Friday, 19 December 2014

Building a G3JKF Magnetic Loop Array

Despite having a plethora of conventional and well-functioning antennas outside, I keep a constant interest in magnetic loops and their various designs.

The attraction of a loop is clear: small, not very prone to electrical noise, wide band coverage, and eminently suited to indoor use.

For a couple of years, I've been in touch with Ken Franklin, G3JKF, who has very carefully developed and tested, over a long period of time, a three loop array.  If you ever catch him on WSPR mode, you'll be amazed at how well his loops do.  A characteristic of Ken's signals is that he reaches just about everyone that any other, full-sized antenna operator reaches, often with comparable (or even better!) signals.  Rarely are signals from Ken significantly down on full antennas.

So, having messed about for ages, I decided I'd make use of a day off, and put together a G3JKF loop for 40m-10m.

Before I start detailing the construction, remember that soldering with a blowtorch can be pretty darned dangerous! This is especially true of this build because you may find yourself soldering 'upside down' fittings, which may drip hot flux and/or solder onto your skin.  Eye protection and some decent heat-resistant gloves are advised.  Flux is also very corrosive, and generates noxious fumes that can give you a belter of a headache.  So plenty of open windows if you are working indoors.

You need pre-soldered, 15mm fittings as follows:
14x 90 degree elbows, 4x 45 degree elbows and 6 tees.  

You need a total of about 13 metres of copper pipe, but there is scope to recycle some you may already have. 

You need a pipe cutter, soldering flux (this is essential), and a gas blowtorch.  You can also use an electrical pipe heater, but these are too slow for me!  Also, some coarse wire (steel) wool.

Remember to clean each joint immaculately with wire wool before soldering, and add a liberal amount of flux to the pipe to be fixed. 

Start by making the complex-looking top joint for one half, as per the image.  This first one has to be assembled in a vice, all the fittings made square, and all of them soldered together in one go.


The first soldering is the most complex.  The arms are cut to 130mm each.


When soldering, heat the pipe ahead of the joint first, then move on to the joint itself.  If you heat just the joint, solder may run, but the underlying pipe may not be quite hot enough.  When you see a complete ring of solder appear at the joint, stop heating.  

When soldering pieces of tube, you may need to support them to make sure they're reasonably square and not sagging - there is some play in the soldered fittings.

With care, you should be able to control the heating well enough to melt one side of the connectors whilst the other just about remains solid.  When one joint is done, move straight to the nearest next connection to be made, until completed.  Give a good 15 minutes or more to cool down, as copper retains heat for a remarkably long time.

If, for some reason, a joint looks a bit suspect, you should first heat it to get rid of any blobs of solder, then clean it thoroughly with wire wool.  Add some flux at the joint, and heat the joint until solder melts into the join.  This should fix most failed joints, which will happen sometimes.

You then need to add the 45 degree elbows, and two pieces of copper 525mm long.  Solder these to the arms coming off the central connecting section.


The completed top joint for one half of the loop, plus the 45 degree bends.  The longest, topmost tube is just a connection to the capacitor, however you configure it.

At the end of these pipes, you add a further 90 degree elbow, and a 1m long piece of copper pipe.  Continue adding elbows and 1m copper pipe until you get to the point where it needs to join the second top joint section.  Repeat for the other side, at which point you will have nearly finished two complete loops of the array.

To join the bottom elbows, which you can't really access at times, use a large wood saw's blade near the handle, to act as a reflector of heat, a hint sink with an air gap to stop the floor being burned(!) and  a catcher for hot solder or flux.




You then need to make a second top joint, exactly like the first, and eventually join its arms to the outer loops.


The central loop vertical tubes have to be a bit bigger - 1100mm - to allow for the 'up and over' as a result of using a tee at the top joint.  The arm from the elbow at the tee to the second, down-going 90 degree elbow is 395mm long, of which you need two, of course.  Complete the loop all the way round.


My support is a simple lattice of light timber, held aloft at about 7 feet with a timber stand.  PVC clamps hold the loop firm. Details as per the photo.




You now simply need an air spaced or vacuum variable capacitor.  Anything in the range 10 - 1000pF should do, but mine is about 10 - 250pF for the moment, which may not tune 40m; I haven't tried as yet.


The feed point is a simple SO239 connector to a gamma match.  A very short wire runs from the outer to the centre of one of the outer loops. The centre pin is connected to a piece of drawn copper or similar, which is soldered about 80% of the way up one of the sides. 


Et, voila!  You have what must rank as the pinnacle of magnetic loop design.  Being 1m cubed, more or less, I can't actually get mine out of the kitchen where I built it without chopping the loops in half!  Eventually, I plan to house the loop in a timber frame with fibreglass panels and 'Coraline' corrugated roof; that' much easier than weatherproofing the capacitor, and avoids the effects of our heavy winds.



The completed G3JKF 3 loop array.







Monday, 15 December 2014

Compact Magnetic Loop

Loops are the order of the week!

Having proven the small loop for 17 and 15m works OK, but that my interests currently lie lower down the bands, I added a second loop to make a new, 60m - 20m compact magnetic loop.

Now, you can call this a two-turn loop, or a two loop array.  I prefer the latter because the loops are separated to be more of a cross than two turns side-by-side.  This design is neater if you have a pipe bender or the appropriate fittings; I just made-do with what I had, hence the Heath-Robinson appearance!

The banana gives you a feel for just how compact this 60-20m two loop magloop array is!


The arrangement of the loops is a bit difficult to depict, but here goes:

If we start at the top, the first loop has an open end, to which you connect one side of the tuning capacitor.  That loop then goes down, around and back up until it nearly meets the first, open end.  At this point, it turns about 30 degrees - that's as much as I could manage by bending gently and doing some fancy soldering.  It then continues around a whole loop again, crossing but not touching the first loop at the bottom, until it comes back to the top, where you have the second open end and the last connection to the capacitor.

Coupling the RF is currently by a small Faraday loop.  You can use a gamma, twisted gamma or all sorts of other ideas you can find online.  Generally, a loop antenna is more stable if you use a gamma match of some form.

I used the usual ad hoc mixture of cable ties and timber to mount the thing.

With my 10-250pF air spaced capacitor, tuning is twitchy at 20m, so I've taken some time (since first posting this article) to test it more fully at 30m.  As I do not have a permanent antenna for 30m, this is an useful thing to do here!

Testing on 30m looks very good.  This is an extremely compact antenna, even for a magnetic loop.  It takes much less space than a single turn loop for the same bands, although it packs a similar length of tubing in total (about 4-5m, depending on your own design.)  Due to its size, its minimum height above ground can be doubled, compared with a single, large loop.

WSPR reports over the past couple of days show this antenna to be broadly comparable to a single loop of about 5m, even though that loop is made of 28mm pipe, and the compact version just 15mm.

Tuning is very sharp and manual tuning, whilst certainly not impossible, is prone to capacitance effects when you stick your arm into the complex twin loop structure, which is less of a problem with the single loop.  So a slow-motion motorised capacitor will be needed at some point.  You could simply use a non-conductive shaft to bring your body away from the antenna for easier manual tuning, of course, and many loop builders do exactly this.

This is not an antenna I recommend you build, however.  Instead, do as I am doing and move on from the succesful concept it represents to the proven performance of the three-loop array built and operated regularly by Ken, G3JKF.  It may be worth adding up to 5 loops, according to some builders, though it becomes an exercise in diminishing returns very quickly, apparently.

If you do experiment with multi turn or array-type loops, do leave a comment to let us know how it went.  After all, there are an awful lot of people who could do with a small, efficient loop, and a large amount of money to be saved over commercial units costing anything from a few hundred to well over a thousand pounds.




17m + 15m Magnetic Loop

Loops are always under test in one form or another up in the old shack.  Winter is a great time to build small antennas, because it can be done comfortably, indoors!

In cutting up an old design that resonated too low for what I needed, I recycled some copper tube, including recycling the expensive bend fittings, into a 17m loop.

A very hastily cobbled together test loop for 17m and above.  Just 0.55m a side!

The initial experiment was two loops in a figure of eight fashion, each loop being just 0.55m  on a side.  The antenna worked fine, but I found that it behaved much like a single loop of the same total perimeter.  So, that being the case, it was more of a 20m and below loop.  Handy, because it's more portable than a single 4 or 5m perimeter loop, but not what I was after that day.

So, chop off one loop, use just the other.  At 0.55m a side, the loop is at the lower limit of an ideal size for a 17m loop; this was a product of making the loop such that it would work well on higher bands, too.

The capacitor is a wide-spaced, roughly 10-250pF air spaced type, manually tuned.  The loop being borderline size, it's a little fiddly at 17m to tune, but much easier on 15m.  However, the old 'by ear' tuning system works easily enough!

The very simple Flexweave primary loop, just 0.44m long.  Matching is easier with a primary that overlaps the secondary.


Result?

It's working indoors, so a clear-site, outdoor test will have to wait.  But, it was picking up a JH station nicely on the grey line at 17m this morning, and is putting out a very strong signal across the EU.  It's certainly working properly.  As I write this, R0AU had picked up my 5 Watts at 3534 miles, at -27dB.  Weak, but it's early yet...

Remember that we're not trying to out-do SteppIR beam twiddlers here.  But we are offering a cheap, very simple to build loop for 17m that gives those living in apartments, HOA and other antenna-hostile locations a chance to go live and work the world, albeit more likely on low power digital and CW modes, on a band that otherwise needs a fairly extensive piece of wire.

Oh, and ignore those folks - and models - that say loops are not efficient.  They are.

The next step will be to revisit the multi-loop development, closely following the fine work of G3JKF, who has the most efficient magnetic loops I have ever seen.

Thursday, 11 December 2014

RSGB Volunteer

A strange, knobbly letter arrived in the post, franked 'RSGB' this morning.

What on Earth?

Turns out it was a standard letter, with text identical to that of an article in the current (Dec. 2014) edition of RadCom, and a smart little pin badge with 'RSGB Volunteer 2015' inscribed upon it.

I'm an RSGB Volunteer!

This morning's post...

I presume this is as a result of enquiries with Graham Coomber as to whether the society might like me to continue keeping abreast of planning issues across Wales, something I've been doing anyway for a number of years.

Nothing in the letter tells me what the RSGB thinks I'm doing as a volunteer, of course!  Neither have I had any response from the planning committee, who were meant to be consulted on volunteer involvement.

I suppose I can read it as a 'gauntlet down' situation, in that, if I'm so convinced I can make a contribution well, get on with it!

Fair enough, I'd say.

So there.  I will have to network like mad and do some proper work now!


Tuesday, 2 December 2014

Hellschreiber

Hellschreiber, more commonly known simply as 'Hell', is a great mode, with a speed that allows you to have a nice chat, and plenty of time to edit a macro or just live-type.


Sadly, there aren't very many users.  Or so it seems if you scan across the bands.

Old school Hell.


This morning, RA6BG was sending Hell on 12m; it's the first time I've heard the mode on 12, so I gave him a call.  I sent a spot to the cluster, to find a remarkable number of people joining in the Hell fun!

So, it seems there is plenty of interest out there in Hell, it's just that someone has to jump in first!


Saturday, 29 November 2014

CW Test

All I can say this weekend, during what appears to be a CW contest, that amateur radio tests are ludicrous, self-interested pursuits that give not a damn for established protocol, civility or respect for other band users.

WSPR - a sensitive beacon mode with well-established, fixed frequency spots.  Wiped out by the CW Test.

JT65 - a sensitive weak signal mode with well-established, narrow frequency spots, regarded as being of scientific value to propagation studies (see rationale for 60m access.)  Wiped out by the CW test.

If you think test interference on the bands is OK, it isn't.  One can only wonder at the abuse an SSB or wider band OLIVIA operator would suffer if he started transmitting in the established CW portions of the bands.  But, somehow, it's fine for CW operators to push aside everyone else.

At least it dismisses the long-held fiction that CW operators are somehow superior and privileged...

WSPR Upgrade

Thanks to M1AVV for news of the latest upgrade to the superb WSPR beacon mode software, which can be downloaded from here (15.7Mb.)

Whilst I've only run WSPR 4 for a few minutes thus far, it's immediately clear that it's a big improvement on the WSPR of old.

For starters, received signals are processed and displayed within a couple of seconds, rather than the several tens of seconds it could take the old version if used on older PCs and what now seem like the very anachronistic 'netbook' computers.

The new WSPR also seems to decode pretty much all it hears; sometimes, the old WSPR struggled with signals afflicted by strange propagation effects.

Adjusting the RX noise is cleaner and easier now, and a handy new 'make the next cycle a TX cycle' button avoids having to slide the TX slider to 100% and then back again.

All in all, almost perfect.  It even adopts the existing interface settings, though it did take a couple of tries to get the TX to work properly - for some reason, RTS had changed to VOX.