Monday, August 12, 2013

RG-6U Warning

In my previous posting on ¼-wave transformer accuracy I mentioned that I'd bought a small roll of RG-6U 75Ω coax with which I planned to build the transformer for my experimental delta loop. Impedance transformation is needed since the loop radiation resistance is well over 100Ω. Today I cut into the cable to see how I might attach a UHF connector on one end and make a pigtail connection at the other.

What I discovered is not good. The attached picture is not particularly good (curse those auto-focusing smart phone cameras!) though I think you'll be able to see the important bits.

As with most RG-6 cables I've read about there are a few features that I found:
  • Foam insulation between the inner and outer conductors
  • Primary outer conductor is foil
  • Braid surrounds the foil
There's nothing wrong with that, though as I mentioned before I had some concern with making the needed connections at either end.

The problems are with the details of the cable construction. To be blunt, this cable was designed to be used as-is, complete with connectors at both ends. The connector I cut off is shown in the picture. The cable is made by a major manufacturer and was bought in a local big-box store.

Here are the difficulties with this cable:
  • The braid is no more than 10% coverage and made from easily-broken strands. What you see in the picture is a fair representation of it. There is nothing to connect to. I don't even see the point to its presence. Well, ok, there is one: as one side of the "sandwich" for the machine-driven connector insertion to form a bond to the outer conductor.
  • The foil is bonded to the polyethylene foam. It cannot be separated from the foam without damaging both. If that had been possible I might have been able to at least made a pigtail of some type, or soldered to it.
  • I tried to solder to the foil. No go. The attempt also melts the foam.
  • An RG-58 adapter for a male UHF connector is not a friction fit for the outer conductors. If it had been I could have probably made a workable connection. The F-connector the cable comes with is fine, but I have nothing compatible. I don't want to spend more money purchasing female connectors. Besides, that does not really solve the problems with this cable.
With enough time and ingenuity I could eventually make this cable useful. However my time is worth more than the small amount I paid for it.

The lesson here is that if you do go for some variety of RG-6 when you require 75Ω coax you should first ensure that it can be put to use without excessive effort. There is no RG-6 standard to rely on; every manufacturer's product is different. I suggest it is worth paying a few dollars more for RG-59 or RG-11.

For the purpose of this one antenna experiment I will use RG-62. Since it is 92Ω cable the SWR of the delta loop models at 1.5, which is good enough. Using 50Ω cable all the way puts the SWR above 2, and that could effect the results of the experiment. If all goes well with the antenna I'll purchase some RG-59.

Baluns also work but the 1:2 ratio required is not commonly available and would have to be built. That is another item for consideration, but not now.

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