Do fires effect UHF signals?

[VK4WDM]

Hi All

I have been looking through one of my old notebooks when I found the following entry: "hams could be involved in research on the effects of bush fires on radio signals, JCU Professor." I puzzled over these words for some time until I remembered a conversation I had with one of the Professors in the Physics School at James Cook University during a North QLD AR Convention about four years ago.

Some firefighters had mentioned to him that they when they were attempting to communicate across grass fires on UHF there was marked surging and fading in their radio signals and that sometime the signals dropped out altogether. He thought it could be caused by changes in air density due to the heat and it was worth some research but could not find the time. He wondered if the local hams were interested in doing a study using 70cm gear across cane fires. I promptly forgot about the suggestion until now.

I know that some of the ham family are firefighters and I would be interested in hearing of your experiences. Would it be worth doing some experiments as the Professor suggested? Perhaps comparing several bands, may be 2m, 70cm 23cm?

73

Wayne VK4WDM

[VK4QB]

Wayne,
I have been involved in ducting with 1ocm radar, and this is caused by the sudden temperature change areas of the atmosphere causing refraction of the radio waves. Usually for refraction to take place, the area that the temperature change takes place is usually kms in length. However the greater the temperature change then the greater the refraction so short range propagation may be possible through fires. However the temperature changes in these areas are not uniform and smooth as normal temperature inversion is but very broken so it may not be as good as normal atmospheric ducting . in fact I would suggest that any transmissions through fire ares would be very intemittent and broken I think that it would be a good experiment.
My 2c
Brian 4QB

[VK5ZKR]

Hi Wayne,

unfortunately I cannot offer you any scientific information other than the following from my previous experiences. For several years I was an active member of the Country Fire Service (CFS) here in VK5, which also included a position as a communications officer for several CFS brigades. Before the introduction of the SAGRN, which operates on the lower 400Mhz freq's, we predominantly used freq's in the 163Mhz range and also used limited UHF freq's as a backup, dependent on the type of incident and our location. During my time the 163Mhhz freq's served us well and it was common conjecture that UHF could be a problem. This was a concern highlighted during the initial design and rollout stages of the SAGRN by those in the service who had some previous radio experience. I did conduct some experiments with a good friend of mine (also a HAM) who at the time was an active member of the local CFS regions communications brigade and we did find that UHF was certainly limited in it's capabilities. As an example we had several large areas of pine plantations within our area of responsibility and found that these areas, even without the involvement of fire, made UHF communications almost impossible when we were deep within the pine plantations.

Within a couple of years of the SAGRN rollout, and the removal of all 163Mhz equipment, they reinstated txcvrs in the lower 160Mhz region to be used for the purpose of fireground communications only. I would suggest that the powers to be would justify this as a method of relieving the overload of transmissions on the SAGRN, which is now used predominantly as a tool for command and logistics now, however I believe that the active personnel on the fireground have far greater confidence in the VHF simplex communications.

My 2c worth Wayne..

Regards,

Rod...
VK5ZKR

[VK3OP]

Given the time of year and the imminent fire season this discussion is very timely. It might be worth trying to coordinate
some sort of testing using the VKlogger across a fire region and comparing the results from band to band, ie, 6, 2, 70, and seeing
if there is any measurable difference. If it is true that the VHF comms are more reliable, and because of the terrain Rod desribes in
the pine plantations, perhaps low band VHF would be the best tool for trucks in the middle of a fire situation.

I certainly noticed the problems with the high band VHF comms here last year during the black Saturday fires. Some of the trucks
depending on their location and situation could not be copied by the controller or dispatchers, and the status of some of these crews
was uncertain.

Food for thought

Frank VK3OP

[VK4QB]

Hi all,
I think the big trouble with UHF is that the operation is line of sight predominately. This shows up where operation on the highway by truckies is monitored .
However VHF does bend about 10%, also the attenuation of UHF due to vegetation is quite a lot higher than VHF. We are missing the point. Re fires !!! The only way, as I see it, is to get a known path which is line of sight and check it out, and tabulate it. Then fire up the middle and see the difference. Hmmmmm. That's a bit awkward during a fire ban ???
Any suggestions ??
Brian 4QB

[VK2HRX]

Brian,

Maybe co-ordinate the test durign a hazard reduction burn. These are generally well controlled events so you would be able to plan it out fairly well. Different fuel sources, different heat, different types of smoke. Would be interesting to try it.

Compton

[VK4WDM]

As I recall it, the incident in question was a grass fire on lightly-timbered flat land so line of sight, or trees were not a problem.

I am going to contact the Prof and see if he is still interested. During the cane-fire season it would be easy to set up transceivers on each side of a burning cane field.

73

Wayne VK4WDM

[VK3OE]

During the early 70's Telecom Research Laboratories conducted a test on just such issues in Queensland. The test was extensively instrumented and involved burning a several large windrows of cleard trees that had been left to dry. The fire was intense but almost NO measured changes in the signals propagated across or through the fire were noted. I do not remember the number of the Research Labs Report. It may still be hidden away is some archive.

So the principal issue is "could fading of radio signals happen is fires"? Fading does happen if an inversion layer forms or can be enhanced by the fire. I have personal experience of this in 1968 when I was operating a 400MHz fixed link between Exmouth and Learmonth in WA. The link completely faded out when a grass fire was lit to clear grass from around the Learmonth Airport. An inversion layer was clearly visible from the trapped smoke. The resulting second signal path just happened to cancel the direct signal completely, an effect often seen on fixed links worldwide particularly when ground based inversions are present. The elevated inversion created by the fire must have been very stable as the loss of link was evident until the fire was extinguished and the input to the temperature inversion dissapeared.

To the issue of loss of communication during fires... I gave evidence to the Linton Inquest when there was an apparent loss of radio communications and several CFA volunteers lost their lives. I was able to model the effects of an inversion layer created by the fire to see if a loss of signal clould happen in such circumstances, given the knowledge of the earlier TRL report. It appears that multipath signals could lead to some loss of signals but over only very small distances given the geometry of the paths. All that is required would be to move less than 0.1m to restore the signals something that we are all accomplished at when we use cell phones.

It would thus appear to be that the loss of communications during fires is due principally to the amount of radio channel traffic and consequent interference and not due to the effects of fire. This was also noted in the Ash Wednesday fires when a mayday call was lost in the radio traffic and not found until several day later, too late for the volunteers involved!!!

Andrew Martin
VK3OE.

[VK2JDS]

G'day, as i am the equipment officer for our brigade this interested me

"I know that some of the ham family are firefighters and I would be interested in hearing of your experiences. Would it be worth doing some experiments as the Professor suggested? Perhaps comparing several bands, may be 2m, 70cm 23cm?"

yes, many of us are RFS volunteers, my wife(also an amateur) is the deputy captain.
UHF on firegrounds is always hit and miss as the terrain we work in is very hilly usually. here in NSW we are being issued with 161 meg fireground radios for use truck to truck. we as 2m operators know this band walks all over uhf pmr and cb.
to determine if there has been any drop or lift in signals would require a dedicated team to make scientific measurements. i wouldnt even like to guess. the melee at a firergound of trucks, helicopters with long bucket lines and fixed wing bombers keeps you very busy, and chasing the firefront across paddocks, cutting fences and gates means a team of volunteers in a rfs truck really dont have time to make any proper measurements, but uhf handheld to truck and hh to hh comms is usually interupted by flat batteries , operator error or too much noise generated by pumps etc.
the 161 meg radios are great, the uhf cb sets are just for use from hose-end to pump operator. comms back to firecomms control is handled by a PMR network of 4 repeaters interlinked on 700 megs and in-out is 403/413 freqs.
personally i havent seen any sign of fire induced decrease or increase away from the normal line of sight path issue with uhf cb handhelds, often 'just a jump to the left' will get a signal through in a forest situation provided you are high enough up the hill. the cb handhelds are cheap and almost disposable if you loose one.. but vhf is by far much better on a fireground.
73's from vk2jds Dave

[VK2SWL2]

A somewhat higher frequency, but where I was working in early 2007 we distributed a couple of different brands of licenced microwaves. One of our customers was based in Sale VIC, and had quite a few 13 GHz links around the area. I don't know the details of the links involved, except being 13 GHz they were probably fairly long haul from microwave tower to microwave tower on top of hills.

At the time of the large scale fires through the Victorian High Country region in January 2007 a lot of the links were dropping out when there was fire in between them. Now going from hill to hill one would assume that most of the time the link path would be well above the flames (unless it was that hill on fire). It appears that the smoke particles and also possibly the water vapour being released from the burning material and maybe firefighting operations was attenuating the links, but it could have been a refraction problem as well.

[VK3OE]

It is that the fire forms a STABLE inversion above the direct line of sight of the 13GHz beam so that a second signal arrives at the antenna via the fire induced elevated layer. Depending on the phase of the second signal the direct and second signal can cancel at the antenna causing a loss of signal. Variations in the height of the fire induced inversion layer will cause a change in delay of the second signal resulting in the links dropping in and out.... Just as you experienced. If you were able to observe the smoke it would have shown where the inversion was. The principal reason for having high gain antennas on line-of-sight links is to discriminate against the second signals arriving from the ground, ground based inversion layers or elevated inversion layers and hence minimise the amount of fading. Forget about "k-factor" fading, it never happens !!! It is all multipath!!!

Andrew Martin
VK3OE

[VK2AAH]

Hi all,

I think this discussion is drifting worse than some of my early homebrew radios...

The question related to UHF and smoke. I think VK3OE may have raised an important factor and that is the impact of fire generating heat layers which in turn cause ducting. I am not so convinced that ordinary smoke, without particularly dense particles, has a significant effect at 400MHz. What I have seen is the impact of specific objects- an example is salt laden River Red Gums around Deniliquin- which cause dramatic signal drop which when mobile is observed as severe "picket fencing" despite signal levels clear of them being in the order of -80dbm.

I think that the "line of sight" approach to UHF is a bit exaggerated and I don't think the difference when compared to VHF Highband is as great as some have portrayed. What is true, beyond question, is that to cover the same physical area at UHF requires greater numbers of bases than it does at VHF Highband. What that statement ignores is that it is far easier to optimise base station coverage at UHF than it is at VHF (due primarily to antenna size) and secondly ignores that it is actually better to have more base sites than to depend on a single site- particularly for an emergency service concerned about the impact of outages on operations. It is not a crime to use 3 base sites to cover an area, and it is not something to be proud of in using just one. The issue is reliability, and should be nothing else.

My background, for those who wish to know, is as a radio system designer & implementer for one of the larger NSW emergency services (involved with fire!). Incidentally, pardon the pun, they have always used 400MHz for simplex incident ground use... Prior to that I did a lot of VHF system design for one of the agencies that is very anti-UHF. I am frequency agnostic, but I am very anti dodgy-design regardless of what part of the spectrum it falls in.

Cheers,


Richard
VK2AAH

[VK2AAH]

Andrew,

Totally off topic but if you want to hear about another pearler of a fade...

I first came across a 1.5GHz microwave link from Jindaboine Hill to a passive repeater across Lake Jindabyne. This link employed space diversity as the impact of temperature and water level regularly induced almost total fades on this link. Both ends of the link are very close to the water's edge. Imagine my surprise when at my next employer I implemented a 900MHz link from Jindabyne fire station to Kalkite Trig, a very similiar link path at almost right angles to the first. Little did I think this would suffer from the same fade... you bet it did! I actually witnessed a fade from -65dbm to nothing, almost but definitely not instantaneous, only to return 5-15 minutes later! And space diversity was a little difficult to implement...

Sorry for straying off topic!

Cheers,


Richard
VK2AAH

[VK2XSO]

They sure do.
I had some fantastic bushfire enhanced propagation with the big fires in the Blue Mountains a couple of years ago.
I was driving down the Pacific Highway from Kempsey when the radio stopped scanning and I heard familiar voices from the
Blue Mountains 70cm repeater. The signals were no different in quality compared to driving around Sydney except for a slight
bit of fading every kilometre or so. I was about to talk via the repeater all the way down to Raymond Terrace where the it faded out.
I am usually able to pick it up under normal conditions as Hexam and this was still the case, before dropping out again on the F3.

The location of the fire close to the 70cm repeater is what I believe to be the cause of the enhancement.
The fires were hot enough to produce pyrocumulous clouds and I suspect the refraction between the column of hot rising air and the cooler
surrounding air is where the enhancement occurred.

Some interesting notes where that no other DX 70cm repeaters were heard and the VHF repeater at the same site was also not heard.

I haven't seen any other similarly large fires in the past couple of years to test the theory again.

[VK9WBM]

Hi,

Vk3OE made a comment that shows the difficulty in replicating the effects artificially, and the danger of relying on research without field observations to check it.

He quotes a paper showing no effect but the wood used for the fire was "dried", a natrual fire will burn eucalypts which release several 100 kg of water or commercial evergreens like radiata pine which release over a tonne of water due to twig and branch burning. Just look over any fire to see the clouds forming from evaporated water (these clouds are called pyrocumulus).

I have seen variation in radio when portable walking through deciduous forests in the winter to the same path in the summer. The water in a tree DOES affect VHF/UHF propogation so I would expect the same water in a hot air column to do the same. The water condenses out at the cloudbase where the temperature and the dew point are equal (saturation of the air by thermal decline due to pressure reduction for the technical minded). Due to this I would expect any effect to disappear over distances of more than a kilometer or so.

Actual tests across a true bushfire wouls be the only way to find out.

Dave, VK9WBM,
OIC Willis Island Weather Office.

[VK3BA]

To the issue of loss of communication during fires... I gave evidence to the Linton Inquest when there was an apparent loss of radio communications and several CFA volunteers lost their lives. I was able to model the effects of an inversion layer created by the fire to see if a loss of signal clould happen in such circumstances, given the knowledge of the earlier TRL report. It appears that multipath signals could lead to some loss of signals but over only very small distances given the geometry of the paths. All that is required would be to move less than 0.1m to restore the signals something that we are all accomplished at when we use cell phones.

It would thus appear to be that the loss of communications during fires is due principally to the amount of radio channel traffic and consequent interference and not due to the effects of fire. This was also noted in the Ash Wednesday fires when a mayday call was lost in the radio traffic and not found until several day later, too late for the volunteers involved!!!

Andrew Martin
VK3OE.

Good work Andrew. I've been a volunteer member of the CFA for approx 20 years. I sat through most of the Linton Inquest and I'm aware of most of the evidence put forward. Prior to that, the general belief within the CFA was that smoke from fire was a major attenuator of VHF signals. Of course your evidence since, has helped to quash any of these old schools of thought. There are so many variables in communications involving fire fighting operations... mainly some of the radio users. This has been evident again during the events of Black Saturday last year.

And yes, the larger fires do create their own localised weather system. All sorts of weather behaviours can occur within this mini-system that isn't often apparent from the outside.

But, whilst this is a very interesting topic, I may be veering off in another direction. So enough for now

Cheers,