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KD8CEC ‘s SWR senser

Joel Trenalone
 

Greetings All,
I have been working on my UBiTx V5 for a few months now and have added KD8CEC update with nextion screen and the I2C update. I had all the features working (CW decode etc) except the SWR meter. 


I built a kitsandparts SWR bridge and hooked it up to the second Arduino, but the meter never operated correctly. I traced the problem to the bridge after hooking an oscilloscope up to the forward and reverse outputs and noticed that the reverse voltage would bounce up to 4 volts or so before settling down in a few seconds (this was while hooked to a 50 ohm dummy load. While the probe was hooked up, the meter on the Nextion display would go high and then settle down to a good swr reading. But when I removed the oscope probe, the meter would just stay pegged at very high SWR. 


I ran across a blog here (http://www.kk5jy.net/swr-meter-v1/)     about building a digital SWR meter and noted that he stated  that “In order to prevent stray RF from getting into the A/D circuits, I placed a 0.05uF ceramic disc capacitor between each line and the GND pin.  Since the A/D inputs are very high-impedance, and the SWR sensor outputs were meant to drive current devices (e.g., analog meters), I also placed a 75k resistor across each of the 0.05uF caps, to drain the accumulated charge when no RF was applied”. 


After I did this it the SWR meter works great. I was just wondering if anyone has hooked up KD8CEC’s meter and had similar issues. In an effort to learn more, I  also wondered if anyone can better explain why the resistor and capacitor fixed the situation. 

thanks,
Joel
KD6AGW

Joel Trenalone
 

For information, here is a sketch of what I did. The home brew Stockton bridge that Ian (CEC) used apparently did not have this problem. 

Mark Hatch
 

Joel

thanks for this!  Have you measured watts out with and without the bridge?  Read somewhere about a watt penalty of 1 watt when it was in circuit. 


mark

 

Thanks for the info Joel

Joel
N6ALT

Gordon Gibby
 

If you build a reflectometer or other system reasonably  well the losses should be extraordinarily low.  Otherwise they would burn up when used with higher power rigs!



On Dec 1, 2019, at 13:15, Joel Caulkins/N6ALT <caulktel@...> wrote:

Thanks for the info Joel

Joel
N6ALT

Joel Trenalone
 

OK, I  checked the output with the bridge in and out and there is no perceptible difference in power out (on the oscilliscope 10 volts per division, reading about 45 volts on each with a 50ohm dummy load):

Joel Trenalone
 

Also, here are the schematics for both the kitsandparts bridge (I’m using), and the home brew Stockton bridge (that Ian Lee uses in his video and description http://www.hamskey.com/2019/04/installing-swr-and-rf-power-sensor-in.html  )

I see some differences, namely the 220k resistor to ground from the reverse/forward power outputs on the Stockton bridge, but is not on the kitsandparts bridge. Note sure why the difference, and how it affects the operation to the Arduino. 

kitsandparts bridge:


Homebrew Stockton bridge:

Joel Trenalone
 

Just to clarify, the test above is a test of the transmitter output with the SWR bridge inline (in) and bypassed (out), in response to Mark Hatch’s question :)

Tom, wb6b
 

HI,

Here is an interesting site that shows building a digital SWR meter using a Stockton bridge connected to an Arduino. Includes some explanation on how the bridge, meter and software work.

https://sites.google.com/view/kn9b/digital-swr-meter

Also has links to other explanations of Stockton bridges and SWR. Such as this Youtube video on Stockton bridges, by w2aew.

https://www.youtube.com/watch?v=byF1FLdbUiA

Tom, wb6b

Jerry Gaffke
 

Kits and Parts assumes you are using an old school micro-ammeter.  
That would present more of a load than the tens of megaohms presented by the Nano's analog input pin.

The TandemMatch/StocktonBridge circuit has RF going into the diodes that is proportional to the forward and reverse power.
That power can vary from second to second, the 220K resistor discharges the 0.1uF cap when the power decreases.
The value of the resistor is not critical, but without it it might be several seconds before the cap discharges.

Jerry, KE7ER

 

On Sun, Dec 1, 2019 at 01:45 PM, Joel Trenalone wrote:
Also, here are the schematics for both the kitsandparts bridge (I’m using), and the home brew Stockton bridge (that Ian Lee uses in his video and description http://www.hamskey.com/2019/04/installing-swr-and-rf-power-sensor-in.html  )

I see some differences, namely the 220k resistor to ground from the reverse/forward power outputs on the Stockton bridge, but is not on the kitsandparts bridge. Note sure why the difference, and how it affects the operation to the Arduino. 

Joel Trenalone
 

Jerry and Tom,
Thanks for the links and information. Each time I deal with an issue like this I learn a bit more.

Here is a pic of my setup, solar charges battery operation. It’s really working great on with the SWR working  

Vic WA4THR
 

I read your post that you have the SWR function working on your uBitX. I have a V4 that I built a year ago and added the Nextion display, then added the second I2C Arduino and I am able to do all functions with that including CW decoding, audio spectrum, S-meter and so forth. All has worked well for many months. In order to eliminate taking a separate SWR bridge for use with my loop antenna when going portable, I decided to add the SWR bridge in KD8CEC’s website. I built up a Stockton bridge and I am measuring good results with itI, with FWD power in the 1.7v range and maybe around 0.5v in the REV pad in a typical check. SWR caculations by hand match actual SWR pretty well. I wired the FWD and REV pads  to pins A2 and A3 on the second, I2C Arduino and on transmit I see nothing…no deflection of the S-meter graph at all.

Did you have to do something different that just connecting the 2 wires? I didn’t run a ground wire as the SWR bridge is already connected to ground through the antenna connector, and the second Arduino is connected to its own ground as well. Is there a switch in software?  I am using version "UBITX_CEC_V1.100_NX_S" of the KD8CEC sketch and “dspmeterv1” dated 4/1/2019 on the second Arduino. Thanks for your help.

=Vic=

Vic WA4THR
 

Whoops! My error, now discovered. Nothing like making a post for help to dicover the problem on you own!

I assumed I had the latest DSPMeter sketch (v1) but it appears by looking at the upload dates that (v0.8) is a later version by about 11 days. I guess I didn’t understand the naming convention. I have now installed v.08 and I am getting good SWR indication.

=Vic=