Friday, 18 July 2014

Building a DC to 2.5GHz stepped attenuator

Whilst sorting out some of my late fathers electronics, I came across an attenuator he was planning to build. Unfortunately his illness didn't allow him to start the project let alone finish it. In his memory I have built the attenuator and hope that he would have been pleased with the finished item.

The unit is based around a Weinschel 3007-100 stepped attenuator. It can apply 0 to 10dB of attenuation with an accuracy of plus or minus 0.3dB. It will work from DC to 2.5GHz and handle an input of up to 1 Watt. It has a 50 ohm impedance and should give a SWR of 1.3:1. The connectors of this sturdy unit are SMA. I managed to find some suitable coax and a couple of BNC chassis sockets that would take this rather thin cable. A trip to Maplin provided a suitable box. As the attenuator is connected using coax it was possible to select a plastic box rather than having to drill and file holes in a cast metal one. The hardest part of the build was wiring the SMA connectors. The miniature coax proved problematic on the eyes and fiddly with the fingers. The first cable was a pain but the second one seemed to be going well (may be too well). A quick test with a multi-meter showed that it had a short in one of the plugs. Fortunately I hadn't cut the coax too short so was able to cut it and start again.

After spending a few minutes each day for four days, the attenuator is now finished and fitted neatly in its box. The final job is to print the details on to some paper which I'll laminate and stick to the lid of the box. I had fun making this and hope to try it out once I complete my next project, a digital VFO.

Here are some pictures of the attenuator. I had to change some of the parts my father had sourced as the coax I used wouldn't fit his connectors.

The parts
The first lead completed

The finished attenuator

Monday, 24 June 2013

Building an inverted “L” antenna

The other day, quite by accident, I realised that my end fed HF wire antenna had some issues. For a while now I have noticed that the bands appeared to be very quiet. Where once the PSK frequencies had been busy they now appeared dead. It so happened that I was tuned in to the 20m band but needed to change the antenna leads about. As I started to unscrew my HF antenna the band burst into life. It appeared that when the shielded side of the PL259 plug touched the rigs socket, the signal strength vanished. If I positioned the connector so that just the centre of the plug was used, I could hear lots of PSK signals. I experimented by connecting my 2m antenna to the HF socket on the rig and I could still hear everything. I therefore concluded that my wire antenna had developed a fault. I had been thinking about changing this antenna for a few months now so here was the opportunity I needed. 

I have decided to build an inverted “L” with the aim of making it resonate at one frequency. Although I use a tuner, I’d like to get maximum performance on at least one band. My initial approach was to get something working with the view to cutting it to length at a later date.

I needed to drive in a copper earthing point so that was my first job. I used a section of copper water pipe and hammered that into the ground near to the antenna feed point. I didn’t want to damage the top of the copper pipe so I used a jubilee clip to give some strength and support and then fitted a wide headed bolt to the top of the tube. I rolled some paper around the thread of the bolt so that it was a tight fit. This hammered into the ground quite easily as this area of ground never gets the sun so is always damp. I then connected the shield of the coax to the earth and the centre to my antenna wire via a terminal block. The “L” shape of the wire has a slightly longer horizontal length over the vertical length. For now this is fine until the second stage which will be to tune the antenna. I plan to screw a box to the wall which will consist of a copper back plate connected directly to my earth rod. I will then attach some sockets to this to enable me to easily connect my inverted “L” and any other experimental antennas. For now I have water proofed the earth and antenna connection as shown in the picture below.

Back at the transmitter, I can once again hear the signals I would expect to hear. At the moment the vertical part of the “L” is next to my house which is far from ideal. If this antenna proves itself I will look to feed it from the other end, reducing possible interference.

The earth rod prepared for being hit with a hammer

Temporary connections made waterproof
A view skywards following the vertical path of the “L”

Tuesday, 18 June 2013

Building a sliding mount for my in car radio - Part 5

Just had lots of fun running the 12v supply from the engine bay of my car to a point just under my dashboard. Modern cars really know how to squeeze every last inch of space. After an hours hard work and some cut hands I finally got the cable to the correct location. I was happy to see my rig powering up and quickly grabbed a SWR meter to test the antenna. With an output of 50 Watts I have a near 1:1 SWR across the repeater and main transmission frequencies. A quick audio check confirmed I could contact Kent from the Danbury repeater. My next test is to try out the rig whilst mobile, probably on my way to work. The rig sits nicely on its slide mount and can be easily fitted and removed. I’ll probably run a short length of coax so that when I fix the mag mount I won’t have trailing leads in the foot well. Let me know if you plan to make your own slide mount and need any advice.

Tuesday, 28 May 2013

Building a sliding mount for my in car radio - Part 4

Now that I have my car back after its accident, I was able to attach the fixed plate that makes up the mounting point for my slide mount. I may trim the screws so that they fit flush but they don’t get in the way at the moment. The plate fits well giving a sturdy base to slide the radio onto. When the radio is in position it is very accessible and doesn’t interfere with my driving position or getting in and out of the car. The next  job is to run the power lead from the battery through the bulkhead. The end of the lead will then be stored in the dropdown tray behind the radio. I will then do something similar with the antenna lead so that it is always available at the radio end and easy for me to connect the lead from the mag-mount at the other.

Top of the bracket fitted to the car. I’ll remove it and give it a coat of matt black paint so that it blends in better.
The radio slid into position.

Thursday, 2 May 2013

Building a sliding mount for my in car radio - Part 3

I have now soldered the two channels onto the brass base. I held the channels in place with some bent paperclips which actually did a very good job. I cleaned the brass and applied some flux before heating it. The solder ran smoothly between the parts and made good clean joints. The top part of the mount slides smoothly in the channels and should hold the radio securely to the underside of my dashboard. The next job is to clean up the brass and remove any flux and excess solder.

Paperclips did a good job of holding the parts
All the parts soldered in place

 The finished parts showing the top mount slid into place

Tuesday, 30 April 2013

Building a sliding mount for my in car radio - Part 2

The main part of the bracket has now been drilled and the holes countersunk. I cut down four screws and will be using both flat and crinkle washers. The preformed channels have had one end folded over and silver soldered. The next job is to clean up the brass and position the first channel ready for soldering. I plan to hold the channel in place with some bent paper clips. Hopefully this will do the job without the parts moving about. Once the first one has been soldered in position I’ll arrange the second channel so that the top part of the bracket slides in without binding. After both have been soldered I’ll have the fun of cleaning it all up.

Thursday, 25 April 2013

Building a sliding mount for my in car radio

Now that the weather is getting better, I am looking to fit my mobile radio into my car. I don’t want it to be a permanent fixture as it could be stolen if left in unattended. I have decided on where I want to fit the radio and have devised a bracket design that will allow me to slid the radio in and out. I’ll still have to connect the power and antenna but this will only take a few seconds. I am going to make the brackets out of various pieces of brass soldered together. I have cut out the bits I’ll need and a photograph of these is shown below. My next job is to fabricate the channels so that they are blocked at one end. These will then be soldered onto the larger sheet of brass which I’ll attach to the radio mount. The small sheet of brass will be mounted in the car where the radio bracket will slid over it. When I get some spare time to do the drilling and soldering I’ll post the next set of pictures.