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Author Topic: Directivity of HF dipoles  (Read 4245 times)

Offline Jock Wilson

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Directivity of HF dipoles
« on: June 25, 2019, 0357 UTC »
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)
« Last Edit: June 26, 2019, 1927 UTC by Ray Lalleu »

Offline The Ether Hacker

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Re: Re: Radio Merlin Int. 6305 AM 2046 utc 24 Jun 2019
« Reply #1 on: June 25, 2019, 0642 UTC »
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. ;)
« Last Edit: June 25, 2019, 0654 UTC by The Ether Hacker »
Antennas: Multi-band doublet for 80 to 10m HF and vertical groundplane
RX: Kenwood R-2000
QTH: Northern England

Online Ray Lalleu

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Re: Directivity of dipoles (was R. Merlin Int. on 24 Jun 2019)
« Reply #2 on: June 25, 2019, 2022 UTC »
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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+/- : about 0.02 offset, ++/-- 0.03/0.04 offset
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Offline The Ether Hacker

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Re: Directivity of dipoles (was R. Merlin Int. on 24 Jun 2019)
« Reply #3 on: June 25, 2019, 2030 UTC »
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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Offline Harmony

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Re: Directivity of dipoles (was R. Merlin Int. on 24 Jun 2019)
« Reply #4 on: June 26, 2019, 1034 UTC »
Here you go, radiation plots of a 48m half-wave dipole at heights:

Black 6m
red 12m (quarter wave)
Blue 24m (half wave)




« Last Edit: June 26, 2019, 1457 UTC by Harmony »

Offline Harmony

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Re: Directivity of dipoles (was R. Merlin Int. on 24 Jun 2019)
« Reply #5 on: June 26, 2019, 1327 UTC »
Here's a more traditional view, V field only. IN the left plot the dipole is running N-S.



« Last Edit: June 26, 2019, 1457 UTC by Harmony »

Offline The Ether Hacker

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Re: Directivity of dipoles (was R. Merlin Int. on 24 Jun 2019)
« Reply #6 on: June 26, 2019, 1408 UTC »
Here's a more traditional view, V field only. IN the left plot the dipole is running N-S.



Sorry, but your diagrams are not displaying, ... at least not on my browser
Antennas: Multi-band doublet for 80 to 10m HF and vertical groundplane
RX: Kenwood R-2000
QTH: Northern England


Online Ray Lalleu

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Re: Directivity of dipoles (was R. Merlin Int. on 24 Jun 2019)
« Reply #8 on: June 26, 2019, 1535 UTC »
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.
D/E/F/G/It/Sp : Dutch/English/French/German/Italian/Spanish
+/- : about 0.02 offset, ++/-- 0.03/0.04 offset
Balanced wire antennas, wire lines and ATU
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Offline ThaDood

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Re: Directivity of HF dipoles
« Reply #9 on: July 15, 2019, 0409 UTC »
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.
I was asked, yet another weird question, of how I would like to be buried, when I finally bite the big one. The answer was actually pretty easy. Face-down, like a certain historical figure in the late 1980's, (I will not mention who, but some of you will get it, and that's enough.) Why??? It would be a burial that will satisfy everyone: (1) My enemies will say that it will show me where to go. (2) On the same point, I can have my enemies kiss my butt. (3) It will temporarily give someone a place to park a bicycle. See??? A WIN / WIN for everyone.

Offline Josh

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Re: Directivity of HF dipoles
« Reply #10 on: July 16, 2019, 0342 UTC »
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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Offline ChrisSmolinski

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Re: Directivity of HF dipoles
« Reply #11 on: July 16, 2019, 1151 UTC »
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.
Chris Smolinski
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Offline i_hear_you

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Re: Directivity of HF dipoles
« Reply #12 on: July 18, 2019, 1456 UTC »
I didn't see this article listed and personally found it very helpful:

https://www.qsl.net/aa3rl/ant2.html

Offline Brian

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Re: Directivity of HF dipoles
« Reply #13 on: July 19, 2019, 1751 UTC »
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.

Offline ChrisSmolinski

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Re: Directivity of HF dipoles
« Reply #14 on: July 20, 2019, 1554 UTC »
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.
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 400 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree