Another word - “Diffraction”
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There is always a take off angle, even if it is zero or negative.
My thoughts are that on flat ground there appears to be an advantage in getting the antenna as high as possible. On a summit with sloping terrain it is possible that the far field plot may be inclined to the extent that extra antenna height reduces gain as the takeoff angle approaches the first null.
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well that is not the way it works,
An angle is only important if the signal is reflected back off the ionosphere
on 2 meters that rarely occurs. Just get it up above obstacles
you may not agree. that is your prerogative
my experience and education tells me something different than you want to believe
FREQUENCY is part of the equation. It rarely will be involved in SOTA
sorry to disagree but that is my opinion based on Science and experience
it the signal is not reflected you do not use the analysis of a HF signal
Jim W9VNE
James, when you find yourself in a hole, the wise move is to stop digging.
You really need to go and read about troposcatter because you are treating 2m as if it was 20m. It’s not, the propagation modes are very different.
Tropospheric scattering, please read about it. Troposcatter is what gives a small 2m station, 10W SSB + 10dB antenna gain or 100W ERP (5 ele Yagi about 1.5 x 1m in size) a daily repeatable range of around 350km. I used to run about 1700W ERP on 2m and would expect 500km range on average.
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Thanks for sharing Kevin.
That’s why I take my 10m pole for my VHF activations.
I know my chasers like reaching summits on FM they haven’t reached before 
Regards
Shane
G6WBS
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scattering and take off angles are not involved
i will stop attempting to share how it works
After so many attempts it is a waste of time
Thank you Gavin and @G4OIG. Highlighting the benefits of antenna height on reducing the negative effects of the fresnel zone is a very good point that I had neglected.
In your opinion do you think that ground conditions on a summit make it unlikely that the radiation pattern will be inclined to an extent the first lobe is below the horizontal?
you use the term TROPOscatter
Tropo occurs rarely
You assume it is there all the time
it occurs when the weather causes it to occur
yes it does but angle of take off is not involved
Probably 98 % of the time there is no TROPO involved
you ignore that
Tropospheric ducting != tropospheric scattering
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you asked a question and you do not like the opinion i gave
that is OK with me but you ignore reality
- SOTA uses low power. My several Thousand Qsos on SOTA the other station is running about 3 watts up to 15 watts from a batter operated rig
- I have been on 2 meters for decades using High Power and big antennas
3.I have bounded signals off the Moon (EME) have had decades of experience with 2 meter propagation. Meteor Scatter, Aurora, Tropo and Line of Sight - Some one inquiring about an antenna for SOTA operation is probably running low power in the area of 5 or 10 watts. I would say with strong certainty that someone using that power level would have any success with TROPO SCATTER. Why would they not be successful. It take a lot of ERP in the area of several hundred watts with TROPO SCATTER. Likely not a SOTA station. Then TROPO occurs a very small amount of the time. Most of the time (90 to 95 percent) it is LINE OF SIGHT.
In LOS propagation there is no reflection off the Ionosphere. The analysis you used with HEIGHT etc is only appropriate when there is a FORWARD REFLECTION off the Ionosphere. That Geometry determines the reflection angle. When there is no IONIZATION there is no reflection. Ionization occurs infrequently.
You asked a question about someone using a 2 meter antenna on SOTA
Unlikely to use TROPO and HIGH POWER which is a requisite for SCATTERING when there is TROPO
you may believe differently OK with me. I guess it was a rhetorical question anyway
W9VNE
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Hello James,
I did ask a question and I am very grateful for your response. It was certainly not rhetorical. I agree with your comments that SOTA is mainly low power and line of site and respect your experience. I have not made any reference to Tropo, Ionospheric reflections, diffraction or scattering, and have not joined in the discussion on these subjects as this is not the point of my question. My posted was intended to share my learnings incase they may be of use to others, to gain a view on the validity of my analysis from people like yourself, and seek guidance on where I may find information on the effects of terrain on propogation. I am sorry for any offence that has been taken and am genuinely grateful for your and the other contributors comments.
MW0KXN
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Re line of sight.
The “2m fm festival” run in Spain by ea2bd is a good example of how the usual limits can be extended and enjoyed. Many of those contacts reported were way beyond line of sight.
I have also used 2m ssb to obtain contacts with relatively low power, 5 and 25w, over more than 300km, using fairly simple antennas like loops and small yagis like the arrow 3el. That is very far past line of sight.
Tropo ducting and refraction occurs for low power levels, air has no power threshold, but the signal from a low power tx will be very weak at an over- horizon distance. How can this be improved? With better antennas, at better heights above ground. And also, in my experience by using horizontal polarisation instead of the HT and repeater standard of vertical.
73 Andrew VK1DA/VK2DA
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Thank you for directing me to look at ground conductivity and psuedo-Brewster angles. I performed my modelling with the default ground conditions which obviously may not be appropriate for all summits, but you have to start somewhere There doesn’t appear to be much on Google on this subject relating to elevated dipoles. Are there any article that you could recommend. I will keep reading around the subject in the meantime.
As others have stated performance is one of several factors for choosing an antenna type and its height.
I use three antenna options for 2m FM activations: 1) Diamond RH-770 ½-wave telescopic whip [mid height ~2.5m when standing, ~1.5m when sitting], 2) roll-up J-pole on 3m pole, 3) roll-up J-pole on 6m pole.
I’ve had 100’s of excellent reports using the RH-770 and it’s blindingly quick to deploy and to stow away. This is my default choice in very cold weather when doing a 2m-FM-only work-through-the-chasers-and-go activation. I won’t use it in high winds as it stresses the HT’s SMA connector. The only disadvantage is when it’s raining or in a thick Scottish mist [same thing really!], moisture drips down the whip and onto my FT1D (yes, I know it’s IPX5 rated but it alarms me anyway).
I use the J-pole on a 3m pole, not so much for Dx, but when I want to hunker down in the heather or sit on the leeward side of some rocks to avoid adverse weather whilst the antenna is out in the open.
I use the 6m pole for the J-pole only when I’m taking that pole anyway for my HF inverted v or EFHW antennas.
Local chasers (let’s say less than ~50km) notice little or no difference between the whip and the J-pole. Further afield the antenna height can make a big difference. For example, Mark @M0NOM and I did a joint activation of Place Fell G/LD-027 last summer. Mark used a Slim Jim from the top of a 10m pole whilst I stood using my RH-770. We took it in turns to work the chasers using our 5W 2m FM HTs. Mark had no problem working the most Dx stations whilst I could barely hear some of them or they hear me.
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At VHF the main processes that enable signals to be received beyond Line Of Sight are:
Reflection Which results from tropospheric ducting (in the part of the atmosphere where we have weather) usually caused by cold air below warmer air, however the actual boundary conditions which appear to cause reflection are actually more likely to be total internal Refraction. Variation in received signal strength (QSB) is often observed with ducting.
Sharp edges (mountain ridges) also have an ability to allow Diffraction to take place, here signals bend very slightly towards the earth as they pass over the knife edge. In lectures I have given, I have used a laser pointer to demonstrate this effect conclusively to the amazement of the audience. Yes light and all of the electromagnetic spectrum, can be bent, but on a terrestrial scale, not due to gravity!!
Forward scatter (also rain or back scatter) only plays a part at UHF and above where high TX power and aerial gain is employed.
I believe the answer to the original question is: as high as practical, but taking full account of the feeder losses / height advantage.
In reality I think for a mountain top, 6M is the best height compromise for an end fed vertically polarised aerial with 0dBD gain. This will provide a range of over 100miles . For range beyond that we need to start looking at gainy aerials.
Polarisation does matter, Tests in the 1930’s proved that horizontal polarisation out performs the vertical polarisation range. However to avoid high cross-polarisation losses, the aerial at TX and RX must be the same polarisation.
I am in no doubt that on Sota, at VHF we regularly achieve ranges well in excess of LOS.
Regards
David
G0EVV
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One should not neglect aircraft reflections as a useful mode at VHF. I have completed quite a few contacts at a few hundred miles, for instance Birmingham to Scotland, in flat conditions using aircraft reflections, though one has to be quick as the enhancement without beating is short-lived (but there is always another aircraft if you wait!) The trick is to know where the target aircraft are likely to be, and if you use a rotatable beam, orientate it accordingly. Aircraft are like mountain sheep, they keep to well defined paths!
Going back to modeling, I think the results from modeling can be misleading because the assumptions about where the earth plane is can break down. On a rocky summit the effect of frost shattering and solution widening of joints can lead to the ground water being metres below the surface, and the solution of minerals from the rock can lead to that water being quite high - or low depending on rock type - in pH and thus more conducting. This is not to say that modeling is useless on a rocky summit, but the results in the field may not be in accordance with expectations.
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I have worked over 600 miles on so called Line Of Sight in many VHF contests from summits. Due to curvature of the earth these paths are not LOS… as the earth itself is in the way.
These repeatable Tropo paths are due to bending mostly I figure…non reflection stuff…just bending or enhancement of some sort. These paths occur almost every night/morning between well equipped stations on 2M and higher freqs.
Even on 2M FM I work many stations where our antennas have a mountain range in the way (we can’t see each other). So Line Of Sight does not always really mean Line Of Sight…
For VHF: as high as possible is better, high gain directional antennas are better, polarization is critical, and more power is better. It’s all about overcoming path losses…
Pete
WA7JTM
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Pete,
my field day experiences are probably less impressive than yours but still along the same lines.
From my usual VK1 field day location my best dx in tropo conditions is around 1000 km or 600miles but usually 400-500km is readily achieved, using 100w and an 8 el yagi at 6m AGL. But when sporadic E conditions extended up past 144 MHz, I worked 2000 km with ease, s9 signals etc. Sometimes a duct gives such strong signals you cannot help trying 432 and for the 1000 km path it also works well on 432. In one case the same station asked about 1296 and the only antenna I had was a whip lying on the roof of the tent. Yet we made a contact on 1296 over that 1000km (approx) path. In this part of the world 1000km is definitely about 950 km beyond literal line of sight.
The question we have to consider is that line of sight at the wavelength of visible light ie. when looking with our eyes to the horizon, say 450 nm wavelength, is not necessarily the same as what happens on a much larger wavelength (2 metres), which is roughly 4e8 (400,000,000) times the wavelength. The Fresnel effects near the antenna are due to the wavelength in use, and an analysis I saw concluded that an antenna for wavelengths of 2 metres would need to be at least 6 metres above ground to be above the interference caused by the ground. This effect is reduced when the ground in the desired direction falls away rapidly, effectively making lower antennas still effective.
Antennas at lower heights than 6m still work, but signals are attenuated. Masts that can hold horizontally polarized antennas at 6m above ground are not really feasible for a backpacker. However that does not make it impossible: at moderate heights of say 2-3m above ground, an antenna for 144 MHz is still directive, still has some gain and makes contacts possible that are not going to happen with a stubby helical on an HT.
And that’s all we need, lots of contacts. A distance record attempt would not be a backpacker project.
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It is common mistake that everything that appears on computer screen is real. No, it is not. It is model. Model is not real world. Don t let one tenth of dB to make your biggest problem.
What is real world? Here in Europe look at Alpe Adria vhf/uhf results. Compare real world antennas, height, power, asl. Then, if you want model that.
Any yagi is better than dipol. Any quad is better than yagi. No matter the model. It is experience.
Reading the comments I come to the conclusion that neither my 144 MHz SOTA station used on a regular basis (8.5 dBd @ 6.5 m or 11.5 dBd @ 5.5 m) nor my 144 MHz SOTA/GMA log (> 120 squares from my home zones JO42/JO50) nor the distances of > 500 km crossed regularly exist in reality. This all is just my magination. 
Back on topic, it’s not only modelling but also experience that an antenna height of 6 - 20 m seems the best on 144 MHZ. 25 m agl even seems to be a distadvantage as the radiation pattern shows too many lobes with deep nulls. Therefore I usually avoid viewtowers higher than 20 m for VHF activations.
Ahoi
Pom
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