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What is the best antenna height for 2m FM – ‘Modelling will give you answers'?

Why did I bring this big, heavy, pole when life is so much easier with a SOTAbeams Tactical Mini?

I read through the SOTA Reflector and found Richard G3CWI’s response to a similar question:

So, I set about modelling a centre-fed half-wave vertical dipole at 145.500mhz in MMANA. As is often the case, each answer prompts more questions.

The gain and far field plot of a dipole is well known, with it beautiful doughnut shaped bubble of RF, radiating to the horizon, and a gain of 2.14dBi, but my dipoles are not in free space. I chose to model the dipole over ‘Real Ground’ using the default MMANA parameters (Dielectric Const = 13.0, Conductivity = 5.0 mS/m).

The chart below presents theoretical antenna gain versus antenna centre height above ground level and shows that there is 4.25dB increase in maximum gain by lifting the antenna from 2m to 9m (the top of a 10m pole).

But this isn’t the full story. The far field elevation plot below presents 2 antennas: 2m height above ground (blue), and 9m height above ground (red). As the antenna height increases the far field pattern splits into multiple lobes, and the elevation angle of the lowest lobe (max gain) drops.

Next, I plotted the theoretical gain at specific take off angles versus height. Raising the dipole from 2m to 9m creates a predicted 12dB increase in low angle radiation, so height is important!

The far field plots present the radiation relative to the ‘ground’ and it is worth noting that the angle of the dipole to the vertical appears to have no significant effect on the far field plot.

What I do not know is the relationship between the far field plot and the terrain on a specific summit, or the actual take-off angle during a VHF contact between 2 stations. Looking back at the elevation plots, there are not many degrees difference between the elevation angle for maximum gain and the first null. The higher we place our antenna, the lobes become narrower, and the actual take off angle becomes more critical. Similarly, achieving gain with a colinear or 5/8 antenna creates a narrow lobe and the same risk. Is there a sensible limit to the benefits to be gained for the height of a dipole on a SOTA summit?

Is it worth taking my 3 element Yagi to a SOTA activation? Choosing an arbitrary 2 degree elevation angle for comparison, I modelled the predicted maximum gain of the Yagi versus a dipole at various heights.

In still to calm conditions, with good guying, I can support a Yagi 7m above ground level on a SpiderBeam 10m pole. Comparing this to a dipole at the top of the same pole (9m), the Yagi offers a potential 4.2dB advantage at 2 degree take-off, much less than I had hoped. I only have to drop the Yagi to 4m before the dipole offers comparable low angle gain.

Comparing a handheld Yagi at 1m with a dipole at 5m, there is a 3.6dB penalty for using the Yagi, when modelled at a 2 degree take off angle. There may be other reasons to use a directional antenna other than just to increase gain, maybe nulling out strong signals, maximising signal to noise ratio, or direction finding, (Edit: but I can’t see a reason to use a handheld Yagi on a SOTA summit) but there does not appear to be a benefits from using a handheld Yagi at 1m height with the ground conditions used in this model. Many highly regarded activators, with lots of real world experience, can testify to the benefits of using a Yagi at low heights. This may be due to ground conditions and topography on a summit being very different from what was modelled. (End of edit)

Interestingly, the front to back ratio of the Yagi actually increases slightly when it is close to the ground, reinforcing that reassuring and satisfying impression that it is working as you rotate it towards the other station.

Putting all of this into perspective:

  • Fantastic antennas are not an essential part of a great activation.
  • I have not struggled for 2m contacts on the summits that I have visited.
  • I have been able to complete several QSOs with my pole collapsed on the floor.
  • 6dB is only around 1 S point on the radio.
  • I admit to enjoying antenna modelling just for the sake of trying to understand what is going on.

My thoughts are so far:

  • I will use a simple dipole (Flowerpot or SlimJim) at the top of a 6m pole as my default antenna for 2m FM.
  • I will only take my 10m pole if I am attempting S2S or DX, and think that I will benefit from a potential 3-5dB of additional low angle gain.
  • I will leave my Yagi at home except for calm, sunny days on summits with a short walk in.

What are your thoughts?
Can anyone point me to an article that explains the effect of terrain on take off angle (edit) for vertical polarisation?

73 Kevin

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Read this, half way down we get to VHF as opposed to HF

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I seem to recall that HFTA gave useful results for 2m. I’m not sure if it’s still available. http://www.arrl.org/files/file/Product%20Notes/Antenna%20Book/hfta.pdf

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Thanks for the link to an interesting read. The link that you and @G3CWI have recommended takes me to Operating Instructions for HFTA which looks like an amazing resource. Unfortunately fairly early on it tells me 'Please note: HFTA does not work with vertical polarization, only horizontal, and it
works best with directive arrays such as horizontally polarized Yagis or quads.’
It would be great to have even a qualative overview of the effects, on a vertically polarised signal.

We (my contest group) used it to experiment with height for single and dual Yagis on 6/4/2m using CW/SSB. The lack of support for vertical antennas is not an issue for us. You’ll find that operating a 2m omni from a steep slope/cliff will give an improvement as well.

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If you’re on a summit with some real prominence, such that you’re looking down on the would-be chasers, and you’re willing to handle the extra weight and complexity, consider a vertical array of 2 or more elements, with down-tilt.

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The first fresnel zone is very important. As you say, the slope / cliff definitely gives an enhancement. Flat top summits generally produce weaker signals - it is very evident in practice. Being at the highest point may not always be the best position.

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The question arises whether an elevation angle approaching zero is always optimal. The reflection in the troposphere can take place several times like in a waveguide.
For example, if the reflective layer is 15km high and the first reflection point is 60km away, the elevation angle should be round about 15 degree.
73 Chris

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Morgen Chris,

2 km height and 250 km away would be more likely figures for 144 MHz tropo dx.
Anyway, if you want to dx SSB and H-polarisation would be the way to go as H gives abt. 3 dB ground gain over V and SSB (3 kHz bandwidth) 6 dB over FM (12 kHz).

Ahoi
Pom

Hi Kevin,
Thanks for the information. I hope to add VHF to my SOTA kit and your post has have given me a great place to start. Nicely done!
73, Fred

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Very interesting. But

I will always take the best antena I can. Some kind of vertical is better than handheld helix, yagi is better than vertical, quad even better than yagi.

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This looks like a good topic for @K0NR to be involved in as he has literally written books on this topic.

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You asked for an opinion. Two Meters is line of sight. Rarely will you have use of the type of conditions where a take off angle is involved, So the higher the better for 144 mHZ
I have 15 DXCC entities and 33 States on Two Meters over a very long period of experience,

73 Jim W9VNE

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Hi Kevin,

as your finding out in your studies Vertical vs Horizontal polarisation are very different beasts. Horizontal you have ground reflection gain, you can study the effects of this with tools like HFTA to use it to your advantage. Its well understood by HF & V/UHF contesters and dxers using antenna stacks and optimum heights to their advantage.

For vertical polarisation you need to explore ground conductivity and pseudo-Brewster angles. Ground conductivity (on hilltops will mostly be poor or very poor) has an effect even with vertical antennas 5 or 10 wavelengths over ground. Also as pointed out by Gerald & Andy you are likely to impinge the Fresnel zone on flat topped summits.

You will come to the conclusion that for 2m FM higher antenna is always better, even if the main lobe narrows / splits as you go higher and you get high angle lobes forming. All that really matters is the gain of the lobe near the horizon, everything above 5 degrees can be discounted unless you want to work aircraft. Plus higher gives you more Fresnel clearance on flat summits.

73 Gavin
GM0GAV

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Well you are chasing Windmills as the saying goes
there is a difference between the 2 stations when one is Vertical and the other is horizontal. But it is still line of sight 95% + of the time. So take off angles are not part of the analysis. Take off angles involve the Ionosphere which has nothing to do with LINE OF SIGHT. If the signal is not being reflected what difference does it make. It is LINE OF SIGHT.

Jim W9VNE

No.

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you deny basic propagation
if the signal is not reflected then geometry is not involved
so take off angle does not exist
simple logic

Radio waves almost always travel through space in a straight line. There are two exceptions. One is that radio waves are pulled and turn slightly because of gravity when they pass by large masses. The other exception is that radio waves can be reflected by certain substances, like the way that light is reflected by a mirror. The angle at which a radio wave is reflected from a smooth metal surface (also called the angle of reflectance) is always equal to the angle at which it approached the surface (also called the angle of incidence). Radio waves can be reflected by the Ionosphere. But most of the time on 2 meters they are not. They continue on a straight line out into the Cosmos.

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Radio waves below 40 MHz are significantly affected by the ionosphere, primarily because radio
waves in this frequency range are effectively reflected by the ionosphere. The E and F layers are
the most important for this process. For frequencies beyond 40 MHz, the wave tend to penetrate
through the atmosphere versus being reflected.

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2 words you should lookup: “Tropospheric Scattering”

You have not been charged for this information.

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