Wednesday, April 28, 2010

250 mW Tuna Tin into a 730-ft. doublet

This item caught my attention when skimming KB6NU's Ham Radio Blog. See the April 14 article here and scroll down to the topic "Another QSO of note".

W3TIM is the operator of that QRP rig with broadcast-style balanced antenna tuner. His page on includes the same photo of the tuner and a nice photo of his Tuna Tin rig.

Tuesday, April 27, 2010

SSB Audio Compression

This morning George KW7GAM asked for reports about how his audio sounded on 3740 kHz with and without compression. Click here for the audio recording made from my transceiver. The screen shot above shows the audio waveform as displayed in Cool Edit 2000, with text labels added to identify the two compression states. Click on the image for a larger view.

The appearance of more black space in the right-hand, uncompressed portion of the audio waveform supports the expectation that average audio power is lower for that state. The software allows for selecting a portion of the waveform for statistical analysis. Here are some data for the two portions of the overall waveform:

Compression ON OFF
Minimum RMS Power: -37.89 dB -39.54 dB
Maximum RMS Power: -10.5 dB -11.23 dB
Average RMS Power: -17.56 dB -19.63 dB
Total RMS Power: -16.48 dB -18 dB

The two histograms below also show the difference. The graph for compression ON (lower left) shows a greater number of samples near the high amplitude (right-hand) side of the graph, while the graph for compression OFF (upper right) shows more samples spread across the amplitude range. The X-axis of these graphs provides the amplitude scale in dB relative to some reference not explicitly stated by the program.

Tuesday, April 20, 2010

W3DZZ Multi-band Antenna 80-40-20-15-10m

This antenna has been discussed several times since W4BXI tested it at his antenna farm. Click here for a nice short article by G8ODE that covers the physical and electrical properties, including some radiation pattern plots. A detailed analysis of the antenna by ON9CVD (with discussion of trap component values, impedance and performance) is available here.

Friday, April 2, 2010

Antenna Tuner Matching & Feedline Lengths

This is a recurring popular topic in our morning QSOs on 75 m.  Multiband operation of a single antenna system results in a wide range of impedance at the transmitter end of the feedline. The Hamuniverse web site provides this article with a discussion of some common scenarios that can result in a very high impedance being presented to an antenna tuner. A few good thumb rules are provided along with a table of feedline lengths to avoid and lengths that are acceptable.

The table focuses on the 160, 80, and 40 m bands. It is worth noting that the listed lengths to avoid are computed for one frequency on each band. For example, the 130 ft and 260 ft lengths to avoid on the 160 m band relate to 1.8 MHz (assuming a feedline velocity factor of 0.95). At 2.0 MHz you may want to avoid feedline lengths of 117 ft and 234 ft. Thus, considering the entire 160 m band you may want to avoid feedline length ranges of 117 - 130 ft and 234 - 260 ft. Similarly, on the 80 m band you may want to avoid length ranges of 58 - 67 ft, 117 - 135 ft, and 175 - 200 ft. On the 40 m band you may want to avoid 32 - 33.5 ft, 64 - 67 ft, 96 - 100 ft.

A table footnote says that "some trimming or adding of line may be necessary to accommodate higher bands". If one desires to use the antenna on ALL bands through 10 m, the set of feedline lengths needed to satisfy the guidelines on all bands may become more restrictive. Careful testing on all desired frequency neighborhoods should reveal any problem areas for matching. Then, hopefully, minor adjustments of the feedline length can accommodate the range of the antenna tuner.