TAKE FOUR - Top band Tuesday - feel free to join in

MW0KXN · 22 February 2023 08:35

Hi Andrew,
I have packed the antenna analyser. We will do our best to get some measurements for you this week.

73, Kevin

G4AZS · 22 February 2023 12:51

I’ve alerted for Stiperstones G/WB-003, I’m going to try just laying the dipole along the rocks / heather. I might take some supports as well, in case that is a dismal failure! Please bear with me if you can’t hear me at first, I’ll self spot for info.

73
Adrian

G4OBK · 22 February 2023 13:43

Very good guys! Speaking as a Chaser I should be a fair bit stronger this week having repaired a broken wire at the top of my inverted L. Aerial now nicely resonant on 1830 KHz at 1.15:1 and usable up to 1872 KHz with a 2:1 SWR there. I’ll probably put the ATU into circuit if you are operating on SSB that high up the band. It will be interesting to see if I can hear Adrian on the DOG! (Dipole on Ground).

73 Phil

G4AZS · 22 February 2023 23:03

Maybe I should post under the “dogs on summits” thread

Well, I worked Phil, and several other stations with quite good reports, but rapid and deep QSB.

Stiperstones is a ridge covered in loose rocks, with a patchy covering of heather. I ran out the 80m long dipole lying on top of the heather and protruding rocks, such that the average height above “ground” was about 20 Cm.
It matched reasonably well with the Elecraft T1 tuner, and I used the FT817 at 5W.
Receive noise was lower than last week, with none of the static crashes.
With 7 distinct callsigns in the log, the summit was qualified, and of course the hoped for S2S with Ben @GW4BML/P and Kevin @MW0KXN/P were completed - thanks guys!

QSB was very deep, taking signals from S9 to below the noise at times. Intriguingly, Karl @2E0FEH and Mike @2M0WNA called me together, from opposite ends of the country, and both (strong signals) were affected by QSB in sync eg they both came up and went down at the same time. Presumably the ionosphere above us was changing - D layer depleting after sunset…?
I wonder if I might have managed more contacts if I had stayed for longer into the night, but unfortunately the cloud settled down over the summit, and it started raining.
The terrain is rough, and a combination of cloud, rain, spectacles and head torch is not great! I know the summit quite well, though, and made it back to the car slowly but without difficulty.
Back at the car, the clouds parted for a few minutes, and I was reminded how magnificent Orion is at this time of year - it is visible from home in the town, but nothing like that!

So, on this summit at least, using a DOG seems to be at least adequate to qualify, and is much simpler than getting it in the air with enough tension to keep it clear of the ground…

Thanks to all who worked me, and listened for me, sorry if we didn’t make it.

Wound up and ready to move off:

73
Adrian

2M0WNA · 22 February 2023 23:11

Fine February effort, Adrian. You were clearly workable from Aberdeenshire despite the heavy QSB - and your eventual heavy rain. There’s definitely life in the old DOG …. 73 Mike

G4OBK · 23 February 2023 06:47

Fantastic Adrian - you guys have really started something here! It was a very interesting antenna experiment that worked surprising well. I imagine your ODX was probably Esther @GI0AZA or more likely Mike @2M0WNA Glad that you were able to work the two Welsh Top Band SOTA wizards @GW4BML AND @MW0KXN before it got too wet to continue.

73 Phil

2M0WNA · 23 February 2023 08:32

Hi Phil. You were booming in here 5/9+ as was Esther @GI0AZA, so great work by One Man and his DOG on Stiperstones. It’s 328 miles from my QTH. He was well workable on CW, too, And I worked the Welsh double-act on Top Band despite the tricky conditions so a fascinating evening all round. Well done all. 73 Mike

MW0KXN · 23 February 2023 14:10

Hi Andrew,
We managed a successful activation without any antenna collapses last night. The images below show VNA plots for the full length 160m dipole with the center supported at approximately 6.5m above ground level and the ends of each 40m wire approximately 1m above ground level. The feed point has a current choke and the feed is 10m of RG174. Ground conditions were wet peak, it wasn’t raining or misty, the temperature was around 3C, the dewpoint must have been around 0C, and there was no dew on the antenna wire at the time of measurement.
The vertical scale is set to SWR 2 units per division.

I hope that this answers your questions. We are happy to take more measurement on future activations if you have any further questions. I will post a more detailed activation report later.

73, Kevin

IW2OBX · 23 February 2023 20:33

Hi Guys, I attended the 160mt event from San Primo I/LO-191.

@GW4BML Ben thanks for the s2s qso in 80 meter. I worked with a 42 meter wire inv V configuration, I fixed the middle of wire to 10mt telescopic pole and one end to 4,5mt telescopic pole. on Top band I connected directly this wire on kx3 and two long wire as ground. for 80meter I connected a Guanella transformer 16:1. On Top Band I worked three stations, all three stations 1500km far; I heard two weak signals unworkable. on 80meter I managed a Little pile up. Great fun evening!!
See you next time!!!

G4AZS · 7 March 2023 17:09

My top band dipole was hastily constructed for the 90% Eclipse in UK in 2015.
Report here by John G4YSS: G4YSS: G/TW-004 on 160m for the Solar Eclipse, 20-03-15
I bought a 100m reel of hookup wire, unrolled it and folded it in half, paced out 40m from the loose ends and tied it there for the dipole centre. That gave me the dipole top, and the remaining “loop” was turned into twisted pair for the feeder. My Elecraft T1 tuner forms a match, but it has never managed much better than 3:1.

It is far too big to work on at home, so today I took it out onto the Long Mynd G/WB-005 (not in the AZ) to set up more correctly.
I decided to write up my findings here rather than start a new topic as it seems to fit in with the discussion.
The objective was to cut it to the correct length for resonance, and to measure the impedance and any change to resonant frequency when close to the ground. It will always be used quite low, so I started with the centre at 3m, and the ends at 1m above ground. Wire sufficiently tensioned to keep it fairly sag free.
I had to cut 4m off each end to make it resonant, so that will involve slightly less walking in future!

Using an MFJ analyser, this was the result for 3m centre, 1m high ends:

And this with centre self suspended about 15cm above ground, 1m high ends:

Putting the ends of the dipole close to the ground (eg removing the walking pole end supports) took the resonant frequency well below the range of the analyser (1.7MHz lowest tuning point), such that there was no hint of a dip as I tuned down towards the limit. Changing the centre height made no noticeable difference to this.

With the ends at 1m high, altering the height at the centre made little difference to the electrical characteristics between the test values of 15cm and 3m.

The ground was damp, flat, grass covered earth with a loose rocky substrate based on how the pegs pushed in.

So, on this basis I think in future I will try to keep the ends clear of the ground, and not worry about the centre.

I did connect up the FT817 and call CQ a few times on CW, but the band was very quiet, and I got no spots on RBN - there seemed to be no skimmers in or near UK reporting any signals at all, which is perhaps un-surprising just after mid day.

The test site, looking across to Caradoc G/WB-006

Calling CQ, the feed point is self supporting just above the yellow kite winder. T1 tuner no longer required for top band, I tried it on 80m but it didn’t want to know…:

Looking forward to the next outing but tomorrow, Wednesday 8th March, is loooking a bit too cold for me by dusk. Hoping to be out during the day, but not on top band!

73
Adrian
G4AZS

G8ADD · 7 March 2023 19:36

You might need the tuner, Adrian! All my 160m chases have been between 1.843 and 1.88 phone so if you try and work S2S to someone near 1.88 who is also using a dipole then either you operate with a wide split or one of you must have a tuner - top band antennas tend to be rather narrow. My 40m long doublet tuned for 1.84 at 1.1:1 gives an SWR of 5:1 at 1.88.

MW0KXN · 7 March 2023 20:24

We hope to get back out on a hill on 160m on the evening of Wednesday 15th March. I can’t get out tomorrow 8th March.
73 Kevin

VK1DA · 8 March 2023 03:09

Hi Adrian,

The SWR and apparent input impedance shown for this antenna at a very low height above ground (even 1m height is 1/160th of the wavelength, or .006) is mainly telling you how big the ground loss resistance is. The following discussion explains why, I think.

The impedance of a dipole is usually displayed on a graph like this:

Compare this graph with the bland statement that “a dipole in free space has a feed impedance of 70 (or 72, or 72.777) ohms”. So what is meant by free space? The graph indicates that the impedance wobbles about an average of somewhere above 70 ohms, and at a height of say, 10 wavelengths would not vary much from about 72, and in real Space (between Earth and Moon, say) it would probably be stable at around 72-73 ohms. Free space means the formula conveniently ignores the effect of nearby ground.

As you can see, there is no impedance plot detectable for .006 wavelength, the lowest actual plot is for 0.1 which would be 16m above ground, not 16 cm.

To account for ground losses, the following graph is usually found next in the handbooks.

The Perfect Earth plot is similar to the previous graph, but the Imperfect Earth is what we are dealing with on poor soil, rocks etc. So the predicted impedance for 0.1 wavelength would be about 20 ohms of radiation resistance and another 25 ohms of loss. Efficiency is 20/45 or a bit less than 50%. Not too bad.

But at .001 of a wavelength we have about 1 ohm of radiation resistance (interpolated aka guessed from graph) and 70 or 80 ohms of total impedance, so about 70 or 80 ohms of loss resistance. The efficiency is now 1/70 to 1/80 or not much above 1%.

This is why height is so important especially on low frequencies where wavelengths are long.

The above analysis is found in all the antenna handbooks and is regarded as standard by the low frequency experts. Add to that the issue of these horizontal antennas being effectively cloud warmers and the rarity of dx contacts becomes apparent.

This is why I was curious to know the feed impedance of the dipole when it was up in the air. Its impedance when lying on the ground is almost immaterial, it’s just a measure of DC resistance and ground losses. Measuring the impedance at the operational height is essential if something is going to be taken from the experiment for reuse on another day.

All the above comments are just that, comments. I admire the effort taken to put up that large antenna. I’m not an engineer, I just read a lot of books and articles over the last 55 years. So if others disagree with my comments, I don’t mind.

73 Andrew VK1DA/VK2DA

GW4BML · 8 March 2023 08:57

Hi all, there will unfortunately be no Wednesday evening madness tonight on top band due to the wx we have received in GW - Kevin @MW0KXN and myself do not want to be stuck on a high mountain in the freezing cold, it has been known though we plan to resume next Wednesday evening all being well.

73, GW4BML. Ben

M0JLA · 8 March 2023 09:00

Wise move gents. I’m very glad we had no plans for this week. Next week might still be a problem.
73
Rod

G4AZS · 8 March 2023 10:18

Hi Andrew, thank you for your comments, clearly presented and very interesting.
I haven’t researched the subject in detail, and my tests were aimed at finding what is practical for me to erect alone in the dark, probably in the winter months, that will radiate adequately to make a few contacts. I’m under no illusion of creating a DX magnet!

Perhaps shorter, loaded elements and a higher support would be better (as used successfully by John G4YSS) - but avoiding a strong central support if possible seemed an attractive option for me.