Practical antenna tuning using pi network question - Page 1
Forum section: electronics / projects

author: nickajeglin
author_rank: Newbie
timestamp: May 21, 2020, 12:40:06 am
content: Hello,I just got ahold of a nanoVNA, so I'm rebuilding all of the old UHF antennas that I made in the past. I had been making like 5 of each, then A/B testing, but now I can actually dial things in right.I've been learning about the smith chart from thisthisexcellent webpage, andthisawesome whitepaper. After working through both of those I have a solid handle on how it works, including the section on using inductors/capacitors to move the complex impedance along lines of constant reactance/suseptance. I'm a ME with a strong interest in engineering acoustics, and it's amazing how similar the fields are. I guess waves be waves.I'd like to take my old antennas and play around with inductor/capacitor tuning to learn more about how it's implemented in a practical way.Here's my question:How do I actually set up a test bed for hooking up components?I'm picturing a small metal enclosure with coax panel connectors on opposite sides. Then I'd run the center conductors to a little breadboard pasted to the inside of the box. I'd also attach a couple rails of the breadboard to the metal box with screws so that I have a connection to the shield. Once I found a tuning network that did what I wanted, I could fab up a tiny PCB with smt components, and do a similar enclosure but use a small metal tube with endcaps as the housing.Will this work? Is there a better way?P.S. WTF is up with hams and non-descriptive jargon? I spent half an hour trying to figure out what they were talking about with 1:1 baluns/ununs before I realized it was just induction to filter common mode noise on the coax. Also seems to be a lot of the cargo cult mentality of just using the same designs over and over without ever checking assumptions. I'm pretty sure that the air core inductor everyone insists on blobbing on the bottom of their antennas can just be replaced with a clip on ferrite at UHF. But even if you look up plans for antennas in the 1GHz range, everyone is arguing over the number of turns to make around a PVC pipe like it's some kind of voodoo. It's hard to google up good resources.

author: cdev
author_rank: Super Contributor
timestamp: May 29, 2020, 08:43:47 pm
content: There is a discussion on the nanovna going on now and joeeqsmith has uploaded pictures of his own test jigs and there are a fair number of examples showing how its used - there are actuually several threads going on now, I will come back and add the links in an edit.You would likely have the best luck making up a test jig PCB and having it made for you ready to be chopped up, and ordering it from China or some inexpensive local PCB house.-----I dont have a nanovna and am hoping to get one fairly soon. I'll them probably find out how badly Ive been screwing up .. I have never had good luck using breadboards for RF because if you look at them inside they are a nightmare as far as impedance and parasitic capacitances especially.For years Ive been making various test boards for filters, which have generally worked but one of the main things I am looking forward to is getting a better intuition on controlling impedance in construction. My construction techniques have evolved but I have no doubt could use a lot of improvement.I usually use a little piece of PCB but sometimes I will use cardboard or foam core and copper tape. Which works great. Foam core and cardboard are both basically the same as air, so their dielectric constant is 1.Quote from: nickajeglin on May 21, 2020, 12:40:06 amFor HF, I've used DIP sockets and plugs - to make a way to have changeable bandpass filters. That works well. DIP sockets and plugs have a modest capacitance and inductance, for HF low enough to work well. It might even work for VHF, depending on how you lay them out.For HF that works fairly well. I have not used that for anything higher than 30 MHz yet although it might work.If the filter is just for receiving you can actually use copper tape and cardboard with copper tape ground plane to mock up circuits well up into UHF. Use a high quality flat, shiny piece of cardboard and the right width of tape for the impedance you want. This actually works better than many other kinds of prototyping for RF. You can use SMT components and Kapton tape to make it easier to solder them successfully. Just make sure they are protected from bending. You can also use PCB and standard SMA connectors and mini coax for transmission lines, soldering the SMA connectors on their sides to the PCBCut up a bunch of small squares, put SMAs on both ends and use the space on the top of the PCB for your circuits, for a long time Ive wanted to get a pad cutter to make small circles in the PCB which can be used as binding posts. Until I do that I just use little tiny squares of PCB super glued to the board (but be careful to solder as quick as you can using small size solder because super glue has nasty fumes when heated, you should ventilate it outside.You can also buy solder posts. But that kind of construction is fine for HF but not much higher. ) You will be surprised how well cardboard and copper tape works if you try it.In a box you can use a particular kind of SMA connectors that have both edge mount and a long threading and nuts. They will allow you to get good shielding. Using a standard metal box. Make sure to think it out well in advance and put it in sideways so you can remove the PCB when you need to. You can either use a PCB in the regular way or put the PCB on the top leaving you some space for the filter on top. These are the habots Ive developed from not being able to have lots of vias made by a PCB house, I instinctively try to avoid what you describe with the rails on the sides because vias made my way - by drilling holes, putting copper wires through and soldering both sides, have historically given me problems.