LoG antenna trick

[Andrew (grayhat)]

Forgot, if someone is interested in antenna modeling using NEC, or wants to play with the free 4NEC2 modeling program, here's a NEC model for the LoG antenna

Code: Select all

CM ---------------------------------------------
CM File: LoG.nec
CM Loop on Ground - http://www.kk5jy.net/LoG/
CM antenna length changed using optimizer
CM feedpoint impedance set to 450 Ohms
CM ---------------------------------------------
CE

' symbols
SY freq=1.800             ' test frequency (MHz)
SY totl=21.336            ' loop perimeter (mt)   :: 18.288
SY leng=(totl/4)          ' square loop side (mt) ::  4.572
SY wire=0.00075           ' antenna wire radius (mt)
SY hght=(wire*3.3)        ' height from ground (mt)
SY segs=25                ' total segments for each wire
SY fnum=1                 ' feedpoint wire #
SY fseg=1                 ' feedpoint wire segment #

' -------------------- antenna wires ---------------
'  #  segs  x0    y0   z0      x1    y1    z1    rad
'  -- ---- ----  ---- ----    ----  ----  ----  ----
GW  1 segs    0     0 hght    leng     0  hght  wire
GW  2 segs leng     0 hght    leng  leng  hght  wire
GW  3 segs leng  leng hght       0  leng  hght  wire
GW  4 segs    0  leng hght       0     0  hght  wire

' ground parameters
GE  1
GN  2  0  0  0  13  0.005

' wires loading
LD 7  0  0  0  2.1  wire    ' insulator
LD 5  0  0  0 58000000      ' copper

' antenna feedpoint params
EK
EX 0 fnum fseg 0 1 0 0

' test params
FR  0  1  0  0  freq  0

' end of model
EN

to use it, just copy/paste the above in an editor and save it as LoG.nec, then load the model in 4NEC2 and you'll be ready to play with the simulations; notice that in the model the antenna wire is sligtly raised from ground, since otherwise NEC will complain about the fact that the wire is too near to ground, nothing particular, just to let you know; also, I slightly changed the loop length after running the NEC optimizer, the original sizes are after the "::" in the inline comments

[13dka]

Thanks for the .nec file! That gave me an opportunity to finally install 4NEC2 and be utterly confused by it!

I changed the geometry to reflect my plain 20m circumference loop and I was particularly curious on A) the influence of ground conductivity / dielectric constant on gain and B) elevation angle at C) higher bands like 14 and 18 MHz. I compared "sea water" with "polar cap" (worst case) and the simulation seems to say...

- Unexpected by me, the "better" the ground is, the lossier the LoG will be? At the North Pole, it will have very little losses and even a little bit of dBi gain beaming straight upwards, but it will lose some 30dB over the ocean?

- It seems to confirm my evidence-supported theory that the better the ground, the flatter the lobes will be. This is because the LoG is by all means a vertically polarized antenna of course, even though it boggles the mind when you look at a LoG and fail to even see it in the high grass.

The LoG on polar permafrost (18 MHz):

Dike_LOG_18068_Polar_Cap_All.jpg (76.22 KiB) Viewed 983 times

On Saltwater (18 MHz):

Dike_LOG_18068_Saltwater_All.jpg (77.71 KiB) Viewed 983 times

Interesting cloverleaf pattern! Now my LoG is not really over saltwater at the dike, it's just very close (50m) to it and I guessed that "very wet ground" (which proves true after only little rain when I can't park my car on the meadow close to the dike because the tires may sink into the ground) would be a serviceable approximation to the conditions there. This has some odd effect on the horizontal pattern in the simulation for 18068 kHz:

Dike_LOG_18068_HighlyMoist_All.jpg (78.39 KiB) Viewed 983 times

Dike_LOG_18068_HighlyMoist_3D.jpg (102.6 KiB) Viewed 983 times

Looks like it becomes almost unidirectional! Now I know that this will change with a lot of parameters and conditions and can't be generalized or even predicted much, it will look very different on 14 or 21 MHz in the simulation (but very similar on 28 Mhz!) but still, maybe it's worth anticipating this when deciding where to put the feedpoint of the LoG?

That sure would make it worthwhile to have an easy way to shift the feedpoint around!

[Andrew (grayhat)]

Ollie, a bit of warning here; NEC modeling may be addictive and you may start modeling an antenna and adding to the model wireframe models for buildings, trees and so on

Seriously, as you correctly pointed out, the program allows to SIMULATE antennas, and while the results will change in real world, a good model will allow one to have an idea of how a given antenna will behave and/or to find some ballpark values to start building the real antenna; for example, here's a model for a resonant dipole

Code: Select all

CM ----------------------------------------------------------------
CM File: dipole.nec
CM resonant dipole model, change the calc frequency as desired and
CM the model will adjust the other parameters including the optimal
CM height from ground; the impedance will be around 50...75 Ohm and
CM will depend from height from ground and arms slope, to feed the
CM dipole use a 1:1 balun, see http://www.karinya.net/g3txq/chokes/
CM to find infos about winding such a thing
CM ----------------------------------------------------------------
CE

' symbols and calculated values
SY freq=14.170                  ' calc frequency
SY wire=0.00125                 ' wire radius
SY fact=0.964                   ' shortening factor (end effect)
SY wave=(300/freq)              ' wavelength
SY arms=((wave/4)*fact)         ' dipole arm length
SY feed=(wave*0.55)             ' feedpoint height
SY term=feed                    ' arms end height
SY segm=51                      ' wire segmentation

' calculate the endpoint (for inverted V dipoles)
SY vert=(feed-term)
SY endp=(sqr(abs((arms^2)-(vert^2))))

' arms structure
GW 1 segm  0 0 feed -endp 0 term wire
GW 2 segm  0 0 feed  endp 0 term wire

' ground parameters
GE  1
GN  2  0  0  0  13  0.005

' wires loading 
LD  7  0  0  0  2.1  wire       ' insulation
LD  5  0  0  0  58000000        ' copper

' feeding
EK
EX  0  1  1  0  1.0  0  0 

' test frequency
FR  0  0  0  0  freq  0

' end of model
EN

now, such a model will give some good indications about arms lengths and height, but then it will be a good idea making the dipole arms longer and then pruning them, since the environment will affect the dipole, yet the values given by the model will give pretty good ballpark values, the same goes for other antennas, for example, this is the KK5JY "SRL" antenna

Code: Select all

CM File: SRL.nec
CM Small Receiving Loop antenna
CM https://www.kk5jy.net/rx-loop/
CE

' symbols definition
SY freq=7.100               ' test frequency
SY hght=3                   ' height of base corner from ground
SY side=0.762               ' length of one side
SY diag=(sqr(2)*(side/2))   ' half diagonal
SY wire=0.00250             ' wire radius
SY segs=11                  ' number of segment in wires

' wires geometry
'  ID seg    x0     y0   z0         x1     y1     z1           wire rad
GW  1 segs     0     0  hght      -diag     0   hght+diag       wire
GW  2 segs     0     0  hght       diag     0   hght+diag       wire
GW  3 segs -diag     0  hght+diag     0     0   hght+(diag*2)   wire
GW  4 segs  diag     0  hght+diag     0     0   hght+(diag*2)   wire


' ground parameters
GE  1
GN  2  0  0  0  13  0.005

' wires loading
LD  5  0  0  0  58000000

' feedpoint
EK
EX  0  1 1 0 1 0 0

' initial test frequency
FR  0  1  0  0  freq  0

' end
EN

now, in this case, the model would allow to experiment with loop size, height from ground, conductor size and then... some more

[13dka]

Ollie, a bit of warning here; NEC modeling may be addictive and you may start modeling an antenna and adding to the model wireframe models for buildings, trees and so on

I couldn't stop playing with it all afternoon, good point! OTOH, I understand very little of it, likely not enough to use it in any meaningful way, not to mention modeling the environment. However it obviously proved already useful with your help!

Even though I either dropped the LoG or put up the 10m vertical monopole at the dike and never compared them directly yet, it always puzzled me how similar the LoG performed to the vertical, using crazy weak VOLMET signals from AUS and ZL I recorded with both of them (at different days of course) as an indicator. With both antennas 14 and 18 MHz never "close" at night unless there's some bad radio blackout conditions, typical telltale effects of flat elevation angles. When testing horizontal wires and SMLs at the dike the opposite effect can be observed - the steep angle signals drop off with the MUF decreasing after sunset but the flat angle signals don't really kick in - > dead high bands.

BTW, here's a nice demonstration how this works both ways:



Edit: OMG, there is so much to learn from this! IIRC some post(s) in some discussion somewhere suggested that minimally elevating the wire would be advantageous. According to the simulation, only a few centimeters dramatically decrease the influence of the ground and turn the LoG into a "cloud warmer". It also decreases the losses so I think I get how this could be perceived as beneficial, stations coming in from high angles are a little louder.

Left: 5cm high wire. RIght: (wire*3.3)

Dike_LOG_18068_HighlyMoist_comparison2.jpg (151.77 KiB) Viewed 960 times

[Blubby]

I have heard great things about LOG antennas, but I can't use one here because my entire usable backyard is full
of radials for my sloping wire antenna to work against. On the upside the sloping wire works really well!

[Andrew (grayhat)]

Edit: OMG, there is so much to learn from this! IIRC some post(s) in some discussion somewhere suggested that minimally elevating the wire would be advantageous. According to the simulation, only a few centimeters dramatically decrease the influence of the ground and turn the LoG into a "cloud warmer". It also decreases the losses so I think I get how this could be perceived as beneficial, stations coming in from high angles are a little louder.

Just a note, the LoG with a perimeter of (about) 18m will mantain its pattern up to about 10MHz, willing to go further up in frequency you'll need to reduce the loop size, the image below shows the patterns at 18MHz for two LoG antennas

log18.png (102.93 KiB) Viewed 954 times

to the left you can see the pattern of the 18m LoG, to the right the one of a 3m one, as you see, by reducing the loop size the pattern is restored, the price to pay is the gain which, by the way, gets lower and that's also why Matt (KK5JY) recommends the use of the LoG for frequencies below the 30 meters band, since, while it's possible to reduce the size of the LoG to go up in frequency, the resulting antenna won't offer the same performance as the larger one at lower frequencies

If you want a low angle antenna for 18MHz, here's one which may fit and which will work really well near salt water, the model is set for aluminium pipes, but it can be easily modified to copper wires (in such a case you'll need some non conductive support structure)

Code: Select all

CM --------------------------------------------
CM Double T-Bar antenna (impedance abt 100 Ohm)
CM --------------------------------------------
CE

SY freq=18.000                ' calc frequency
SY test=18.000                ' test frequency
SY vrad=0.015                 ' vert section radius
SY hrad=0.010                 ' horz section radius
SY cfac=0.965                 ' length correction factor
SY wlen=(300/freq)            ' lambda
SY wave=(wlen*cfac)           ' correction factor
SY hght=2                     ' height from ground
SY hats=(wave*0.085)          ' capacitive hats arms length
SY vert=(wave*0.272)          ' vertical section length
SY toph=(hght+vert)           ' height of antenna top hat
SY segl=51                    ' segments for long sections
SY segs=11                    ' segments for short sections
SY fseg=1                     ' feeding segment

' vertical section
GW 1 segl  0 0 hght     0 0 toph vrad

' top hat
GW 2 segs  0 -hats toph 0 0 toph hrad
GW 3 segs  0  hats toph 0 0 toph hrad 

' bottom hat
GW 4 segs  0 -hats hght 0 0 hght hrad
GW 5 segs  0  hats hght 0 0 hght hrad

' ground
GE -1
GN  2  0  0  0  13  0.005

' loading (58000000=copper, 37700000=aluminium)
LD  5 0 0 0 37700000

' feeding
EK
EX  0  1  fseg  0  1.  0  0

' frequency
FR  0  0  0  0  test  0

' end
EN

the resulting antenna will show the following characteristics

dtbar18.jpg (207.09 KiB) Viewed 954 times

as you see it offers omnidirectional pattern and very low launch angle w/o the need to be placed high (the antenna in the model is installed with the bottom at 2m from ground)

[edit]

Forgot; if you didn't already see it, Martin (G8JNJ) experimented a bit with the LoG and wrote about it

https://www.g8jnj.net/loop-on-the-ground

now, while I don't agree with the idea of raising the loop, I believe that the above may be worth reading

[Andrew (grayhat)]

I have heard great things about LOG antennas, but I can't use one here because my entire usable backyard is full
of radials for my sloping wire antenna to work against. On the upside the sloping wire works really well!

Well, you may always put together a LoG and use it away from home all in all it won't take too much; a 9:1 BalUn offering galvanic isolation, that may be wound using the indications found on Matt's site http://www.kk5jy.net/LoG/ or, not willing to build it, one may just pick the NooElec 9:1 BalUn v2 https://www.nooelec.com/store/balun-one ... bones.html and add a weatherproof enclosure; the remainder is... well, just a spool of insulated wire, nothing special

If you'll decide to build a LoG here's my suggestion: measure the wire to the desired length (60ft), remove a bit of insulation from both ends and add whatever connector you used for your BalUn box, done so, fold the wire in two and mark the middle with some tape, fold the wire in two again and mark the two side corners with some tape, this will give you a reference for three square corners (the 4th being the balun connection), done that, wrap the antenna wire on some support starting from the feedpoint and ending with the top corner (the one opposite to the feedpoint and which we marked with the first piece of tape); now, when deploying the antenna you'll just need to unwrap a length and hold the top corner with a stake, stone or whatever is available, next you'll walk and unwrap the remainder of the wire in two parallel lines, at end you'll have the feedpoint in your hands, connect the BalUn to the feedpoint and lie it on ground, now walk to the sides markers and spread the sides letting the balun slide on ground, lock the sides with stakes or whatever fits and your LoG will be deployed and ready to work

[13dka]

Just a note, the LoG with a perimeter of (about) 18m will mantain its pattern up to about 10MHz, willing to go further up in frequency you'll need to reduce the loop size, the image below shows the patterns at 18MHz for two LoG antennas

log18.png

to the left you can see the pattern of the 18m LoG, to the right the one of a 3m one, as you see, by reducing the loop size the pattern is restored, the price to pay is the gain which, by the way, gets lower and that's also why Matt (KK5JY) recommends the use of the LoG for frequencies below the 30 meters band, since, while it's possible to reduce the size of the LoG to go up in frequency, the resulting antenna won't offer the same performance as the larger one at lower frequencies

I think I'd be more interested in utilizing the bent pattern than keeping it uniform over the entire frequency range. Of course I've read the statement about higher frequencies but likely not very thoroughly, that's why I was initially a little baffled by the fact that it works really well on 18 MHz and higher. I was thinking about trying smaller loops but I like I said, the results are surprisingly good on higher frequencies and I like that I can zoom down to MW to harvest some potential DX there. Also, my passive LoG had a former life as my regular loop and should I end up around some trees with it, I can still use it this way.

Thanks for the double T-bar model! I was contemplating trying a vertical dipole array for 17m (because that would fit on my 10m pole), which could be made with a single T-bar and a fiberglass pole but then I started my experiments with a simple monopole antenna (with optional single radial) at the beach and that kept me already completely entertained with previously unheard DX until now.

[edit]

Forgot; if you didn't already see it, Martin (G8JNJ) experimented a bit with the LoG and wrote about it

https://www.g8jnj.net/loop-on-the-ground

now, while I don't agree with the idea of raising the loop, I believe that the above may be worth reading

Thanks! The interesting part in this is that his simulation ends up with the opposite results (raised wire = flatter elevation angle) than mine and I wonder what the difference is.