A bare random wire end fed antenna vna measurement - Page 1
Forum section: electronics / rf-microwave

author: iMo
author_rank: Super Contributor
timestamp: July 19, 2020, 08:00:27 am
content: I want to measure impedance of an 8m long end fed wire in 2-30MHz range with the nanovna, CH0/S11.There is none balun/unun, nor antenna tuner or any feeder connected.I've seen somewhere in similar case the vna should be terminated with 50+j0 load, and the wire should be connected to that termination. It would also require a de-embedding of the 50ohm load afterwards, imo.Is that something which may work?Is there a better way to proceed?PS: another option could be to use an unun, say 1:4, and a) to measure the unun itself with a 200+j0 load, b) remove the load and attach the antenna, c) de-embed the unun's data such I get the bare wire's data. Any experience with that?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 19, 2020, 02:54:29 pm
content: If the VNA is terminated in 50ohm, you can't hook something else up to the VNA. That is what a termination means.I don't get what you are trying to do here - you should just calibrate the VNA, then hook up the antenna, and you know what the antenna s-parameters are.

author: cdev
author_rank: Super Contributor
timestamp: July 19, 2020, 03:06:39 pm
content: You should also pick and use a ground for your measurement. Whatever the antenna is going to use. Aununwill have a ground connection.Quote from: imo on July 19, 2020, 08:00:27 am

author: radiolistener
author_rank: Super Contributor
timestamp: July 19, 2020, 03:07:58 pm
content: with no balun and RF choke it is impossible to measure just your wire. You will measure your wire include coax cable, VNA, your body, and maybe other equipment if you power VNA from external power.In order to measure wire, there is needs at least good RF choke near the wire feed point.You can make simple RF choke with coax cable coil.For example:More inductance is better RF choke. For low frequency there is need more inductance.For low frequency band, like 160 or 80 meters, it's better to use ferrite for RF choke, because air core coil is too large.It's better to place RF choke on both side of coax cable - near the feed point and near the receiver (or enter to the building).RF choke prevent use of the coax cable braid outer side as a part of antenna. So, you will have less noise from home equipment and less influence on home electronics at TX time.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 19, 2020, 03:24:37 pm
content: Pretty short antenna.I have no idea of why one would connect a 50 ohm load then, on top of it add the antenna, (if that is what you are saying).I think maybe they are talking about a 50 ohm "termination" which converts high impedance to 50 ohm. These are used on stuff like scopes with a high impedance input(and no 50 ohm input). The "termination" makes the "load" (oscilloscope) appear as a 50 ohm load. These 50 ohm terminations have significant loss and their own problems with reflection. In this case the termination would make the "high Impedance" antenna appear as a 50 ohm load to the VNA. Theoretically reducing the reflection from and to the antenna. What good would that do? I dunno.Here is what I did:Impedance of your antenna will change depending on how and where it is hung, probably the best way to check it is to hang it and feed it with a proper 50 ohm line. Then you can come up with some impedance for the antenna and perhaps make an appropriate balun to feed it (balun put at the end of coax at antenna feed point). SWR will vary wildly across the 2-30 MHz spectrum however.You might get a few Ham bands in some livable SWR, Is this what you are attempting to do??If you measure the antenna with a VNA directly (at the feedpoint), you cannot be near the antenna because this will change your results, perhaps significantly. Bluetooth would be cool here.Wally

author: A.Z.
author_rank: Super Contributor
timestamp: July 19, 2020, 06:25:37 pm
content: Given that the wire Is short (and assuming you have a good ground or counterpoise) my wild guess is that you'll be facing a pretty High impedance; at least on HF bands and down

author: iMo
author_rank: Super Contributor
timestamp: July 20, 2020, 07:48:44 am
content: My very intention of this post is to get the data for the "wire only".Imagine yourself a wire "W", with fixed mounting somewhere, and a fixed banana socket marked ANT - "random end fed antenna" where the end of the wire is connected, and a fixed banana socket marked GND.The question here is "how to measure the impedance (R+jX) from A to B MHz at the ANT banana socket with the nanovna" such the resulting R+jX data for the naked antenna wire "W" are as close as possible to the reality..The use case is therefore:1. I carry the nanovna (or any other VNA) in my left hand, calibrated with reference plane at the S11 connector,2. there are the ANT and GND banana sockets in front of me3.. ??4. I want to get the R+jX data of the wire "W" from A to B MHz on the VNA screen, or, I want to save the data on the vna's sdcard and do a data postprocessing if required (in nanovna-saver or SimSmith or Matlab).Q: what needs to be done (procedure, additional hw) ?PS: this is a measurement with direct connection between the antenna socket (ANT, GND) and the nanovna.