Sunday, May 31, 2009

Attenuator, RF probe and some matematics

Made an rf attenuator for an radio front end. I decided on copying values from one of the radios that I like, the FT-1000 (no I don't own part of Vertex :P) so I decided on 6, 12 and 18db attenuation because if it's good to Yaesu it should be good to me :)

Went to an online calculator for attenuators at:
and got myself some resistors values... I chose the "Pi" attenuator because.. I like the "Pi" shape...

The values for the resistors are:
Att ----- R1---------R2---------R3
6 - 150.47 ---- 37.35 --- 150.47
12 - 83.54 ---- 93.24 ---- 83.54
18 - 64.40 ---- 195.43 --- 64.40

Schematic is on the pictures or in ascii art:

| |
R1 R3
| |

Then was time to test.... how do they do that ?....

Having brewed an rf probe I though... power on a 50 Ohm load gets smaller if we increase attenuation before gets there.... so applied power to an dummy load and measure the voltage across it in the different levels of attenuation...

In the paper schematic is missing the dummy load that is connected to the output of the attenuator pad's; rf probe measures voltage across the dummy load.

Labels in the various parts of the testing...

If my calculation is right, values are this ones:

For 6 Db I got 6.7, for 12 I got 14.17 and for 18 db I measured 22.29.... honestly for something measured with a frequency generator, and all the rest homebrewed, I am quiet happy with the results....I think the values are correct, anyhow I am open to sugestions.

Formula for relation in dB is => "20 * Log (V/V0)"
My V0 is my value without attenuation to the load.

p.s. There are lot's of online calculators here:
case you need further info.

Sound test of the airband receiver

Just a small video (for those who don't believe my word...) It's not airband (which in fact also receives) but packet on 144,800Mhz


I now must add squelch, signal meter, rebuild the pre-amplifier (not working good) and make a beter bandpass filter....and a box :)
IF of 10.7 whas allready set on the Si570 kit.

From DC to Daylight....

I was looking for past circuits built and found my first buildup and secound and third... it was an power supply (on the left image) an batery tester and an FM transmiter (bellow).

The power supply only worked after I correct the initial circuit that came in a magazine, it was something to do with dropdown voltage for an zener that had the series resistor too low, not giving the output voltage wanted.... these were simple circuits but giving lot of fun and a sense of acoumplisment, they were done by the age of 14/15. The power supply box was also made and was part of a school project for one of the electricity lectures.

The batery tester, red for low, yellow for medium and gren for 13.8v

The FM transmiter bellow, range was around 100m from inside the house and with an small wire as antenna.

Probably will rebuild the power supply, after all it's allready a classic... and the 2N3055 used is more tham 20 years of age!

Saturday, May 30, 2009

Tuna Fish ( Atum )

Tuna Fish or in Portuguese "Atum" it's one of my favorite fish's ! See the pictures for better understanding....

Some years ago I found the "pixie" an simple transceiver with an simple performance but giving a lot of fun to build....

Some had put the litle transceiver in tobaco boxes or "altoides" boxes... I went a litle further and built the whole tuna can's; the "pixie" (in the center) the tuna power supply (near the cat), the tuna speaker (on the right of the image) and the tuna frequency counter...bellow

The counter is from kd1jv and the pixie design came from the net... similar at:

Never quite make an qso with the litle think but sure was lot of eat the tuna fish.

Circuit was built, I think, in 2002.

Cat name is "Pipas", is not mine, and lives now in the north part of Portugal hunting birds and sleeping.

Wednesday, May 27, 2009

New "kit" in town

Got an frequency generator kit from a fellow ham locally (That was king enough to let one part go) which in turn he had got from "Kees"
This kit is based on the Si570 chip and has an LCD, it can be programed to show offset or multiplier (for softrock radios). Mine is an CMOS version.

The idea was to have an stable VFO for the VHF airband receiver.

After powering up and verifying it was on frequency it was time to connect it to the TA7358, first mixer on the receiver.

I sent some emails to people I know have this kit but they only used it on the Softrock receiver without "normal" mixers like SBL-1, SA602 or TA7358, technically it should drive the SBL-1, but SBL's are on the expensive side and I had none in the junk box. So after thinking how to connect and many calculation later I found a way....(was joking about the calculation part). I decided the try and error method.
For the first time it worked at first.... just connected an 50 ohm resistor in series with an 10nF capacitor between clock + output on the kit and pin 8 on the TA7358.

This was on a little spare time I got so no further testing was made, but, I did listen to packet on 144.8 and some broadcast on 104Mhz. If your are trying to duplicate this please don't connect the varicap part on the airband receiver schematic! Obvious I think!

I still can only listen to FM modulation, no AM, I will check further to see what is the problem.

p.s. Found the "AM" problem at all, it's lack of sensibility on the filters (I think). If the signal is strong enough, it copy's good but noise to audio signal relation is low...

There is 3khz difference between the generator and the frequency counter.... so... which one is right ? :) reminds me that one of this days I have make an frequency standard generator for calibration.

Monday, May 25, 2009

VHF - Aircraft RX

Why would you make an aircraft RX when you allready have 2 receivers that can do the same?

..because you can? Because it's cheaper? Because you want get rid of any commercial equipment that in a few years it's more expensive the repair than buying an new one? All of the above!

After searching the net I found an simple receiver that suited me, it's relatively cheap to make, "it should work" just by looking at the schematic from my experience and it uses components that in 200 years should be around :)

This is the design:

After initial problems with the VFO (read: diferent components than the original design) I now have an more or less stable VFO. Also some problems with de audio circuit now sorted. It works but due the VFO nature it's dificult to get AM because of frequency drift. I am now studing the inclusion of a more stable and external VFO. I can get FM due the nature of the design but only stations on high power so theres is sone issue on the gain/bandpass of the bandpass and rf amplificator filter.

The ceramic filters (10.7 and 455) were removed fron the low budget frequency counter radio, so their bandwith is too large ence the AM receptions has no great gain/selectivity relation. The VFO varicap I used it's an BB109 (I think, since there's no marks) laying around in the shack.
The front panel has one rectangular hole that was to get the low budget frequency counter in FM mode but since I allready broke 2 I have to find other "cheaper" alternatives... There's the big (red) an small button for tune and fine tune. I allready left provision for the squelch circuit and pot.

I also tryed with the schematic (basicaly the same) from: see
There are some subtle diferences in the way biased is done on the TA7358 from the posivite line to the 2.7uH choke and on the TDA1072, I think this last schematic (from PY2MG) is near the datasheets for the above IC's.

The box if from an old computer power supply the was broken. These make fine boxes for small projects and are dirty cheap!

Monday, May 11, 2009

Very low budget frequency counter

How to get an frequency counter for 6 Eur, and a bunch of electronic components.

At some point in time I needed an fm receiver for testing conditions for an receiving station for network radio broadcast. So being far from home I went to an Chinese store and bough one of those little radios, still works as off today. Then on the internet I found some articles about conversion of those litle radios to amateur use.... one of this days I went to the local low budget Chinese store an bougt one small radio. Here is the result.

Can you see the 10.7 offset? On AM de offset is 455Khz.

I destroyed the FM input after much soldering an testing on the "frequency counter" and bough another radio, the secound one only lasted the first test. the AM input is working on both...

The power supply are some 1N4001 diodes in series after an 7805 to get the 3v supply by droping in the diode, not elegant but it works.

There are also some nice componetes you get when buying this radio, 10.7 ceramic filter and 455Khz that I used for an aviation band receiver (but they are to wide, better use for wfm), some ferrite bar, some capacitors, etc.

Case you ned the cable connection information I can provide to you altough I don't have all of them maped.

Low budget helping hand PCB stand

One little and simple "project"

The photo speaks for itself...
The design could be improved but more than 15 years of electronics soldering without "helping hands" there's no point for now :)

The "hand" come from some design store and was really cheap.

Tuesday, May 05, 2009


After making the pc programing interface for the TH-F7E I remembered to test against Linux in my Feodra Core 6 box before upgrading to FC10. I remember having tested the USB to Serial cable and not working with my TM-D700 in TNC mode.

Connected the cable to the Kenwood and to the USB plug an called "grig".... nothing.... tested again but powering on the radio after the usb cable, voila.... the bloody thing works... here is the command line "grig -m 220 -r /dev/ttyUSB0 -d 2"

"-m 220" is for model 220 wich is the Kenwood TH-F77, you can list like this:

"....[root@work2 ~]# grig -l | grep -i th

***MEMORY-WARNING***: grig[26426]: GSlice: g_thread_init() must be called before all other GLib functions; memory corruption due to late invocation of g_thread_init() has been detected; this program is likely to crash, leak or unexpectedly abort soon...
217 Kenwood TH-D7A 0.2 Alpha
220 Kenwood TH-F7E 0.2 Alpha
223 Kenwood TH-G71 0.2 Beta ..."

"/dev/ttyUSB0" is the interface created when connecting the usb to serial cable.

"-d 2" is debug level and my version is "grig 0.7.2"

At the moment funcionality is basic but it works, probably with new version of hamlib it could make something else.

My idea was to put the TH-F7 as an scanner connected to the computer to receive some unatended digital modes but I forgot the the audio ouput is used when connected to the computer.... dam!!!