Tuesday, 27 September 2016

ABSC Nostalgia

A recent comment on an earlier post got me thinking about other experiments I did with kits of parts from the American Basic Science Club so I opened up a box and extracted these items.

Tuesday, 5 January 2016

Loop Comparison

LW, MW and HF listening did not become practical at my current location until I got my first Wellbrook active loop antenna (an ALA1530). Even when I just had it sitting out on the wood-frame balcony outside my home office it was a huge improvement over the indoor random wires I had been using up to that point. The noise level was low enough that I could rely on it for reception of much more than WWV and CRI. When I moved it away from the house to be mounted NE-SW on a nearby fence the noise level dropped a further 5 dB to 10 dB on most of the frequencies I tested. I call this Loop A.

A few years later I added a second Wellbrook loop (an ALA100) with four turns of wire wound on a wood frame and mounted on a different fence panel so that it favours NW-SE. I call this Loop B. It's centre is about 5 metres away from Loop A.

The plan was for Loop A to favour Europe and the South Pacific on HF while nulling the MW broadcasters in northwest WA. To compliment that, Loop B was to favour Asia, Alaska, the Lower 48, and the Caribbean on HF while nulling the MW broadcasters in the lower mainland of BC. That has all turned out as expected and the spacing between them has also been adequate for creating a deep electronically steerable null on MW using a DX Tools RF+ Quantum Phaser (not nearly as lethal as it sounds). I also hoped that on HF either loop could serve as a reference for the other when connected to an MFJ-1025 noise canceller. Unfortunately that turned out to be a complete waste of time and money because I did not find a single signal that benefited from use of the device.

After a recent post to the Shortwave Radio Station Listening group on Facebook I was asked about how well Loop B works. Such things are difficult to quantify so the best I can add to what I have said above is to refer to the following two frequency response curves from yesterday afternoon.

Loop A

Loop B

At almost all frequencies the output of Loop B is lower than that of Loop A but this is to be expected because, even with four turns, Loop B is built with much less "capture area" than recommended by the manufacturer. What else do the two images reveal? As expected, my only local MW broadcaster is clearly weakened by being in the null of Loop A and multiple Vancouver stations are brought down by being near the axis of Loop B. Aside from that, the two curves show differences in the sizes and locations of their peaks and valleys but there is no way to know, from this experiment alone, the extent to which that is due to differences in their frequency response or to differences in their location and orientation. For example, the electromagnetically noisiest place in my house is the service panel where AC power, cable TV and ADSL internet all converge and it is almost directly in the null of Loop A while the orientation of Loop B favours reception of noise from my nearest neighbours.

Switching from Loop A to Loop B

This last image shows what happened in the vicinity of a broadband noise source when I switched from Loop A to Loop B. I am sure that the frequency shift means something but I currently have no clue about what that might be or how to use the information.

Saturday, 26 December 2015

Signal Tracer and Injector

On Christmas Eve I finished building this simple piece of test equipment from QRPkits. It's a combined signal tracer and injector. It doesn't do anything that I cannot accomplish with an oscilloscope and a function generator but it's much more convenient to use.

As with the Accuprobe described in my previous post, this device uses heat-shrink tubing as a case. At Joanna's suggestion, instead of using a hair dryer as a heat source, I simply place the unit on top of a space heater (as shown in photo) with the air flow directed vertically. This worked much more quickly and provided more uniform heating.

Tuesday, 22 December 2015

Accuprobe for Measuring RF Voltages

Today I finished building my latest piece of test equipment -- the N2CX Accuprobe Plus from Pacific Antenna. It was an easy, high-quality kit. The only odd bit was using heat-shrinkable tubing in lieu of a tubular case. Without a proper heat gun I had to improvise with a hair drier but it worked well enough.

For testing, I compared the results obtained with it against the RMS values I calculated from the peak-to-peak readings off of an old Kenwood oscilloscope when using a function generator feeding my Elecraft dummy load for weak signals and a Yaesu FT-817 as a source for QRP power levels. The accuracy was what would be expected from the specs and much better than the readings made with my old Heathkit probe. However the latter would still be the device to use for voltages over the Accuprobe's 35 V limit.

Tuesday, 15 December 2015

Elecraft DL1 Dummy Load

Around the same time that I got the ZM-2 shown in my previous post, I also bought an Elecraft dummy load kit that you can find on this page.

One nice thing about it is that it includes a diode, capacitor and two convenient test points for measuring power with any high input impedance voltmeter. The manual comes with a graph for converting the voltage readings to watts. Using an inline meter to measure forward power as a reference, I got considerably more accurate results by using an old Heathkit RF probe across the load. However he test points are convenient and for my needs would be perfectly adequate to get relative readings for peaking the output of a QRP transmitter.

Emtech ZM-2 Antenna Coupler

Several years ago I bought this kit but only got around to building it last week. The manufacturer calls it an antenna tuner but I insist upon referring to it and other such devices as an antenna coupler because it doesn't actually tune an antenna. Aerials can be tuned by changing the sizes of their elements or the values of their lumped reactances but this device is not part of any antenna. It's function is to couple a feedline to an aerial so that energy can be transferred from one to the other. I am also willing to call it a transmatch because it does its job by transforming or matching impedances.

The ZM-2 is a nice little unit that if very easy to adjust without the need for an external SWR meter. The empty holes in the corners are for screws that I still have to buy to hold the faceplate to the plastic box.

Tuesday, 8 December 2015

ABSC Regen Meets Ocean Hopper

After receiving generous constructive feedback from members of the Regenerative Receiver Initiative Facebook group and others, I have decided to abandon the idea of reviving my ABSC regen in its original form and, instead, to merge it with the even older Knight Ocean Hopper (OH) design. The above circuit is my current idea of how to create something similar to the OH while still reusing as many components as possible from the ABSC radio.