Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com
 

I've been evaluating a series of incremental improvements to VLF reception with the Z1501D Active Antenna to reduce the 60/120 Hz interference level as well as computer birdies and the like.  The changes are not to the Z1501D, but rather how it is installed. I've had significant success, but there's no single silver bullet fix. Rather, it's a matter of several improvements, each responsible for several dB reduction in noise. The  result of making all the incremental improvements is remarkable.

Let's see how the VLF spectrum looks now. Click on either of the images below for a larger version.

The first image is the spectrum from 0 to 100 KHz, whilst the second image is an expanded view from 17 KHz to 27 KHz.

I should add that while my emphasis on this page is for 100 KHz and below, the improvements are equally effective at higher frequencies.
 

At the same time, I also made a recording (3 MB, MP3 format) of the VLF spectrum that you may listen to by clicking the here: . Sounds\22 Apr 2010 Scan 01.mp3 The audio is from a Harris RF-590 receiver connected to my Z1501D test antenna. CW mode, with the BFO set at 800 Hz, medium AGC decay speed and 300 Hz selectivity.

The scan starts at 17 KHz and tunes up to 100 KHz over the space of 3 minutes. The table below indexes the recording time to frequency and station. The recording shows a near total absence of 60/120 Hz hum and computer birdies. Indeed, the normal background noise and static crashes are the dominant noise source.
 

Installing the cable and outdoor choke

I bought a 1,000 foot spool of RG-6, quad shielded CATV cable, direct burial style, and a Thomas & Betts "Snap-n-Seal" installation tool and "F" connector kit. In retrospect, I wish I had not gone with quad shielded direct burial cable but instead had bought normal flooded RG-6.

The problem with quad shielded cable is that the F connectors are difficult to install compared with standard RG-6. This is due to the cable and connector design. F connectors have an inner stepped cylinder that slides between the center conductor insulation and the shield, expanding the outer layers of the cable as the connector is slid into place.

The drawings below are from the Snap-n-seal patent application. Part 30 is the inner cylinder.

In the complete cross sectional drawing below, the  cable enters from the right side, with the inner cylinder (30) fitting between the inner foil and the inner braid. Part 60 is the plastic compression sleeve, which is detached from the connector during installation. Part 14b is an O-ring to seal the connector from moisture.

The figure below shows the connector with the plastic compression sleeve (60) detached and ready to be slid forward into the connector body. (A special tool is used for this purpose.) When slid into the connector body, the plastic compression sleeve forces the expanded coax layers against the inner sleeve (30) thus making tight 360 degree connection of the shield to the connector.

A quad shielded cable has four shields, hence the name. They are:

1. Inner foil (adhesive bonded to the center conductor insulation)
2. Inner braid
3. Outer foil
4. Outer braid

Over the outer braid is a thin transparent film and then the jacket. The inner cylinder of an RG-6 QS F connector slides between shields 1 and 2, i.e., between the inner foil and the inner braid. (The inner foil is adhesively bonded to the center conductor insulation and cannot easily be removed.) When the F connector is slid onto the cable, the inner cylinder has to expand the inner braid, the outer foil, the outer braid, the clear film and, finally, the jacket. Forcing the F connector onto the cable sufficiently far to be properly seated is extremely difficult, even when the cable is prepared with the correct tools and to the correct dimension. (Quad shielded cable has a slightly larger outer diameter than standard RG-6. It's important to use a RG-6QS connector, or a "universal" connector suitable for all forms of RG-6.)

Why go with RG-6 CATV cable? First, it's readily available in direct burial construction, unlike RG-58. Second, it's relatively inexpensive. Third, properly installed, F connectors are weather resistant. (And the T&B Snap-n-seal connectors are excellent in this regard.) That the cable is 75 ohms impedance instead of 50 is not material for an active antenna. One other difference is that most RG-6 cable has a copper plated steel center conductor. This increases the  DC resistance and loss at lower frequencies. These concerns are more theoretical than practical unless your coax run is extremely long.

In addition to relying upon the weatherproof nature of the Snap-n-seal connector, I also used a length of 1/2 inch adhesive lined heat shrink tubing.

To bury the coaxial cable, I use a lawn edging tool. It cuts a narrow slit in the soil, which I enlarge slightly by working the tool back and forth. I cut perhaps 10 feet (3 m) of slit at a time and push cable into it.
 

It took me about 2 hours to bury 150 feet (45 m) of cable with this technique. I also ran into about one buried rock every five or six feet.
 

Before burying the choke, I wrapped the connectors with Coax-seal.  The common mode choke is inside a short length of 2 inch diameter PVC pipe.