Measure Velocity Factor, An Alternate Method (050)

[@dennisqwertyuiop]

have to watch and follow along with my vna in shopagain to see what i come up with,,,are you using rg58u ?,,thanks for doing great videos ??

[@eie_for_you]

Yes, this was RG-58'ish stuff. Actually somewhat unknown and very, very ancient. I used this coax back in the early 1970s to connect to my antenna when I was in High School and it was used coax someone gave me even then! So, I truthfully did not know how the VF would work out. I have some other RG-58 coax that measures out to a VF of 0.6523. So, you never can tell. But, if you are going to do the "cut long and trim to tune method" and you want to just use the datasheet VF value, cut extra long and trim to tune. All of this measuring the VF becomes unnecessary.

[@StephenKangas]

Can this technique be used to measure the VF of insulated wire used for antennas? I am planning on cutting several End Fed Random Wires for use with a 9:1 unun I recently made. As you may know, there's nothing really "random" about EFRW, especially when trying to do many ham bands. There are so many lengths to avoid, especially when trying to cover the 60m band in the mix with the usual suspects. A difference in as little as one foot can be bad. SO, I'd like to adjust the table of EFRW lengths that assume 100% uninsulated copper wire and recalculate for my silicon jacket insulated copper wire. Your video removing some braid from coax suggest it would work for the entire stub being unshielded but insulated wire? My bet is there are others out there using various other wire gauges/insulations whose inquisitive minds want to know.

[@eie_for_you]

First of all, all of the methods that I have presented in my videos on measuring velocity factor assume a transmission line ... two parallel conductors of some sort. A single conductor hanging out there is a bit of another thing. I suppose the return path would have to be the ground beneath it. You are right, the VF of an insulated wire is lower than a bare wire and this is dependant on the particular material used to insulate the wire. The higher the dielectric coefficiant, the lower the VF. If I were faced with this conundrum, I would be tempted to use an empirical method. I would string a known length of the insulated wire up and then see what frequency it is resonant at - Return Loss dips. Then back calculate its VF. I would try this with the wire in the environment that it will ultimately live. For transmission lines a quick estimate of the VF may be calculated with VF=1/(sqrt(dielectric coefficient of the insulating material)). Whether this holds with a single wire traversing through space ... I don't know. I did a quick ask of the Internet about this and got a mish-mash of uncertain answers. 😞

[@StephenKangas]

@@eie_for_you Thx for the reply. Yeah, I was hoping for some unfounded reason to avoid doing that, but it makes sense. I have some scrap unusable length of the stranded copper 20AWG silicon insulated wire I can measure, I just need to find some bare stranded copper 20AWG to cut to the same length to compare and calculate VF based on my nanoVNA measurements of those. I'm like you presently in that I don't have two supports for elevating the wire away from the ground to minimize ground effects, and using a counterpoise wire off the unun may unduly alter the measurements (I use a choke placed back on the coax as counterpoise on my EF Half Waves), so I'll plan on measuring the wire as a sloper with one end up 50ft. While I'm at it, I might as well measure some of this cheap stranded 14AWG THHN wire from Home Depot I use for higher power wire antenna applications, which I already know from past experience that I cut a surprising amount off for tuning (it has two layers of different plastics). BTW, your videos as not only excellent topi coverage but very well produced; keep up the good work!

[@eie_for_you]

@@StephenKangas Thanks man!