The dBm calc

I am sure conversion from mW to dBm is not an everyday occurrence but never the less I wanted to try this blogspot feature of creating your own mini application. So to remove some rust from my programing skills made a simple program in php to help convert those mW to dBm. Named "mW2dBm" is the first app on the right panel of the blog. Formula for computing dBm is: 10 times Log base 10 of mW that is 10 * log10(mW).

Here's the simple code, case you want to put on a webserver. You will need php and to name the file "mw2dbm.php".
It's an image cause I had no time to check how to put code in a blogspot post.
License is GPL, you can do whatever you want.

Internal impedance measuring

Decided to measure internal resistance on the Si570 kit. It's a somewhat impossible mission without proper test equipment, eg: an oscilloscope or similar.
One way somewhat acceptable of measuring the impedance is... to place another impedance and measure the voltage drop. If it drops to half the open voltage then the impedance is the same of the one placed in the circuit. That's a simple ohms law (voltage divider)

Now, you don't need to put an exact value impedance...because you don't know the internal impedance.... in the books an carbon resistor has the same impedance of it's resistance.... in real live if you don't go far on the frequency it's almost the same.

So....measure the rf voltage of your oscilator in open circuit, that will give you "Vosc" then place an resistor on the ouput and measure the rf voltage on that resistor, will give you "Vtest".

The formula for the internal impedance is one of this:

Rinternal = Rtest * ((Vopen/Vtest) -1)

or

Rinternal = Rtest * ((Vopen - Vtest)/Vtest)

You can assume R = Z for resistive circuits.

The formulae are equal just one mathematical pass more on the first one.

or if you are lazy then go to this online app to make some calculation:
http://www.sengpielaudio.com/calculator-InputOutputImpedance.htm

Now my results: 7 Ohm, 16 Ohm and.... -10 Ohm with 3 diferent Rtest resistors....will have to check again my procedure because something is wrong, I should never had a negative inpedance. The diference in 7 and 16 Ohm can be traced to resistor tolerance and measuring error.
Life is not linear!

A box for the Si570 kitt

Decided to put the second Si570 kit in a box, here's the outcome.
The box aluminum was very thin and I didn't placed a protection in when driling so got some marks, nothing that a nice front plate won't disguise.



Doing some house clean up, found 2 multimeter. My first ever multimeter (my father ofering circa 1988) and a digital one, placed a new battery on the digital one and it works. My first one never worked very well in AC voltage range, it went two times for repair under warranty and never returned ok. Incidentally my broken scope is of the same brand. So, just in case, don't ever buy "Kiotto" brand equipment (I think it's some rebranding for local market)

Over 12K visitors

Since I placed statistics online there were already over 12K visitors to this blog, I only hope that means 12K new projects for those who visited this site.

I try to keep the blog on the democratic side...well not exactly democratic because I am the only one who writes and review the comments (there are many spammers around) but I am open to suggestions on how to improve this blog. Maybe more technical subjects, more general news, more nude girls? Leave a comment or send me an email on your ideas.
I am not a native English speaker so I always try my best writing but I am open to suggestions on how to improve communication.

Now let's see the top 10 visitors and the less frequent:

United States - Where the biggest ham radio population lives, so it's logic
United Kingdom - Probably the second bigest ham population
Portugal - This is me reading this fine blog :)
Italy - I wasn't expecting, since I have no 5KW amp project in the blog...:)
Indonesia - Had no idea there were so many hams there.
Germany - Lot of hams there, from what I could see when I went to Friedrichshaffen
Brazil - Os nossos irmãos do hemisfério Sul
Romania - Not expected
France - Oui, ça va bien?
Netherlands - I exchanged some emails with Dutch ham's so they probably see the blog...

Now the less frequent ones...but also important.

Guatemala
Senegal
Netherlands Antilles
Andorra
Maldives - wan't to exchange QTH?
Albania
Armenia
Saint Lucia
Tunisia
Mongolia

Thank you all for stopping by.

Ricardo, CT2GQV

World Amateur Radio Day - April 18

...Today, case you didn't noticed is the world amateur radio day, the date matches the foundation of IARU.

"...Created in Paris, France, the International Amateur Radio Union has been the watchdog and spokesman for the world Amateur Radio community since 1925. The IARU Constitution, last amended in 1989, organizes the Union into three Regional Organizations that correspond to the three radio regions of the International Telecommunication Union (ITU). The IARU Constitution also provides for an IARU Member Society to serve as the Union's International Secretariat...."

Source: http://www.iaru.org

Member societies at: http://www.iaru.org/iaru-soc.html

Some member societies:

REP (Portugal) - Founded 1926
ARRL (USA) - Founded 1914
RSGB (UK) - Founded 1913

73 de Ricardo - CT2GQV

USB SW receiver Si570 based

...that's Uper Side Band and not Universal Serial Bus


Some time ago I started assembly of a future SW receiver based on a 10Mhz IF (there was a 10Mhz crystal promotion at the local electronics shop so I bought 10 for 1 Eur, I think). The idea was also to have some modules in a pcb for making some tests.
Since my second Si570 kit is already working I decided to use it as vfo.

Schematic is not complete (part module diagram, part schematic) but you can see them in some receiver schematics around the net:




DBM is from here: http://www.qrp.pops.net/images/2008/2008-larger/big_dbm.gif
and was made without matching the diodes (1N4148) and with diferent wire sizes in both coils...

Crystal filter and if preamplifier is from Bitx: http://www.phonestack.com/farhan/bitx.gif (see Q3 and the crystal filter) The crystals were match at no more than 80Hz difference from a reference value.

NE602 is...well...is an NE602 runing with an external oscillator.

The antenna used in this quick test was the Mini-whip with the antenna spliter (see the schematic somewere in this blog), nice signals were coming in considering there's no bandpass filter before the DBM.

Video is on 20m band with some French hams chatting. So I guess at least 1200Km range from here (didn't checked callsigns). I was also listening to Shannon volmet station and that acounts for at leat 2000Km. Not bad for some components in a PCB and a piece of cooper as antenna.

UHF Yagi Uda antenna

Addendum, here's the complete antenna (overstatement)...built in 30' with coat hanger wire.
Does it work? Look's so but no QSO so far with Arecibo radio telescope...

Heading to the moon...
Soon as I heard the some crazy guys (KP4AO) are going to frie the moon via EME (http://dxinfo.ea3bhk.com/Special-events/KP4AO-Arecibo-Observatory-Amateur-Radio-Club-on-432-EME.html) I had to do something, at least for some reception.
I like making antennas so here is the oportunity to make one and probably operate something (don't expect that 5w will be enough but since they have big ears we never know) since I never operated SSB on 70cm band... I hope both my capable radios are still working :)

Here's my drawing:


Driver element is feed "live" on the lower side and "ground" on upper side (in the drawing)
and made in #10 copper wire... whatever that means :)... I will get the first wire that look's easy to work and has some style... which is very importan in antenna design

Elements are made of 1/8" diameter which is 0.31cm (3.1mm)

Boom will be made of wooden.

Antenna is made to operate at 432Mhz.... don't try at 20m

Made conversion from "inches" to "cm" based on 2.54cm/Inch, enough accuracy for me.

Original design from here: http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf

Here's the same design in html format:
http://www.fredspinner.com/W0FMS/CheapYagi/vjbcy.html

Here's another with dimensions rounded from the original article and some construction details: http://personal.inet.fi/koti/olavi.vahamaki/LowCost_432MHz_Yagi.jpg

There's small diferences in sizes... I based my design in original article.

Testing was not done yet...use at your own risk