Hi Joe,
Thanks for the comments and the thorough testing you did, and what a great starting point to further test and improve these designs! It's amazing to see how well these antennas are already performing, given the vast number of variables and designing these things at such little cost.
Both antennas are a tiny bit too small, the resonance of the 433 Mhz one is at ~443 Mhz and the 868 Mhz one around 875 Mhz.
I can think of a few things that could add to a probable cause, but without further testing, have nothing substantial yet to back this up.
Some pointers (in no particular order):
* fault in the gerber files
* different production materials at the PCB manufacturer
* different measurement equipment
* different measurement techniques/environment
* something else..?
I have also just received my batch of antennas this week (only the 433MHz ones) and should be ideal for re-confirming (or not) what you see. Thus far I have only held them up next to my first prototype antenna and they seem to be within 1mm of accuracy of each other. A rough calculation shows me that a frequency of 10Mhz higher (to 443MHz) should be the result of a difference of about 2.3mm shorter on each leg! That definitely isn't the case with the new antennas I just received so something else must be going on.
on spot at 433 and 868 if I hold the dipole by its feedline in my hand during measurement
Interesting.
You probably already know that this is not how I tested these antennas with the VNA (my testing procedure is described a few posts earlier) but it does show that environmental changes can easily affect the performance.
I'm open to any other interpretations others may offer as it seems there are some other variables that have come into play.
Then, things impedance..
why it is so much off the ~75 Ohm a dipole should have.
Some pointers:
* The effects of ground are quite substantial on a dipole.
* Add (almost lack of) thickness of the antenna elements to the equation as well.
* Then there's the feedline that can make all the difference. (is it really 50 ohms characteristic impedance?)
Obvious question, but I have to ask since it's not specifically mentioned: you did tune the feedline in each scenario for electrical half wave(s)?
Mind you, my setup could have been just as prone to all sorts of misinformation and variances in equipment etc.. so I'm definitely not saying who's right and who's wrong. However, with all the information presented I think we're getting very close to coming up with a low-cost, simple and effective design.
In short, I think the resonance is the thing to chase, I wouldn't worry too much about impedance.
I will hook up my new 433MHz antennas to the VNA and post here what it reveals. It could very well be that changes in the design are required. In the next version I also want to: 1) include the modification to change the right angles into smooth bends to avoid reflections and 2) re-do the tiny pcb transmission line (from SMA connector to antenna elements) so it's closer to a 50 ohms characteristic impedance.