if I had 2 generators being buffed by amplifiers, what would be the best way to combine the signal into an antenna?
signal gen -> PA -> isolator(s)
signal gen -> PA -> isolator(s)
Would combining them in a regular T-junction work (N-type) or do I need a special microwave circuit element here?
signal level ~ 10 W from each amp.
What if I also wanted to connect a spectrum analyzer to tihs circuit, can I just put an attenuator and another T junction? Is there a 'proper' method? (all to 1 antenna to measure reflections/NLJD).
Circulators are NOT an option because I see that the minimum order quantity for multi octave circulators is 5, at a price of 400-800$ each, I won't be catching the 3rd harmonic with the 'cheap' mass produced options.
Surprisingly high frequency (mid C band +) multi octave power circulators are available for ~+600$ in single quantity... but some cursory experiments showed that measurements above 4 GHz in a NLJD seemed to be unstable (then again, I just had a PA (with 80db isolation) and SA (with 20 attenuator to protect maximum power input of SA) connected to a T junction with a SA. This requires a very broad band ridged horn
I think for this experiment to work with higher frequencies maybe its enough to use zip tie holders on the microwave cable (i am using high grade microwave cable... good to 26ghz)... but I found during my experiment that just standing next to the spectrum analyzer (a few feet away from the horn being aimed across the room) it was enough to just turn my body to significantly effect the harmonic readings being taken on a test subject (in this case a organic material with a blue-tooth radio tire pressure gauge inside of it)... any kind of motion on the cable had a great effect too. seem to be able to detect it somewhat even with 2 inches of wet organic material (cantaloupe) infront of it.... but only small difference. I assume the fruit absorbs ALOT of energy. Consistent readings with 2, 3 and 4 GHZ stimulus signals (but the 12Ghz of the 4ghz stimulus is difficult to measure.. very wonky).
it seems that commercial NLJD have a full scale of around 3db.
also, a trick, if you want to do it yourself, to equalize the 2nd and 3rd harmonic energy in the 'null' reading is to use a band pass filter and play with the frequency until the readings are equal. they will equalize in amplitude because there will be modeing (seen as a 'comb like' transfer function on the 2nd and 3rd harmonic of the pass band. if you get lucky with the filter you can operate on the edge of its pass band and have the modeing actually do you a favor.... (i found this on a 2 ghz microwave waveguide filter). I had two of the, one was disassembled (due to being 1900 mhz and the other one 2044), and it had little disks/rods in it (kind of like the tower in the first men in black movie where the alien bug hides his space ship in new york city).
I don't see my horn going above 18GHz, meaning 6GHz stimulus signal maximum. And I have no idea if this is even true, I need to do an experiment with 2 horns ( i have two identical ones) to determine 3db points. Perhaps I will try with zip tie holders first, then hardline.
I also have a very high frequency horns (18 ghz and 24 Ghz) set (lens horn, round conical shape (not rectangular prism pyramid)... I am interested in finding mixing products of semiconductor devices, as mentioned by another EEVBLOG user in my thread about NLJD. However, these horns only have a bandwidth of like 6GHz, as they are meant for communications purposes, but do have a high gain, I would need a smaller ridged horn to deal with higher frequencies in broad band...
I kind of see using two different microwave frequencies in a NLJD similar to doing geneology on a circuit (like mendel squares)... perhaps it will have a higher specificity.