Analog Computing

I recently had the pleasure to visit Prof. Dr. Bernd Ulmann, leading Guru of Analog computing, an avid collector of aged computing devices and electronics paraphernalia. I had an older HP Spectrum analyser which  I wanted to donate and I was invited to see the beautiful collection of analog computers.

Unlike the recent devices that all of us use, an analog computer does not execute algorithms in a step-by step fashion but uses precise analog circuits that are wired to solve the problem in question. Interestingly, for many kinds of problems this approach is more suitable than digital, sequential processing. Calculations happen in real time by definition and parameters can be varied by the turn of a potentiometer, lending a tactile element to IT. One area of interest is the simulation of dynamic processes such as the control of a self propelled projectile or the suspension of a car. In fact, the A4 rocket was controlled by an analog computer called the “Mischgerät”.

Pictured below is a RA770 computer by Telefunken

There are some youtube videos detailing the systems operation:

Bernd also owns plenty of more recent digital computers, such as the lovely HP9820 (aka Model 20) on which I learned programming.

In case you are interested in these areas, check out his websites at: (more on the digital side of things)

and (the analog systems)

I can definitely confirm, the man has the knack! It’s time to get him into ham radio.

RF Field Strength Indicator

Here is the latest toy that I finished. It is an active RF field strength meter. So far, I had a passive one (a detector which feeds into a microamperemeter). This one here has an op-amp stage plus a nifty VCO that outputs a sound that varies in pitch with the strength of the signal that was received.

The circuit follows the one published in Funkamateur 10/2010 by DK6UU. The circuit contains a bug as that is is published with the wrong type of chip (74HC4046 should be: CD4046).


Endfed halfwaves (EFHW)

Some years ago I began to experiment with endfed halfwave antennas. My friend Eric WL7CMT suggested many years ago that I experiment with those. As this is an antenna which is a bit off the beaten track, I did not trust it. Then I built one for 20m and was surprised!

The one shown in the video is matched with a low pass, now I built one using a broadband ferrite transformer using two stacked Würth Ferrite cores (2 primary windings, 15 secondary windings, primary compensated with 110pF). The core material is a bit like Material 43

The results are not in yet but I am under the faint impression that the low pass is more efficient than the transformer. Certainly, using a single core drives the same into saturation.  FT140-43 would be the traditional building material.

Another insight was that two primary windings are more suitable for high frequencies than one. It is difficult anyway to get the cap to compensate for the primary inductance. I might add some screenshots of the VNA plots.

Hamfest Score

I recently was so lucky to run into a gentleman selling two vintage Fluke differential voltmeters, a Fluke 891 (DC only) and 893 (AC and DC) for 10 Euro each. What could go wrong?

Both instruments were in a fairly questionable state so I took them through a thorough cleaning cycle and re-calibrated them. All knobs were rinsed ultrasonically and one of the meters had to be disassembled and oiled to bring it back to life. I could not find a manual for the 891 but since the 893 is practically the same instrument, all the information is there.

Luckily I had seen W2AEWs excellent Video review, so I knew what to expect.

What a fabulous instrument! How amazing is it to have an instrument which is almost as old as myself that can measure a voltage to the nth decimal digit?

What had to be done was:

  • Thorough cleaning
  • Fixing the meter on the 891
  • Recalibrating both instruments
  • Adding a power cord plug to the 893

SE6861 Manpack

I met this Gentleman, Benno DK7GB, recently during Hamradio in Friedrichshafen and we found that we both own one of these fabulous SE6861 radios.

Mine, I was able to score at a very decent price at a tiny hamfest. Interestingly my best hamfest bargains were found at the smaller venues, not the big ones. Anyway, all that was needed is to replace the long gone battery packs with new ones.

The new ones are 3500 mAh cells that I found on eBay (and which surprisingly really hold that capacity). They were soldered together, arranged on a piece of FR4 fiberglass board, wrapped in heatshrink tubing and attached to the bottom of the battery compartment. Power is supplied to the radio through standard 4mm sockets. The charging circuit was slightly modified by increasing the charging current to the battery to 350 mA (for a C10 charge). Charging power is supplied by an old Dell laptop power supply with an attached Mil power plug (that alone has cost almost as much as the radio)

The entire modification is fully reversible.

The radio itself is nice to work with as it is fully self contained, runs USB, LSB and CW with 20 Watt, has a lovely antenna tuner and can tune any wet string.