Tuesday, 20 November 2018

Geomagnetic anomalies, 30m

My single turn magnetic loop has been out in the strong and very cold easterly winds over the past few days, doing very well on 14MHz.

Yesterday morning, I decided to re-tune the loop to 30m and see where the 200mW WSPR signal got to.

I am not sure what antenna other people are using at 10MHz, but my loop is doing better than any others I can find from the UK, by quite a big margin.  This is surprising, as a 10MHz antenna is not so large - just 7m per side as a dipole, or 7m tall as a vertical - as to preclude most people from having a full-sized, efficient radiator.

It's too early for a full analysis of spots yet.  But I did note that geomagnetic disturbances overnight generated interesting propagation enhancements throughout the dead of night.  Curiously, as the early morning period advanced, a periodicity between spots of an hour, or multiple of an hour, seemed to become apparent.  I will have to think about that one, but this paper is an interesting place to start, not least because the proposed modulation period occurs in agreement with my spots:

Magnetometer stackplot, 19-20/11/2018, Norwegian line.

Monday, 19 November 2018

Standing the test of time...

A quick review of antenna durability for the late edition post today.  I thought this would be useful, because there are plenty of antenna designs out there, but very few that report back some years later, to say what is left of them!

Yesterday, during rather cold weather, I decided to do some maintenance on my 6m 2-element quad.  This was unashamedly based on this web site, way back in around 2012.  Since then, it's been a superb antenna for me - so much so that I have always resisted the temptation to buy an all-aluminium quad to replace it.

I immediately knew that making the antenna out of timber would not last very long in our windy, wet environment.  So I made a spider support from aluminium 90 degree pieces, and (eventually) the arms from spare fibreglass elements from a 7m fishing pole.

The good news is that, if you make the antenna as I did, plus a few minor alterations, then this will last you many years without trouble.  Mine is going into its seventh year now, still with the original components.  The feedpoint has had to be redone a few times, but only because I used TV 75Ohm coax with an aluminium outer for the 1/4 wave stub; this reduced to dust in our salty air in no time.  It now has an all-copper, heavy-duty coax stub.

Originally, for a short while, I recycled a Pfeiffer, 15m quad experimental frame in PVC tube to hold out circular wire loops for the 6m, 2-ele quad.  Of course, it worked very well, with maximum enclosed area.  One day, I hope I can try the linear-loaded Pfeiffer quad at 15m again, because, as you can see, the saving in size is quite considerable:
Early days of my 6m quad.
The trouble with the PVC is that is has a high wind loading area.  Worse, the glued 'tee' pieces very quickly deteriorate under UV light, to the point that they fail in about two years or so outdoors.  Despite this, you see an awful lot of US-based antenna designs using PVC tube.  My advice: avoid it completely!  And for intolerant neighbourhoods (my neighbours know to leave me alone!), that bright white plastic is not very stealthy!

I then made the quad look like this:

Second iteration.

Since this photo was taken in early 2013, the only change I've made is moving the coax down from the boom (which then stupidly ran down the lower driven element spreader again) so that it simply feeds directly the same lower corner (horizontal polarisation) from the mast itself.  This reduced the interaction between coax and antenna considerably, and reduced the length of feedline by nearly 3m.  You can see the arrangement in the last image of this post.

Here's the quad spider detail.  Tape and cable-ties were OK for those nearly 7 years, provided one changes the cable ties every two years or so, but the top arms slip through eventually.

What you  need to do - and what I was doing yesterday, it cut notches with a rotary tool in the 'L'-shaped aluminium, so that the cable tie (or, preferably, stainless steel hose clamps), make better contact with the fibreglass arm.  Typically, the arms are slightly smaller than the depth of the 'L' bed, so there is only slight contact between them and the securing fastener if you don't make notches.

Also, it's a good idea, especially for the topmost bracket, to drill a hole and fix a stainless steel bolt, to physically prevent the upper spider arm from slipping down.  You can do this for all arms if you like, as the tension of the antenna wire does tend to pull all of them inwards.

Also cover the exhaust clamps in plenty of grease to make them last longer.  In an ideal world, I would suggest you use stainless clamps for a square aluminium boom, not a round one.  This avoids slippage and rotation as one could get with round tube, though it has not been a problem for me.

When you think this antenna has gone through nearly 7 years of very strong sunlight and even stronger winds, then, for the peanuts it cost to build, it really is a true winner!
Serviced and ready for another several years.

Figure-of-eight vs. standard magnetic loop.

Many days of data are now in from WSPR transmissions from my 'Meight' (figure-of-eight) loop and a standard, square loop.

Now, the figure-of-eight loop, for reasons of not having to spend money because I had the tube to make the loop already in the shack, and for its ease of forming into shape, is made of 10mm soft copper tube.  The standard, square loop is 15mm diameter tube.

The difference in tube diameter can be expected, if both loops were the same design, to yield a +9% advantage in favour of the wider tube.

And, after many days, we do indeed find, even by eye, that the single loop significantly outperforms the figure-of-eight loop.  A comparison with MX0PHX shows that, whilst the figure-of-eight loop was running (shaded area, left), it was only very occasionally able to outperform that station.  On the switchover to the standard loop, we find MX0PHX is usually under performing the loop:

Indeed, the single loop did admirably well against the UK's typically top-performing 14MHz WSPRer, GI8YJV:

I can't say whether a figure-of-eight loop would perform better or worse than a standard square loop of essentially the same total perimeter, because the 15mm tube, compared to 10mm, does make a difference to efficiency.

But what I can say is that a 15mm square loop is considerably easier to make than a figure-of-eight loop.  My own view is that I would prefer to have a wider-bore standard loop of, say 22mm pipe in practice (the increase from 15 to 22m gives roughly +7% more efficiency).  A figure-of-eight loop in 22mm pipe might slightly outperform the single loop (but of course, I don't yet know), but it would become quite top-heavy to the point of necessitating a more substantial mounting system to tolerate strong winds.  A permanently-located, stout, treated timber mast of about 4m height would do the job well, easily and fairly cheaply.

One other thing in concluding is that I realised the figure-of-eight loop needs significantly more tuning capacitance to bring it to resonance than the standard loop.  The answer as to whether or not this affects overall antenna efficiency is something that will have to await another day. The tuning range of the single loop is also significantly different, tuning about 7MHz up, from the figure-of-eight loop, which tunes from roughly 10MHz up.

Friday, 16 November 2018

All change!

The figure of eight loop, as earlier reported, has done very well in WSPR performance, and showed itself able to tolerate strong winds up to, so far, about 110km/h.  Tuning is stable in all weather - a satisfying vindication of my simple, very cheap approach to remote tuning).

Having gathered about a month's worth of data with the 10mm pipe, figure-of-eight loop (total tube perimeter of 3.67m), I decided I might as well see how a single loop of 15mm tube, 3.9m total perimeter (1m per side, minus 10cm gap where the cap sits) compares from exactly the same location and height (noting the single loop base is the same height, but its top is only half as high as the top of the 'meight' loop).
Single loop test.

In fact, I had stopped running the 'meight' loop a day or so previously.  I had been running my vertical delta loop at 1W from the rig overnight (shaded period in the graph below).  At 08:06UT, I switched over to the single magnetic loop running 200mW.  The outcome is surprsing:

Of course, my delta loop might well have seen its spots graph increase after this period; it usually slightly outperforms GI8YJV most of the daytime.  But that doesn't really matter, because the magloop running 200mW is certainly reaching as far as the delta running 1W, and doing as well at this moment as the full trapped wire dipole at GI8YJV, who is one of or often the best WSPR performer on 14MHz from the UK.

At the time of writing, as it's possible to see on the extreme right, the magloop has started - and has so far maintained - a significant distance advantage over the wire dipole.

I will now continue to run the magloop for at least several days, to see how it compares to that figure-of-eight loop, which is of almost identical perimeter (though, again, the single loop is of 15mm, not 10mm pipe as in the case of the 'meight').

Other than that, this Sunday, it's time to go flying (using VHF AM radio). 

Image: Wiki Commons.

Thursday, 15 November 2018

Propagation anomaly, 2018 November 15

Last evening's Norwegian line geomagnetic stackplot looked like this, showing a relatively minor disturbance in the early evening, and some much smaller disturbances later in the morning:

The last that was heard of WJ1I anywhere other than the US was at 21:02UT - except for here at MW1CFN, where a single reception was made at 01:20UT (now reverted to the vertical delta loop antenna):

The fact that only one US station made it across the Atlantic at this time, and that he was not then heard again, shows there must be very localised and extremely transient propagation changes related to the passing of the auroral oval at mid-Atlantic longitudes.  Again, the anomalous spot comes at the time of field restoration.

Wednesday, 14 November 2018

60m - Wow!

It's been a bit quiet here at Copper Mountain HQ of late.  Normal operations have been largely suspended in favour of long-term beaconing on WSPR, using the figure-of-eight loop.

But, as the commercial FM radio blurted out the latest shouting about Brexit from the circus that is the UK Parliament, I turned over to 60m FT8.
Busy on 60m this evening!

I was amazed to get an immediate reply to my 'CQ' from BH4IGO.  Wow!  China on 60m.  That's a first for me.

60m is really coming alive now, as more and more countries approve allocations for their amateur population, and the weaker higher bands propagation encourages its use.

Certainly, it's worth trying a few calls around sunrise and sunset, so why not give it a go? 

Wednesday, 7 November 2018

LED Security Light - no RFI!

In the past, so-called security lights were incandescent units with bulbs that typically blew up after a few months.  Due to the cost of the whole lamp being close to the cost of the bulb, most people would not bother replacing the light, leaving it to rust on the side of the house.

But now, we have LED lights!  These offer about the same light of a 250W incandescent unit in a package that is small, lightweight and typically only uses 10W.

Of course, as someone involved with astronomy, I would very much encourage you to fit a PIR-operated unit, and ensure it is pointed downwards, where it won't uselessly light up the sky, disturb neighbours, and cause glare that diminishes, not aids visibility.

For us hams, LED lights are yet another potential source of RFI, often from unfiltered mains connections that allow the switch mode PSUs in these lights to inject interference directly into our wiring. 
No detectable RFI!

I bought a new LED unit earlier this week, for a demonstration of light pollution at the Snowdonia National Park headquarters.  It cost me just £8.   Even better, it produces no discernible RFI on any of the ham bands.  All the others I've tested have been so incredibly RFI-noisy that I put them in the bin immediately.

If you want one of these in the UK, you can find them here.  Elsewhere, you can probably find the same thing from the product reference number on the box pictured above, as they are mass-market things likely sold under a plethora different names.