As you can see from the infrared photo below, the delta, on the right and propped-up by a pole off an old chimney, is hardly in the most isolated situation possible, being within about 5m each of a 12m beam and tower, a 2-ele 6m quad, and a 20m vertical!
As soon as I put up this delta one rainy Christmas holiday morning, it became clear my experience of radio had changed dramatically in comparison to the earlier, sloping long wire from which the delta had been made. The DX was now easy, whereas it was more hit and miss with the LW.
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| The 15m delta is visible at right, apex supported off a pole on an old chimney. Hardly an ideal environment! |
Recently, I haven't been on 15m much, but conditions there were quite good this weekend, so I ran some WSPR transmissions towards the end of the day. The time range in question spans 18:52UT to 19:08UT on 17/05/15, so well before sunset at any point in the UK, so the greyline enhancement would not have occurred yet for any of the stations in question.
What happened next was remarkable - even for a station based on an old copper mine ridge, 320 feet up in the air.
The receiving station was K9AN. By any standards, K9AN runs a good quality station from what you can see from Google Streetview, is a pretty clear, rural location where the band is clear of most interference. Anyone who's a regular on WSPR will know K9AN is a reliable, reference station in the US.
Now, my 5W from a delta loop with a base 2m off the ground, and an apex at roughly 6m, was hitting K9AN at +2dB. How were others doing?
Predictably, my friend Ken Franklin, G3JKF, was the second strongest signal to K9AN at the time, coming in at -13dB, also from 5W to a very -carefully researched and maintained magnetic, 3-loop array which is very often equal in performance to full-sized antennas. So, a difference there of 15dB.
When I looked at others, G4KYA was at -21dB (5W), G8VDQ at -23 (5W), GM4SFW at -20 (1W) and G3TXA at -22 (1W). These are enormous differences in signal strength compared to mine, even allowing for the output differences of the 1-Watters. I should also point out my spot was repeatedly as strong, at +1dB, a few minutes earlier. It's a shame I didn't start earlier in the day, actually.
Now, there is no question that this QTH benefits from (a) lack of man-made development (b) ground conditions that are, from the point of view of how many other stations are so situated, unique, and (c) a modest elevation that nevertheless puts it above the rest of the landscape, with clear sea views to the Americas.
According to the ARRL HF Terrain Assessment software, the model I've carefully prepared of the landscape towards north America yields a antenna-plus-ground gain figure of +8dBi at an elevation of 17degrees. Between 2 and 5 degrees, it's about +6dBi. Note that, because the software only deals with horizontal antennas, I've been forced to input a dipole at half a wave up.
Clearly, this model does show a very good gain figure for a dipole, reaching values more indicative of a 2 element beam at very low angles of radiation, and a 3 element beam at the still-low elevations around 12-15 degrees.
What it doesn't do is explain the enormous differences in received WSPR signals by K9AN. One might be tempted to look at 'patchy' propagation conditions, where a cloud of ionisation has fortuitously aligned itself between me and K9AN. But, because of the spread of other stations spanning from the south east of England to the north of Scotland, and that those signals were of very comparable strength at K9AN, this explanation seems to fall flat on its face.
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| How software models a dipole at half a wave above ground, at 21MHz. |
Now, how about we model a vertically polarised delta at the real height, with real (or as best as can be estimated) ground conditions input into the mix? Here's what we get:
With real ground conditions: +4.5dBi gain at 14 degrees elevation.
With perfect ground conditions: +7.63dBi gain at 0 degrees elevation
Free space: +2.71 dBi.
Does this really tell us anything? Well, let's assume that, based on what we know about the QTH here, that my signal of +2dB, coming from an undeveloped, clear site with sea horizon views, is close to what K9AN should receive at 5W, at that time of day. This must mean that other stations are, for reasons of their environment, probably above all else, losing a fantastical amount of energy to their surroundings.
This is why, of course, much cleverer radio people than me made a point of buying houses on hillsides wherever possible; the late Les Moxon, G6XN, was one of them, and often cited SSB contacts with VK using milliWatts of power from simple verticals when working from such locations.
As usual, I'd be interested to hear of any considered input into this. My enduring frustration is being unable to properly assess the ground conditions; according to the ARRL, the highest values for ground dielectric - much higher than seawater - have been found at highly-mineralised places - such as copper mines! It seems they got their measurements right, anyway!
UPDATE:
Just to keep things balanced, here's the ARRL HF Terrain Assessment for my QTH, but to the south, where, at about 42km, lie the Llyn Peninsula (southern Snowdonia) mountains, only a couple of thousand feet high. Even so, and even at that distance where they are only about a little finger's width above the horizon, they have a marked effect on low angle signals, taking them into negative territory for a while.
Although this gentleman's talk is a little long, it does neverthless get to some important points, and worth watching from about 23 minutes on.
So, it's not all good news here. Maybe this effect of hills explains why so many "kiloWatters" exist in California, surrounded by vast mountains to the east? I'm too lazy to build a terrain model for that!
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| Distant hills bring considerable gain penalties at low angles... |
3 comments:
Hello John, from what you describe you really are living at a unique location. Not only very high but also near the Atlantic and on a old copper mine. I would love to visit your station once at hear the loud signals that you must receive. I have a modest station as well. Free to the south and partly to the north. At the north side it is about 4 km to the sea. I have to say that if I hear for instance Iceland or Faroer Isl. it's always easy to work even when there is a pile-up. But nothing compared to you of course. Indeed if you want to work a modest station with less then perfect antennas and still want to have a good signal you should choose a location near water to live if possible. 73, Bas
Hi John, I could not agree more - location is critical. I don't hear that much dx from my QTH in the city.
I am blocked to the West and East by houses + qrn from the city.
From the hill tops or the beach is a different story...
73 Angel
Thanks for the comments, folks - and for taking it for what it is - not a bragging session, but something to remind us about what may be possible away from all those highrise buildings and plasma TVs! A good reminder that becoming a slave to the home QTH isn't always necessary - or even a good idea!
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