My new 2m Power Amplifier

Now that I am using my Wouxun handheld unit in my car, I feel that its 5 watt output is a little on the weak side. Recently I spotted a 2m amplifier on eBay and decided to try a few bids. The device was described as untested but the seller said the lights came on and there was a circuit diagram available on the Internet. I kept my fingers crossed as the minutes ticked away but I eventually won the amplified for just over £30. It soon arrived so I set about making up some new patch leads so I could test it.

This amplifier produces around 30 watts of output from 5 watts in. It also has an adjustable RX amplifier. The unit is made by "Tokyo Hy-Power" and the model number is HL-37Vsx. I first decided to power it up and ensure it didn't do anything strange such as smoke or draw too much current. In its idle state all seemed well. I then connected it to my rig and antenna so I could test the RX amplifier. I found a suitable signal which was a conversation on the local Cambridge repeater. As you will see from the short video below, moving the sliding potentiometer adjusted the signal strength. This RX amplifier can be left at a suitable level or turned off altogether. As I tend to use the Cambridge repeater when mobile, I will probably set this to a reasonable level as it'll be hidden away and not accessible whilst driving.

Next on the "todo" list was to check the SWR whilst transmitting 5 watts into a dummy load. I did this with the amplifier switch off and then switched on. With the unit off the SWR was about 1.3:1 and with it on it was just about 1:1.

The final test was to see if the amplifier was living up to its name and actually amplifying. My transmitter was configured to output 5 watts. With the amplifier turned off and my power meter set to 20 watt fsd, the meter showed an output of just over 3 watts of FM.

I then changed my meter to display 200 watts fsd and switched on the amplifier. The meter showed an output of 40 watts of FM.

The manufacture's specification says I can expect an output of around 20 to 35 watts with a maximum input of 5 watts. With my meter showing an output of 40 watts I am happy that this is probably about right, give or take a few watts. At this point my dummy load was getting rather warm so I completed my basic testing. The next test will be to try some on-air contacts to see that the audio isn't being distorted etc. I will then need to buy some additional coax to enable me to set this up in my car. So far this amplifier seems like a good purchase.

Building a new matching transformer

Having a recent antenna failure I decided to rebuild the matching transformer that feeds my end fed wire. This design comes from the “Emergency Amateur Radio Club” based in Honolulu (http://www.earchi.org). This project creates a trifilar wound 9:1 UNUN (unbalanced to unbalanced) toroid matching transformer. This will match the high input impedance of an end fed wire into a range that most antenna tuners can handle. The designer says it should handle 100 watts of power.

The key parts are: 

• A small plastic box (something that can be made waterproof)
• A powdered iron toroid (EG T106-2)
• Three 20 inch pieces of 22 gauge solid insulated copper wire (in three different colours if possible)
• An RF panel mounted connector (BNC or S0-239)
• You will also need the relevant screws, nuts and solder tags etc, to fit the package together.

I first drilled the box and fitted the connectors to ensure that everything would fit correctly. 

 

Next job was to wind the toroid. As I didn’t have access to different coloured copper wire, I instead fitted coloured sleeves to the ends of each cut length.

 

 

It is important to keep the wires in the correct order and not cross them as they are wound. They should be appear in the following order (left to right), Green, Black and Red. Wrap these round the toroid 9 times as shown in the picture below. I found that the first few windings were a little loose so I added a couple at the end and unwound the loose ones from the start. That way I still had 9 turns but a much better spread and even tightness. After winding, ensure that the wires on the last turn are in the correct order. If they aren’t then they must have twisted at some point.

 

It is important to get the next sections correct else you will have to start again with new wire. Holding the toroid as in the above picture, solder the “Left Black” wire to the “Right Red” wire. I found the best way was to first scrape off the insulating coating from both wires where they would touch. Then twist both wires together with a single turn. This will pull both together and make a sound mechanical joint. Solder the two wires and remove the trailing ends. 

Next you need twist the “Left Green” wire and “Right Black” wire together. First clean off the insulation before twisting together. This joint needs to be attached to the centre connection of the RF connector fitted to the box. Before cutting off the excess wire. Place the toroid into the box to see where the cut should be made.

 

The (Left Red) wire should be cleaned and soldered to the connector on the box that the wire antenna will be connected. The remaining (Right Green) wire should be cleaned and soldered to the outer ground connection of the RF connector.

 

Now inspect the solder joints for any dry joints and ensure the box is free of any solder splashes etc. I used a silicon grease spread around the lip of the box and lid before screwing the two together. In the past I have used a silicon sealant, the type used in the bathroom. If you use this type and it has the vinegar type smell, keep this away from and metal as it tends to promote corrosion.

In the next few days I hope to get up the ladder and fit the unit to my mast. I will take additional pictures and post these in the next week or so. Please ask any related question and I’ll try to assist. Further details including a kit of parts are available from the EARC website: http://www.earchi.org

Matching transformer failure

The other day I attempted to work PSK only to find that I had a very high SWR. My antenna tuner was clicking away but after a few seconds it was clear that something had failed. Up the ladder I went and fort off the accumulation of spiders to retrieve my matching transformer. Once opened, it was clear that there had been some heating and probably arcing. I have taken a picture and indicated the problem area with a yellow arrow. Fortunately I have plenty of spare wire so I’ll wind a new transformer. This will be a temporary measure as I have plans to replace my end fed wire with something less likely to cause interference.

Going mobile

It has taken a while to finally get this far but at long last I’ve put together my mobile setup. I checked the SWR between the antenna mounted centrally on the roof of my car and my Wouxan handheld. It is very close to 1:1 across the frequencies I plan to use so no extra adjustments were required. I plan to build a proper mounting bracket so that I can more easily access the controls of the transceiver. So far I’ve just been listening to see what the reception is like on my journey to and from work. I seem to get a reasonable signal on a couple of the repeaters and the one at Danbury seems to pick-up my signal, even though I’m only pushing out 5 Watts. As long as the reception is acceptable I may look into the possibility of making a 2m amplifier to boost my output.

Here are some pictures of my current in-car configuration.

Sharmans NR-770H and mag mount

I have just purchased a Sharmans NR-770H dual band mobile antenna and six inch mag mount. The build quality of these items appears to be good with no obvious areas for concern. Not too sure when I’ll get some spare time to set these up on my car but hopefully it won’t be too far away.

Digital interface working PSK31 on 17m

Here are some screen shots of my homebrew digital interface working. I am running the Ham Radio Deluxe Digital Master 780 software which provides lots of handy functions. The waterfall display and macro options are far better than similar software I have used. As you can see, I have configured the application to send CQ on the 17m band. On the waterfall you will see that I have chosen a low noise area of the band and well away from the strong signal which is left of centre.

Here is my radio gear listening on the central frequency of the band. My digital interface has been labelled so I don’t need to remember the function of each knob. I recently added the Avair SWR/Power meter in-line to provide an analogue view of these values. The LDG antenna tuner also provides similar information but in a less granular digital way.

I have my rig configured so that the ACL is only just starting to activate. This is keeping the TX power output at around 20 to 30 Watts. Over driving the rig causes some interesting issues which I think are due to RF pick-up on the leads. I need to do some further testing with a dummy load and some ferrite cores to prove my theory. My HF antenna is an end-fed wire which isn’t ideal and could be causing some of my problems. For now I’ll keep the power low so that I don’t cause interference.

USB digital modes interface

The September 2010 addition of the RSGB RadCom magazine, has a simple project for a USB digital modes interface. You can purchase back issues of this magazine from the RSGB website. The article was written by Dave, M5TXJ, and some follow-up information is available on his website HERE. I can’t reproduce the article on this blog due to copyright but here are some images of my construction of this project.

I made the PCB myself by printing the tracks using a laser printer and then ironing it on to a thoroughly cleaned copper clad board. I bought some acid crystals from Maplin and made up the evil concoction in my shed. The etching process took about fifteen minutes.

I was unable to source the relay used by the designer so I used a different type but mounted it under the board.

The USB sound card was sourced from eBay. Because I was using a double sided USB socket in my box, I needed to modify the soundcard to fit in the confined space. The USB plug was easily de-soldered and a short length of ribbon cable inserted.

I’m not a fan of hard wired cables so the box has plenty of sockets.

Here are some images of the internal wiring. I have kept the runs neat and tidy to make maintenance easier in the future.

I found this project easy to build and found most of the components easy to source. The only problem I had was a duff Op amp which once replaced allowed the circuit to burst into life. I hope to upload some video footage soon of this device being used, so keep watching.

Building a dummy load

My first self-build project was a dummy load. I collected various circuit diagrams and reviewed various designs but shown below it the one I went with. The box came from Maplin and the remaining components from Farnell and my junk box.

The load described here is capable of handling up to 10 watts of RF power for a couple of minutes, and is designed for the widely used 50 ohms impedance. It consists of ten parallel connected 560 ohms 1 watt resistors, R1 through R10, a voltage divider, R11-R12, and a rectifier D1-C1. Apart from loading the transmitter output with a minimum of reflected power, the dummy load also provides a direct voltage output to which a voltmeter may be connected to measure the RF power. If the dummy load is used for power levels higher than 10 watts simply use more or higher wattage resistors to give a total of about 50 ohms. For instance, by using twenty 2 watt 1,200 ohms resistors instead of R1-R10 and 150 ohms resistors for R11 and R12, the dummy load is turned into a 40 watt version. The diode may be almost any Schottky type. Types like BAT85 and HSCH1001, for instance, are also suitable. Even a germanium type like the AA119 will work, but then for low powers only.

As illustrated, the ten 560 ohms resistors are soldered in a circle around the centre pin of the BNC socket. Their ground terminals are soldered flush to the inside of the case. Capacitor C1 is a feed through type for which a small hole must be drilled. All resistors should be mounted with the shortest possible lead lengths to keep the reactive component of the dummy load as small as possible. After mounting the parts, the cover is fitted on to the case. Do not drill ventilation holes because that will defeat the purpose of making a non-radiating load. The case may get quite hot when transmitter power is applied for a while but that is no cause for concern. Allow the case to cool between tests.

Avair AV-601

As I’m hoping to go mobile in the near future, I thought it would be a good idea that I buy a suitable SWR meter. I wanted one that would cover both low and high frequencies. The one I went for was the Avair AV-601. I’ve only just unboxed it so once I can find or make a patch lead, I’ll be trying it out. First impressions are that it is well made with no sharp edges or cheap plastic trim. It would have been nice to have had N-Type connectors for the VHF sockets as I’ll have to use a converter. Here is a picture of the new device.

First post

I’ve decided to create a new blog that concentrates on amateur radio experiences. I plan to upload images of my home brew equipment and details of my latest projects.