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Hello all,

I don't have much experience using TDR's in general and am wondering if they can accurately measure cable length if the cable is coiled rather than deployed in a more typical manner? At my work QTH we have a length of heliax on a big wooden spool and no one knows how much of it there is, nor can we easily count the number of coils on this stuff. It is quite rigid and we'd like to not have to unroll it.

Thanks/73,
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John AE5X

I am no expert, (I have only used TDR with Nanosaver on short lengths with multiple connections -every single connector showed up with a slight blip) but I see no reason for a TDR measurement not to work. It does not matter that the wire is coiled although there might be some uncertainty at the far end ; hopefully it is open circuit at the far cut end, but the last inch might be crushed. What you hope is that there is not a perfect 50 ohm load at the far end! You need to set the vna to the highest possible upper frequency for short lengths (1mm to a metre) but have the lowest start frequency for long lengths.

The biggest difficulty will be to get a decent connection to the heliax. I do not know how you connect Heliax to sma. I suppose very short wires soldered to the sma connection should be ok for the far end measurement. You might even get a clean enough signal to confirm that there is no damage somewhere on the coil.

Steve L

On 11/8/20 12:17 PM, John AE5X wrote:

Hello all,
I don't have much experience using TDR's in general and am wondering if they can accurately measure cable length if the cable is coiled rather than deployed in a more typical manner? At my work QTH we have a length of heliax on a big wooden spool and no one knows how much of it there is, nor can we easily count the number of coils on this stuff. It is quite rigid and we'd like to not have to unroll it.
Thanks/73,

that is the perfect use for a TDR.

Yes, at the factory they are typically tested on the spool.
You are looking at internal fields, coiling the coax up makes no difference.
73 Kent WA5VJB

Thanks for the info, guys. We use heliax to feed a transponder antenna and the indoor end is connected to a special N connector made for the cable. We have a ton of N-to-SMA adapters. We also have a $$$ VNA at work but I also want to test it with the nano for comparison, just didn't know if the cable being coiled would matter.

Thanks again,

--
John AE5X

John

You don't state which manufacturer of cable but I know some years ago that Andrew used to number their coax every meter so if you're able to ascertain the right number on the cable [easy] and access the other end, you should be able to confirm your nanoVNA measurement!

Best of luck and I, for one would be interested on how accurate you find the nanoVNA compared to the expensive VNA.

73, Bob
G1ZJP

TDR measurments are not significantly affected by wire being coiled. If it had ferrite sleeves on it maybe, but not just coiled, and certainly not a large diameter coil. Youll likely have a greater error from temperature.

On 11/9/20 8:08 AM, Mark Sedutto wrote:

TDR measurments are not significantly affected by wire being coiled. If it had ferrite sleeves on it maybe, but not just coiled, and certainly not a large diameter coil. Youll likely have a greater error from temperature.

I can't imagine anything on the outside of the coax (ferrite sleeves, iron pipe, anything) having an effect unless it's coax with a leaky shield. (they actually make deliberately leaky coax, for things like distributing radio signals in mines and tunnels)