HFU HF Underground
Technical Topics => Equipment => Topic started by: ChrisSmolinski on May 04, 2020, 1543 UTC
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I'm actually considering setting up a vertical antenna, now that I have cleared some more space. The plan is first a single vertical, and then perhaps parasitic reflector and director elements for some directionality towards Europe. Ideally they could be easily raised and lowered so I could go back to an omnidirectional vertical if desired. I am planning on using just insulated wire for the vertical element(s), held up with a support rope over tree(s). This antenna would be for receive only, BTW. Ideally for both 48 and 43 meters, but if I had to choose one band it would be 48. But if a compromise design for say 6600 kHz has enough bandwidth to cover both, that would be ideal. I'm thinking a regular 1/4 wavelength with similar length radials.
That's the easy part. My main question has to do with the radials, the bane of every HF vertical antenna installation it seems. Obviously the FCC recommendation for 120 radials as used for MW stations goes out the window. Looking at some papers with actual test data, it seems that around 16 radials is a sweet spot, with diminishing returns above that. 16 radials would be do-able, so OK there.
The next questions are their length, and elevation. From some papers it seems that even a small elevation of the radials can be beneficial in reducing ground losses and increase gain. Even just 6 inches above the ground. I don't want them very high for various reasons, laying a few inches above the ground might be best (they would be in a mostly wooded area so they could lay on top of the various twigs, small branches and leaf matter on the ground).
But it also seems that (maybe?) this creates some resonance effects, which might reduce the useful bandwidth of the antenna? Or is this only a consideration for transmitting (if it is even real at all). If this is a real effect and possibly an issue for even just receiving, then would cutting the radials to a range of values, say resonant between 6200 an 7000 kHz be useful?
It also seems that I can get away with fewer elements for the parasitic elements, perhaps only 4 or so?
Next is radial material. Lots of options here, some cheaper and easier than others. Insulated wire? Fairly cheap electric fence or aluminum wire? I assume you want to avoid anything that has too low of a resistance like plain steel/iron wire?
Finally how to feed the antenna. It seems the feedpoint resistance may be in the 40 or so ohm range? I use RG6 for my feedlines, so a simple matching transformer can deal with that. Hey, as it turns out I make and sell them myself :)
I know some folks here have extensive experience with verticals, so any and all comments/feedback/suggestions/warnings welcome. I am still a few weeks (at best) away from being able to start doing any actual construction work, so now's the time to formulate a plan.
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Well, here's what I've been wanting to try for years, but the closest that I've gotten too was to shunt-feed a tower. My plan is to build a raised VERT dipole on 40M in an +80ft pine tree. However, I don't have an 80ft pine tree where I am here, but did up in WNY. The idea was to run the VERT radiator and radial mounted to the whole trunk of the tree. The few problems that I foresee here was the GND affect of not having that elevated GND radial high enough. That should actually be more of a problem for TX and not RX. S/N Ratio would be increased going VERT, so ideally having such an antenna as far from the power lines and the noises of the house would be good. NVIS closer-by stations would be harder to DX, so the HORZ dipoles for them, but European, Pacific, and Latin American, DX should improve significantly. (Lazer Hot Hits would be a good tester for this.) To reduce static build-up, a 1K-Ohm resistor between the radiator and radial would be a good idea, albeit it may go POOF with a close lightning strike. RG-6 is certainly not a problem for RX'ing, albeit your feed point would be elevated +40ft in a dipole situation. Sounds like you have a Balun already in mind for the slight miss match. (For RX, I wouldn't even worry about that.) If you went the 1/4-wave over GND based radials, the question is on RX only if the more radials, the better? I can tell you that just using 4 radials on the tower shunt-feed may not have been enough on TX'ing. Stations on MW, 160M, 75M, and 40M, were very good, like S+20/9 RX'ing, but getting back to them was doable, but I was told that I was weak. So, more elements for GND radials would have helped me there, but in your case 3, or 4, for just RX'ing maybe enough. As far as GND radial wiring to try? I was using AWG#10 solid copper, leftover from a 500ft spoil that I bought in the early 1990's. (Yeah... I've made a lot of antennas from that, as well as gave away a lot of it.) Aluminum should work fine for the GND radials. Never tried steel, but I don't see why that wouldn't work. And, a parasitic reflector? Hmmmmm... Do you have a close-by tall tree to run some metal element up and mount too? (Wishful thinking, but...) Finally??? Even though you want to dead-nuts match this VERT at 43M, you may find that it should be decent from MW to 30MHz, especially with an antenna tuner, but even without. So, good luck on this project! Just let us know what your results are after you get that going. I'm still going to wish for that +80ft tree to use for mine. Just my $0.02 worth.
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I'm more involved with transmission aerials than reception so I'm not 100% qualified to answer you but I doubt whether the number of radials, length and on top of or above ground will make much difference. One good earth rod may be sufficient.
Impedance is going to change depending on frequency anyway so I wouldn't be bothered if it's around 40 Ohms. (Unless you're secretly planning on putting on a TX ;))
Considerations for transmission aerials are much stricter than for reception. The earth system is equally important as the radiator which is why they usually use copper wire, several earth rods and 120 radials. Also the matching is much, much more important as the TX must see the correct load.
Heck, even my HF aerials give excellent reception on bands other than what they are tuned to.
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IMHO, for a receive only vertical you may not need to be so detail oriented. A 1/4 wave vertical with 1/4 wave radials cut for somewhere between your frequencies of interest
should work fine. My approach would be to staple the radials to the ground with pins made for that purpose like those available from DXE or fabricate your own. This will keep the radials
at or slightly below grade and thus not present a tripping hazard. Even if the area is out of the way, various critters and especially deer will tear up any even slightly elevated radials.
Don't use steel wire radials for any permanent type installation as they will quickly rust and especially stay away from aluminum wire as it will deteriorate even more
quickly especially in soil in the woods that is probably acidic. Consider insulated copper wire such as THHN building wire for the radials. You may find that use of a typical off the shelf MFJ
antenna tuner will enhance your reception.
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Some food for thought;
https://www.okdxf.eu/lankford/Measurements%20Of%20Some%20Loop%20And%20Vertical%20S%20to%20mmN%20Ratios.pdf
"LF AM stations from Eu.. 160 Khz to 200 KHz. Any evening I can hear 3 or 4 of these on the Lankford vertical array, sometimes even 6 or 7 of these stations, and often at S6 or 7 comfortable listening levels. I CAN NOT EVEN DETECT THEM ON ANY OTHER ANTENNA ! You want a demo of low noise receive methods ? This is it ! BTW, my receivers are an IC7700, K3 and mint condition R390A, although the R390A and K3 don’t go down to the LF band. This is with 2 small 15 foot whips, 60 feet apart, fed with speaker wire, on a small urban lot. Power lines all around, crap ground, lots and lots of negative antenna influences. The array is influenced by nearby antennas, something I cannot do much about. But it still works in a highly remarkable fashion.
160M, 80M and 40M ham signals.. very good reception with the Lankford verticals. In all cases, there is less noise and improved intelligibility on weak signals compared to any other antenna. On 160M, it compares favourably or better than the K9AY, although it is more convenient to use the K9AY because of the simple K9AY switching direction system. My Lankford array requires tweaking the phaser controls, plus the null from this 2 element array is bi directional and can only be moved through 180 degrees. The SWA has the capability of moving a 360 degree very deep null towards the noise source you don’t want. This can be effective, but takes quite a bit of fiddling to make it work. "
http://www.ve1zac.com/Lankford%20Antenna%20References%20.htm
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According to the late Les Moxon, a quarter wavelength is just the length to avoid for radials of ground plane antennas. Rather use shorter wires, and even take into account the speed factor (for buried wire, it can be as low as 66%).
Also a long study by the ARRL gave the result that shorter (buried) radials increased the transmitting efficiency. Maybe not so important for receiving, but buried wires are a lossy part of an antenna system. Moxon strong advice: if needed, tune the antenna by adding a coil (or capacitor) on the vertical part. Well known too: elevated radials are so much better that only a few of them are needed (of course, they'd better be 2.5m/8ft high for obvious reasons) . Think of the radials as the 'hat' of the bottom of the vertical antenna.
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Transmitting and receiving are reciprocal. What benefits one, benefits the other.
Its been my experience that in a practical sense one will SEE the benefits of more radials in the reduced bandwidth of the antenna in use as a transmitting antenna (as losses decrease) more than you'll hear the difference. You'll also SEE that adding more beyond a certain practical point (8-10) becomes less and less impactful to the efficiency . (It's kind of like asymptotically approaching a limit- there's a big diff when you start, but that really tapers off quickly). You can see this with the narrowing of low SWR bandwidth. Its pretty impressive ! :-) Short, long or in between length radials, it's hard to tell much practical difference given all the other variable involved in setting up an antenna in all but a uniform, idealized antenna range.
I'd not overdesign on paper chasing that limit, but put up the 1/4 wave with 6-8 radials under it, then play with the spacing, position and length of the director/reflector. I think you'll notice FAR more of an effect messing with that , than the radials (beyond a few). I'd get the geometry correct, then add radials, as the directivity and S/N will come from geometry of the array , while the radials will "just" affect the overall, broad band loss.
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.... and I can show you aluminum electric fence wire that has been in/on the ground in my pine/hardwood forest floor for 20 yrs. It's still fine. :-)
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FWIW, all my transmit antennas use the same basic configuration, 8 radials. I settled on 8 as any more add setup time and inconvenience, while offering no discernable performance improvements. As part of the setup proceedure, a sweep of the antenna is made and the antenna adjusted as necessary. There is a balun designs feedline choke at the feedpoint, nothing else. Based on the theoretical feedpoint impedance of 36 ohms, this implies a ground loss somewhere in the region of 14 ohms.
This sweep is form the a previous show setup (05/10).
(https://i.imgur.com/iIaIXlI.jpg)
This one from a few weeks later, same antenna and locale.
(https://i.imgur.com/3BZWBLm.jpg)
+-RH
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Probably doesn't matter, but ran into this while researching how to insulate my hanging discone...
http://www.repeater-builder.com/antenna/tree-antenna/tree-antenna.html
There's some fellow near Glenelg that has one of those fancy 4-corner vertical setups, along with a whole host of other antennas
The countryside in that part of MD is thick with folks from (ahem) places w/ lots of RF experience......
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There's some fellow near Glenelg that has one of those fancy 4-corner vertical setups, along with a whole host of other antennas
The countryside in that part of MD is thick with folks from (ahem) places w/ lots of RF experience......
Yes, when I used to commute home along MD-97 I noticed an antenna farm or two.
I've shifted gears and am now considering a V-Beam instead of a vertical, as it will let me get some directivity/gain towards my intended target area, Europe. I need to finish clearing some brush/trees and see what my location options are first.
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Simple vertical antenna (modified 11 mtr/27mc antenna)
(http://www.coolam.nl/antenna-vertical-48m.jpg)
drawing by MRF/Netherlands
André
CoolAM Radio - Shortwave
the Netherlands
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Don't forget to weatherproof the elevated end like crazy (and a drip loop might still be a good idea) if you're using F-connectors and RG-6. Ask me how I know.....