The impact of trees on radio propagation, 8th December 2016

The Impact of Trees on Radio Propagation

John Dawes VK5BJE/VK5PF

This paper came about because Paul, VK5PAS, was told by a European amateur, who in responding to a photograph of Paul’s portable station, suggested his antenna was too close to trees and that this would impact negatively on propagation. Paul asked me to write something on this topic for Out and About.

I should declare at the outset that I do not have formal qualifications in physics and simply hold an AOCP (1977).

However, I said I would tackle the topic as an amateur radio operator.

Trees belong to a class of organic objects which include shrubs and ground cover plants, such as native grasses, which might all have a potential impact on radio propagation. The factor of interest is absorption, but could include other factors such as directivity. If trees and other foliage absorb radio frequency energy (RF), is this a serious matter and likely to adversely affect a portable amateur radio station? Many objects are able to absorb RF, for example, people, cars, rocks, hills as well as foliage.

In my experiments on 23 cm with Brian, VK5BC, Brian noted that an increase in signal strength at his home station occurred if he turned his beam towards the local silos at Gawler. I know from Amateur Radio Magazine, that Justin (VK7TW) reported that amateurs in Hobart use Mount Wellington as a passive reflector for 23 cm transmissions around the city and environs. Those of you with two metres and 70 centimetres transmission capability in your vehicles will know that antenna placement is critical in determining the radiation pattern. Roof top centre-mounted antennas are more likely to result in a donut-shaped radiation pattern and perform better. You are also likely to have experienced ‘flutter’ on mobile VHF and UHF, as well as with FM broadcast transmissions caused by objects, including plants, in the propagation path.

When it comes to absorption of radio waves by foliage this has been researched for several decades (see Goraishi, Takada & Imai, 2013, ch 6).

More recently telephone companies have driven this research on the impact of foliage on their UHF and microwave transmissions because they want to know how foliage might change the radiation patterns from their phone towers, especially when foliage is suggested as a factor in persuading the local neighbourhood to accept a tower, as was the case recently at Bridgewater in the Adelaide Hills.

An internet search revealed a plethora of articles, some scientific, of absorption at UHF and microwave frequencies and many articles by amateurs about foliage and HF. The latter were more subjective and observational as one would expect from amateurs who do not have the resources available to research institutions, government and military.

When I later conducted a search using Google Scholar I found well over 100 refereed journal articles. A meta-analysis of these is beyond my capacity and time. So returning to observational approaches to understanding absorption has been the field of amateurs.  Carefully recorded observations can be useful. I know from my own experiments on 70 cm using ATV (AM), SSB and FM foliage has an impact on signals. Flutter noise and fluctuating signals resulted, especially when the foliage was wet (see Meng et al, 2009) and the impact on ATV was more noticeable because of the 7MHz wide channel. With digital television signals, at home if I walk through the signal from channel 44 the picture pixilates.

At the empirical level many of the reported experiments targeting the interaction of radio waves with vegetation in a spatial sense used directional antennas (beams). This is the simplest approach (see Ghoraishi et al, 2013). Their conclusion ‘is that the airy spaces in the vegetated area can have a crucial influence in directing the signal toward specific directions, to be re-directed by foliage with the line of sight towards the receiver”. Or, in my words, such a situation can have unintended consequences in terms of the target area for your signal.

What about foliage at HF frequencies?

Should we give up using trees as antenna supports for our stations in the field or at home? My answer is a strong NO! Experiments at HF are much more difficult to construct. There are simply too many variables to control, for example, the sun (the K and A indices, the solar flux, the time of the year and the season), antennas, structure, type, direction, the skill of the radio operator (the art of communication) and equipment. What we can say is that trees are small, relatively in terms of wavelength. Australian trees, especially eucalypts, are generally less dense than trees that grow in colder climates, Europe and North America for example. And our portable experiences would be poorer if we couldn’t find a magnificent shade tree to keep the sun away in an extended activation while the tree also supports one end of the antenna. My view is that the losses in miss-matched antennas, batteries with reduced voltage and lack of operator skill are more likely to impact on the success of your day out.

Just beware that some Australian eucalypts can drop branches, especially on a hot day after wind. So be careful!

References

Giles-Clark, Justin, VK7-News, Radio and Electronics Association of Southern Tasmania (REAST), Amateur Radio Magazine, October 2016, pp 62-63

Ghoraishi, Mia, Jun-ichi, Takada and Imai, Tetsuro, (2013), Radio Wave Propagation Through Vegetation, Chapter 6. Accessed 6th October 2016. http://dx.doi.org/10.5772/52571

 Meng, Y. S., Lee, Y. H. and Ng, B. C., 2009, STUDY OF PROPAGATION LOSS PREDICTION IN FOREST ENVIRONMENT, Progress in Electomagnetics Research, B, Vol 17, 117 – 133

This article first appeared in Out and About, Issue 26 December 2016

 

4 thoughts on “The impact of trees on radio propagation, 8th December 2016

  1. Paul always booms into my station, that link dipole does wonders. I agree with the analysis. At higher frequencies objects become troublesome, but at 7MHz I’d be surprised if one could repeatably measure the impact, even at an antenna range.

  2. Hi Jarrad,
    Thank you for the comment. At HF it is highly unlikely that Australian trees would cause any noticeable attenuation of signals. The overwhelming focus of studies seems to be on UHF and microwave frequencies for obvious financial reasons as described in my piece. Paul, of course, operates mainly at 40 watts and I use mostly 10 may be 20 when needed. That makes a small difference and I can get away with using smaller batteries. What I did notice reviewing the articles that some of the recent HF focused work explores short distance tactical use of HF by military and Near Vertical Incidence Skywave transmissions. It is interesting that the short wave (MF) ABC broadcasting stations in the NT are closing (ABC news on line 8th December 2016). They used NVIS, but one frequency I could quite frequently hear in Adelaide. I am pleased you like my piece.
    Cheers
    John D

  3. Hello John, in a time prior to SOTA, I was once ‘told’ by a local VK1 that portable HF operations in the Brindabella Ranges through to Mt Ginini would be a waste of time as eucalypt trees absorb radiated RF energy. This chap claimed it will be impossible for you to communicate with other radio amateurs when you have a simple antenna among the trees. So wrong! …fast forward, last Saturday operating from Devils Peak, Brindabella National Park in a dense forest of eucalypt, wattle and tee trees, not only did I work VK and ZL chasers on 40m, 20m and 10m, I also bagged a 6m contact with VK3WE over 302 km and all on a simple inverted V linked dipole with the apex at 7m. Thanks for the post John, your research is backed up by evidence and the practical application of HF antennas in the Australian bush. Well done!

    73, Andrew VK1AD

  4. Hi Andrew,
    From a subjective point of view I have never noticed any attenuation of signals from operating in the Australian bush. Such a judgement would be more difficult to make if you were setting out to record your observations at present because of the variable and often poor band conditions. And my comment is also predicated on the other variables we have no control over, like brother Son (St Francis of Assisi)! When it comes to VHF and UHF I would be more cautious. Thanks for the comment Andrew. With a bit of luck I might soon be back on Mount Ainslie to finish it off (44+). I will let you know. We might be able to have a DMR simplex contact.
    Cheers
    John D VK5BJE/VK5PF

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