HFU HF Underground
Technical Topics => Equipment => Topic started by: ChrisSmolinski on May 11, 2020, 1840 UTC
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I went outside to make a quick repair to the sky loop that, three hours later, ended up re-routing one side of it and enlarging it from 670 ft to 750 ft.
While repairing it I ran a SWR plot with my RigExpert and noticed that there was an SWR peak right in the middle of the 43 meter band. While probably not a huge deal for an antenna used for receiving, I decided that as long as I was working it, I could try to address that. The easiest way seemed to be to add some more wire to move the peak.
Here's a plot showing the SWR before and after making the change, the red plot is before, blue after. The resonant frequency shifted from 1220 to 1090 kHz. There's an SWR minimum now at 6840 kHz. I'll need to do some actual listening over time to decide if there's any improvement. But it's 80 ft longer now, so it has to be better, right? ;D
(https://i.imgur.com/McjcxUD.png)
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Well scratch that. It's not 750 ft long. I was perplexed by the resonant frequency being around 1090 kHz, vs the 1300 kHz you'd expect if it was 750 ft.
The 750 ft figure was based off the original construction, then adding/subtracting lengths as I repaired/modified the antenna over a decade or so. Errors add up. So I measured it again, no doubt still with some error due to the wires sagging and not running exactly in straight lines, etc. I came up with... 900 ft. Which gives you a theoretical resonant frequency of... 1080 kHz. How about that. So the antenna is now 150 ft longer, without doing any work ;D
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I like that kind of work-less work! :)
I'm trying to get a good sky loop going here, and it looks like I will be able to get about 500-ish feet in the air (well, the trees), but only 20 to maaaaybe 25 feet above ground. I have higher trees than that but my physical/tech ability to get it there (and properly mounted/pulleyed/bungied/counterweighted, etc.) is very limited.
Still, my QTH is atop one of the higher hills in the region, so my antennas can sometimes have at least that elevation advantage. Trade-offs, as always.
Is your now-900' sky loop still on your KiwiSDR? I listen to it often, and really enjoy the s-n-r with the loop and the TTFD as well. Nice, quiet antennas, especially compared to the active loops and OCFs and even G5RVs so many other KiwiSDR stations seem to use. Not knocking them, those are all worthy antennas in the toolbox, but they're just usually noisier, at least as implemented by some of the stations I've heard online. Your SDR seems to be kept very clean in comparison.
Mike
N0TLD
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I like that kind of work-less work! :)
I'm trying to get a good sky loop going here, and it looks like I will be able to get about 500-ish feet in the air (well, the trees), but only 20 to maaaaybe 25 feet above ground. I have higher trees than that but my physical/tech ability to get it there (and properly mounted/pulleyed/bungied/counterweighted, etc.) is very limited.
Still, my QTH is atop one of the higher hills in the region, so my antennas can sometimes have at least that elevation advantage. Trade-offs, as always.
Sections of my sky loop are around that high. I am in the process of raising some of them now in fact, but performance is still excellent even with the lower height, so I would not worry. With 500 ft of wire the resonant frequency should be around 2 MHz. You'd probably get decent reception from the upper end of the MW through much of HF. My experience is that once you get below about 0.75 wavelengths, the performance of the loop starts to drop off - quickly. While not completely deaf at the lower end of MW, mine is certainly hard of hearing.
Other parts of the sky loop much higher, 50 ft or more in some places. No idea what the radiation pattern is like, but I am sure it is quite convoluted.
Is your now-900' sky loop still on your KiwiSDR? I listen to it often, and really enjoy the s-n-r with the loop and the TTFD as well. Nice, quiet antennas, especially compared to the active loops and OCFs and even G5RVs so many other KiwiSDR stations seem to use. Not knocking them, those are all worthy antennas in the toolbox, but they're just usually noisier, at least as implemented by some of the stations I've heard online. Your SDR seems to be kept very clean in comparison.
Mike
N0TLD
Thanks. I try to run a clean operation :D
No, the KiwiSDR that used to be on the sky loop ( http://sdr.hfunderpants.com:8073/ ) is now on the 120 ft T2FD, which also works very well. The sky loop actually provided too much signal at night, causing overloading on the KiwiSDR. The second KiwiSDR ( http://sdr.hfunderpants.com:8074/ ) is presently on the 43/48m dipole (sometimes it is on the 500 ft beverage).
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Yes indeed, that T2FD of yours is a signal-to-noise beauty. I made a (decidedly shorter!) T2FD for upper HF RX (about 10 to 30 MHz) many years ago and in most instances it gave me quieter, cleaner reception than any TX antenna I had at the time (including resonant dipoles for 15 and 10). The 120' version on your first SDR just sounds great.
Good to know about the sky loop height, thanks. I know there is a point where returns begin to diminish with length (certainly if I want fuller MW range), but I will not concern myself too much then if I can only get it mounted so high. It will not be a perfect loop anyway of course -- it will be more like a skewed septagon?-ish shape, with not all anchor points the very same height... so what's a little more imperfection? :) I just want to get the largest area of... area, I guess!
Mike
N0TLD
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Yes indeed, that T2FD of yours is a signal-to-noise beauty. I made a (decidedly shorter!) T2FD for upper HF RX (about 10 to 30 MHz) many years ago and in most instances it gave me quieter, cleaner reception than any TX antenna I had at the time (including resonant dipoles for 15 and 10). The 120' version on your first SDR just sounds great.
Thanks! I am quite happy with how it works, even on some of the higher bands. It's actually my go-to antenna/receiver for the 22m (13560 kHz) beacon band for example. I'll just open up a browser tab and periodically check for any beacons.
Good to know about the sky loop height, thanks. I know there is a point where returns begin to diminish with length (certainly if I want fuller MW range), but I will not concern myself too much then if I can only get it mounted so high. It will not be a perfect loop anyway of course -- it will be more like a skewed septagon?-ish shape, with not all anchor points the very same height... so what's a little more imperfection? :) I just want to get the largest area of... area, I guess!
Mike
N0TLD
Yes, I think maximizing your area (within reason, don't go crazy) should be the goal. My height philosophy is to use the highest practical trees I can, but again don't go crazy.
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Well, too late to keep me from going crazy -- in general. ;D
When I get it going well enough I'll post about it, as I'll definitely want some feedback along the way, at least from people who know the subject well. I've been building antennas of all kinds for decades so I'm not too green, but I find it inspiring and motivating if I know others are offering their informed advice and even their own crazy ideas. Well... within reason. Not wanting anything TOO crazy. :)
Mike
N0TLD