Estimating an Antenna's Impedance at a Distance: Impedance Transformation in a Feedline (039)

[@dennisqwertyuiop]

great information

[@eie_for_you]

Glad it was helpful!

[@izzzzzz6]

This channel has to be one of the best resources for HAM's where is everyone? Is it possible to have a video summery on this series but tackling the same scenarios with open wire feedline? For example. How would i calibrate the vna at the end of a length of 170ft of ladderline? Would i need a 1:1 balun at the shack end to convert the VNA to balanced and then another 1:1 Unbal at the feedpoint to convert it back to unbalanced for the calibration loads / stubs? Many thanks.

[@eie_for_you]

Hmmmmm ... having never really worked with ladder line ...I'm going to have to do a little research on this.

[@Welshwizzard1]

I made up that random impedance in a small box as well , I used sma connectors like on the tinyVna, so we have two different values. This is good for exercises , matching into an impedance at a specific frequency.

[@eie_for_you]

Cool! I used a N connector because I most often us my Tektronix VNA which sports N connectors. It is nice to have a "known bad load" for various testing scenarios. You should see what you get measuring your "known bad load" with various off the shelf antenna analyzers. An interesting exercise. 🙂

[@Welshwizzard1]

yes the version i built and used after seeing your idea, had three pairs of SMA connectors, this allows me to have three frequencies for the bad load ,I then sweep the load so I use my tinyVNA to sweep the Unknown load from say 7mhz to 30 Mhz then select three different frequencies, noting the Z ohms and Xs at each frequency i can then use SimNec to calculate the required inductor and capacitor values for each freq. then insert these into the path to get the match into our unknown load. i built another small box to go between the VNA and the unknown load, here we can put in the C and L values. This unit is then good for testing analysers and the difference they give on measurements. I find the MFJ analysers the worst. Also using this with Simsmith or Sim NEC to find the L and c network values for a given frequency. So we could put a marker on any part of the simsmith chart and make a load value to simulate the antenna. I was going to get the Rigexpert Zoom , but the Tiny VNA does it , it just that its not so portable in the field , as laptop and cable required to measure stuff. I used SMA connectors as i have loads of this type , i can send you some if you want to play for with these . for free just email me from QRZ page ( GW4LWD ) I just put up these images .into my drop box , follow the links to load them www.dropbox.com/scl/fi/qcof5qvo6exwju10l5mme/Will-image.jpg?rlkey=mi7apw1c14tkk2ow7d59q6qja&dl=0 www.dropbox.com/scl/fi/gcnawk4cdizzw5ydnr5me/28mhz.jpg?rlkey=ccg45z82kn02k0l3ig9vwmii1&dl=0 www.dropbox.com/scl/fi/q2io8vwxm6evi9t3hdg01/series-L-and-C-unit.jpg?rlkey=b94c3z0hn56ehote9pemccjbz&dl=0 www.dropbox.com/scl/fi/q262letegqx7awv56kubh/tiny-and-unknown.jpg?rlkey=gp84zzhwo755qcfnwe9bwrwv2&dl=0 www.dropbox.com/scl/fi/1icton2m1boahn2uuw3et/dual-unknown-load.jpg?rlkey=5438ftnb5xmig3bn3ha5eqhwv&dl=0 www.dropbox.com/scl/fi/9z5bxoiyntk2f2fmd4mps/15Mhz.jpg?rlkey=9tzt37pmslosdurbphsgwkesr&dl=0 www.dropbox.com/scl/fi/p6wfcs31ifqjwqvd41j87/unknown-load.jpg?rlkey=9kvngdq6wurwrismr2k6e2w4e&dl=0

[@Welshwizzard1]

Here are some images drive.google.com/file/d/15n9QB4EHS8dYb_CJi3Si01xRYpxcLrF9/view?usp=drivesdk Also drive.google.com/file/d/1VLunNUTmuW_aahJ6AhAymU9xJ1ybypVd/view?usp=drivesdk The 10Mhz Smith chart drive.google.com/file/d/1SiNOpGmpA0n6E38KVT7D9hNhT5sUfarE/view?usp=drivesdk

[@eie_for_you]

YT had relegated this to "Held for Review." I don't often check there (cuz it's *always* empty). Sounds like a neat idea.

[@shortwavelistener]

Thank you very much! This really helped me and was so useful. I did your example in my case with 50ft of RG-8X with a vf of 0.82 with an unknown external antenna with unknown feed point impedance. Is the assumption that the calculated SWR based on the radius of the circle in the smith chart (and VNA) is the actual SWR at the feed point as well? I'm fairly new to this - if the coax isn't acting as part of the antenna, is the SWR constant independent of coax length? Nevermind! I see someone else asked the same question and you answered it!!! Perfect! PS - my first calculation mistake was not looking at the VNA units correctly - ugh! The reactances were in uH and uF vs ohms. Once I figured that out, all calculations worked! Plus, doing the smith chart on paper with a compass is quite enjoyable.

[@eie_for_you]

So cool! 🙂

[@marianduval5270]

Excellent video. In my case, I don't have a VNA and I don't know the impedance of my load (the antenna). How do you suggest to calculate it if I don't have tools to measure it? As per what I can see on the video, it is necessary to measure it with a VNA in order to normalize it and plot the SWR circle.

[@marianduval5270]

Also, where did you find the values of resistance and reactance for the software?

[@eie_for_you]

See my reply to your reply ...

[@eie_for_you]

@@marianduval5270 It used to be that VNAs cost tens of thousands of dollars and required a two-man lift. Not anymore. The nanoVNAs are very, very inexpensive these days. If you go with one of the cheap, clone knock offs they can be had for under $100 (nanoVNA). It won't be as good as the real deal, but it will be sufficient for anything that the average person might need to do. If you decide to get one, I have a whole Playlist on how they work and how to use them. I would ALMOST say a good antenna analyzer will give you the values you need, but my experience with these has been less than stellar (especially the MFJ model 259) ... except with the higher end RigExpert (see my video on improved impedance measurements with the RigExpert - I borrowed one to make that video) The values of resistance and reactance come from the measurement made with the VNA. I hope this helps! 🙂

[@yo4aul]

Do you have any tutorials about measuring the impedance of an efhw antenna with the help of nanovna? Or in general the impedance at the end of any wire?

[@eie_for_you]

The tuning of this antenna has to be done at the coaxial connector on the 9:1 BALUN which is part of the whole assembly. The nanoVNA (or any VNA or antenna analyzer) is looking to see a transmission line which, by their own definition, is two conductors. There must be a definitive return path. Regarding the tuning of any single wire, you are not the first one to ask about that ... its velocity factor and, therefore, its electrical length is highly dependent on its physical surroundings. The simple use of insulated wire as opposed to bare wire for an antenna shortens the antenna because the presence of the insulation changes the velocity factor of the wire. You put it in the air as opposed to being near the ground or the building...different. This is why we cut purposely long, trim to tune in the orientation and environment that it will live in. 🙂

[@PBenetton]

This is an amazing channel! I am not shure if you ever addressed what I'm going to ask, but I coudn't find it. My question is preety commom, and I'm not shure if its a myth or not. Does the lenght of coax between the rig and the antenna matters in terms of simple SWR to the radio? As one can use coax as an impedance transformer I'd think that yes, but is it just a constant SWR value along the coax even though the complex impedance is changing? In the end, does it matter or we should just use the minimum length to cut the losses? I'll be sure to write this down as too many experts seem to be divided on this one. Thanks in advance.

[@eie_for_you]

First of all, a big thank you for the encouragement! You have a very good question! The answer is a decided yes and no. I know, this doesn't help much. But, let me explain. First the "no" and then the "yes" and then the "maybe." No, it makes no difference. If we consider the coax to be lossless and look at the Smith chart, yes, the impedance continuously and cyclically morphs, repeating every electrical 1/2 wavelength. Throughout this 1/2 wavelength, the reflection coefficient and, thus, the SWR, remains constant. The components of the impedance change in such a way that the calculated SWR from the existing complex impedance at any given point is the same as it is at an other given point. Yes, it does make a difference as viewed from the transmitter end of the coax. Coax is NOT lossless. If we consider a piece of coax which has a 1dB loss over its length at a given frequency, this means that a 100 Watt transmitter's power at the other end of the coax is about 79 Watts. If the antenna has an SWR of 1.6:1, it will be reflecting back around 4 watts of the 79 watts that it is getting from the coax. This reflected power travels back through the coax experiencing the same 1 dB of attenuation. Back at the transmitter end we get 3.2 watts coming back at us from the antenna; the remaining 0.8 watts is dissipated in heat in the coax. Comparing this to the 100 watts sent to the antenna, this *looks* to the transmitter like a 1.41:1 SWR. The signal loss in both directions fools us into thinking our SWR is better at the antenna than it really is. One other factor ... the antenna itself. Some antennas are **really** weird about coax length and grounding and such (I know, I have one of those!). In that case, yes, the length, type, grounding, and just about anything else you might think of make some weird difference not covered by conventional theory. So, like I said, yes...and no. Now that I have thoroughly muddied the waters, I hope that I've shed some light on the question at hand. 🙂

[@PBenetton]

​@@eie_for_you Ralph, first of all, let me thank you for the most complete and record-fast reply I've ever seen. Much appreciated. I truly love that most answers in the Ham Radio hobby are definitive "it depends"; it makes everything much more fun and approachable for the inquisitive experimenter. The Nano VNAs and Tiny SAs are amazing and only add to the fun and democratization of the craft. You, sir, have indeed shed a lot of light on the question, and the "yes and no" answer is clearly the right answer. Many die for either the "yes" or the "no"; a true master knows it is neither. I'm a Brazilian Ham and Production Engineer and a true fan of yours, really honored by your reply. Thank you, 73 DE PP2PB (by the way, are you a Ham, sir?)

[@eie_for_you]

@@PBenettonWell, thank you very much! When we are dealing with the black magic world of R.F., it is often a lot of "it depends." The best we can do is know as much as can know and then learn more. None of us have the whole story perfectly! Yes, I am a ham and have been since 1970 (WN2PUX, then). I am WA2PUX continuously licensed since 1970. 73! 🙂

[@tomstrum6259]

Trying to figure out how any LC "L" or "T" network antenna Tuner could protect a Final output device from Antenna Swr reflected power effects using what Intuitively Should be a simple Bi-Directional impedance matching network that should Pass rf power (Forward or Reflected) equally Both directions easily passing any antenna Reflected power thru towards Rf final device.....Normally, only a transmission line "Isolator or Circulator" can effect 1 way Directional rf power continuity flow...What part of this video or transmission Line "Impedance transformation" enables a simple "LC" matching network to protect a rf final from antenna Reflected power ??

[@eie_for_you]

Man! I am just missing things here! I didn't ignore you, it is just that I have had a very FULL last few months and some stuff just fell through the cracks. :-( R.F. is a mystery in so many ways. :-)

[@Welshwizzard1]

Hi there , do you show how you made that test impedance load ?.

[@eie_for_you]

Actually ... no I never did. It is quite simple, though. I put a 33 Ohm carbon composition resistor in series with a coil of wire that I just threw together from some scrap. This across the connector. Then, in parallel with that combination, I threw a 27 pF ceramic capacitor. No calculations or goal apart from wanting something that was purposely non-ideal. I wish I could put a picture in here. Here is a link to a picture of the innards. drive.google.com/file/d/10_aOotowIvmFgyKeHUh8Im4PcXFO55_C/view?usp=sharing

[@Welshwizzard1]

@@eie_for_you I’ve subscribed , interesting topics .

[@eie_for_you]

@@Welshwizzard1 Welcome to the "family!" 🙂

[@Welshwizzard1]

@@eie_for_you Hi there , after thinking about your unknown load, I made up another version , but this one is completely variable. It had a variable inductor, variable capacitor, and variable resistor, I can now create an unknown load anywhere on the smith chart , then use the VNA to read it and calculate the L/C to match to it.

[@eie_for_you]

@@Welshwizzard1 Wow! Getting better and better! 🙂

[@M0XYM]

Excellent! Just found (and subscribed) your channel. Thank you. Kevin M0XYM

[@eie_for_you]

Welcome aboard! Glad you liked the video!

[@sleeve8651]

Ever notice in most Ham circles, you seldom ever hear a discussion on the Smith Chart ! And if you do, it's typically from someone trying to impress others with how intelligent they think they are ? Obviously, present company excepted ! 😂🤣😂

[@eie_for_you]

And yet ... it is such a totally handy tool! 🙂

[@sleeve8651]

@@eie_for_you Lol.....! Indeed ! I think the ARRL made the mistake of placing it too far in the back of the Hand Book ! And actually, I think it could have been represented much better, as to how critical a tool it could be ! But like most everything else, wait long enough, and Technology will give you an escape ! Pointing to the now popular VNA's, as an example ! Addendum : I forgot to say that the Human Animal is naturally lazy !

[@eie_for_you]

@@sleeve8651 I have a whole play list on it (9 videos). 🙂