HFU HF Underground
Technical Topics => Equipment => Topic started by: Jock Wilson on June 25, 2019, 0357 UTC
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Yep, your reply is informative, and I agree totally with your analysis.
I think it's worth adding that the physical characteristics of his half-wave dipole, namely 4 metres off the ground and slung in an east-west direction must necessarily make it directional as a north-south RF radiator for 6 megs.
I reckon if said radiator were slung higher, it would be less directional up to a critical vertical height when it would become directional again above said critical height.
My memory ain't the best these days, but it tells me for a non-directional dipole, it should be non-sloping and about a quarter of a wavelength off the ground.
If the station were located in the UK, the op would be well advised to sling his half-wave dipole in a north-south direction were his target listenership eastern seaboard North American DXers, of course located across the pond from him.
Any thoughts on what I have added?
Over and out.
(From thread R. Merlin on 24 Jun 2019, subject edited by moderator Ray)
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Yep, your reply is informative, and I agree totally with your analysis.
I think it's worth adding that the physical characteristics of his half-wave dipole, namely 4 metres off the ground and slung in an east-west direction must necessarily make it directional as a north-south RF radiator for 6 megs.
I reckon if said radiator were slung higher, it would be less directional up to a critical vertical height when it would become directional again above said critical height.
My memory ain't the best these days, but it tells me for a non-directional dipole, it should be non-sloping and about a quarter of a wavelength off the ground.
If the station were located in the UK, the op would be well advised to sling his half-wave dipole in a north-south direction were his target listenership eastern seaboard North American DXers, of course located across the pond from him.
Any thoughts on what I have added?
Over and out.
There's an interesting article about dipole height and it's affects online: https://www.qsl.net/aa3rl/ant2.html
Johnny Tobacco also uses a low dipole at this time... BUT with his kilowatts of power, he still gets out an impressive signal, including over the pond to the US.... just goes to show what brute RF power can achieve.... but Radio Merlin's power being a modest 20 watt, the signal is generally quite impressive too, considering it's low power.... and it also travels to the East Coast of the US.
Basically, from what I understand, a low dipole will radiate most of it's power straight up in the sky at high angles so "IF" conditions permit NVIS propagation, Radio Merlin will get most of it's power radiated straight down on the UK and surroundings, and I understand that the directionality of such a low dipole is not as great as when it is put up half a wavelength up from the ground, which for 40 meters would be a height of 20 meters above ground! Such a high dipole would be more directive east-west and the lobes of maximum radiation would be at lower angles benefiting long distance DX when and if the ionosphere allows for this.
Considering how well Radio Merlin's signal is received over Europe and in the UK when conditions allow, with only 20 watt, the relative low height of the dipole might be the most desirable, to be honest, but it should not be much lower or you get more ground losses.
A low dipole (5 meters above ground) is a "cloud warmer" at long hf wavelengths, but can be desirable if you want to try and get a strong "local" NVIS signal when conditions on the band allow for it.... when they don't you'll be sending maximum power to your local flying saucer in space or your friendly nasa astronaut on the space station or moon. ;)
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Not agreeing with Jock Wilson about low dipoles: they are not directional.
Confirmed again reading the reference from aa3rl ginen by the Ether Hacker :
a low dipole is equally well (or equally bad) in every azimuth, with the maximum of radiation direct to the clouds. For receiving, a low dipole is sometimes jammed by noise in the clouds just above, even hours before any T-storm. Most of the time, it's a very good receiving antenna.
At low HF frequencies, that idea of low take off angle seems a bad one. Rather look for 'chordal hop' !
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Not agreeing with Jock Wilson about low dipoles: they are not directional.
Confirmed again reading the reference from aa3rl ginen by the Ether Hacker :
a low dipole is equally well (or equally bad) in every azimuth, with the maximum of radiation direct to the clouds. For receiving, a low dipole is sometimes jammed by noise in the clouds just above, even hours before any T-storm. Most of the time, it's a very good receiving antenna.
At low HF frequencies, that idea of low take off angle seems a bad one. Rather look for 'chordal hop' !
It's difficult to say what is a better antenna or not. There's no correct answer. I think it depends on your (local) situation and what you wish to do and the frequencies on which you wish to do it. Certainly a dipole, monopole or related array antenna is good for radiating... for receiving I think low noise antennas like Beverage antennas are probably the best I've come across... but they take up a lot of space... nice, if you have it.... and they are not suitable for transmitting, just receiving.... but very, very impressive results
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Here you go, radiation plots of a 48m half-wave dipole at heights:
Black 6m
red 12m (quarter wave)
Blue 24m (half wave)
(https://cdn1.imggmi.com/uploads/2019/6/26/a2c9c403ba9b2aae9522ed5a35dbf96d-full.png)
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Here's a more traditional view, V field only. IN the left plot the dipole is running N-S.
(https://cdn1.imggmi.com/uploads/2019/6/26/23254683f3c04bea2dc6f2c10b328930-full.png)
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Here's a more traditional view, V field only. IN the left plot the dipole is running N-S.
(https://cdn1.imggmi.com/uploads/2019/6/26/23254683f3c04bea2dc6f2c10b328930-full.png)
Sorry, but your diagrams are not displaying, ... at least not on my browser
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Anyone else not seeing them? I am.
Links:
https://cdn1.imggmi.com/uploads/2019/6/26/a2c9c403ba9b2aae9522ed5a35dbf96d-full.png
https://cdn1.imggmi.com/uploads/2019/6/26/23254683f3c04bea2dc6f2c10b328930-full.png
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The directivity diagrams are for far fields.
For far fields, V and H are linked by the free space impedance...
That may be wrong for receiving antennas, when the oncoming wave is strangely disturbed, with unknown polarisation.
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Kind of related here, but you can also implement NVIS Antenna pattern, http://www.w0ipl.net/ECom/NVIS/nvis.htm
HAM's are finding that the newer 60M, (5MHz), band is great for this. Great from regional coverage.
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Unless you're up a quarter wave or more you get nvis ie omnidirectional pattern, but mostly up, rather than classic dipole radiation pattern. I suppose a vee or long wire of a few wavelengths or even a bev will be about the only way to get real directivity on hf low to the ground. I know a guy who has a wire beam on 40m and he has an insane sig but I think he has them up at phonepole height.
This is a neat design and quite directive;
https://www.w8ji.com/ham_universe_curtain.htm
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Yes, as Josh noted unless your antenna is up fairly high, really close a wavelength, your pattern will not be anything close to the classic dipole figure eight.
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I didn't see this article listed and personally found it very helpful:
https://www.qsl.net/aa3rl/ant2.html
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Personally I don't over analyse this things. I have 2 inverted Vs which are both approx 1/4 Lamba high and get out well into the target area. I have made small adjustments over the years and not really noticed any difference in pattern when checking the online SDRs.
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Personally I don't over analyse this things. I have 2 inverted Vs which are both approx 1/4 Lamba high and get out well into the target area. I have made small adjustments over the years and not really noticed any difference in pattern when checking the online SDRs.
There's enough known unknowns, plus all the unknown unknowns, in the typical hobbyist antenna setup that antenna patterns rarely approach anything remotely like theory or models. Get it as high as you [safely] can, and the correct dimensions (if a dipole or other resonant antenna). If a sky loop, make it as large as possible, more wire usually wins.
VHF/UHF is even easier, there's only three factors to worry about: height, height, and height.