[K3PZN-List] Watch out for cheap coax

[Peter Morton]

Just read an interesting article in the March/April 2010 issue of TCA (The Canadian Amateur [Canada's QST]) by Nicholas Shepherd, VE3OWV.  He bought two types of cheap RG-6 type coax from local stores in Canada for use in feeding his antennas.  He noticed that the SWR bandwidth of his 40 meter inverted-vee was very broad and not what he anticipated.  So he measured the insertion loss and was disturbed by the results.

Normally, true RG-6 solid polyethylene dielectric cable should have an insertion loss of about 1.9 dB/100 ft. at 50 MHz per The RFC web site and Times Microwave.

His two cables measured significantly higher.

The first cable, Omega OVK-30, purchased at a local Home Hardware store, measured 6.0 dB/100 ft. at 50 MHz.

The second cable, GE RG-6, purchased at a Canadian Tire store (product code 20638), measured 3.25 dB per 100 ft. at 50 MHz.

As a check, Nicholas measured some scrap RG-6 CATV/Ethernet cable (Comspec 2000896), and it's loss was 1.5 dB/100 ft. at 50 MHz, less loss than the 1.9 dB typical for solid polyethylene dielectric RG-6.

In his article, VE3OWV states: "Physically there is very little external difference between the two types of cable.  The braiding on the Comspec cable is slightly denser than that of the Omega cable and the aluminum foil is very tightly bonded to the inner dielectric.  In the Omega cable the foil can be readily peeled away from the dielectric.  The dielectric and inner conductors themselves appear identical.  It is likely that the differences in the HF RF performance relate mainly to the quality of the dielectric insulating material so there is no reliable way of visually differentiating between good and poor cables of this type.  The DC resistances measured at around 2.5 ohms per 100 feet for both the inner and outer conductors of either cable type so simple resistance measurements don't help much either."

He also states in the article:  "The Omega and GE RG-6 cables are made in China, whereas the Comspec cable was almost certainly of North American manufacture, but appears to be no longer available."

-Pete, W3GVX