Home » Blog » Radio » Shortwave Radio » Loop Transmission Line Shielding

Loop Transmission Line Shielding

loop transmission line

Proper connection of my CAT7 provides loop transmission line shielding for lower noise reception at my location. My loop now performs properly on lower HF and below. 

I have been experimenting with different ways to use CAT7 cable loop transmission line. When I first set things up, I did not have any connection to the CAT7 shield or any transformer matching between the line and receiver input.

My initial testing showed that the transmission line was acting as an antenna. Even when disconnected from the wideband loop, my receiver picked up lots of noise and signals. Not good.

In the improved circuit, shown above right, I have inserted a current balun between the twisted pair and receiver antenna input, and tied the grounds to the CAT7 shield. This is a huge improvement.

My transmission line no longer acts as an antenna. Noise pickup has pretty much disappeared. With the shield grounded at the station end only, the noise floor has dropped down to -125 dBm, or roughly the receiver’s internal noise. When the active loop power is turned off, no signals are picked up except on Medium Wave broadcast where signals are very strong.

To be honest, I am not sure whether or not the current balun is making a difference, but I will leave it in anyway. The CAT7 I am using is S/FTP. This means that each of the four pairs is within a foil sheath, and all pairs are within a shield.

Loop Transmission Line Shielding Makes a Difference

Attaching the shield to ground at the station end, and leaving it floating at the antenna end, keeps the antenna system balanced, which is what I want with a wideband loop.

You expect lower signal levels with a wideband loop. But, ideally, you also get lower noise levels and improved signal-to-noise ratio, compared to a full-size antenna. As you can see above, this has been achieved on 40 meters. The picture compares my Flex 6300 with a full size, tower mounted 40 meter dipole (above) with the Perseus/Loop combination (below.)

Signal levels are around the same, but the noise floor is 10-12 dB lower. I wish this was true on every band.

Also, I now have a decent receiving antenna on 80 meters, for which a full size antenna is not available at my QTH.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.