How does modelling antennas help us

[northern35s]

I see a number of issues on this particular section of the forum with regard to antenna modelling software and real World results. Let me start by saying I am a rank amateur when it comes to modelling software, I use freely available software and play at it, and maybe learn something along the way.

I know we can't account for everything real World that may affect an antenna, but we can use modelling to help us understand what's happening and plan our installations accordingly.

Here's a post I made on an anateur HF mobile yahoo group I'm a member of, I've added brackets and italics is to help explain the post a little :

I'm pretty sure we've discussed this before, about height above the sea and the anecdotal benefits some have observed. Well tonight I decided to model and compare a 20m 1/4WL ground plane antenna at 1m and 10m high respectively, now considering the sea makes an almost perfect groundplane I was surprised to discover the large difference between the two heights, I've uploaded an image to the forum under my callsign for you to see, and below is a link to the image on photobucket:

The lobes and pattern of the antenna at 1m AGL are those that I'm more familiar with, that is, when I've been experimenting with the antenna modelling software I've always placed the antenna within 1m, usually 0.5m, above the sea as a good number of us operate from the beach, now there's nothing wrong with this, the antenna produces gain, has a very low take off angle and the benefit is supported by the many hours people have spent on the beach DXing. However it doesn't go unnoticed that the raised location close to Dave (My friend G4AKC) and myself has an advantage, I've experienced this numerous times, both with others and by myself when I've walked onto the beach and to the waters edge, in the hopes that I may squeeze a little more decibels out of the salt water location, however I've never seen an improvement, in fact I always thought it was worse, not by much, but enough to know I prefer operating from the high wall location.

Well the comparison in the image would seem to back this up, not by a huge amount and not necessarily something you'd see on an S meter, but a bigger difference than I had assumed. If we want to squeeze every decibel out of the system and our signal then the high ground takes the trophy, however in the image and the data below there is a flaw with being so high:

1m AGL, at 1 degree elevation 1.5dBi, 4deg elev 4.4dBi, 24deg elev 3.8dBi, 48deg elev -0.9dBi

10m AGL at 1 degree elevation 3.2dBi, 4deg elev 5.9dBi, 24deg elev -24.9dBi, 48deg elev 4dBi

At 24 degrees elevation the 10m AGL antenna has a whopping -28.7dBi disadvantage in comparison to the ground level antenna, this is because the pattern of the 10m high antenna has split into two distinctive lobes, a very low high gain lobe, a very deep null, and a high gain high angle lobe. Now there have been many times when I've been in my car at Fairhaven lake and absolutely swamped by an EU station close by the frequency, and Dave on the high wall can hear the station, but isn't bothered by it too much, I've just checked some arrival angle statistics and can see that signals from the EU arrive into the UK between 7 and 25 degrees elevation, at 16 degrees the antenna at 10m AGL has almost 0 gain, whilst the lower antenna is still around 4.5dBi, if we now factor in the clear sight to the East from Fairhaven lake, and the cluttered take off from the high wall, it's easy to see why the high wall location wins again, in fact the high wall is beginning to look more and more like the perfect location, as if we didn't already know it.

What the model has done is help me understand why one backpack location suffers such high levels of splatter from EU stations and the other doesn't, they are both on the same section of coast, no more than 5 miles apart North/South, but the difference can be day or night when it comes to QRM. For info our backpack pedestrian mobile kit is almost identical, 100W, 1/4WL antennas, small batteries and a great take off for Oceania / USA

 

[W5LZ]

"I know we can't account for everything real World that may affect an antenna, but we can use modelling to help us understand what's happening and plan our installations accordingly."
I think that pretty well sums it up. Antenna modeling programs are only approximations, they are never, -can- never be extremely accurate. That's simply because the information entered in the programs can never be extremely accurate or complete -for the average user-. They can certainly produce 'close' results, and maybe the important thingy, they can give an approximation of the antenna's radiation pattern, where it 'puts' a signal (or hears one).
Modeling programs have about the same 'probability' of being correct as the average person has of winning a bet. Just depends on how much information that person has before making that bet, right?
- 'Doc

 

[M0GVZ]

When I was running an inverted L, modelling it showed some very interesting characteristic changes as I altered the vertical:horizontal ratio. On 20m I found a change of 8ft in the horizontal length changed an apple shape pattern to a X which allowed me to null out Europe but gave me four lobes, one for Oceania, another for South Africa, a third for South America and a fourth for North America.

They're also useful for seeing what elevation patterns are at various heights for dipoles and yagis so you can decide whether that extra 5ft is really worth all the grief or not.

 

[Henry HPSD]

Modeling (when done correctly)
Is more accurate than 99 percent of the antenna users can measure.

Often one will read, well...yes but in real live things are different !
And yes most certainly "things' will effect SWR etc.
But if we have modelled two different antennas, wouldnt for example..that big mountain blocking your signal, be equal in both cases ?

Now, im not saying all antennas have the same effect on near by obstacles, however the "negative" point often made....
"real" versus "software" is in most cases provided with a litlle bit too much enthousiasme...


Kind regards,

H.

 

[northern35s]

Conor one of the concerns voiced on this section of the forum is the reliability of modelling, whilst I use it to explain simple antennas, simple installations, it's true that it's nigh on impossible to factor in all the variables.

In the example I discussed above, I wasn't looking to see why one location favoured EU and the other didn't, it was just an aha moment when I looked at the results. My friends and I had considered the difference to be associated with different terrain / buildings blocking the signal in one location and not in the other. Whilst those may well be a factor, although impossible to be sure, modelling highlighted a much more tangible reason why.

Do you still have your inverted L or have you ditched it now you are using the Hexbeam?

 

[W5LZ]

"...one of the concerns voiced on this section of the forum is the reliability of modelling."
That reliability thingy is entirely dependent on how much data you can plug into the program. In most cases, modeling isn't very dependable. It can certainly get you to the ball-park, you never to the right seat, sort of, and ain't gonna buy you no hotdog!
- 'Doc

 

[Shockwave]

Even though I've worked with particular antenna designs that have challenged typical modeling software, I have to say by in large that modeling can be very accurate so long as the model represents the antenna and the environment it's mounted in accurately. That's where 99% of the variables we see between the modeling and real world come from and obviously can be eliminated based on the modelers skills.

Software modeling of antenna patterns is common place today in every application from military to cell phone site arrays and this would not be the case if it couldn't be done accurately. These users don't just trust the software either. It's "trust with confirmation" to borrow a quote since there are always precise field tests conducted to confirm their results.

 

[M0GVZ]

Conor one of the concerns voiced on this section of the forum is the reliability of modelling, whilst I use it to explain simple antennas, simple installations, it's true that it's nigh on impossible to factor in all the variables.

In the example I discussed above, I wasn't looking to see why one location favoured EU and the other didn't, it was just an aha moment when I looked at the results. My friends and I had considered the difference to be associated with different terrain / buildings blocking the signal in one location and not in the other. Whilst those may well be a factor, although impossible to be sure, modelling highlighted a much more tangible reason why.

Do you still have your inverted L or have you ditched it now you are using the Hexbeam?

Fully agree with you. When I modelled my inverted L, there was quite a few changes over just modelling the antenna once I started putting in the metal guttering on the house it went over, the metal pole supporting other antennas and the other antennas themselves. I didn't even include things like the house wiring.

As Shockwave said, "its trust with confirmation." With the length altering the pattern it was easy to confirm by the fact the Italians were peaking around S7 instead of S9+20 showing they were indeed in the 20dB or so null that the modelling software showed. It was quite good in so much that it ended frustration of wondering why I couldn't hear certain areas well. I reckon anyone using a dipole on multiple bands would do well to model theirs just so they can see where the lobes are heading. They may find that the way they've orientated it results in a whacking great -30dB null in a direction they actually want.

The inverted L sadly is no more due to losing the tree supporting one end of it to disease. Its a shame because it contributed a lot to my contest scores over the years it was up. At some point I'm going to get a vertical up for 40/80, just deciding on what at the moment.

 

[jazzsinger]

We have locations here very close to sea front with 200m hills right behind, I can work dx from both, but there's been many a time the mobile couldn't hear skip at sea level yards from the water, yet climb that hill its booming in, both have clear take offs to the west and NW/SW and the high ground always wins, although on one occasion further along the coast at Ardrossan I could hear a station on Blackpool sea front S2 working 29 div, neither myself or a boat halfway between here and N.Ireland could get inbetween signals straight across sea from 29 div, neither of us had power that day which may have made a significant difference, went up the hill and he just wasn't there, some form of ducting I guess as far too far for line of sight and no propagation running whatsoever, his signal was steady for over 4 hours on a flat band.

I think modelling software gives an idea, but it certainly ain't the bible, come to that the bible had more than one edition when mankind decided to add shit that wasn't true either, maybe the more info you tru to squeeze out of modelling the more innacurate it becomes, simply because you ain't putting in FACT to begin with. Just my take

 

[northern35s]

George if you ever visit Blackpool, genuine offer, get in touch before you come here and I'll show you what happens when you add salt water to the DX

 

[M0GVZ]

George, I suggest you don't. It'll just leave you completely disheartened when you go home.

 

[The DB]

One thing I haven't seen mentioned with modeling is comparisons. You can compare various designs without purchasing them first, or compare the same design in multiple situations (stacked yagi's at various distances apart, or a single antenna at various heights above ground).

Another thing I've noticed since I started playing with modeling (and I to am still a rank amateur) is patterns people post or advertise with begin to look "right" or "wrong". Plus, if you know the general dimensions you can model the antenna yourself for a comparison...

Modeling is not exact, but if you take precautions, and learn the limitations of the program you are using, it can be reasonably accurate. Also, as has been mentioned above, any variances the local geography has will affect all antennas, and the effects will be largely similar among those antennas...

Plus, sometimes it is easier to show something with a graphical representations of the subject at hand. A picture is often worth a thousand words, and modeling helps with that.


The DB

 

[jazzsinger]

George if you ever visit Blackpool, genuine offer, get in touch before you come here and I'll show you what happens when you add salt water to the DX

I've dx'ed at the seafront many many times, both right at the shore, also from Mull of Galloway, all down west coast and from all the hills behind it, i've also dx'ed from the many sea lochs in the Clyde estuary, and believe it or not from Blackpool prom too as my aunt owned a guesthouse in south shore area, i still prefer the hills, I have no doubt even very close proximity to the sea helps, and sea ducting happens, but ducting happens up high too and its far superior when working high locations.

 

[jazzsinger]

George, I suggest you don't. It'll just leave you completely disheartened when you go home.

,

I live 500 yards from Clyde Estuary at most, can walk in 10 mins and be on shore, could have an antenna up within 30 mins of leaving house, i can also access hills 200m high in similar timescale, where i live the sea and hills are very very close, but tbh, when it comes to dx, if i get my antenna up, no matter where i am i will blast out anywhere in the world, i'm used to doing it from street in a car with hp4000, with a clr2, 20 foot mast on a 30 foot building and a kl 203p, i ain't ever going to be disheartened, i adapt to surroundings

 

[Marconi]

Jazz, I fished the Gulf of Mexico almost every week of the year, weather permitting, while I was working for nearly 30 years. We use to take an old Big Stick set horizontal, and a CB with us even if we were only bay or jetty fishing.

There were times that we made big DX contacts, mostly into Europe, and times we didn't. We didn't find just being near or over sea water help us work DX...unless DX was working. We figured we could not make conditions, so if that is what you are saying...I can't make the same claims as you.

My biggest thrill was many times being able to make contacts back to mobiles in Houston over 70> miles away, and surrounding the Galveston area and even into the Louisiana coast line just using a 4 watt CB radio.

I also remember talking to shrimp boats all along the Gulf Coast from my base early in the mornings before DX rolled in...weather DX was working or not. And, when I was back home at the base...sometimes we could not talk local for all the shrimp boats talking early in the morning 60> miles away. When this boat traffic really got bad though was when the Vietnamese started taking over all the shrimping along our Gulf Coast.

We use to ride ferries back and forth from the mainland at times, and again we could almost always talk back to mobiles/bases in the Houston area and beyond. That was virtually easy all the time if we were over sea water.

DX was only easy when DX was working...and I've been thru several 11 year cycles in my years in two way radio.