Wednesday, December 26, 2012

Linux and the future of desktop

These days I have been reading articles and opinions in the net about the new Windows 8. Why? Because Windows 8 is the starting of the end of the desktop as we know it. Windows 8 is the first step in Microsoft roadmap to unify all the current platforms (Home computers, tablets and phones) into a single interface.

But using the same interface into so very different devices means you can only use the lowest common denominator of all of them, in this case, phones. And this also means you must use your desktop computer in the same way you use your phone.

Tuesday, October 30, 2012

24GHz the old way

In the 80's wide band FM was common in the 10 GHz band. When 24 GHz devices were available at surplus, wideband FM was overridden by narrow band modes (SSB), so there was very little wide band FM activity at 24 GHz. Even today there is almost no wideband activity in any microwave bands (Except for some ATV signals). Why?

Monday, October 22, 2012

Dissection of Icom FA-S6270A antenna

In this photo you can see a dissection of a quad band Icom FA-S6270A rubber antenna who suddenly stopped working:

This is the antenna supplied wit the Icom IC-T81 quad band walkie talkie. The symptoms were very evident: The antenna suddenly become deaf, giving very poor signals, so it was sacrificed in the name of science.

Tuesday, October 9, 2012

Looking for the Square Law region (II)

After the experience in Looking for the Square Law region (I) it was clear the only way to find the Square Law region was to find a way to measure microvolts. The first idea was to use an operational amplifier in a DC non-inverting amplifier with a gain of 1000 (actually 1001). This will allow me to measure microvolts as millivolts. My first try was a OP07, but input bias currents were too high, so I ended using a JFET operational amplifier, a TL081.

Sunday, October 7, 2012

Looking for the Square Law region (I)

One of the basic tasks in RF work is to measure power. There are many ways to measure power but the simplest ones use a diode as peak rectifier. Peak voltage (over a known impedance) is a measure of power, because P = (Veff)/ R and Veff = Vpeak x √2. Combining both expressions you get P = (Vpeak)2 / 2R, where R is usually 50 ohms.

Thursday, September 20, 2012

Using mini iPhone chargers with Android devices

It's possible you have find these little USB chargers in eBay and similar places. They are very cheap, you can get one even under one euro (shipping included). But, Can they be used with your Android device?

Wednesday, September 19, 2012

What means "guaranteed" in a datasheet?

This last summer I spent some time rebuilding my workbench power supply. I chose the LM350K regulator as the heart of the power supply because its 3A guaranteed output capabilities and short circuit protections. I bought two units, just to have one lying around in case something went wrong.

Wednesday, September 12, 2012

Using the V2000 triband antenna on 70 MHz band

During the first year the 70 MHz band was authorized in Spain some spanish hams were active in the band using PMR transceivers in FM. Curiosity made that I connected my IC-91 walkie with general coverage receiver to the V2000 antenna just to see if something can be heard. To my surprise, I was able to hear a lot of G stations in the first days, with some locals also. Signals were quite strong, too strong for a no 70MHz antenna so I stated to wonder if the antenna could be usable on 70 MHz also.

Sunday, September 9, 2012

UGP: Interfacing Geiger tubes to computers in a easy way

Universal Geiger Platform (UGP)

Sooner or later, any geiger user feels it's time to connect a Geiger tube to the computer to do some data logging. Some Geiger counters has computer interface, but most handheld or pocket Geigers does not have any way to connect them to a computer. Let's see how to accomplish this task:

JDM programmer

If you look in Internet for a PIC programmer, you will find tons of pages with info about homemade programmers. But you must be cautious. Most programmers were designed for the 16F84/16F84A microcontroller, and today, the 16F84 is considered an obsolete device. So if you want a working programmer today, you must look for one capable to program current PICs.

The M3FD19 photomultiplier

If you are reading this, I'm sure it is because you have brought a M3FD19 photomultiplier on eBay, but you can't find its pinout. Or maybe you have seen a cheap M3FD19 photomultiplier and you are wondering if it is useable for your experiments.

Detecting alpha particles with a modified webcam

After some interesting messages in CDV700Club, I decided to do a quick experiment with a spare webcam. Is it possible to detect alphas with a DRAM memory? What about other devices?

α, β and γ homemade Geiger counter with UGP interface

With the principles shown at UGPplatform / protocol / program, I build this simple but effective Geiger counter:

One Gigaohm high voltage probe

Measuring Geiger's power supplies is a difficult task, because usually the voltmeter is a huge load to the supply, therefore the measured voltage is unexpectedly low. For a good and realistic measurement, the used voltmeter must have a very high input impedance.

Wednesday, September 5, 2012

Radex 1503 sound mod

Maybe you have noticed the Radex 1503 beep is far too loud to be comfortable. It is ok for alarming, but very uncomfortable if you want to have audible indication of the counts.

Repairing the Radex 1503

After more than two years of almost continuous operation, I started to notify my Radex 1503 reacts to vibrations and fast movements. It was very noticeable while operating the counter in a car, and later I noticed that shaking it, specially in its vertical axis, the effect becomes obvious, getting readings in the 300 - 400μRem/h in just a few seconds. So it was clear there was a problem with my unit.

Radex 1503 review

The Radex 1503 from Quarta is a low end digitally controlled Geiger counter. I acquired mine from annakozub seller on ebay for about 100 euros. It is a very small unit, only 6 x 10.5 x 2.5 cm, very lightweight (90 grams without batteries) and fits perfectly in your pocket. It's perfect if you need something more than a "beeper Geiger".

DRSB-88 review

The russian DRSB-88 Geiger counter is maybe the cheapest Geiger counter you can afford. I received my unit just for under 20 euros including shipping from annakozub seller on ebay. It is a very small unit, only 13 x 3.5 x 3 cm, very lightweight and fits perfectly in your pocket. It's perfect as a first Geiger counter.

Visual MS

This article was created in 1997 and I have published it here again just for historical purposes. This was the starting point of a road that finished at the fantastic WSJT software.

Tuesday, September 4, 2012

KP4AO: Our first EME experience

On April 17, 2010 a small group of operators (EA4AAE, EA4EOZ, EB4EMM and EB4GLE) prepared a quick and dirty station to "try" to work KP4AO via moon, who was operating the impressive 300 meters radiotelescope at Arecibo.

Playing with Gunn diodes

You need to know how Gunn diodes work if you want to get the most of them. Gunn diodes are devices from the 70's but they are quite useful these days. They are the cheapest and maybe the easiest way to get some microwave power. This is the test I made to one of these units trying to characterize its behavior.

LNB and its ham radio usage

The letters LNB means Low Noise Block, but we must call it LNC, this is Low Noise Converter, because a LNB is only a receiving frequency conversor. These LNB-LNC have become quite popular in roofs and balconies, and can be purchased new as cheap as 5 euros. They are quite interesting from the point of view of a ham radio experimenter, because they are the cheapest and quickest way to get into the 10 GHz band.

Recycling the SilverCrest WE-2100 digital dial

The SilverCrest WE-2100 is a cheap portable radio with digital clock available in some shops for a price about 10 euros. In Spain, it have been sold by LIDL. This radio is based on the Sony CXA1691BM circuit and includes a clock/dial digital readout based on the CD3610CR circuit. The radio features FM, MW and SW bands.

The noisy PC power supply

Some time ago I was experimenting with indoor HF antennas. My goal was to receive some digital communications. But as soon as I started to experiment, I noticed a big S9+ noise in the receiver, so the combination of indoor antenna and S9+ noise makes the experiments useless.

Yaesu VX-2 jumpers

This walkie is sold without the wide band receiver enabled, this is, only 144 and 432 MHz band. This is really a problem with a walkie that has a complete receiver from 500 kHz up to 999 MHz. It is easy to find in Internet how to modify it, but most modifications are "free band" or MARS and sometimes this is not preferred, because you usually lost functions, like Automatic Repeater Shift, or even a completely different frequency coverage.

Technical considerations about Yaesu VX-1r

The handheld is about 10 - 15 kHz off frequency

This handheld has a little mechanical problem: The speaker touches physically the small diode shown in the photo. So with a fall-off or a big shock, the diode could be broken or get de-soldered.

Converting a Yaesu NC-29 charger to 220 volts operation

In a recent transaction, I got an american version of the well known Yaesu NC-29 quick charger. Logically it was constructed to be operated at 125 volts AC (to be more precise, at 117 V AC). Here in Europe we use 220 volts, so the charger stayed some days in the junk box. But the charger would be very useful to me, because I had some Yaesu batteries laying around, so I decide to change the charger's internal transformer to a 220V version.

Technical considerations about Yaesu G-250

The Yaesu G-250 is a very common rotator in small size VHF and UHF stations but this rotator has a little problem: Sometimes, the rotator starts to oscillate quickly left and right from the settled position in the indoor unit. Those oscillations are very dangerous, for the rotator itself and for the antennas. The only cure to this is to turn off the indoor unit when not in use.

FT-817's noise blanker

The FT-817 is one of the most popular portable rigs. In the size of a notebook, there is a complete multimode rig from 160m up to 70cm. But nothing is perfect in this world, and the Yaesu FT-817 noise blanker circuitry is one of these things

Modifications to the Diamond V2000 and similar antennas

This kind of antenna has grown in popularity over the last years because it gives you a decent performance and triband capabilities. But its 50 MHz design is far from optimal. Here you can learn how to improve its 50 MHz performance in a very easy way.

The amazing portable all-band receiver

Some time ago, I was searching in Internet and I found this interesting link:

I have used many times in the past diodes as unconventional RF detectors with great success, and sometimes, connecting the diodes to audio amplifiers many interesting things was heard. So after reading The Amazing All-Band Receiver, I decided to construct my permanent portable all band receiver.

Handheld 2.4 GHz Spectrum Analyzer

After visiting some web pages about 2.4 GHz ISM band spectrum analyzers based on the CYWM6935 module, I tried to build my own analyzer, but with some improvements. The references I found on the net used the parallel port or a serial link to a host computer. I want the analyzer to be portable, and easy to transport, so I will use a microcontroller and a graphical LCD.

I had also some old Nokia phones, so maybe I can use phone plastic box and build in LCD to make my own portable analyzer, but... Does it can be done? Let's see:

The Turnstile antenna

omnidirectional and horizontally polarized VHF antenna

Some time ago my antenna rotator got faulty and I replaced temporally a 9 element Tonna with a small Diamond X-50 vertical antenna. This antenna works nice, but is very limited in tropo VHF DX work because its vertical polarization. Of course sometimes 2m opens widely and you can work some tropo DX with a vertical antenna, but of course it's not a every day occurrence... neither a every year occurrence.

Once inspected, the rotator was dead, so I decided to try a small horizontally polarized omnidirectional antenna. There was some antennas of this kind available, so I started a small research to decide what antenna to build.

144 MHz Halo

This halo antenna was built to have an omnidirectional coverage while working in VHF contests. The idea was to have a small but useful antenna for receiving, but it has demonstrated very capable on TX also.

Construction of OZ2M 70 MHz transverter

In this post, you will find, in a blog-like way, my experience mounting this transverter for the 70 MHz band.


A simple field strength meter

This circuit is maybe the simplest RF sniffer you can find: Only three components: An RF choke, a germanium diode and a microampmeter.

137 kHz antenna preamplifier

One of the principal problems an amateur operator finds when tries to receive in the 137 kHz band is the really poor performance on typical amateur transceivers. Almost any modern transceiver tunes down to 500 kHz, many also tune down to 150 kHz, and some of them tune under 100 kHz.

Build your own HF balun

A balun is a MUST for dipoles or similar antennas when they are feed with coaxial cables. Many hams connect the center conductor of the coaxial cable to one side of the dipole, and the shield to the other. Wrong!

Welcome to my blog


My name is Miguel A. Vallejo and this is my new blog. I opened this blog as a successor to my old website ( ) with the idea to have a more interactive tool.

My main hobby is electronics. Another related hobbies I have is ham radio ( licensed since 1991 ) and observational astronomy since I was a child. In electronics, one of my favourites fields is detection, with a special emphasis in radiation detection.

I have worked also with microcontrollers ( 8051 family and currently ATMega family, mainly ATMega8 ) and in the PC I program in some languages, like Delphi, Java, Lazarus ( FPC ), C ( GCC, MinGW ). I have had also some incursions in Android programming ( HamGPS ).

As ham radio operator I'm interested in VHF and higher bands, including microwaves and optical communications, although I have made some things in HF and LF/MF bands.

The first posts in this blog will be articles currently in my web page with some additions and upgrades and then, the normal cycle of life of a blog.

I hope you will find useful the information contained in this blog.

Miguel A. Vallejo, EA4EOZ