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April 2010 Archives

 

27 April 2010

Collecting signal data in action. 7850 KHz (CHU) with 100 second span (10 seconds/division.)
 

 

26 April 2010

When I developed the Z1501D Active Antenna, one purpose was for Elecraft K3 owners to have a a broadband antenna for diversity reception. (The K3 allows the sub-receiver to be used in diversity mode, with the phase and frequency matched to the main receiver. The two signals, presumably from different antennas, can be routed to the left and right sides of a pair of headphones, which allows the operator to mentally select the signal with the best signal-to-noise ratio at any given instant.) This brings up the obvious question - how much diversity gain does a second antenna provide with the spacing between antennas one might find in a reasonable size suburban lot?

To get a sense for the realizable gain, I've written a program to control two Advantest R3463 spectrum analyzers, operating in zero span mode. The two analyzers start sweeping at the same time (within 75 milliseconds) and collect 1001 signal strength measurements. I've run all measurements so far at 100 second sweep, so a signal strength measurement is made every 100 ms. The 75 ms skew, amounting to one data point offset between the two spectrum analyzers, is negligible considering the relatively slow observed fade rate.

The two antennas compared in the sample plots below are a Z1501D Active Antenna and my 80 meter band inverted V. The inverted V's center is 80 feet above ground and the end of the radiating elements are about 40 feet above ground. The Z1501D is located approximately 75 feet horizontally and 75 feet vertically from the inverted V's center, or about 106 feet straight line.

The normal rule of thumb is that appreciable diversity gain requires antenna spacing of at least one wavelength. At 7850 KHz, the frequency used for these plots, one wavelength is about 125 feet (38 meters) so the straight line distance between the inverted V center and the Z1501D is not too far from one wavelength.

I'll have more to say on the subject of diversity gain later as I collect more statistics and have a chance to analyze the data. However, the data shows the potential advantage of diversity reception, even with as little as one wavelength spacing.  There's probably some element of polarization diversity mixed in with this data as well.

The data shows two fade classes. First, there's a general trend tracked by both antennas. Runs 4 and 8 both show clear correlation in the general signal level change. But, critically for the possibility of diversity gain although the general signal level change is similar, the specific fades are in many cases strikingly different. For example, Run 4 between 64 and 66 seconds, or at 85-87 seconds. Run 8 shows one very deep fade at 44 seconds. It also shows several fades of similar shape, but displaced by one to two seconds between the two antennas.

 

 
23 April 2010

I've added text and photographs to the VLF improvement page showing how I installed the buried coax cable.

I've also added a second photograph to the Z10050A 3-dB coupler page.
 

22 April 2010

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.

I've added spectrum analyzer images and a 3 minute recording of the 17 KHz - 100 KHz spectrum after making these improvements. The images and recording are available at a new page: VLF Signals with Improved Setup
 

18 April 2010

I recently acquired a Harris RF-590 receiver with a memory backup battery of unknown age, but still holding a charge. Rather than wait for the battery to fail, and risk damaging the printed circuit board, I decided to replace it with a new battery. I've added a page showing how the battery is replaced. Click here to view the new page.

 

17 April 2010

I've added photographs to the Z10050A 3-dB hybrid coupler kit page showing how two customers have built their couplers.

 

16 April 2010

I've been working on a common mode choke kit/assembled option for the Z1501D active antenna. To be effective, the rule of thumb is that a common mode choke should have an impedance of 4K ohms or greater at the frequency of interest. The design I've put together meets this requirement from 60 KHz to 13 MHz, where I stopped collecting data. I have a couple ideas for further improvement and will see if the low frequency limit can be moved down to 30 KHz or so. Extending the upper end is not difficult as only a small amount of inductance is needed at 10 or 20 MHz.

The important element in the plot is impedance, Z, in the thick blue line. For our purpose, it matters little if the choke impedance is inductive, capacitive or resistive, if the magnitude is 4K ohms or greater. In fact, the impedance is predominantly resistive above a couple hundred KHz. This is because the ferrite core material selected for the choke is intentionally low Q.

My current thinking is that the common mode choke will be housed in a short length of 2" PVC pipe. BNC, UHF or F connectors will be available options.

I've also moved my Z1501D test antenna to be further from my shop and also plan to bury the coax cable. I'll have more on that when I've finished the job - only about half the coax is underground now.

 
11 April 2010

I've added an excerpt from the Steward 40T0501 core data sheet to the Ferrite Core Sample-to-Sample Variation page.
 

08 April 2010

I've occasionally wondered how repeatable inductors are when wound according to instructions along the line of "wind 20 turns on an FT-50-75 core." So far, none of Clifton Laboratories kits require accurate inductance from similar instructions, but the question still bothered me. To address this question, I measured 118 ferrite cores of two types today to get a feel for core-to-core variation in permeability. Details at Ferrite Core Sample-to-Sample Variation

 

04 April 2010

I've made a small revision to the Z10040B manual, as a consequence of a customer problem. The builder wound the T2 and T4 transformers with the correct number of  turns, but in a mirror image of the manual instructions, with the theory that the important point is to make the two transformers identical but that the winding direction could be either "left hand" or "right hand." Making the two transformers resemble each other as close as feasible is, of course, beneficial. But, the winding orientation in the manual must be followed. The reason is that the winding polarity ("dot notation") is critical for the proper operation of the circuit. The orientation of windings A-B is determined by the printed circuit board layout. Unless windings C-D and E-F follow the orientation in the manual illustrations, the windings will not be properly polarized and the result will be oscillation and a non-performing amplifier. I've added a caution to the manual along with a brief explanation of why the winding orientation must follow the illustration. The revised Z10040B manual can be viewed by clicking here.

I've also made a number of small fixes to the Z1203A DC coupler manual based upon builder feedback. The revised Z1203A manual can be viewed by clicking here.

 

 

04 April 2010

As usual, last month's Updates are now in the archive. You may view the March 2010 updates by clicking here or by the links in the  table at the top of this page.