Pigtails, Coax and Velocity Factors (044)

[@elmoreglidingclub3030]

Thanks for a great video. I’m a new ham and am finding that antennas are profoundly interesting. After erecting a 40m insulated copper wire inverted V dipole, and trimming for minimum SWR at 7.034, I began searching to learn why the legs were shorter (at 29’6” each) than the calculations suggested. That has taken me down the very happy rabbit hole of velocity factor! So thanks for this.

[@eie_for_you]

Well ... welcome to the hobby! There is sooooo much to explore. There is a LOT of magic in antennas. Yup, an insulated wire antenna is going to be shorter than a bare wire antenna. How much shorter depends on the type and thickness of the insulation. Like I said ... magic! 🙂

[@brendandenney]

Great video

[@eie_for_you]

Thanks! 🙂

[@milesprower6641]

How would I measure antennas and devices that use 75 ohm coax with the nanoVNA? I always get tons of ripple measuring swr

[@eie_for_you]

Really good question ... and it is easier than you think! First, please note that your connectors have a characteristic impedance. If you are using 75 Ohm coax, use 75 Ohm connectors. 50 and 75 Ohm connectors are mechanically different than each other. You can buy 75 Ohm UHF (AKA: PL-259/SO-239), BNC and N connectors. F-connectors are 75 Ohms. The easiest way to do this is to convert the 50 Ohm connectors of the VNA to 75 Ohm connectors using a mechanical adapter. With that done, you need a 75 Ohm calibration kit for the VNA. This means the short, open, thru and load items have to be all with a characteristic impedance of 75 Ohms. You say, "A 75 Ohm short???" Well, the short uses a 75 Ohm connector, but it is still a short. 🙂 Now, you simply use the 75 Ohm calibration kit to calibrate the VNA on the 75 Ohm side of the adapter the same way you would do with your 50 Ohm kit. Then make your measurement. An alternate way still will require 75 Ohm connectors and a 75 Ohm calibration kit. You can create a 50 Ohm to 75 Ohm "L-Pad". This required 2 resistors. An 86.6 Ohm resistor in parallel with the 50 Ohm side and a 43.3 Ohm in series between the 50 Ohm side and the 75 Ohm side. Looking in from the 75 Ohm side, it "looks" like 75 Ohms. Looking in from the 50 Ohm side, it "looks" like 50 Ohms. The downside is that it has 7.5 dB of loss in both directions. You would insert this between the VNA and the 75 Ohm system. Calibrate the VNA using the 75 Ohm calibration kit on the 75 Ohm side. Then make your measurement. The VNA will "calibrate out" the 7.5 dB loss. Hope this helps. 🙂

[@dennisqwertyuiop]

what vna are you using ? i have the nano,,the only thing i dont have is a vna like HP,,its on my list

[@eie_for_you]

I *did* have an HP VNA ... two man lift version; man, it was nice. I have since sold it. The one I used for this video was the Tektronix TTR503A. I also own a miniVNA tiny and a nanoVNA. For bench work and the nanoVNA or miniVNA tiny, I *always* connect it to my computer and use the software.

[@elmoreglidingclub3030]

@@eie_for_you. Very cool. But the price of those…..WOW. I’ll just have to do with my Nano as I want to stay married….

[@eie_for_you]

@@elmoreglidingclub3030 I can relate. For what MOST hams do, the nanoVNA is quite sufficient. There are places where they cannot go or go well, anyway. Like tuning duplexers (not enough dynamic range to do a good job) or measuring the input impedance of a receiver front end (cannot handle the very low stimulus levels). But, for most everyone, it is a way awesome tool and terrifically affordable!🙂

[@Brenda-jf2pe]

Imagine the slide rule guys doing this, like 50 years ago! Thank computers thank you AI! JohnBoyUtah KJ7TBR

[@eie_for_you]

LOL! I was one of those slide rule guys many moons ago! 😀

[@danieljohnson8437]

Damn Ralph?

[@eie_for_you]

That's me! 🙂