How to tune a 433MHz uni pole antenna

[ozpos]

Hi All,

I have discovered that one can solder a phosphor bronze lower E acoustic guitar string and thus create an excellent 1/4 wave uni pole antenna.

I have cut it longer than 164.5mm and now that it is connected and installed in a waterproof plastic case could anyone advise me (or direct me to the right source) on how to go about trimming it in order to get the best performance.

Thank you in advance,
oz.

[Felix]

Guitar strings as antennas, haven't thought of that! 
Any reason why the lower E is best? I would guess thickness?

Anyway, if you already soldered it, without any special equipment, I would start LONG and start trimming it half a mm and observe RSSI on a receiver. The setup should be first optimized for best RSSI with the longer antenna, and then not move or change anything BUT the trimming. Ideally the physical orthogonality of the antenna should also be identical between trimmings, but given how guitar strings are not stiff, do your best on that I guess.

So this first one would be experimental, then once you observe RSSI peaking and then degrading you know where you should be.

[ozpos]

Hi, thank you for a quick response. 

You are right in that the lower E sixth string is the thickest but not all strings have a phosphor bronze outer winding.  Really cheap strings can be bought with this covering and as a result they are quite resistant to corrosion.

If anyone is interested here I solder a gold plated turned pin socket (as used with 0.1" DIL ICs.) to the pcb.  It must be perfectly aligned vertically otherwise these sockets tend to shear easily if stresses are exerted by insertion and removal of the enclosure lid. A better solution would be a spring loaded probe tip as used in ATE production jigs. I drill a hole through the lid for the guitar string.  On the end of the guitar string I solder a gold plated pin from a high quality pcb header (that would be used with ribbon cable) , 3M are good ones and the pins can be pulled out of their housing quite easily without damaging their smooth surface especially near the rounded end of the pin that will enter the socket. Then I insert the string with soldered gold pin into the socket and screw down the waterproof lid before applying fast setting epoxy resin to seal the hole and support the string. When firm I remove the lid and apply epoxy to the under side for more support.  The enclosures I use are cheap waterproof AA battery box sold by Adafruit. https://www.adafruit.com/?q=waterproof+battery+box&sort=BestMatch

I would be very grateful if you could please answer a few more questions regarding antenna tuning.

Does one start with both the tx and rx at the same longer length ?
How much longer should I start beyond the 1/4 wavelength ?
Does one trim the same amount off both tx and rx ?
Should I be outdoors ?
and finally what should the separation be ?

Many thanks in advance and best regards,
oz

[Felix]

Maybe the 1k words can be replaced with 1 image 
Outdoors is best and keep everything the same except the 1 variable that you change (the antenna you're trimming).
The RX can be a dipole or helical antenna, or just a 1/4 monopole, doesn't need to be perfect. Once you determine the trimmed antenna, you can perhaps repeat the experiment with the trimmed antenna as the RX and another one as second trimming to see if anything changes.
Common sense will get you pretty far and good results.
I would start with initial separation that yields RSSI at least 20dB above the best expected signal (about -20dBm), to ensure that trimming will give you enough room to play with. This will depend if you're indoors or outdoors. No worries, it will not have to be 1km 

[ozpos]

Thank you for the info.  Sorry about the long post I do not own a digital camera or mobile phone that is why I took to words.  It takes much longer to write than read and I did my best to try and keep it as concise as possible.

Best regards,
oz