Monday, April 23, 2012

Wilkinson Power Combiner / Splitter For VHF

This technology was first used in the FM broadcasting industry wherein most of the solid state rf amplifiers were combined to arrive a higher power levels.  The Wilkinson network is reversible that means that it can be used as a power divider to split power equally and it can be also used as a combiner. On this project, it uses a pair of Daiwa 80W rf booster (LA-2080) modified for "class C operation" and was only set at 70W output. Note that each rf amplifier should be identical so that there will be no phase difference on the signals arriving at the combiner network. For 144MHz, 4 pairs of RG-59 was used for the quarter wave section and several RG-58 cables for the patch cable used for connecting the radio transceiver to the Wilkinson input and from the Wilkinson output going to the power meter. Note that the RG-58 cables were cut at half wave of the operating frequency.

The 100 ohm resistor in the splitter section is rated 10W same also to the combiner section. During the test, the 100 ohm resistor at the combiner generates some heat and should have been increased in wattage to 50 or more for safety purposes. It is also necessary to match each amplifier and should have an equal rf output. This can be done by tuning each rf amplifier one at a time while keeping the other unit off. Please be aware that when one amplifier is off, the output power of the combiner is just a quarter of the original power output when all of the rf amplifiers are working together.  Once rf output of each rf amplifier are matched then the system is ready for use.





For the input drive of 5 watts I was able to measure 135W output from the combiner network. Please also observe the 100 ohm resistor at the combiner, too much heating means there is an imbalance between the two rf amplifier and should be equalize again.  ---73 de  du1vss

Saturday, October 29, 2011

Icom IC-2200H Adjustment Mode Jig




In order to adjust the power output of the IC-2200H transceiver it must come to "Adjustment Mode" and we need also the adjustment jig. The jig itself is made of 22K resistor connected to Pin7 and Pin2 of the RJ45 connector. When the jig is inserted to the microphone jack, "Adjustment Mode" can be initialized by holding the keys "SET" and "MONI" simultaneously while pressing "POWER ON"
The transceiver screen should look like this;


Adjustment items are displayed in the lower right corner of the LCD and these can be toggled by pressing the "BLANK OPT" or the "V/MHZ" keys. When the transceiver is on "Adjustment Mode" you can now remove the jig and reconnect the microphone. Please refer to the IC-2200H service manual for more details on the adjustment parameters. Note that the power output of the IC-2200H can be adjusted to 85W max by setting the PO parameter in the "Adjustment Mode". When done please perform a "Master Reset" to take effect the changes made on the transceiver.   ---73 de du1vss
Attachment: 2200H.pdf

Monday, October 24, 2011

Turnstile Antenna for FM Broadcast






Above photo is the hand drawn illustration of my own version of Turnstile antenna used for my fm broadcast test station. It is basically two dipoles feed using a 75 ohms phasing harness but intentionally one of the dipole is 90 degrees delayed from the other dipole in order to have a circular polarization. You may find my design differs from the usual turnstile antenna posted in the internet anyway, in my project the two open dipoles is known to have an input impedance of 75 ohms. Feeding the these dipoles using a 75 ohms phasing harness will not transform the impedance therefore two 75 ohms phasing line in parallel will yield 37.5 ohms. You may find the 37.5 ohms a little mismatch with our 50 ohms transmission line but this will just raise the standing wave ratio to 1.3 : 1 which i think not too bad for the transmitter. For those who would like a perfect match, you can include a matching transformer out of coaxial cable but be reminded for the additional loss it would create. To avoid feedline radiation, pawsey stub was installed to each dipole using a #16 stranded wire cut to a quarter wave length of the operating frequency. When computing the phasing harness, include the velocity factor for the coaxial cable used. The signal polarization of this antenna is circular and purposely chosen to minimize the multipath fading. The theoretical gain is only -3dBd reference to a vertical dipole therefore multiple stacking is recommended.   ---73 de hevir


Wednesday, July 20, 2011

Homebrew 30W FM Broadcast Amplifier





The original diagram was taken from the data sheet of Mitsubishi 2SC1946A designed to operate at 175MHz. Both input and output tuning network was modified to work between 88 to 108MHz very well intended for our main purpose. Large heat sink is required since the 2SC1946A must operate at 100% duty cycle. Forced air cooling aids in dissipating the heat produced by the rf transistor. The use of high voltage trimmer capacitor is a must and the use of ground plane technique is best suited for this project. RF input of 1 to 2 watts coming out from the exciter is enough to drive this amplifier to produce 20 to 25 watts of rf output assuming that the DC supply is at 12 volts. Take note that this rf amplifier is operated in class C therefore, the need for low pass filter in the output must be installed to avoid interference caused by the harmonics generated by this amplifier. --- 73 de hevir

Monday, June 20, 2011

Collinear antenna for FM transmitter



This collinear antenna works perfectly after the installation. The antenna is basically two dipoles excited in-phase and theoritically exhibits 3 dB of gain when compared to a dipole. 3/8" aluminum tubing was used here but i suggest 1/2" will do the job even better. The quarterwave stub is made of #12awg stranded wire and is grounded at the center(null). When properly tune, this antenna should have SWR value of 1:1.1  to 1:1.2.   73 de hevir

Sunday, November 7, 2010

Amateur Radio Exam Result, October 2, 2010

 

Feels good to have a new ticket. I would like to give thanks to my fellow hams (DX1CW) who have provided me the reviewers that made possible for me to pass the Class A amateur exam.   ---73 de DU1VSS

Friday, July 9, 2010

Automatic Battery Charger





Here is a simple battery charger circuit intended for 12 Volt gell-type battery. Current is limited by the 7805 regulator IC and the limiting resistor (62 ohms) to approximately 250 milliamperes, anyway most small sized gell-type battery capacities ranges from 2.5AH to 7.5AH so charging time should take several hours. When the battery is full, the regulator adjust its voltage output from 15 volts down to 5 volts automatically terminate the charging process.  ---73 de hevir