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July 2008 Archives

 

24 July 2008

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.
 

As I understand it, these stations use a NATO standard system at 200 bits/sec with four logical 50 b/s channels. All the signals are, of course, highly encrypted so there's not much point in further analysis. It is, however, worth of note that at 200 b/s, the occupied bandwidth is rather modest. The images below show NAA and NML4 in more detail. At the -20 dB point, the occupied bandwidth is about 280 Hz. There's a notch and then the modulation sidebands pick up a bit, but bandwidth is traditionally defined as the 99% energy or -20 dB point.

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.)

WWVB at 60 KHz is also easily detected, as shown below. This plot is of WWVB signal strength in dB (10 db/div) versus time, 500 ms/division X axis. WWVB uses an amplitude modulated data transmission method, with full carrier level and reduced (17 dB) carrier level. WWVB's time code is summarized at http://tf.nist.gov/stations/wwvbtimecode.htm 
The WWVB data was captured at 1332 EDT, or 1732 UTC. NIST provides the image below showing predicted WWVB coverage at 1800 UTC, with red being 100 µV/m field intensity or better. (For all 12 plots, go to http://tf.nist.gov/timefreq/stations/wwvbcoverage.htm) The east coast of the US is at the edge of  the 100 µV/m countour.
 
17 July 2008

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.

 

13 July 2008

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.

 

11 July 2008

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.

 

10 July 2008

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.
 

10 July 2008

A year or so ago, I built a couple of active antennas as developed by PA0RDT, described at http://www.aldomoroni.it/pa0rdt-Mini-WhipGeneralDescription.pdf

It's a quite decent performing active antenna, but suffers to some degree from intermodulation products when used in an area with strong AM broadcast signals. This is a common failing with active antennas. It's desirable that the active antenna has no selectivity ahead of the input stage. A wide open input stage however must be extraordinarily linear if it is to function in a high RF environment.

I've been working on my own version of an active antenna off-and-on over the last few months and finally laid out a prototype printed circuit board  this week. It's still a long way from being a real product, but preliminary measurements show some promise. One part of  the design is an AM broadcast band rejection filter, and I thought the filter results worth sharing. The design is a 6 pole band reject filter, for the US standard broadcast band, Chebyshev design.

The plot below shows the predicted attenuation (LTspice) and the measured attenuation. The main divergence is, not unexpectedly, the finite stop-band attenuation of around 75 dB in the breadboarded filter. The predicted attenuation shows 220 dB attenuation or some such utterly unrealizable value in  the stop band.

As the prototype's photo reveals, there's considerable room for improvement in layout to reduce stray coupling. It is encouraging, however, that the general agreement between measured and predicted is, except for the stop band depth, quite good. And for the purpose, 75 dB stop band attenuation is more than adequate for the task at hand.

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.


The plot below shows the frequency response of the complete active antenna with the band reject filter in place, over the range 10 KHz - 100 MHz. (log horizontal axis). The lower 3 dB point is 24 KHz and the upper 3 dB point is above 100 MHz.
 
08 July 2008

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.

 

02 July 2008

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.