Monday, 29 September 2014

Twisted Gamma Match

Attention has turned again at this station to the wonderful and intriguing magnetic loop.

Having been doing some more technical reading, I thought I'd try out a twisted gamma match, brought to our attention by Prof. Mike Underhill probably more than anyone else.

Now, most people who make a magloop use a primary 'Faraday' loop, which couples to the secondary, larger loop.  This works well, especially if the Faraday loop is squashed into an oval, which tends to yield easier matching.

But I was curious whether the gamma match would work as well as - or better than - the two-loop system, although the overall efficiency of the antenna was already superb.


Solder shield to one side of the loop, and the gamma wire to the centre..


So, out came the blowtorch and solder, and off came the Faraday loop!

I connected the shield of the coax directly to the bottom of the large loop, and then soldered about 2 metres of relatively heavy duty (30A) insulated equipment wire to the centre conductor.  I twisted this, with no idea what might work and what might not, until the wire wrapped up half one of the bottom pipes, up one whole side, and a few inches along one half of the top pipe.  Prof. Underhill seemed generally to do much the same, loosely wrapping one whole half of a circular loop with the gamma match wire.  I used a crocodile clip to connect the end of the twisted gamma match to the top rail of the magloop; you eventually solder this in place.

I tuned-up by ear with the magloop's 0-200pF air-spaced capacitor, to pleasantly find the SWR a perfect 1:1.  Running on WSPR at 5W in poor daytime conditons on 20m showed the system was working as expected.  Comparison of my received signals by US and Canadian stations showed I was level-pegging with other transmitting stations known to have very efficient beacon systems.

More tests needed, but the twisted gamma is very easy to do, seems non-critical as to design, and is less prone to being disturbed when knocked or moved than a Faraday loop.  You can find lots of information about loops and matching systems here.


Friday, 19 September 2014

RSGB: Up The Swanny?

The latest edition of RadCom just dropped through the letterbox.

Fair play, the editorial team are making what is a fairly obvious attempt to change the content so that it appeals to mere mortals as well as career engineers.

On page 7, half-yearly and unaudited accounts are presented.  They make for troubling reading, indeed.

Having ridden a significant but very short-lived wave of sales during the centenary year (2013), 'normality' is now coming home to roost.  This month's RadCom heavily promotes the new Handbook, whilst trying to get members to boost the society's income with a £5 book token. 

Whilst I could go through a lot of detail, the bottom line is that the RSGB has taken a huge, 9.2% drop in income as compared with the half-yearly results for the same period last year.  Its income from interest has halved in one year.  The RSGB states it expects a "roughly break-even" position for the full year.

Now I don't know much about business, but when faced with a situation like this, a subscription fee hike would appear to be wise, if unwelcome.

One detail that I think the membership ought to severely criticise the Board for is its continued operation and financial support for its ill-conceived National Radio Centre at Bletchley Park.  This accounts for an overhead of £18,414 - or a staggering 25% of the RSGB's total non-activity overheads.  I've said it before and I'll say it again - the NRC has no meaningful worth to the society or its membership, and is entirely unaffordable.  It has been the focus of much criticism and needless expense.  Why it isn't being closed down as an expensive luxury is something that will become a louder cry in coming months, I think.

Urgency should be setting in amongst the Board members and all members of the society alike.  If the society maintains its current, quite static line of activity, then it has only a few years left before hitting the hard rocks of financial unsustainability.

Many have commented in the recent past that the RSGB has only "about ten years left."  On the latest results, that would seem to be rather optimistic.  Membership, and so income, is still falling, and the realisation that nobody's been doing anything to attract newcomers to the hobby has come way, way too late.  Many of the RSGB's members have been only too enthusiastic to play their part in the downfall, in their attempts to keep radio an exclusive man's club.

The RSGB now runs, unless it shows exemplary management direction, the potential risk of making mistakes in an effort to improve its future prospects.  With a break-even outcome, it has no latitude to invest in improvements to services to its membership, many of whom seem simply to join "for the magazine".  It has reached a point where it can only - just - keep its head above water.

The time for a total revamp of the society, how it does things, what it throws money at, and what services it provides, is now.  Not tomorrow.  Not the next time a new Board is appointed. Now.  If not, then during the next couple of years, we'll be seeing accounts with little more than increasingly red numbers.

On the positive side, we can take a glass half-full approach and say that the RSGB's time may have passed, and that a new kind of representative for the radio community should come into existence.  Many would welcome that, and I would be one of them.



Friday, 12 September 2014

Switch Mode Power Supply RFI

Just when you thought we had enough QRM on the bands, I seem to have found a new culprit yesterday - domestic CCTV systems.

Now, before we begin, CCTV isn't rare.  It's used by a large swathe of the neighbour-harassed population, so there will be one near you.

Because of their popularity, and developing HD technology, CCTV is subject to huge competition and cost pressures.  As a result, so is the quality and design of the various components.

The facts:

I bought a 'Home Guard' CCTV system with a 1TB HDD and 2 cameras.  It cost £189 at Argos, a very popular UK retail outlet.

After a full and careful installation (well, there are only a couple of basic BNC and PSU connections, after all!), I powered up to see whether there was any RFI.

Sadly, there was.  A lot of it.

The 12m band went from a totally quiet, noiseless environment to one with a S5 hash.

Here's the evidence:

So, if you buy one of these, which are probably the same kind of unit sold under endless branded guises, then chances are it will have a cost-cutting PSU, and RFI as a result.

I'm taking mine back as not fit for purpose, and a breach of European Directive 2004/108/EC, the material part for consumers being Article 5 (hat tip to UKQRM for this):

"Equipment shall be so designed and manufactured, having regard to the state of the art, as to ensure that:

(a) the electromagnetic disturbance generated does not exceed the level above which radio and telecommunications equipment or other equipment cannot operate as intended".

Argos were exceptionally good with me - they accepted the complaint and refunded me with no quibble whatsoever.  They even called me back the same day when I told them about the wider issues with selling this product. 

But, many other shop assistants will look at you with a blank face if you present them with this complaint as a basis for returning and claiming your money back.  It's understandable, but not acceptable.  It's probably best to ask for the manager, if there is one, and explain it is a piece of equipment that is probably illegal due to its RFI-generating nature, and that the problem of power supplies like this generating RFI is very well known and the reasons for it understood (omission of isolating components.)

Tell the shop you are also the holder of an OFCOM radio licence, and that you will be reporting the product to both OFCOM and the local trading standards office.

DO NOT ACCEPT the shop telling you to contact the manufacturer, even if the manufacturer asks you in their instruction leaflet to contact them 'in the event of a problem'.  This is not an operating problem for the maker to help you with, it's a basic manufacturing flaw.   It is the seller's legal obligation to deal with complaints and refund you (a replacement isn't appropriate here, because it's likely all their CCTV systems will use the same PSU - you should tell them this if they dig their heels in.)

If the shop, as often happens, really digs their heels in, you can try and ask the manager to write down their reasons for refusing to accept the return and/or refund you, and then go to ask for help from your local trading standards office at the Council.  Inevitably, these are rather stretched and much less able to help than once was the case.





Saturday, 6 September 2014

Antenna Switch

Like most hams, I accumulate antennas over time.  I have one commercially made antenna switch, which can handle QRO operating, and so has some pretty robust switching.  However, it only has three antenna ports, and I never use more than 100W, usually much less.

Here's what my commercial unit looks like.  You may wish to note that a four port switch sells for anything from £60 - £90, depending on maker.  That's a lot of money for what is actually an extremely simple device.

Knife switch on a commercial, higher-power 3 port antenna switch.  Simple, but expensive.


As I have five antennas in active use at the moment, and always a couple under experimentation, I decided to make my own, seven port switch.

Now, a note of caution: if you use more than 100W output, you may want to select your switch unit with consideration to how robust the contacts are, and whether their arrangement might be too close for higher powers.  At 100W or below, the following should be more than adequate, but it's up to you and your training to check!

I obtained my 11-position, single pole switch from a well-known online auction site for about £2.50 each.

At the same time, I bought a smallish aluminium box from RS Components, who seem to have a more sensible size range than other outlets.  This was about £12.  I bought a lot of twenty SO239 panel-mount connectors for £8 - all the way from Texas - via the same online auction site.  They took a mere week to arrive.

The 11-position, single pole switch, and all the connections.  The SO239 on the left is attached to the common pole, to which the transceiver is connected.  The remaining seven are antenna ports.  Don't forget to add a grounding post as well.


I then bought a reamer, which cuts larger holes in metal to allow the SO239s to fit through.  This was a one-off tool outlay of £11.

Making the switch is easy-peasy, lemon-squeezy.  Just drill a number of modest sized holes where you want the connectors, and then use the reamer to widen the holes until the SO239s sit snugly against the box.  Then drill some smaller holes for attaching the SO239s to the box using M3 or thereabouts bolts and Nylok nuts.  Two is enough for each connector.

Drill a hole in the bottom of the box (NOT THE LID!), which is where you stick the switch through to attach it to the metal case.  The lid goes on last, and becomes the bottom of your antenna switch box.

The rest is just a case of connecting each antenna port to the switch.  You also need to attach a bolt through the lid somewhere out of the way so that you can ground the box directly (and not to another piece of equipment like an ATU, etc.)

And as I can hear the purist elders ask: "Ah!  But what about the inter-port isolation, my boy?", I ran a test using two rigs, one on a connected port, the other on a disconnected port.  According to this simple test, the isolation is very comparable to the commercial unit, being of the order of 60dB.

So there you go.  Apart from the fact that you rarely see antenna switches with more than four ports, this project saves you a lot of money.  Sure, you can do a nicer, less hurried soldering job than me, and maybe use a chunkier switch, but that's another story!

Tuesday, 2 September 2014

ARRL DXCC Award

For some reason, I went to do battle with the LoTW site this afternoon.  Like every other time, I ended-up going round in circles, trying to find somewhere that was user-friendly.

Eventually, I once more figured-out how to apply for a DXCC award, this being rather belated by now, and not really of any importance to me, to be honest.

In total, the certificate would cost me £38 at today's exchange rate.  £38 - for a piece of paper?  I can get a walnut-effect mounted brass plaque for a bit more off E-QSL, a system that, whilst admittedly much more open to abuse, is infinitely easier to use.

I can only imagine that, if QRZ.com get their skates on properly, they could sweep the floor with a reasonably-priced award program linked to their very nice new logbook, which is now double-blind until confirmation by each station.  It's clear such a program is in development, and needs only to be launched.

So, the ARRL can get stuffed, because only egotistical fools with space to fill on their walls pay £38 for a piece of paper. 

Friday, 29 August 2014

0% Compliance for USB Chargers.

During ongoing discussions within the radio community about EMC issues arising from the flood of Chinese USB chargers, my attention was drawn to this important article.

Given that most of us have several of these damned things around the house, this is certainly worth a read:

http://www.electricalsafetyfirst.org.uk/electrical-professionals/product-safety-unit/plug-in-chargers/

Suprisingly Useful Tip of the Day

The other day, I accidentally soldered a PL259 to the free end of a roll of RG-8X.  This proves to be surprisingly useful, because I'm always chasing a brainwave when I am connecting-up coax, usually a brainwave in bad weather, or when there are other things to do as well!


By making sure I always have a PL259 nicely finished off at the end of the reel, I can just cut the right length needed for the job, connect-up at the antenna, and pass the other end through the wall for some comfortable indoor soldering to finish the job!