RF (Radio Frequency)

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Phase 1 of testing antenna radiation patterns.  The intro to this journey is Antenna Radiation .

I plan to share the good, the bad and the ugly in the evolution of execution of this project.  Sometimes the journey is more important than the destination.

 

The Phase 1 plan is to verify my ability to measure RF signals using a dipole antenna.  I have built a few iterations of RF Field Strength detectors.  See Field Strength Meter for an example of how they are put together.

 

With the field strength meters working, it was time to get a recurring transmitter set up so the meters had something to receive.  This would also allow me to sort of calibrate them.  This was supposed to be the easy part.  Silly me.

Original & Improved DMM Field Strength MetersArduino Field Strength MeterMore Portable Field Strength MeterPortable Field Strength Meter Top View

 

I opted for an Arduino controlled automated transmitter setup that amateur radio folks call a "fox".  It is used in hidden transmitter hunts using different radio direction finding techniques.  Mine would do a tune and give my call sign in Morse code.  Only...

1.  I decided to use some of my Arduino Mini's.  Not the Mini Pro.  I had to relearn that the Mini gets programmed as a Nano - using the Old Bootloader.  Slight delay.

2.  My radio cable didn't seem to work.  Get a new one.  The new one doesn't work.  Try a different radio.  That one sometimes works.  The "key the mic" relay works but the radio circuit doesn't trigger reliably.  It takes a lot of fiddling with.

3.  Let the magic smoke out of a Mini by holding down the reset button too long.  Get another Mini and start over.

*  The root of my issue seems to be in my cheap Baofeng radios.  The Arduino part is working fine.

4.  Take a radio apart trying to see if I can direct wire to it.  Break the radio.  Order repair part.  Buy different cheap radio.

 

Update:  Receive new cheap radio.  It doesn't respond to the relay that is supposed to key the transmitter either.  Setting this part of the project to the side for now.  Go to Plan B:  Key the mic by hand.

 

Update 30 DEC 2019:  I have healed the fox.  The schematic from the Internet was wrong.  Imagine that.  I used the information from Whitecourt Amateur Radio (Fox Hunt Transmitter – Whitecourt Amateur Radio) but have provided the corrected schematic - and a picture of the Arduino setup that will soon reside in one of the plastic ammo boxes I have.  It's a shame that I killed a $40 radio in the process.  Still... I got to take it apart and see what's inside.  I guess I'll never grow up that way.  Arduino Mini RDF Fox

Arduino Mini Hidden Transmitter Assembly SchematicArduino Mini Fox & UV-5R Radio

 

The next step was to attempt to somewhat calibrate the DMM meters to each other and the Arduino meters to each other.  After much trying, I abandoned this idea.  To get as consistent measurements as possible, I'll stick to one meter.

During this process, I had an antenna concept reinforced.  It's called "Take Off Angle".  The field strength can vary quite a bit (in the near field) over a few inches of elevation - meaning at which height I held the meter.

So... the field strength meter in the plastic ammo box is destined to be re-purposed.

 

On to the testing!

2m Dipole

This is the 2 meter dipole antenna I used.  It is vertically polarized and the feed line is run horizontally out from the feed point to decrease the SWR (Standing Wave Ratio).  If the feed line is allowed to just hang, it increases the SWR or reflected power in the antenna.  The white post in front of the base/bucket is to attach the string that insures I stay at the correct distance when making measurements.  I have included the video.  Thankfully, it got up to 0C that day.

I tested the field strength a half a dozen times before the weather drove me back indoors.  By reviewing the aggregate readings, I determined that the radiation pattern for this dipole was the one below.

2m Dipole Plot

I used a radar plot because the distance scaling was easier to use.  The view is top down.  The antenna feed point is the center of the circle and the feed line extends from the center toward the 180 degree mark.

This plot revealed some interesting characteristics of my dipole antenna.

1.  It is not omnidirectional.  The radiation pattern favors an opposite feed line direction.  A vertical dipole is supposed to have a uniformly circular radiation pattern.

2.  The feed line side of the horizontal support has a slightly weaker signal than the non-feed line side.

Both of these phenomenon point to the feed line influencing the radiation pattern.  Even with a common mode choke on the feed line, it still has influence.

My opinion is that the feed line is also broadcasting RF, resulting in a more directional pattern than expected.

 

Even with this less than perfect radiation pattern, I have still verified my ability to measure relative RF field strength.  Phase 2 will be to verify the radiation pattern of a three element Yagi antenna and to attempt to measure the nulls.

Phase 2:Antenna Radiation - Phase 2