Friday, February 7, 2020

Magic Reel

Since I began planning this station, long before I went property hunting, I opportunistically purchased large items I'd need. One of these items was low loss transmission line. By the time I moved in late 2016 there was 2000' (600 m) of Andrew Heliax in my hoard. Since then I have continued to buy and scavenge Heliax so that I have far more, deployed or in storage.

I have never bought Heliax for anywhere near full price. It's all NOS (new old stock) or used. The used stuff varies from gently used to very nearly dropped from towers.

Magic reel of LDF5-50A
The last case is an amusing example. A friend in the business did me a favour by having his crew save what they could of a long Heliax run on the tower they were decommissioning. They had to be quick to avoid annoying their client. After cutting out the kinked sections I had 150' of perfectly good LDF5.

The reel of LDF5-50A on the right is NOS. I don't know the original capacity of the reel but it would at least 1200'. When we agreed on a price per foot the seller and I had to decide how many feet were on the reel. This is not as easy as you might imagine.

With both of us working with a tape measure and calculator we used our fingers to probe down the layers to estimate the number of turns. The task is near impossible without unwinding the reel, and you can't do that without great effort and risk of damage.

Reasonable people will be reasonable. Other times the calculation will be more contentious: the seller wants a higher number and the buyer wants a lower number. We were both reasonable people and after our best effort we agreed on 400' and shook hands.

When the picture was taken I had already taken two runs from it: 125' and 250'. Do the arithmetic and glance again at the picture and you'll notice that something must be amiss. There cannot possibly be that much left after removing 375'.

With the reel now significantly depleted I was able to reach all the remaining layers and confidently estimate that there are 250' to 300' on the reel. Obviously the deal was in my favour. I call it my magic reel for that reason, seeming to grow more Heliax when I'm not looking!

The deal could easily have gone the other way and it has on other occasions. Even an open reel of used Heliax, such as those on the right, can be difficult to measure. The turns are usually tightly taped (as they must be to keep the springy stuff intact) and are of variable diameter. Errors of 10% or 15% are easy to make. The same can be true of smaller cable like LMR400 when it is tightly taped.

One of the people I've bought NOS and used Heliax from uses a simple mental calculation to estimate length. For cable on the reel it's 6' per turn and for used coils it is 12' per turn. This works well for LDF5 size cable since the typical diameter on the reel is 2' and 4' in an open coil.  

C = πd and π is approximately 3 so we have 2' × 3 = 6' and 4' × 3 = 12'. For metric length skip the multiplication by π. Hence, 2 meters and 4 meters, respectively, for diameters measured in feet.

For a simple rule it works well enough to usually achieve an accuracy ±15% in my experience. The magic reel is an exceptional case.

If you only ever buy one reel of hard line you may be sensitive to errors. When you buy many it is less concerning since the errors tend to average out. On some deals you win and on others you lose. The key is to be reasonable. Yes, be happy when you eventually discover you've been lucky (as with my reel above) but you should not be angry when you find you bought less than you paid for.

After all, what is the correct price anyway? When you paid $1/ft that is a fraction of the new price. As you unroll the reel later and you find the true price was $0.80 or $1.25 per foot does it really matter? It's still a good deal if the cable is in good condition. What is important is to inspect the cable for kinks and jacket cuts. Those are far more important to the price than an exact length.

Often there is a connector on at least one cable end. That's alone can compensate for any shortfall. Scavenging for deals should be a fun part of the hobby, not a chore or a source of aggravation. Be vigilant but relaxed when money changes hands. It's better to assume the best in people rather than the worst.

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