gordon




On Aug 20, 2018, at 12:39, V Zecchinelli <n1vin@...> wrote:

I use this one.

http://www.4sqrp.com/4stuner.php

ince, N1VIN



On 8/20/2018 11:31 AM, Mike KK7ER wrote:
So far I have been setting the antenna tuner using my Kenwood and then swapping in the uBITX.  But now we want to take the uBITX on the road.  Problem is that the uBITX does not put out enough power for full deflection on our MFJ versa tuner II so we cannot get an accurate SWR reading to tune the antenna.  Does anyone have recommendations for a QRP SWR meter?  Thanks!

73, Mike KK7ER
(and son KI7ZZS)

Gordon Gibby
 

Here is an example of one of the little swr indicators  where you just dim the diode


I’ve used that kind of a circuit as well as the two torrid system  others recommended which also works fine but you’ll need an external meter or voltmeter or something. 


On Aug 20, 2018, at 13:14, Bill Cromwell <wrcromwell@...> wrote:

Hi Mike,

The tuner itself does not care how much power you use to 'set' it as long as it isn't *more* than it can handle. It is not impressed if your TX can pin the needle or not. If your uBitx will only drive the meter to half scale in it's "Forward" configuration then set the "Cal" knob full right. Flip the switch to "Reverse" and adjust the tuner for minimum as per usual. If you can get it all the way to zero - well zero is still zero. And less is still less. So what if you can't read that it is 1.25 to 1 if it is 1.25 to 1. A zero in "Reverse" is 1:1 no matter how high the TX didn't drive the needle in "Forward".

I built an SWR bridge that could be pinned with less than two watts (maxed at about 30) and discovered that it doesn't make any difference as described above. What does make a difference is a tuner and SWR bridge built for QRP power levels can be made much smaller and lighter:) Put the tuner and the bridge in the same box together for even more convenience out in the field. Like a scaled down version of your MFJ. If you build it into your uBitx you will forfeit using it with other radios but will be even more convenient when taking the uBitx to the field.

Keep us posted with progress.

73,

Bill  KU8H

On 08/20/2018 11:31 AM, Mike KK7ER wrote:
So far I have been setting the antenna tuner using my Kenwood and then
swapping in the uBITX.  But now we want to take the uBITX on the road.
Problem is that the uBITX does not put out enough power for full
deflection on our MFJ versa tuner II so we cannot get an accurate SWR
reading to tune the antenna.  Does anyone have recommendations for a QRP
SWR meter?  Thanks!

73, Mike KK7ER
(and son KI7ZZS)

--
bark less - wag more



Jack, W8TEE
 

Tom:

You can get a marginal speed improvement by moving the data definitions out of the loops. For example:

#define ELEMENTS(x)  (sizeof(x) / sizeof(x[0]))  // Gets rid of "magic numbers" in the code
float scaledReadin;
for (int pin = 0; pin < ELEMENTS(_analogValue); pin++) {
   if (_analogActive[pin]) {
     scaledReading = analogScalingFactor * (float)analogRead(pin);
     _analogValue[pin] += (scaledReading - _analogValue[pin]) * _analogFilterValue[pin];
   }
}
The macro ELEMENTS() allows you to determine the number of elements in an array automatically a compile time, thus eliminating the need to hard-code array sizes into the body of the program. That way, you can simply change the definition of the array in one place in the code, recompile, and the new size is automatically changed everywhere in the code. The really nice thing about the macro is that it's completely typeless...that it, it will work with any aggregate data type.

Moving the definition of a variable before and outside the loop saves the compiler the instructions necessary to adjust the stack pointer on each pass through the loop in the "in-scope, out-of-scope" aspect of a data definition within the loop. Quite honestly, on 8 iterations of the loop it will not likely be noticeable. Still, in other programs where SRAM is pretty tight, it could mean the difference between running and a stack crash.

Jack, W8TEE


On Tuesday, August 21, 2018, 1:02:39 AM EDT, Tom, wb6b <wb6b@...> wrote:


Here are some code snippets from a project I did. It calculates SWR from the voltages read from analog pins A6 and A7.

It does a number of other things, so here are the basic pieces of code for SWR extracted out into small snippets.

This first code segments captures selected analog pin voltages and applies an exponential smoothing filter to them.

for (int pin = 0; pin < 8; pin++) {
    if (_analogActive[pin]) {
      float scaledReading = analogScalingFactor * (float)analogRead(pin);
      _analogValue[pin] +=
        (scaledReading - _analogValue[pin]) * _analogFilterValue[pin];
   }
}


This next slice of code captures the max voltage and applies a slow decay to the max value.

  // Capture the peak analog values with a slow decay time.
  // The decay time is a starting point. May want to adjust it to taste.
  float decayValue = 0.001;

  for (int pin = 0; pin < 8; pin++) {
    if (_analogActive[pin]) {
      float analogValue = _analogValue[pin];

      _analogValueMax[pin] = max(_analogValueMax[pin], analogValue);
      _analogValueMax[pin] += (analogValue - _analogValueMax[pin]) * decayValue;

    } else {
      _analogValueMax[pin] = 0.0;
    }
  }


And here the SWR is calculated from the Max values.

    // "SWR"

    float vf = _analogValueMax[6];
    float vr = _analogValueMax[7];
    float vd = (vf - vr);
    vd = max(vd, 0.002);  // Limit max SWR reading before divide by zero
    // Calculate SWR from directional coupler voltages.
    result = String((vf + vr) / vd);


You can see the full code here:
https://github.com/mountaintom/SmartLCDandIOexpander_I2C/blob/master/examples/SmartLCDandIOexpander_I2C_Backpack/SmartLCDandIOexpander_I2C_Backpack.ino

Tom, wb6b

Lee
 

If you want SWR only I just got a nice cross needle MFJ-822.   It covers 1.8-200 MHz, 30 or 300 Watts.  On the 30 watt range half scale is about 7 watts.
--
Lee - N9LO  "I Void Warranties"

 

Tom, wb6b
 

Jack,

Very interesting. Thanks.

Tom, wb6b

Tom, wb6b
 

So to elaborate on my previous really short reply. I really like the macro, it handles the hardcoding the number of elements issue well.

For the definition inside the loop. I may be trusting the optimizer too much, the definition outside the loop is technically more correct. Out of curiosity I'll see if I can get an assembly output to look at, as with these tiny processors that type of code checking is still of value.

In programming teams, as optimizers get better, the holy wars have started to move from coding style vs resulting assembly code to each individual programmer's perception of what must be easiest to read for them must therefore apply to everyone else. So, may have been beat-up in one of those wars. 
Readability and coding is a manner that is consistent with the style of the team are good things, though. 

Tom, wb6b