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VHF propagation question

Singularity35

Singularity35

DU7DVE
Jan 16, 2014
425
90
138
Small City, Philippines
I'm just wondering in an intellectual curiosity way about this. I put up a 2 meter station with a 6 element, twin stack aerial about 50 feet high. I can make contact with stations in a location about 80 Km away on another island about 80km across an uninterupted stretch of sea. Here's my question, contacts usually start out good and strong with 5-6 S units but sometimes fade out to nothing in less than a minute or two. What could I attribute the fadeouts to? Your thoughts are much appreciated.

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I take it you are in The Philippines? Do these contacts do it ALL the time of usually more in the early morning just after sunrise and again late afternoon/evening? Sounds like what is typically called a "lift". A lift is simply a type of atmospheric enhancement that allows VHF/UHF signals to propagate much better. I used to be in the broadcast business and one transmitter we had was leased to the CBC and it's program source was an off-air pickup from another CBC station in Halifax. Often in the early morning and again around suppertime or early evening in the spring/summer months we would loose the CBC Halifax feed and have to switch to CBC Saint John for the feed. CBC Saint John was better during these times. Since you are in a hot humid climate and over a water path you probably see it happen a LOT.
 
Thanks for the reply Captain. Yes, I am in the Philippines and you're correct about it happening early morning and early evening. Some older hams here have told me that it might have something to do with over water and temperature changes and it turns out they were right but being me and my OCD, I do like to know the proper terminology. :)

Thanks again Cap.
 
Using far less impressive antenna arrays, I've seen similar effects on 2 meter marine VHF and on 11 meter CB here in the lower Florida Keys, where water temperatures are relatively warm year round. The night-time "K factor" effect is also very noticeable down here
 
Fresnel nodes?

Singularity35 said:
Thanks for the reply Captain. Yes, I am in the Philippines and you're correct about it happening early morning and early evening. Some older hams here have told me that it might have something to do with over water and temperature changes and it turns out they were right but being me and my OCD, I do like to know the proper terminology. :)

Thanks again Cap.
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Captain Kilowatt said:
K factor is a number used to calculate sunspot numbers and predict propagation. It has a value less than 1 and is dependent on the solar observatory used.

http://solarphysics.livingreviews.org/open?pubNo=lrsp-2010-1&page=articlesu4.html

http://spaceweather.com/glossary/sunspotnumber.html

http://wattsupwiththat.com/2013/01/04/counting-sunspots-and-sunspot-inflation/
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Thanks Cap, I doubt that that is the cause since the conditions vary almost from minute to minute. I'm having two or three back and forths at 5-6 bars, then it fades out to nothing. After a few minutes, propagation is back, might be good for another 5 minutes then out again. I check into the net and sometimes I get fadeout before I have a couple exchanges. Other times, signal strength holds for 30 minutes or even longer.
 
Singularity35 said:
Thanks Cap, I doubt that that is the cause since the conditions vary almost from minute to minute. I'm having two or three back and forths at 5-6 bars, then it fades out to nothing. After a few minutes, propagation is back, might be good for another 5 minutes then out again. I check into the net and sometimes I get fadeout before I have a couple exchanges. Other times, signal strength holds for 30 minutes or even longer.
Click to expand...

No,no....neither K-factor nor sunspots have anything to do with your original question. I was just answering the question. The reason for what is happening with the fading was answered in my first reply.

Rick33man suggested Fresnel zones but I don't think it is that. That usually happens when there is a direct line of sight between stations and the OP is talking a path of 80 Km or about 50 miles. Fresnel zone fading should happen pretty much all the time and not just early morning/suppertime which are the peak times for ducting or lifts as they are often called.That is a great time to listen for distant FM radio stations BTW. My best was WOKI in Knoxville Tenn. when it was stomping all over a local station.
 
Temperature inversions do neat things to VHF, I've been able to make it in to a repeater 220km away with a 5w HT while driving AWAY from it lol
 

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