G4AKW

The first steps in restoring and updating the old G2DAF linear amplifier commenced with the removal of the 2 x 6U4GT rectifier valves and the associated voltage doubler components / 300 ohm input circuit. This will be replaced with a solid state voltage doubler (probably a 1N4148 diode stack) and a 1:4 unun broadband transformer to give as close a match to 50 ohms as possible. Next step was to solder a 5000 ohm resistor from the 813 anodes to ground. Two 10k resistors in parallel were used (see photo) to represent the load that the valves should see under normal operation. An MFJ antenna analyser was then connected to the pi - tank output and the variable capacitors in the tank circuit altered until a 1:1 50 ohm match was obtained on 80m and on 10m. This checked  the output matching circuit.

This antenna is a dipole, each leg approximately 60 ft long (overall length ~ 120ft), fed with open wire feeder (ladder line).  Photo 1 shows the ladder line (constructed from plastic garden plant staples garden centre stuff is usually good at withstanding the outdoor conditions!), and the feed point insulator. A short length of 12 guage wire is used to take the wire tension and support the ladderline on each side of the insulator. The ladder line spacing, around 3 inches in this case, and its characteristic impedance, approximately 500 ohms here, is not important. (Spacing determines the maximum power handling capability (volts) and wire thickness (current)). The ladder line enters the house and is connected directly to the transmatch by two lengths of URM 67 coax stripped of its outer sheath and braid (photo 2).  These two 2 ft lengths take the feedline through two 15mm speed fit Polyethylene pipes through the wall and into the living room.  The impedance at the point ...

On N900 open App Manager and install OpenSSH Client and Server, and rootsh. Open X Terminal and at prompt type 'sudo gainroot' Set the root password by typing 'passwd' at the prompt Enter the new password for root and enter it again. Note: Above assumes that N900 has acquired an IP address on the wireless LAN. Also worth installing Personal IP Address, bash3 and Enhanced Linux for power users. To transfer files (photos into /DCIM) to N900, on the remote Linux PC; Open terminal and in directory of photo type: (with N900 IP address) sudo scp photo_filename.jpg 192.168.1.66:/home/user/Mydocs/DCIM/ Enter the password for root on the remote PC, then enter root (username) and N900 root password (just set). On Windows PC use winscp (username root, password ****)

As both the remote station pc and the control pc external IP address are dynamically assigned, the first step is to get the address of the remote station computer. This python script does the job in combination with a cron job and associated cron mail output. --------------------------------------------------------------------------- #!usr/bin/python # # Program gets your external IP address # # This program reads the title from # the whatismyip.com website # # set up SSMTP then put the following line in crontab: # 0 * * * * python /home/geoff/python/ipadd3_new.py # # Cron job will mail output to nominated mail a/c # on the hour with the above. # # The ip address is written to file # ext_ip_add.txt (overwritten) # # Version 1.0 Geoff Robinson Aug 09 fileobject1=open('/home/geoff/python/ext_ip_add.txt','r') oldipaddress = fileobject1.readline() fileobject1.close import urllib url = urllib.URLopener() resp = url.open('http://www.whatismyip.com/automation/nxxxxxxxx.as...

It's not a great vertical by any standards, but on the plus side, its cheap, fits in a small garden, could be considered a temporary structure, is dual band by just removing the bottom loading coil and capacitor and.... just add more pwr for more erp. The total height is 43 ft - 10m fishing pole up 3 m off the ground, attached to a 27ft ex - army telescopic mast. Centre loading coil is just above the main mast at about 30 ft - 110 turns of 2.5mm square wire on a piece of waste pipe. This matches the antenna without further components at the CW end of 80m. The addition of a further series loading coil and 1000 pf cap at the base matches it on 160m. Currently with just 4 radials the match is 1:1.4 between 1820kHz and 1840kHz - best DX to date is eastern US with CW and 400W.

After some tests on Topband a few modifications were made. The 40A bridge rectifier failed and was replaced with 50A one mounted on a better heatsink, the problem appears to be the ability to handle the peak repetitive forward current charging the 30mfarrad capacitors -the specs for the rectifier don't show this value - only average forward current i.e. 50A. A front panel transmit/receive switch was also fitted to replace (temporarily) the control from the FT817. The FT817 transmit control line currently does not provide sufficient delay (pretty sure it can be programmed for sufficient lead-in) to ensure the antenna relay is properly made before the amp delivers full power. The final mod was to the microwave oven step/slow start protection circuit. Instead of a 22 ohm series resistor limiting the primary current, the two 50 ohm resistors were connected in series giving 100 ohms which limits the primary inrush current to 2.4 amps (24 amps in the secondary). The protection rela...