Amateur Radio Homebrew (Shack Culture)

[PanBiker]

I ordered my 250W resistor on Saturday and when I checked with the supplier although the description said the item was in London the delivery date given was 27th June. This led me to believe it was actually coming from China, it's country of origin!

I had a tracking number from Yodel this time so I punched it in on Monday and it said that the item was waiting to be delivered to Yodel. This reinforced my view that it was coming from the other side of the world. I received an email though this morning that said my item was on the van and due to be delivered tomorrow. Knock on the door at 2.14 this afternoon and I have it in my hand, it's come from Birmingham which I suppose is another outlet for the importers.

Baking this afternoon so I will put a picture up later of the little beast.

[PanBiker]

OK here is the resistor pack, just 1" wide including its integral metal heatsink mount. Amazing that a package as small as this can dissipate 250W. I have no doubts as these same devices are used in Bird Through Line watt-meters which are regarded as top line equipment.



Before we go any further drilling mounting holes and such, good practice says it is best to test the component before installation. My Beckman digital bench meter shows it reading 50.3 ohms, near enough for me.

[Stanley]

Amazing how components have shrunk in my lifetime....

[PanBiker]

As resistors go, small as this one is, it is actually massive compared to the lower power surface mount components. Lots of these are no more than a couple of millimetres in size. Some semiconductor components are now "printed" and actually integrated directly onto the surface of some printed circuit boards.

When I was TV servicing a 100W resistor or dropper stack used in the valve based TV's for multiple voltage rails would be between 6" to a foot long. If one section of it burnt out you chopped out a section of the winding to make sure it was open circuit then attached a replacement high wattage wire wound resistor between the section terminals. My 100W instant heat Weller soldering gun came in handy for jobs like that, you generally had to use high melting point solder for the connections as they ran at such high temperatures. You needed clean contacts and a lot of heat to get that stuff to run, it wasn't flux cored either you had to use separate flux from a tin or tube.

[PanBiker]

This is the external profile of the extruded box section. I need to drill two mounting holes through for the resistor pack. The diameter of the drill needed is greater than the gaps between the flutes so without the benefit of a pillar drill and drill vice the bit is going to jam with a hand held drill.



I decide to start with a countersink bit in order to clear the flutes back a bit, I ran the countersink down to the base of the flutes then drilled the clearance holes that i needed for the mounting screws. The screws are pan heads so I followed the first hole with a bit slightly bigger to open up the base of the countersink. This allows the screws to sit down a bit further in the base.



I mounted the resistor with a thin film of heatsink compound and secured the 3mm screws with nylock nuts. The heatsink of the resistor package is connected to one end of the resistor. The centre tab which is the other end of the resistor is connected to the centre pole of the SO239 using a short length of solid 2.5mm mains cable. I installed the resistor pack with it's tab away from the socket to give a bit more room to solder up the connections. Here it is looking inside. I checked on the meter again that I have not damaged anything using my 100W iron for the joints.



Job done apart from additional internal heat shunting material. The inside void available is essentially 1.5" high x 2.5" wide and a maximum of 2" deep from the back of the case to leave a sufficient gap so as not to foul the "live" side of the resistor pack. Any additional metal does not need to be in one solid piece as long as components are securely fastened to create a thermal link to the outer case.

[Stanley]

Good trick with the CS bit!

[PanBiker]

Needs must Stanley. This is one project that doesn't really need to be pretty just robust and functional. It will only get occasional use as a piece of test kit. I have done a tentative test of the unit from my handheld transceiver which puts out 4W on high power. I shoved a connector adaptor on its micro antenna port to convert it to SO239. Then used a line coupler to attach it to the dummy load. I stuck my finger on the top of the resistor when I put the handheld into transmit. As expected, no detectable warming of the component with full power indication on the transmitter. In this test x 62 overkill, I would expect nothing less.

[PanBiker]

I have passed on my project to Stanley today with a fag packet drawing. This is to make and fit some internal cooling vanes.

[Stanley]

I must be slowing down a bit because even though it is a good drawing I had to have it explained to me. Hardest part will be identifying the threads on the tiny pan headed screws he has given me!
It will be a nice bit of distraction therapy from the valve gear. Ian agrees with me that the drawing is a nightmare!
But first I must cook veggies!

[PanBiker]

I bought a whole range of standard metric thread screws, nuts and washers for the fastenings on the motorbike. As far as I am aware they are bog standard metric thread. I think the pan head ones I supplied are 3mm. I have matching nuts if that would be any help?

[PanBiker]

Furthered the dummy load a bit more by modifying the side panels for the case. I marked one side up then bolted the two sides together using the corner fastening holes before drilling them out for more ventiation to the case. I ran the countersink bit over them to break the edges. When I get the main unit back from Stanley I may put some holes in the top as well, there is a flat panel section that will be easy to drill. It will depend where the internal cooling fins are fastened as to where I put any extra holes.

[Stanley]

No need for the nuts, I'll manage something Ian.

[PanBiker]

I collected my modified dummy load from Stanley this morning so it's back on my bench for the finishing touches.

Unit with extra heat-sink material installed.



There is scope on the small flat top surface for a few more ventilation holes.



Three more holes for extra air circulation.



Side panels on and it's now complete.



Thanks again to Stanley for his metalwork skills.

[Tripps]

Looks like a really nice job - well done both.

Could do with one (and a fan ) for my mobile. I've been using it for Skype calls, and it gets worryingly hot after only about ten minutes. I take it out of its case for calls now and it feels better. Google says it's not uncommon.

[Stanley]

Don't even think about it David! I have an engine to finish.
Ian, keep your eyes open for a nice piece of hardwood 7" X 15" X 3/4" Finished, I'll be wanting a base for the next beam engine in about 12 months and I have run out of wood....

[PanBiker]

Mentioning the HB9CV design of antenna over in the Rolls Royce A R Club thread has turned my attention to a potential next project. I have some of the solid aluminium rod left over from my last antenna build which was the Moxon design. I have tested it in anger and unfortunately it is slightly out of tune on the portion of the band I want to use it on. It's functional but not ideal. The problem with the Moxon design is that there is no easy adjustments that can be made to retune it to my requirements. The portion of the band it covers is directly attributable to the dimensions and as it is a folded reflector and director, that can not be easily altered. So, maybe time to look at another option that will be adjustable.

That comes as the aforementioned HB9CV design. I found also that I had a length of hollow aluminium tube, its internal dimensions allows the solid rod to be quite a good telescopic fit. That got me thinking that the two element design may well lend itself to a bit of slight modification by making the elements telescopic. That would make it more portable. There are commercial versions of portable versions of the HB9CV but none using telescopic elements most are complete breakdown of the director and reflector from the boom so I might have a go. I will need to do a bit of measuring up of the materials and some calculations to see if it is feasible. I might have to buy a length of box section for the boom but that won't break the bank. Tuning of the antenna comes via a variable capacitor linked to the phasing line between the elements which can be replaced by a fixed value capacitor once the correct value is known for a given tuning.

HB9CV

The design is beloved of portable operators and DF hunters as it has a good front to back ratio and a reasonable amount of forward gain for a 2 element design.

[PanBiker]

I had a measure up of my available aluminium stock yesterday and found I had enough to further the design so made a start. I can get both director and reflector out and have each one telescopic. I am thinking now on how I can secure the inner rods when the elements are set to the correct length. I will put some pictures up to show the nature of the problem. I have material for the phasing line / gamma match but I will need to buy some box section for the boom.

[PanBiker]

Pictures are worth a thousand words so here goes. Here are the two elements:



Both constructed using aluminium tube for the centre section and two rods for each to create the telescopic feature required. The tube is nominally 8mm OD and approx 6mm ID or a tad more. I say this as the rods that slide inside are 6.3mm approx. The top element has a 50cm long centre and the front one a 60cm centre. The rods for each end of the tubes are all 25cm long. This will allow total adjustment of the overall dimensions of the elements for fine adjustment.



Here is the close up of the ends of both. The rod is a comfortable fit but not tight so some method of clamping to give an electrical contact is needed.



Rods and tubes shown separated.



All components for the elements.



I don't want to introduce any great degree of extra metal that would interfere with the antenna design.

From the Moxon antenna that I built previously I have this Acetal bar which is 16mm OD. I reckon this could be the solution for a fixing with a bit of machining. It needs the right sized hole poking down the middle to be a good interference fit on the 8mm nominal tube. 4 x bushes could then be cut one for each end of the tubes. The bushes would need a set screw in the side that could be used to clamp the inner and outer sections of the element. I am assuming the the Acetal can be successfully drilled and tapped?



I have these metric screws with various heads and threads that may be suitable. Ideally I don't want to have too use any tools as such to deploy the antenna from it's collapsed to deployed state. I assume a coarser or larger thread would be better for use in the plastic so the hex head could be finger operated. Alternatively the Allen head screw would be suitable if it had a knurled outer to give a bit of grip. I think the longest of the screws is M3 and I suspect the others are M4. I also have different lengths of screw so will use the shortest that will do the job. The width of the bushes could be cut to suit the hole required.

I need a bit of help from Stanley with this I reckon.

[Stanley]

"I need a bit of help from Stanley with this I reckon"
You know where I am Ian, anything I can do is a given....
Email me to make sure I am not in bed!

[PanBiker]

Thanks Stanley. I can drop the materials off and just leave them on the bench. I will leave four each of the screws so you can choose which ones to use. The shorter ones may not be long enough. They are all standard metric threads that I got for bits of jobs on the motorbike which was metric throughout. The width of the Acetal bushes is not critical as it is inert material as far as the antenna is concerned. I will do a fag packet drawing.

[Stanley]

Got them, I shall wake up before I look at them and then get back to you if I have any problems.

[PanBiker]

Thanks Stanley, I didn't think you were up when Finlay and I called round.

When the bushes are made, I will drill a corresponding hole to the set screw in the side walls of the central tubes so that it can capture the adjustable rod.

[Stanley]

They're done Ian and on the bench in the yard. I used the short 5mm socket heads. Tried knurling the hexagon head but too short and even if I had managed it no good for gripping you'll be far better off with an Allen key in your pocket. I wasn't sure if you wanted the 8mm tube drilling so left it, I reckon if you want to do that you're quite capable.

[PanBiker]

That's quick service Stanley thanks, Fin and I will have another walk round to collect. I hope you didn't have a lot of chips to chase.

I could always fasten an Allen key to the antenna one way or the other so it's an integral part, lot's of scope with a square section boom.

[Stanley]

If you leave these small tasks hanging around they become a pain in the bum when they should be a joful half hour. Glad you liked them.... (I saw him and Findlay heading across town with the loot.)