Month: September 2014

http://www.rfcafe.com/references/electrical/smith.htm

There are some areas on the Smith Chart, within which the antenna feed point impedance can not be matched to the system impedance of, say, 50 ohm, with a given network. The figure shows a forbidden region for a particular type of L-section matching network.

It was DU3LA, Luzon island, Philippines. It seems my antenna works well (or only) for the direction of south.

So I should try to work on the 80m band, and perhaps on the 160m band.

With my 40m band dipole antenna I had some QSOs with local stations on the 6m band, and it turned out that the antenna also works with skywave propagation for the distance of about 3,000km. The station was DU7/PA0HIP, Lapu-Lapu city, Philippines.

Bent Dipole with a short Parallel Transmission Line.
Bent Dipole with a short Parallel Transmission Line (2).

This is the output audio signal from a DC receiver, Soft66LC4. The signal flow is:

Audio SG, WaveGene -> (USB I/F) -> IC-7410 (USB Mod Level 25%, RF output power 2 watts) -> Dummy Load -> (pick up antenna) -> DC receiver -> (analog audio) -> FFT Analyzer, WaveSpectra.

The same configuration, but with USB Mod Level at 35%. Since the RF output power is only 2 watts, I suppose the distortion is due to the baseband stage of the transmitter.

The above figure is from the book, “An Introduction to Satellite Communications”. Some part, including this figure, is available at Google Books.

http://www.teledyne-mec.com/appnotes/appnotes.pdf

Because you will get 2dB improvement of the 3rd IMD for every 1dB of input backoff, I thought a linear amplifier stage of the transmitter gives you no problem as far as you are operating in digital modes such as PSK31 since the required Eb/No and hence the RF output power is much less than in the SSB mode.

This is the 3rd order IMD of TS-990 at its full power of 200 watts. (See QST Product Review, for IC-7410.) If you are operating at 20 watts (-10dB to the full power), the 3rd IMD should be 20dB better than in this figure.

So the question is: how much power do you need for your satisfaction when you operate in digital modes? Of course, the answer depends on you.

Since I am a CW lover, and CW having the great virtue of constant envelope and being a nice friend of class-C amplifiers, IMD measurements had never been of my interest…

O.K. This is a PSK31 signal in radio frequency. The baseband signal is generated by MMVARI with its digital output level slider adjusted to have no significant clipping. The signal is fed to IC-7410 through its USB I/F with “USB Mod Level (menu 39)” set to 25%.

Sometimes it is more flexible to use a signal generator program to obtain PSK31 like signals.

This signal is also fed to IC-7410 using the same USB I/F, and what you will obtain is:

Looks almost the same?

Now to measure the two-tone third order IMD of your transmitter or transmission system including the baseband part, you usually use a spectrum analyzer, but since I do not have one (not currently, I mean), I must be satisfied with analyzing the audio signals converted from RF signals by a receiver, perhaps with a Direct Conversion Receiver, Soft66LC4.

This is not a received audio signal, but a loopback audio signal, or monitor out signal, from IC-7410.

This is the original signal generated by WaveGene. The difference is due to the various level adjustment scheme in the “Windows Audio Architecture” and the internal processing of IC-7410.

Here is a RF PSK31 envelope at its preamble. The modulation level is manually adjusted so that no significant clipping is observed with the baseband loopback signal.

The envelope does not look very smooth due to the various sampling phases.

After adjusting the digital output level for several times, I think there is no clipping now.

The monitored signal from IC-7410 with and without clipping. MMVARI -> USB I/F -> IC-7410 -> (monitor out) -> USB I/F -> Audacity.