Building a Low Power RF Attenuator and Some Nano VNA Fun

This Project's main goal is to build a RF ATTN (attenuator) to lower the RF Output level of my Baofeng UVR-5 Handy Talky that will be used as a Hidden Transmitter Hunt Fox Transmitter.

The secondary goal is to get to know and use my newly acquired Nano Vector Network Analyzer to test the attenuator.

The Baofeng TX outputs 1.2 watts on Low power.  Ideally, I want to transmit approx. 200mW to the Fox Antenna.  General License Test math says:  If I place a 6dB (Divide by 4) ATTN between the TX output and the antenna feedline- the resultant RF Level should be approx. 300mW.  Since I used to work for the Government... that's close enough.

Regarding how to build the ATTN, this particular Youtube video inspired me the most:
#91: Basic RF Attenuators - Design, Construction, Testing - PI and T style - A Tutorial - YouTube

But instead of doing lots of math in my Spiral Notebook... An online App was discovered to provide the schematic and calculate the resistor values. 
Check it out: Pi Attenuator Calculator - everything RF

Values of R1 and R2 are rounded off.

In my collection of 1-watt resistors- I didn't have the exact values.  So put resistors in parallel until the approx. desired value was obtained.

Then blob soldered two SMA-F chassis connectors to a piece of blank Printed Circuit Board.

Lastly, blob soldered the resistor combinations to the PCB, accidently in a neat fashion.

How did I do?  Is the Attenuation close enough?  Is the SWR on each side of the ATTN = 50 ohms?             Enter the Mini Vector Network Analyzer...

Important Note:  This my second time using the VNA to make a semi precision measurement.  I had to start over many times until I was satisfied that I was doing the testing correctly.

The VNA was set up and calibrated to measure a 2-port device via Port 1 to Port 2.  And SWR measured on Port 1 only. 

The SWR and Log Magnitude (signal level) displays selected. 

VNA frequency span is set:  10MHz to 200MHz- as seen at the bottom of the screen.  The Green trace is the amount of attenuation measured across the frequency spread.  There is a marker (little box with 1 inside of it) at 141.1 MHz on the Green trace that indicates 5.71 dB loss, Port 1 to Port 2.

Also, another marker that indicates the same frequency on the Blue SWR trace at the bottom of the display- which reveals it is 1 to 1 across the frequency span.   When removing the test cable from Port 2 of the VNA, which unterminated the ATTN of its 50-ohm load... the resultant SWR at Port 1 was 1.5 to 1. 

Another concern is:  Will the 1.2W of RF Power melt down the ATTN's resistors?  After testing the Baofeng HT for 5 hours with the ATTN in series with the antenna- the resistors get noticeably hot to the touch.

As mentioned above, the Baofeng's Power Out measures as 1.2W.  At the output of the ATTN, the Power Out measures approx. 320mW.   Almost as calculated.  Much too close for Government work.

If the Nano VNA sparks any interest for you, gentle readers... I received my inspiration and information from this Youtube Ham Radio enthusiast:  smoking ape nanovna - YouTube - a Smart easy to listen to presenter.    There are many Mini VNAs to choose from.  I chose this particular model because the Port 1 to Port 2 dynamic range was listed as 70dB versus 50 or 60 dB in others. 

End of Blog.  As usual- THX for reading this far.  30, 73 - Ricky KR7W.