Two, or four?

I'm continuing with side-by-side experiments using different numbers of radials on elevated 1/4 wave verticals.

Ready for deployment. The feedpoint is at 1.5m when installed.

 

Last evening, I set up two identical antennas for 14MHz, one with four radials, the other with only two. My previous post compared two radials with only one. The experiment ran between 17:06 and 18:24UT, so it's a snapshot comparison for the moment.

For this work, I created a plate that slips over the fibreglass pole and is a big improvement over trying to mount things 'dangling down' from the pole, which makes for a lop-sided feed point and radial tension is not so easy to equalise. This proved to work very well indeed, though I need to make a better, final version to tidy things up a bit.

Now, before I go on to the results, it's an idea to reflect on Les Moxon's problem, discussed in his excellent book HF Antennas for all locations when he depicted a four-radial vertical and asked, in relation to coupling of the radials with the the coax feedline: 'how does one achieve symmetry?'  He was thinking at the time about low-to-ground feedpoints, the problem being lessened by raising the feedpoint (as I do by default), burying the coax, or shortening the radials. 

Moxon's 'problem' of additional imbalance due to the feedline.

 

In my case, I simply run the coax vertically down immediately from the feedpoint and is of the same length as the feedpoint is above ground. With WSPRlite transmitters, this is great; with real transceivers, not so much. The elevated feedpoint sorts this problem out in that situation, more or less.

Back to the experiment. Everything identical, except no plate for the two-radial antenna (direct connection to an SO239). 200mW out, both antennas on the same moist, rich ground with an entirely open landscape all the way around. Matching was 1.12:1 for the 2-radial antenna, 1.15:1 for the 4-radial version.

This was the coverage, pretty-much identical for both antennas:

200mW goes far.

A simple table (click to expand) shows more efficiently the outcome, rather than plotting graphs and so on. Overall, we get a small advantage for the four-radial antenna, though for several US east coast receivers, the advantage was much more significant - up to +7dB. Also, the four radial antenna is typically heard more often, although there are exceptions. 

A longer test is necessary, but it's a good first indication. The elevation pattern seems much the same, based on more local stations' reports, but that the overall efficiency is somewhat improved.

I've done these tests inland rather than at my usual coastal haunts. Not many people have easy access to the coast, and the ground there is so near perfect at RF that the number of radials is much less important than over soil; often, a simple copper tube in the ground is more than adequate (and much safer, in public areas). 

For portable coastal work, the difference is likely to be much lower or none at all. Over normal ground that most people operate from, it's too much hassle to deploy four radials, at least to my mind. But for permanent installations, especially if the ground conditions are not very good (arid zones, etc) the additional effort of installing more and perhaps many more radials is certainly worthwhile.

One, or two?

A recent article online raised an interesting question about the use of one radial, rather than eight for a 1/4 wave vertical antenna and whether the modelled slight directionality of the resultant pattern for the former would be seen in practice. The author concludes that the real antenna performed in accordance with the model output.

Firstly, it's good to see the model being questioned and not taken as gospel; they are by no means infallible.

I thought it might be an idea to try a side-by-side comparison at 14MHz using WSPR, rather than 'real' QSOs with CW and a very subjective assessment of signal strength by ear or quivering S-meter. The enormous advantage of WSPR is that it is independent of human bias - provided the experimental set-up carefully eliminates other sources of error.

With the fields being rested a little at the moment, I had several acres of free space, devoid of charging cattle and munching sheep, to set-up a peaceful test last evening.

Antennas in position.

 

The test was to compare my standard elevated 1/4 wave vertical (MW1CFN) with sloping elevated radials against an identical vertical but using just one sloping radial (MW6PYS). Both were made of the exact-same wire, mounted at the same height about ground (1.5m), 32 metres apart, had the same slope and final height above ground for the radials, were fed by the same coax type and length (RG8-X, Nevada, 1.5m) and SO239/PL259 connectors. 

Each was fed by separate but identical WSPRlite units sending 200mW 40% of the time. Identical fibreglass poles supported the wires and the ground conditions were a gently sloping short grass field of very moist, rich soil; the water table is about 4 feet beneath the surface at the antennas' location (a well and/or surface breakthrough allows this to be known throughout the year).

The horizon is flat and open, with no buildings or other clutter, apart from a very thin dotting of trees and bushes at a distance. Mostly collapsed wire fencing lies at about 40m from each antenna and seen not to interact to any measurable degree.

Finally, each in-situ antenna was analysed to ensure matching of each was as close as possible to one another. Matching at the 14MHz WSPR frequency for the 2-radial antenna was 1.12:1 (RL 24.67dB) and for the 1-radial antenna, 1.19:1 (RL 21.08dB). For a nominal 200mW output, this translates into 199.32W from the 2-radial antenna and 198.44mW from the 1-radial antenna, a 0.88mW difference. More time would allow for minor trimming of the 1-radial antenna and identical matching, but this small difference remains practically inconsequential.

Unfortunately, the WSPRlites can't be set to 100% duty, which means that two independent units will not very often be transmitting at the same time. But statistics lets us out of that problem.

How did things pan out?  Well, I changed the radial 'pointing' direction from NW to NE during the test, to see whether the claimed directivity of the pattern was seen in respect of the G-to-VK path and, conversely, in the G-to-US path. 

Looking at results to VK7JJ, there's no evidence of directivity. Given the outcome in the final third section of the experiment, when the radial was to the US but the general signal seems unaffected to VK7, it seems the absence of spots for the single radial antenna in the first third and halfway through the second third of the experiment is most probably explained by it having a higher departure angle compared to the two radial antenna, where lower angles were favourable earlier on, when the path was briefly becoming active. I've checked that MW6PYS was transmitting during the 16:45-17:35UT period, and can confirm it was sending every 4-8 minutes or so. Certainly, when the radial was changed to the US direction, there was no sudden drop in the signal to VK7, which in fact increased slightly for a time.

The overall result is a median SNR from the 2-radial antenna of -19.5dB (range of 14dB to -23dB) against a median of -21dB (range of -18 to -26dB). I'd conclude that this is a small but sometimes greater and significant advantage in favour of the 2-radial antenna, especially noting the earlier period of the plot.

RX at VK7JJ

 

Let's check this against another VK station - VK5ARG, a notably good and long-term receiver. Here again, we see no evidence of a meaningful directivity when the radial is aimed at the shortpath great circle direction to VK. There is one point at which the single radial antenna significantly rises above the two radial version and when both transmitters were sending at the same time. But it's an outlier; the overall output is clearly one of perhaps surprisingly little difference between the two antennas, the range of signals being exactly the same for both (-12db to -24dB) and the median received signal at VK5ARG being only 1dB lower for the single radial antenna). I conclude that, for this DX path, at this time, there is no significant difference between the antennas. This is perhaps quite surprising and useful; deploying one wire near the ground is more convenient than two.

RX at VK5ARG

Note from the original article, though, that the author is, like many in north America, interested in north American QSOs. This selects for fairly modest distances of, at most, 3300km, with the majority of QSOs being 1500km or less. I've selected OH8GKP as a modest-distance receiver (2100km) that happens to lie on much the same great circle path to VK. Does a higher pattern and, specifically, a higher peak gain elevation explain the claimed directionality of the 1-radial vertical? Let's see (and don't ask me why Libre Office randomly changes the plot line colours!):

RX at OH8GKP

Well, that's a big 'nope'!  Only changing propagation is leading to the ups and downs (and long absence) of spots in this case.

OK, how about a transatlantic path, over to the eastern side of the US and WA3TTS. Will this kind of distance (~5500km) yield anything useful?

RX at WA3TTS

A cursory look might lead to excitement; the signal seems to drop when the radial is switched away from the US and then rises again when the radial comes back to the US. But the devil is in the detail: the signal has already risen before the radial direction is changed to the US and after a very brief period of broadly matching the 2-radial antenna, drops in strength. This is again probably the result of propagation along different initial elevation angles, although the sustained stronger signal for the 1-radial antenna between ~17:50 and 18:04UT means this is an advantage for that antenna. The overall result is that the median SNR for the 2-radial antenna to this station was -19dB, with a range of -18db to -24dB and for the 1-radial antenna, a median of -20dB with a range of -15dB to -25dB. I'd have to conclude there is no practical difference between the two antennas, though it's again interesting that the single radial antenna performs essentially equally well on this particular path.

A final look, then at N2HQI, which is a shorter period of study as the path only opened to that station a bit later in the experiment:

RX at N2HQI

 Pretty certain there's no directionality of the pattern here, the signal from the 1-radial antenna being heard only briefly. The 1-radial antenna is clearly at a substantial disadvantage in this case.

Overall, then, my conclusion based on this carefully-conducted but rather too brief test is that the 1-radial antenna performs surprisingly well against a two-radial antenna, but that the two radial antenna remains a better choice. For simplicity, space and public safety, and because I operate in moist or wet places most of the time, two radials have never left me wondering if I should use three or more. But these results suggest it's worth trying. That experiment, and the large amount of time analysing and presenting the data it takes, will have to wait for another day...

Evening shortpath - at the beach.

It's that time of year again when summer is drawing rapidly to a close, and the terminator lines up nicely between the UK and Australia.

Time for some beach work with WSPR!

Modelling using Proppy HF showed that 14MHz would exhibit a fairly sharp rise to a peak in propagation between the two regions, which looked like this, based on a 1/4 wave receiver and 8-ele transmitter (the maximum available from the software), beaming shortpath:

The weather was forecast to be a bit rainy, but it looked OK skywards, so off to the east-facing sandflat near high tide, and in went the 1/4 wave vertical, my standard antenna for most operations, and always for WSPR. It has two elevated sloping radials with a near-perfect match over seawater-saturated sand and glacial clay.

The model proved to be very reliable. Peak signals from Australia for me, in the far west of the UK, were at about 21:20UT, somewhat earlier than the modelled peak.

The main interest was the strength of signals over time from Ian's vast, 4x5-ele array, just outside Melbourne and beaming along the shortpath to Europe, with 5W going into the array, supplied by an Ultimate 3S transmitter.

VK3MO's world-class 14MHz array. Reproduced with consent.

With such a system, everybody hears Ian!  But not everybody hears him as well, of course. One reason is that some WSPR operators - it seems to have increased over the years - are using beam antennas. Another is that some are close to the coast, yielding enviornmental gain; this is the chosen situation for me, for example. And the other big cause of differences is local noise levels which, sadly, is far too high for all too many of us.

I've extracted all the stations that heard Ian in the time slot I was operating, and then taken a fairly large sample of those, at random, without first referencing their received signals so as to give a genuine, unbiased spread of what people could hear. This is what the plot looks like (click for larger image):

The remaining problem, one which has afflicted WSPR from its inception, is: how do we know if the best signals (other than mine, which we know is an elevated 1/4 wave), aren't simply the result of a Yagi pointing at Ian's array?  In short, we don't - because a surprising number of operators don't tell us anything about their WSPR setups. A number of those are older operators, seemingly unwilling to respond if contacted and often apt to not publish their contact details. We just have to live with this, unfortunately, or set-up our own, second identical antenna inland, in a quiet place, to run a coast/inland comparison. This is possible for me, but not until the cattle are out of the fields later in the year (cattle love destroying antennas!) I'll be doing this at some point, with a bit of luck.

Certainly, relative to the majority of UK receivers, being at the beach for this particular path at this particular time is very worthwhile, yielding maybe a 20dB gain over those, partly due to the environment and partly due to having no noise at the beach. It's reasonable to assume some of the equally good receivers to mine are using directional antennas; certainly, 2E0PYB has uses phased arrays in the past, but I don't know what his current setup is for sure.

The other take-home message is that the peak signal is generally sustained over a somewhat longer period than other stations. This may reflect access to a greater range of signal arrival angles at the beach, including exceptionally low angles not accessible to inland antennas, not even when they are large Yagis. Overall, I suspect that because there are a fair number hearing Ian at reasonably good strength, that the arrival angle on the evening shortpath is not especially low and seems likely to be perhaps in the 5-15 degrees range, at a semi-informed guess.

Next, it will be a similar test at morning long path. I already have, as many of you will know, the results for a coast/inland comparison with my own antennas, conducted during a Covid lockdown (may they never return!) and that up to 25dB environmental gain is available from the coast for this path at this time, allowing for the 6dB measured difference for this path and this time between the two antennas used for the test:

I didn't just look at VK3MO - using transmit from the beach, rather than receive, there were many stations in the far east and west that were also highly interesting, not least as I was either the only or one of only a couple of stations being heard by them. I'll cover those in another post.