Velocity Factor: Measuring using the Antenna Analyzer Method (00e2)

[@patrick4228]

Interesting and enlightening material. Thank you Mr. Gable.

[@eie_for_you]

Glad you enjoyed it! :-)

[@toddanonymous5295]

Two more great ones Ralph. Feels like back in the good old days of ham radio .. _ . _ ...

[@eie_for_you]

Thanks, man!

[@PatAutrey]

This is a fantastic video thank you so much! When measuring a section of coax does it need to be shorted at the opposite end or is it okay to leave it open

[@eie_for_you]

Thanks, man! Leave it open or shorted it should make no difference for this measurement.

[@PatAutrey]

@@eie_for_you I have a nano VNA I was wondering if you happen to have one, and and can tell me how to do this type of measurement using a nanoVNA

[@eie_for_you]

@@PatAutrey Well, Pat, have I got a deal for you! I demonstrate exactly that in this video: kzbin.info/www/bejne/kHnUhmudgZJrp6c I just watched it again, myself, note that I keep talking about 1/4 wavelength. I mean 3/4 wavelenth when I get to the "Do the Math". With the nanoVNA you have to set the Start and Stop frequencies *before* you calibrate just like a normal VNA (miniVNA tiny is very different, it always calibrated it entire range). The when you set new Start and Stop frequencies to zoom in on that second transistion, you will have to recalibrate 😞which is a nanoVNAism. This assumes an open end. If you short the other end, the phase transition with be from +180 to -180 degrees, but will exist in the same place. I hope this helps.

[@PatAutrey]

@@eie_for_you the video is perfect! I am wanting to determine the velocity factor of MFJ 300 OHM twin lead. I have got so many different answers from people as to what the VF is. Some say 83, some 89. MFJ said 66, but when I measured I came up w 42! Will try again with these instructions and see what I get

[@eie_for_you]

@@PatAutrey First ... MFJ...not a big fan. Yes, I own one, but their accuracy for reporting anything but SWR is suspect, even coming basck from factory calibration (which cost me nearly half the cost of the instrument and took 8 weeks). Aaaaahhh! Now THAT's going to be a trick! Let me explain briefly. With COAX, everything is pretty much self contained (EM-wise). Twin lead and the like, not quite so much. Let's say that you measure this stuff laid down on the ground. You have conductive ground underneath it with air inbetween. This creates capacitance between each conductor and the conductive ground. This changes things. How much these sorts of environment effects will actaully affect the measured VF I cannot say because I just haven't messed with twin lead in that way. But, you could do an experiement by hooking it up to the nanVNA and then have someone move it around, change its environment and watch you VNA to see how the phase moves about with the changes. Let me know what you discover.

[@aaronhope8366]

Will this work on any old conductor? I'm looking at making a slim jim styled antenna out of conductive nylon backed tape and a seatbelt strap. I saw it on a ham channel, and they got the weight down to 40 grams for a dual band for 2m and 70cm bands. The issue the maker was having was that he didn't know the velocity factor of the conductive tape, so he was guessing his way to the right lengths with respect to the known velocity factor of copper ladder wire. But the follow up for antenna building : If copper has a velocity of 0.9 and conductive tape has a velocity of 0.55, it is as simple as a ratio between the two to achieve the needed radiative properties of an antenna with a low and ideally targeted SWR?

[@eie_for_you]

Now THAT is a really good question! I've never actually tried to do this, but let me noodle a bit ... The method depends on the analyzer seeing the "test subject" as a transmission line. It seems to me that a transmission line requires two conductors. If there is not one explicitly provided such as with coax, then any conductive or semi-conductive stuff in the vicinity becomes the second element. This is especially significant as we increase frequency. So, if your assembly uses this conductive tape in conjunction with some other conductive material, you could do this. But I am not sure how well it would work without the other half of the transmission line. You could try it and let me know how it works for you. That way when someone else asks me a similar question, I might be able to answer them. Sorry I couldn't be more definitive. 😞

[@aaronhope8366]

@@eie_for_you no worries! I think you have me on the right track. I'll get with another local antenna enthusiast and see what we can come up with.

[@eie_for_you]

@@aaronhope8366 Just as a note ... environment is everything! Changing from naked wire to insulated wire with an antenna makes the antenna wire's VF go down, shortens the antenna. The basic equation for the VF of coax *only* involves the dielectric coefficient of the insulating material used in its construction. Ya gotta be paying attention to this in your process. 🙂