Monday, May 20, 2013

Antenna Topics: RG-6 coaxial cable for antenna feed lines. Post #193

RG-6 coaxial cable--an alternative to RG-8X, RG-58, and RG-8.

Most amateur radio operators are familiar with the RG-8, RG-8X, and RG-58 family of coaxial cables.  These moderately priced cables are used for antenna feed lines and patch cords around the ham shack.  With a nominal impedance of 50 to 52 ohms, they work well with both contemporary transceivers and the old "boat anchors" we love to hate.  Many amateurs (such as yours truly) also use 300-ohm tv twinlead and 450-ohm ladder line to feed our antennas. With a suitable transmatch and a 4:1 balun, we can use an antenna on several bands.

What if you don't have any ladder line, tv twinlead, or 50-ohm coaxial cable to feed your antenna?  Is there any other cable that can be used?  There certainly is.  RG-6 coaxial cable, the type used by cable companies , can often be used the same way as 50-ohm coaxial cable.  RG-6 usually refers to a cable with an 18 AWG center conductor and a 75-ohm characteristic impedance.  Usually, this cable comes with "F" connectors.  All you have to do is get a "F" to "UHF" adapter from Radio Shack and you're ready to connect the RG-6 to your equipment.  Many amateur radio operators use RG-6 to make matching sections for loop antennas.

As luck would have it, I acquired several 25-foot (7.62 meter) and 10-foot (3.04 meters) pieces of RG-6 when my radio station (KKBG-FM/KHLO-AM) did a studio remodeling job and had some RG-6 left over.     At that time (2010), I was still working as the news director and had a good relationship with the station engineer, who also was a ham.  I secured these "left overs" at no cost...they even had the "F" connectors still attached.  I bought several Radio Shack "F" to "UHF" connectors to convert the cables for amateur radio use.  The cable has been stored in plastic containers since that fortunate day 3 years ago.  Now was the time to modify my antenna feed system.

When I researched the characteristics of RG-6 coaxial cable, I was surprised by the attenuation figures I found:

For 1 MHz, the attenuation (dB/100-feet/30.48 meters) was 0.2 dB.  For 10 MHz, the figure was 0.6 dB.  For 100 MHz, the number was 2.0 dB.  And for 1,000 MHz, the attenuation was 6.2 dB.  Those numbers would serve my purposes very well.

I tested the RG-6 on my recently erected 40 meter delta loop.  First, I replaced the 450-ohm ladder line/4:1 balun combination with 50-feet (15.24 meters) of RG-6 fed directly into the Drake MN-4.  The old Drake was able to get the SWR on both 40 and 15 meters below 1.5 to 1 on both bands.  I didn't try the antenna on 20 meters because the mismatch would be too great for the Drake MN-4.

When I reconnected  the 450-ohm ladder line/4:1 balun combination, I replaced the usual RG-8X coaxial cable from the balun to the static discharge system and the RG-8X cable from the static discharge system to the shack with RG-6 coaxial cable.  As before, the Drake MN-4 transmatch was able to bring the SWR to 1.5 to 1 on 40, 20, 15, and 10 meters.

Following a few on-air tests, I removed the RG-8X coaxial cables from the antenna system and stored them in the garage.  I'll be using the RG-6 coax substitutes for awhile, just to see if I run into any problems.  So far, the Swan 100-MX runs cool, the SWR is below 1.5 to 1, and the loop performs as well as before the substitution of cables.

So, if you want a good, general purpose feed line for your antennas, consider RG-6.  You can buy RG-6 in various lengths at the nearest hardware store or home improvement outlet in a variety of lengths.  You might be able to get some RG-6 for free from your cable company service technician.  If you don't want to worry about converting the "F" connector, just buy some "F" to "UHF" connectors from Radio Shack.

Try some RG-6 for your next antenna project.  You might be surprised how well it performs.

REFERENCES:

en.wikipedia.org/wiki/RG-6.

http://www.broadbandutopia.com/coaxial.html.

http://www.bluejeanscable.com/articles/rg6.htm.

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Thanks for joining us today!

Aloha es 73 de Russ, KH6JRM

BK29jx15--along the beautiful Hamakua Coast of Hawaii Island.