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

E-mail: Jack.Smith@cliftonlaboratories.com

The last week has been devoted to working with the active antenna in a couple of different configurations. Results are quite promising with a short (5 ft) whip as the receiving element, although additional tests are necessary in a high RF ambient environment.

One remarkable feature of active antennas is their low frequency response. And I do mean low, down to 20 KHz and below. The image below is a spectrum analyzer grab of the range 20-30 KHz, taken with an HP3562A Dynamic Signal Analyzer connected to the active antenna.

Frequencies below 100 KHz are widely used for military communications to submerged submarines. Even with WW II technology, NPM's VLF signal was readable at 2000 miles on a loop antenna at periscope depth. http://www.warfish.com/report6.html.

In the spectrum analyzer image immediately below are four signals:

• 21.4 KHz NPM at Lualualei Hawaii [566 kw ERP]
• 23.4 KHz DHO38 at Fhauderfehn Germany [500 kw ERP?]
• 24.0 KHz NAA at Cutler, Maine [1,000 kw ERP]
• 25.2 KHz NML4 at LaMoure, North Dakota [500 kw ERP]

The single image below that is for 39.75 KHz, NAU at Aquada, Puerto Rico [100 kw ERP]

The unmodulated carrier seen in the second image below is some sort of local spurious signal, I believe. Possibly from a computer monitor.

Many VLF and LF stations have been shut down. Just on the east coast, for example, NSS at Annapolis MD is no more, and the antenna array sold for scrap. The Navy's station NAM in Driver VA is also shut down. I also believe the Navy's VLF/LF facility in Iceland is decomissioned. When NSS was operating, it provided an incredibly strong signal into Northern Virginia, even without an active antenna.
 

Of course, at 24 KHz, antenna bandwidth is an issue. (Although physically immense, the antennas used by these VLF stations are still small in terms of wavelength and are heavily loaded with capacitive top hats and base inductors.)

I've received prototype boards of my active antenna layout, as seen below. The actual "antenna" is a copper foil disk, about 2 inches (50 mm) diameter, with a 2 inch long connecting wire. The entire assembly fits into a short length of 2" diameter PVC pipe, with PVC caps at both ends.

At the receiver end is a power injection arrangement, with a great deal of filtering and noise suppression.

One antenna is on its way to K8AQC for comparison testing in an environment with much stronger AM broadcast signals than in my distant suburban Washington DC location.

Frequency response over the range 10 KHz to 100 MHz is quite decent, within 0.5 dB of flat. The actual gain is 3 dB or so, as the 0 reference calibration is a 50 ohm termination in the VNA's receiver. When operating into a high impedance, such as an active antenna's input circuit, the applied voltage increases by 6 dB compared with the 50 ohm case. And, of course, it's sloppy thinking to even use dB in this method, as dB is a power reference. If the input and output have the same impedance, then the standard dB = 20 Log(Vout/Vin) is correct. In this case, the input impedance is in the megohm range and the output impedance is 50 ohms. However, I'll "go with the flow" here and use a dB reference of voltage gain without regard to the classical definition of dB as only a power reference.

Pete, N4ZR, has supplied a Radio Shack 600:600 ohm isolation transformer, part number 273-1374, with the thought that it might be used as an audio isolation transformer with the Softrock receivers.

I've added measurements taken of  the transformer/Softrock combination to my Softrock Lite 6.2 page. In short, this  transformer surprised me with its high frequency performance. Go to my Softrock Lite page and look for the link for the 273-1374 transformer. If you have not read the rest of the page, you may wish to do so to place the 273-1374  transformer into context.

To answer the question "what diode to use in an RF probe," I today measured the response of three diode types, silicon signal diode (1N4148), germanium signal diode (1N270) and Schottky signal diode (1N5711). Data and analysis at my new page Diodes for RF Probes.

My article "Observations on Ferrite Rod Antennas" is in the current (July/August 2008) QEX magazine. If you carefully read the entire issue, you'll find reference to my work in two other articles.
 

This is a reasonably complicated filter, with six tuned circuits. That degree of complexity is necessary if adequate AM broadcast band rejection is to be achieved whilst simultaneously providing essentially lossless performance at 500 KHz and below and above 1800 KHz.

I received an E-mail from Elecraft yesterday that my K3 transceiver is nearing the top of the shipping queue.

Although ordered on 24 May 2007, the second receiver option has delayed shipping  Based on the E-mail, I should receive the K3 around the end of July, about 13 months after order.

As usual, I've moved the June 2008 updates to an archive page, viewable by clicking on the June 2008 link at the top of this page or by clicking here.

I've also added a new page "Radio Intelligence Example" demonstration one small way in which the WJ 8617B surveillance receiver might be used to extract intelligence about a partially known radio source.