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