Sunday, December 30, 2012

A 1:1 Balun

Whilst the cobweb is being dismantled and serviced I've put up a 20 meter inverted V dipole, just so I can use the OHR100A which is set for 20M. The dipole is completed with a 1:1 Balun.  

The ugly balun is very simple to build and does the job of removing any stray rf off the outside of the coax and away from the shack. 
It only took me a morning to make and it's from various bits and pieces I had lying around, an odd length of guttering pipe, some spare coax with an SO239 attached at the bottom of the pipe. The two wires from the dipole attach directly at the top of the balun via a choc block.

It's a bit rough and ready but as long as it does the job I'm not too bothered as it will be up 30ft and no one is going to look at the cosmetics!

The cobweb has as previously mentioned been dismantled and is now lying in the shed with the connection box drying off. Once its dry I will add some more silicone and drill some draining holes in the bottom of the box. 

The replacement fibre glass poles arrived but on closer inspection were just too thin to be used to hold the twin speaker cable, so I've found another supplier and re-ordered. 

The wooden poles are actually in not too bad a condition but I would prefer fibre glass to add strength and to make the antenna lighter. The new poles must be near to an inch in width to fit the metal sleeves and will be hollow therefore lighter.

Wednesday, December 26, 2012

Servicing the Cobweb Antenna

Finally I've got round to begin the service of the home-brew Cobweb. Taking it down from the mast wasn’t as bad as I thought, I decided to do it now as the weather forecast is not good with high winds expected over the next few days, so while there was a break in the weather I zipped up the ladder and began the removal process.

 Once down the initial inspection was that it didn’t look too bad considering it’s been up on the mast for nearly two years and been subject to some pretty bad weather conditions. A bit of rust here and there on the metal tubes that hold the wooden poles, but perhaps through luck or my building capabilities it has managed to withstand all that has been thrown at it! 

  The wooden poles and the box are covered in mildew and slightly bowed, but still sound, the cables are fairly loose which I suspect is the main reason my SWR has been out, especially on 20 meters which at the last reading was 2.5. Interestingly I thought the box would have been pretty rusty inside but surprisingly looks very clean apart from a good ½ “ of water slopping inside, first thought was hmm, I forgot to drill any draining plugs!

 The wooden poles are going to be replaced with fibreglass, I managed to find some ex army surplus which should do the job nicely but I will have to wait as I’ve only just ordered them so it will take a few days to arrive. Since the main box is in such good condition I will do little except give the outside a good scrub and more importantly drill a couple of draining holes. 

The silicon grease has done its job and kept the contacts all clean and rust free, I'm amazed there is no rust on the copper wire or the steel screws. Originally I bought a Cobweb (back in the 90s) for the grand sum of £160 and when I sold it I remember inspecting the box and seeing how badly rusted everything was, so I was expecting the home-brew to be the same, just goes to show what a bit of silicone will do to protect.

Over the next few days I will tighten the cables and recheck the SWR and if needs be replace any that are suspect, luckily I have some spare cable that I got last summer from Maplins, (our version of Radio Shack). 

Looking inside the box it is a remarkable simple system that is very easy to build and yet gives great results. This is easily the best antenna I've ever built and quite honestly if I can build one anyone can. If you want further information look at the Cobweb Revealed website or alternatively please ask and I'll be happy to help where I can (but I warn you now I don't understand the theory, hi hi). I will include as many picture as I can during the service process so you can see just how easy it is put together.

Friday, December 21, 2012

OHR100A Completed

The OHR100 A is now complete, a few QSO's later and I must admit I'm pretty impressed with this little QRP radio especially on the receive side. The Bandwidth takes a little bit of tweaking to get it just right but works well and can quite easily cut out the clutter from other nearby stations. So far I've had some good reports with stations around Europe and I have no doubt that I'll be able to contact across the pond with my 7 watts of power.

I've managed to get hold of a frequency counter off Ebay which has made a big difference, I don't have to guess where I am within the CW side of 20 meters which does speed things up a bit when searching for QRP stations. 

Inputting the IF offset frequency took a bit of thinking as I'd never done it before, but after reading and re-reading the instructions I eventually got there and had it all set up. The casing is a bit scruffy, but I'll give it a lick of black paint or some Hammerite to tidy it up a bit. I fitted an internal 9v battery and also added a small on off switch to save power when not required. It does the job nicely and I amazed how quickly I built it all up, another thing 6 months ago that I wouldn't have had a clue at doing!

I eventually gave up on the old TH-215 E and went shopping on Ebay, I managed to pick up a TH-22E for a few quid. It's an excellent little radio and works well when connected up to the Yagi, only 5 watts but that's all I really need as I'm 750 ft above sea level so can get out pretty well on low power.

The next project I must get round to (having said I was going to do it back in the Summer) is to take down the Cobweb and give it a good service. The SWR on 20 meters is up at 2.5 so I'm having to use the ATU all the time now for the RMs and the OHR. It probably just needs tightening, but I'm going to open the box and give it a good clean and re-grease the connections, after all it has been up there for a long time now. I'll take some pictures so you can see just how easy it is to build one!

I've got 10 days off over the Christmas break so I shall pick a good day weather wise and do the work! Also over the break I am going t have a good think about what to do next radio wise, another kit perhaps, or maybe an old transceiver that needs a bit of TLC? Well see what happens in 2013.

On that note May I wish you all a Merry Christmas and a Happy New Year.

Thursday, December 13, 2012

OHR100A Build Part 2 (The Chassis and Alignment)

After re-checking all the soldering and making sure there were no obvious mistakes, I started on the chassis, firstly installing the SO239 socket and then attaching the optional ten turn VFO tuning pot followed by the AF and RF gain switches and the Bandwidth and RIT switches. The wiring instructions were fairly easy and I soon had everything connected up and ready for a smoke test. 

 All ready for the smoke test

The only problem was I didn't have a 13.8v power supply! The RockMites run off batteries and the FT1000 MP V has its own separate power supply. So on to Ebay and I quickly found an old but unused power supply for the princely sum of £15.

Two days later (a miracle in postal terms) the PSU arrived and after wiring in a 1A fuse I was able to carry out a smoke test. Do you ever have that feeling of dread, fingers on the switch wondering if you really will get smoke or just nothing happens? I must have been waiting for half a minute before I pulled up enough courage to switch the transceiver on. I turned the switch and bingo, the radio came to life ....what a relief!

Once I'd got hold of a power supply it came to life!

A quick play on receive and I found some cw, turning on the FT1000 MP and matching the incoming signal I could see that I was tuned in to 14.045 with the VFO fully turned anti clockwise, so I wasn't far off, the VFO toroid just needed some tweaking which was confirmed when I attached the MFJ 259 and using the frequency counter read that I was at 23.400MHz, just off the mark as the instructions quote 23.000MHz.

 So all I had to do was alter the gaps between the windings on the VFO toroid L114 to bring the frequency down to the required 23.000MHz as per the instructions. At the same time I had to tweek the caps for maximum power output between 100mv and 400mv, but this was fairly easy using my DMM. Once happy with the toroid readings it was a case of pinching some of my XYL's clear nail polish and giving the toroid a good coating and leaving for 24hrs!

 The toroid at L114  that needed a tweak

I must admit the hardest part I have found with this kit is the alignment, I've had little experience of this process but with the help of the instructions I managed to get through but it took time. You have to have a frequency counter and a DMM for this project and to be honest I'd never used the MFJ as a FC so it was all new to me and there are no instructions in the MFJ manual about how to set it up for frequency counting. In the end I attached a short piece of coax fitted a PL259 to one end and trimmed the earth off the other end leaving just the central wire. By chance I had a very narrow metal tube that fitted over the wire giving it strength and hey presto it was an excellent probe.

 The homebrew probe attached to the MFJ

The radio is now virtually complete except for the optional LED that I have to fit. I've had a good play but I haven't attempted a QSO yet, hopefully this will be done over the weekend. It's putting out a healthy 7 watts on the dummy load, but I've reduced this to 5 for QRP. First impressions are that the receiver is excellent especially when using the bandwidth, it certainly matches up to other radios I have owned. The 10 turn pot although smooth gives you little idea where you are within the CW sector of 20 meters so I suspect I will purchase the OHR Digital Frequency counter to make life a little easier.

Overall I'm really pleased; I've come along way since the early days of the RM builds and my confidence as a QRP builder has grown dramatically. Reading the reviews on Eham, the kit is probably classed as an intermediate, so hopefully I've passed with flying colours and now can look towards building my ultimate homebrew kit, the K2 !! 

In the meantime I shall enjoy working some QSO's with my trusty new OHR100A. Once I've added the LED I will post some pictures of the completed Radio.

Sunday, December 9, 2012

OHR100A Build

Finally the OHR100A arrived but in the end I had to nip down to our local postal sorting office to collect it as they were complaining about how busy they were and it was going to take time to deliver. 

The parcel was well packed with individual sections of the kit separately kept apart which made things easy for identification. Marshal certainly makes sure of doing a good job in packaging as everything is nice and secure.

So the build of the OHR100A begins. To start with I thoroughly checked the inventory to make sure I had everything; I ticked off each part as I went and seemed to be all there so I could get on and make a start. I don’t plan to rush it; I’m going to take my time and build in roughly 2 hour slots so that I don’t over do it and make silly mistakes. The first part is to install several diodes and the IC sockets making sure that all are correctly positioned on to the PCB board before soldering.

The instructions seem very clear, certainly as good as the Wonderlabs notes which helps greatly when a newbie like me is attempting something a little more difficult than the RockMites. Amongst the instructions is an overlay sheet with clear diagram of where each part should sit which is very useful.

6 hours later and I’ve installed the diodes, chips, chokes, resistors, trim caps and capacitors, the only problems so far have been a slight issue with soldering as I’m having to re-apply a small amount on a few components to the top of the PCB board as the soldering is occasionally not penetrating through the holes, this is more likely because of the solder I’m using but in actual fact probably helps in double checking that everything is soldered correctly.

Approximately 9 hours later and I've all but completed the PCB board. Just the IC's to slot in to place and add in the Molex connectors. The toroids were a little fiddly but I think that is probably due to lack of practice and experience, but once I had done the first the others seemed to wind OK. 

Interestingly in the picture below you can see my one mistake that I had to rectify, which was placing the transistor D105 in to U104. As soon as I done it I realised my mistake but then had the job of carefully removing the transistor and then soldering it in its correct position. The one thing I've learned is that as soon as you feel at all tired take a break because that's when mistakes are made!

Now I have to assemble the chassis with all its components which hopefully won't take too long and then just wire up and do the smoke test.......More to follow!