Hi Andrew,
The coils are wound tidily and closely packed using the “magic SOTA wire” that I bought while still in Australia from - was it it Peter? who imported it in bulk and then sold off shorter lengths. Great quality wire that I have used in many projects. The scrambled look is from the ice on the coil.
The G5RV / ZS6BKW with twin feed wire may be worth a look - another one for the list to investigate.
With the centre loaded 80m dipole, it could of course be it’s location that impacts its performance and impedance (which seems low at 24 ohms) - I think I might take it out to a summit late afternoon and try it there, however the Balun that it currently has is quite heavy as it was not intended as a portable antenna. Perhaps I should simply build this design again but with lower weight components and try that out.
Now there’s a good idea - thanks Pete. Actually I bought a cheap kite about a year ago to try out a kite held antenna on a SOTA activation, but the work to keep it airbourne while operating, probably would be easier with someone along at the activation (idealy another amateur).
Sorry, but must take exception to a few of Ron’s points:
An 88ft doublet still has two main broadside lobes on 14MHz. It doesn’t get multi-lobed until you’re above 14MHz.
A 44ft doublet is down only a fraction of a dB compared to a full size dipole on 40M and is not “certainly shaded”.
My personal experience on summit activations is that a horizontal dipole will outperform a vertical for dx, The down slope in all directions on a summit means that effectively the dipole is very high off the ground and the elevation angle of propagation very low. I’m sure everybody’s experience will vary but I disagree with it being stated as a rule that the gold standard for dx from a summit is a vertical.
According to the antenna analysis program EZNEC, on 14.2 MHz the 88 ft doublet in inverted Vee arrangement has two good lobes at right angles to the wire with 4 nulls of 30 dB approx at angles of 35 degrees to the wire. Except for the area covered by the two lobes the signal is several S units down cf a dipole. On 21 and 28 MHz there are more nulls. Now I know the program has it’s limitations but the old ARRL handbook antenna patterns support this. I also know that often in the field theoretical shortcomings can be reduced by the nature of the ionosphere and the topography.
The biggest single factor about mountain top operation is the low noise level. You can hear the other QRP guys very well.
On Wednesday I did a A/B check on 7.032 MHz and found the dipole on the same mast in the same alignment to be a good 6 dB better than the 44 ft doublet. How much of this was due to the balun and ATU I’ve yet to find out. However the balun did well in back-to-back testing at 100 W and the tuner is an LG model so it’s unlikely that they would soak up as much as 1 dB each.
The strongest 20 m SOTA signals out of the UK from summits are all from verticals. Only when the dipole is up at 20 m or more - worked a couple of DL activators who have done that - does the dipole do as well or better. For short hop distances the vertical is not as good as the dipole.
Poorly performing verticals IME have problems with the radials. Being on or close to the ground can give a good VSWR but it likely will warm up the worms. ie they have reduced efficiency and the radiation pattern in elevation may be affected.
It is probably best not to argue about the performance of antennas for SOTA on the basis of theory, there are too many uncertainties involved - the effect of the slope of the surface, which will only very rarely be the same in all directions, the depth to the effective ground (both rocks and snow are good insulators but organic soil can be a good conductor) and the geometry of each individual set-up (how vertical is a vertical in practice!) Even a straight-forward A/B comparison may be more about the site and set-up than it would be about real differences!
As one who has performed soil resistivity surveys, I can tell you that the resistivity of soil in a location can vary by a couple of orders of magnitude in a few metres!
If we come back to the 40m inv-V with coil loading so it works on 80m, Ed says it doesn’t work very well. In what way is the performance sub-optimal? Compared to a full size 80m inv-V? Compared to an 88ft doublet? 44ft doublet?
Is it not working as well as something loaded and half sized should work or is the performance bad allowing for it being shorter?
Perhaps investigating if there are some setup/design/construction (*) issues, or installation issues may be time well spent. Moving the apex up or down or altering the height the ends are located may change it from a dummy load to something acceptable.
As for the possibly radiation pattern strengths and weakness of 44 or 88ft doublets. Maybe it would be worth going chaser portable to a local park etc. and setting up the coiled loaded antenna and doublet and trying to chase with them to see if the nulls are an issue in use? Also I should point out the design requirements were for an antenna that was space optimal for 80m and didn’t necessarily mention it had to be FB on 10m at the same time. A radiation pattern that reminds you of a porcupine/hedgehog’s bottom on the higher bands may be totally acceptable if the thing works on 80/60/40 etc.
Just thinking out loud here, please do shoot down any daftness I may be suggesting!
(*) Ed’s has been licensed for many years so I think he can wind a coil and solder a connection OK, but there could be silly errors being overlooked as the antenna may have an acceptable match.
The impedance of 24 ohms is not far off what I would expect for a low dipole. If your dipole is 7 to 9 metres above ground, that’s a good figure because it includes ground resistance. Was the bandwidth fairly narrow, eg <100 kHz for 2:1 SWR?
If you look up the antenna handbooks you will find a graph showing the impedance of a dipole at various heights. The heights are in wavelengths. A 9m height is about 0.11 of a wavelength on 80m and you may find the graph indicates an impedance of 15 ohms or so, the other 9 ohms of your measured impedance would be the sum of the resistive losses including ground and in the coil. My books are still in storage so I can’t refer to the graph here, but I know the convenient 50 ohms impedance of a dipole is shown at a somewhat greater height than 0.11 wavelength.
As others have mentioned, efficiency suffers with low loaded antennas. 80m is reasonably tolerant of an antenna that is only 50% efficient, so if you can achiever 50% you have done well. That’s only a loss of 3db.
If you have access to a Q meter it would be interesting to measure the Q of the coil.
Another approach you might consider is a trap dipole 40/80, Sotabeams have some interesting designs available and I think you’d find the 80m extension past the trap would be only about half the full size extension (best check with others who have tried them), again that’s not much different from a G5RV. The bandwidth can be expected to be half what a straight wire would be, so I’d not expect better than 100 kHz for 2:1 SWR. For ssb only (no CW required) that should permit enough bandwidth to be useful as a SOTA station.
Yes, antenna theory and experiments fascinate me also for more than 40 years.
But to be honest, I can not explain the s2s QSO today at 14MHz with VK2IO/P with my KX3/10W and 80-10m EFHW (42.5m on 12m pole), hi.
The 19 QSO on 80m (HB9, DL, OK, G, SP, F, I, PA) are then more likely to be explained.
Virtually all my SOTA operation is on 80, 60 or 40 and I use an “extended Norcal” made out of computer ribbon cable with 17m legs each side - ie just over a quarter wave at 60m. Matched with an MFJ 904H tuner (though I am in the process of building a smaller and lighter Z match tuner). Seems to be reasonably effective. Not meeting your no bigger than a 40m dipole criterion, but it is lightweight, horizontal, wire, reasonably economical and easily homebrewable, perhaps five out of six ain’t bad
Yes although we complain about conditions, there are some days when they can still surprise us. But I think the 20m experience is a completely different “animal” than the 80m antenna problem. There it is all about reducing losses and obtaining a workable impedance for the radio. 160m even more so.
Have not worked you for a while, no activations here due to a minor injury, now much better. Just in time for cooler weather and the end of daylight saving
I’m a little surprised that the idea of a magnetic loop antenna hasn’t been floated yet. There are a number of designs already being marketed, including the alex loop, For those looking to build their own, there’s some very good information to be found here: http://gridtoys.com/glen/loop/loop_4.html
Strangely the antenna analyser told me the antenna covers most of the 80m band at an SWR of 2:1 or better. I was surprised at that - I was expecting a narrow bandwidth.
With the Impedance, what was strange was adding a 1:2 Balun to the feed point only took the impedance up to 32 ohms, not to 48 ohms as I had hoped for.
As I said somewhere above, the location of the shortened dipole at home is not good - it is simply strung from the (5.5m) flagpole out to covenient points.
My comparisons of effectiveness is against a 40m full wave loop, 3 metres higher. The loop doesn’t work well on 80m but I guess the extra height makes the difference.
The antenna (or a lower weight copy of it) deserves to be tested without buildings around - i.e. from a summit, with the centre up at 9m off the ground (I wish I could get it higher but that is all that is practical with a portable 10m mini-mast, the top section is effectively unusable as they’re too thin to carry even the weight of the balun). The point about getting the ends as high as possible off the ground is a good one as well, however in a portable configuration, I think I will still be setting up in inverted V format (albeit with as long a cord as I can fit in on the ends of the wire, to get the ends as high as possible).
Could you not use your 40m dipole or a 66ft doublet antenna and turn it turn into a vertical style antenna by disconnecting the feeder at ground level and short the conductors together and then connect to your ATU/rig as a single wire and at the other connection point on the ATU/rig connects to a ground rod feeding ground radials. Depending on the ATU or rig a good investment is a BNC binding post always handy for those emergencies.
For 80m I built a OCFD antenna wieght at 1.5lbs including balun, pegs and feedline. I used 135 feet of dx wire ultra lite (not sure of the lengths after trimming)10m of rg174 with 10 turns on a half inch ferrite rod for a line isolator/choke and a hombrew 4-1 current balun splashed with liquid electrical tape for luck and weather proofing.
My plan is to use this on overnight activations for camping trips or on large summit areas with calm weather as it is a bit of a beast to setup and the long element does some swaying on windy weather.I have used it twice from a summit for testing and trimming but looks promising. I used it with a T1 tuner and KX3 ATU on 160m and seems to tune well but will soon find out when I get a chance to dabble on that band.
Hi Graham,
My current SOTA antenna is a Spiderbeam UL-404, (it’s an OCF dipole), so I believe your suggestion to change a 40m dipole into some kind of end-fed wire would not work with this one. If I understood correctly, there would also be a disadvantage of needing radials - which in themselves take up a lot of area and therefore breaks one of my main criteria of a small area requirement.
I know there is no perfect answer and some solutions will suit some amateurs and some summits better than others but I think it’s great that we are all discussing this putting in ideas.
Quite early on a tapped loaded vertical was suggested and I realised I have one of those somewhere and today I actually FOUND IT! And … (drum roll …) it is supposed to work on 80m! How well it works, will remain to be seen. It will need a groundplane as it’s actually specified that it is meant to be roof mounted on a mag mount on a car, hence the bround plain is not large.
I am also encouraged to look at the shortened 80m dipole again as it seems what I am seeing on the one at the home QTH, may just be a bad physical location and the antenna is not high enough. (I am still searching for the lightweight balun that I built, to use to build a lighter version of the shortened dipole.
Even better, off centre feed it, in fact take it to extremes and feed it from the end!
No need for any feeder worth talking about, no balun or feeder up a flimsy mast and only requires a very short counterpoise.
I haven’t done any 80m SOTA activations (yet) but my 60m dipole I feed from the end and raise it up to between 3"-9" (depending on how long the grass is that supports it).
Negates the use of any mast or support.
