uhf radio query

Submitted: Wednesday, May 23, 2007 at 19:48
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i was working at a mine on tuesday night and the uhf radio signal was extraordinarly clear, and it could transmit and recieve a very long way ( 50km straight ). i was under the impression that uhf didnt rely on atmospheric conditions because it dosent use the ionosphere to ' bounce'. can anyone tell me why transmissions improved so much . bearing in mind that the temperature that night was about 8 deg cooler than perviuos nights.
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Reply By: Member - Doug T (W.A) - Wednesday, May 23, 2007 at 19:59

Wednesday, May 23, 2007 at 19:59
I have recieved and transmitted 260 klm I was somewhere between Richmond and Hughendon Qld and me and the 2 trucks I was escorting could hear the Shell McKinlay Roadhouse along other trucks on the Landsborough Hwy , I made contact with the Shell , I imediatly checked the distance to McKinlay with the GPS in straight line and it was just under 262klm when I checked , It would be freak transmissions trapped between 2 layers of the Ionisphere and what is called piped, back in the early 1960s when I lived at Waikerie SA we got TV from Perth under the same conditions, sometimes the picture was bad but sound good, other times sound bad /picture not to bad as we seen the Perth Test Pattern as they use to have before the stations opened for the day.

Doug
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Follow Up By: samsgoneagain - Wednesday, May 23, 2007 at 20:03

Wednesday, May 23, 2007 at 20:03
no. for one richmond is flat geographically and higher than mckinley. and tv would not have been transmitted on uhf in the 1960's.
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Follow Up By: Member - Doug T (W.A) - Wednesday, May 23, 2007 at 20:16

Wednesday, May 23, 2007 at 20:16
samsgoneagain.
You know your beginning to Pi$$ me off mate , I did not say TV was transmitted on UHF in the 60s you clown, I said we recieved signals from WA the same way a radio uhf signal would be piped , Have you studied for an Amature Lic' , was you even alive in 1961/2, you come on here and rattle on about BS you know nothing about .
And you have the hide to tell me I and my family didn't see Perth TV in SA during freak conditions , rack off .
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Follow Up By: Member - Doug T (W.A) - Wednesday, May 23, 2007 at 20:25

Wednesday, May 23, 2007 at 20:25
samsgoneagain
And another thing ....you asked a very good sensible question and when you got a sensible honest answer you shot me down , .I don't know why I bother
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Follow Up By: disco1942 - Wednesday, May 23, 2007 at 20:51

Wednesday, May 23, 2007 at 20:51
Doug

This is a common occurrence. The mechanism is generally the result of a temperature inversion rather than ionospheric. This is what samsgoneagain experienced - generally happens at night time but can happen during the day. The quick change in the density of the air between the cool air and the warmer air that drifts over the top of it. Cloudy conditions can also produce the effect. What time of day did you hear the Shell roadhouse

We used to get herringbone interference on CH8 from Orange at Dubbo when the ducting conducted CH7 from Canberra into our area (TV sets are very poor at rejecting adjacent channel interference.)

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Follow Up By: Member - Doug T (W.A) - Wednesday, May 23, 2007 at 20:56

Wednesday, May 23, 2007 at 20:56
Peter
The time was about 6:30am and cool .Just had a thought ...samsgoneagain might have been on Duplex going through a repeater, the one I was talking about was on Ch 40
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Follow Up By: disco1942 - Wednesday, May 23, 2007 at 20:59

Wednesday, May 23, 2007 at 20:59
Doug

That would be a good time for a temperature inversion.

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Follow Up By: Member - Doug T (W.A) - Wednesday, May 23, 2007 at 21:14

Wednesday, May 23, 2007 at 21:14
Signals with frequencies above about 30 MHz (VHF and UHF for example) are progressively not returned to the Earth's surface, because they penetrate the ionosphere. (This includes most communications with spacecraft and satellites.) Exceptions include rare occasions of E-skip, when FM and TV signals are reflected. Skywave may be disrupted during geomagnetic storms.

Low to mid frequencies below approximately 10 MHz (longer than 30 meters), including broadcasts in the mediumwave and shortwave bands (and to some extent longwave), travel most efficiently by skywave at night. Frequencies above 10 MHz (shorter than 30 meters) travel better during the day. Frequencies lower than 3 kHz have a wavelength longer than the distance between the Earth and the ionosphere. The Maximum usable frequency for skywave propagation is strongly influenced by sunspot number.

Because the lower-altitude layers (the E-layer in particular) of the ionosphere largely disappear at night, the refractive layer of the ionosphere is much higher above the surface at night. This leads to an increase in the "skip" or "hop" distance of the skywave at night.

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Follow Up By: obee - Wednesday, May 23, 2007 at 21:21

Wednesday, May 23, 2007 at 21:21
its called ducting if I remember correctly from when I learned amatuer radio.

Owen
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Follow Up By: Member - Doug T (W.A) - Wednesday, May 23, 2007 at 21:30

Wednesday, May 23, 2007 at 21:30
Owen
SPOT ON MATE

Ducting is the very name I was looking for , just that I didn't make it to the end of amatuer classes . I got a job and couldn't make it to courses , that was about 1976/7 ,
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Follow Up By: Footloose - Wednesday, May 23, 2007 at 22:20

Wednesday, May 23, 2007 at 22:20
Doug, FYI there are various kinds of ducting such as tropospheric and ionaspheric. They affect different bands of frequencies.
We used tropo as secure comms over 800k in asia, by bouncing the signals on ionized meteor trails.
The mechanism is temporary but relatively easy to predict. However it is a specialist field, used largely by radio amateurs for long distance contacts.
Interestingly enough, HF waves can also be reflected back off objects, and this is the basis of our over the horizon radar systems in Australia. So the next time a plane goes over your aerial, listen for a bit of "flutter" .
BTW I heard some South Oz amateurs on the TV around Xmas in 1958. That's what started me interested in getting my license. I also heard Taxis, but I never wanted to drive one :)))
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Follow Up By: Pajman Pete (SA) - Wednesday, May 23, 2007 at 23:42

Wednesday, May 23, 2007 at 23:42
We used to call that surface ducting in the Navy. It results from a low level temperature inversion and can also affect VHF and radar.

I can recall a night in the Indian Ocean in the 1980's when the surface search radar was picking up ships at over 150 nautical miles whe its normal range for all but supertankers was about 30.

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Follow Up By: howie - Thursday, May 24, 2007 at 14:05

Thursday, May 24, 2007 at 14:05
i will not believe it unless someone can provide a recording on a cassette.
would this be called 'duct tape'
seriously, occasionally get bothered on vhf when flying at 1000ft at bunbury by very clear calls from aircraft on downwind (1000ft also) at geraldton (over 500 km away).
mentioned height because as we all know, it greatly affects range so this must be ducting.
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Follow Up By: Member - Davoe (Nullagine) - Thursday, May 24, 2007 at 22:04

Thursday, May 24, 2007 at 22:04
Ahhhhh Doug your bringing back memerys from the Eyre Penninsula now! 40ft high antenae and not knowing what you will get when you turn on the tele - complete snow? intermitant pictures/sound? (yes we still watched it) or crystal clear TV. Discussion all week would be wheather or not there might be a semi watchable reception for prisoner. Kids today get the ABC and Imparja no worries - never used to be that easy
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Follow Up By: Member - Doug T (W.A) - Thursday, May 24, 2007 at 22:26

Thursday, May 24, 2007 at 22:26
Davoe
Yeh you got it mate , We were pushing our luck to get reception at the best of times at Sunlands, near Waikerie/ Ramco b/w , the signal had to come from Mt Lofty , Just using Mapsource it shows 144klm , amazed to see on T4A all the roads in Sunlands, IMPARJA ....that wasn't even thought of in 1961 , Nah we had all the Fab Westerns, The Wrestling , Laural and Hardy Movies. oh and Test Patterns, , then earlier when we first got TV at Eden Valley ....yeh we watched the snow waiting for the transmission to begin and went to bed after the National Anthem, Oh we had Lionel Williams and Kevin Crease on CH9s Adelaide Tonight .



" Now Look what Youv'e gotten us into Stan "

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Reply By: Auntie - Wednesday, May 23, 2007 at 22:38

Wednesday, May 23, 2007 at 22:38
Just for interest, many years ago,1970 in fact, I was serving in the Army in sunny Sth. Vietnam as a radio operator in a Command Post for my unit. All of a sudden I began receiving these 'Pommy' voices over our Unit net. After listening for a while I butted in to see whar was going on. It turned out to be a British Infantry unit exercising in the hills out of Hong Kong ! We conversed for a minute or so,then, as quick they arrived they were gone. The incident was promptly recorded in the Unit log book. We were using a VHF set, but I can't recall the actual frequency.It's a fair way from Southern Vietnam to Hong Kong, but just about a clear path over open sea. It certainly startled me at the time.
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Follow Up By: Footloose - Wednesday, May 23, 2007 at 23:00

Wednesday, May 23, 2007 at 23:00
Common frequencies ranged from between 30-70 meg on VHF AFAIK. Ducting on those frequencies is fairly common especially where temperature inversion occurs.
No doubt the Pommies were using the same kind of radio, and I bet it startled them, also :))
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Reply By: Member - Barry (NT) - Wednesday, May 23, 2007 at 23:02

Wednesday, May 23, 2007 at 23:02
Yep it happens IAGREE WITH THOSE WITH EXPERIENCE as stated above (not wnna be's),,,, in Darwin in 80's we had interferrence on Air Traffic VHF frequency from Timor on a regular basis,,, (VHF and UHF same propagation priciples generally and only arbitrarily divided into "bands" or groups of frequencies). It go so bad the Darwin frequency was changed.
Fair dinkum.
Barry
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Reply By: Auntie - Wednesday, May 23, 2007 at 23:36

Wednesday, May 23, 2007 at 23:36
Doubt if the Poms had the same sort of radios, as we were using all American equipment.The to and froms would no doubt have had their own equipment. Can't be sure on what they had though.
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Reply By: Member - Hughesy (NSW) - Thursday, May 24, 2007 at 10:34

Thursday, May 24, 2007 at 10:34
Sam, You sure it was UHF? A lot of mines run on VHF too these days. 50km is certainly nothing extraudinary though, given the right conditions.

I remember when working on properties in western Qld - you would be on a tractor during the night and have the radio on AM scanning for stations. You'd pick one up clear as day......and it would be a Brisbane station. But typically over 5-10 minutes it would eventually fade away and you'd scan again and pick up another far off station only for it to eventualy fade away. Kept me awake anyway.

I reckon if you could see radiowaves the sky would be black.
AnswerID: 242000

Reply By: Tony MD - Thursday, May 24, 2007 at 18:14

Thursday, May 24, 2007 at 18:14
For those interested in tropospheric ducting, take a look at this site. www.dxinfocentre.com/tropo_aus.html
This is one that we use to predict band openings on VHF and UHF
The Victorian distance record for 144mHz and 432mHz is held by an amateur in Bairnsdale who worked another operator in Albany.
Conditions are normally indicated by a large high pressure system central to transmit & receive sites. In mid winter, it is not uncommon to have good inland paths from Adelaide to central N.S.W
Low band VHF TV signals may also be affected by a propagation mode called sporadic E. - up to 144mHz.
VHF and UHF radio signals may be transmitted over long distances by aircraft enhancement - reflected off the body of a high plane. Typical Melbourne to Canberra, Sydney to Brisbane.
Other modes of propagation are auroral scatter, meteor scatter.
Normal mode of operation for this weak signal work is SSB, CW or Digital.
Cheers, Tony VK3CAT
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Reply By: Olcoolone (SA) - Thursday, May 24, 2007 at 23:57

Thursday, May 24, 2007 at 23:57
What gain was the antenna......going by what other topics have said low gain antennas are great for hilly terain and high gain antennas are better for long range.

SO.....using this formula I have work out you must of used an antenna of 305.7295100 db gain ......am I correct.

OR had a transmitting power of 10254 watts.

MAYBE you were using 27Mhz or VHF low band and you did not know.

OR the conditions were just right.....you can work distance and it happens more then you think.

There use to be a big thing with working TV stations from around the world and also TV Dxing thas a big rage once.

Regards Richard
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