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Author Topic: Strange VHF propagation  (Read 1941 times)
M5AEO
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Posts: 270




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« on: May 06, 2011, 10:50:06 PM »

I have a puzzling observation of VHF propagation.
I live reasonably close to the GB3VHF beacon on 144.3Mhz.  It is usually about S8 on my FT-817 and vertical 5/8 antenna.  (I recently moved my antenna; it is now 6 feet higher at about 60 feet above ground and much more in the clear.)

In recent days I have found the beacon signal strength to vary enormously, from a steady S8 all the way down to zero.  Sometimes it is a slow fade over about a minute, at other times very rapid.  I had never noticed this before, so I'm not sure if it is related to my antenna move or not.
As far as I can tell, these are the factors that may be affecting things:

1) Daylight: the beacon is usually a steady S8 at night;
2) Temperature: when things cool down, the signal strength seems to recover;
3) My antenna is quite close to the metal roof of the appartment block (although I have not observed any SWR issues);
4) I am under the flight path for London City Airport and Heathrow, so could aircraft-scatter be an issue;
5) I am in a built-up urban area; could I be experiencing multi-path reception?

I can hear quite a few FM repeaters now that the aerial is higher, but I don't find much variation in their signal strengths, unlike with the beacon.

Any insights into this odd propagation would be most welcome.

Jonathan, M5AEO, London.
(I am posting this in 'VHF/UHF' and 'Elmers')



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AC5UP
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« Reply #1 on: May 07, 2011, 01:31:42 AM »

Best guess is what you're seeing is probably multipath distortion. I have a local AM broadcast station that at times can almost fade out at twilight. Apples & oranges, but possibly the same principal involved.

VHF line of sight propagation is definitely influenced by temperature and humidity. In the summer late at night when it's humid my VHF range just about doubles. With that in mind, imagine your beacon signal arriving over multiple paths... Direct, bounce from a building, bounce from a hill, bounce from whatever. All the same signal but all arriving at different times due to the length of their particular path. The multipath signals can attenuate the direct signal somewhat as they arrive out of phase and their strength will be influenced by the temperature & humidity on any given day.

That's why the variation and why you notice this after you moved the antenna. Chances are the old position had less multipath from the beacon, move the antenna again and it may improve or could get worse. If you're curious, log the beacon s-meter reading at about the same time every day against the temperature & humidity. Betcha' you'll see a pattern within a few weeks and the most unusual readings will be on foggy days.

BTW: In the case of my local AM station the fade is (probably) caused by a minor NVIS lobe phasing against their ground wave. At twilight the D layer is rising so the path delay on the NVIS side is somewhat like being on a slider. When it hits ~180 degrees out there is a very noticeable loss of signal. (?)
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M5AEO
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« Reply #2 on: May 07, 2011, 02:13:00 AM »

AC5UP, many thanks for such a comprehensive reply.  I am now starting to notice all kinds of things around me that must be causing multi-path distortion!  I think I'll start to plot things a little more scientifically and look for a pattern.  I'm also wondering if heat rising from the metal roofing is contributing to the effect.

M5AEO



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G3RZP
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« Reply #3 on: May 07, 2011, 05:34:43 AM »

I'd suspect aircraft flutter, as the beacon is more  or less under an airway.
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G8DZC
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« Reply #4 on: May 07, 2011, 06:32:20 AM »

I would suspect that you are getting all forms of propagation going on. Aircraft flutter has a characteristic steam train effect on the signal that you are observing, so it is easy to identify that.

GB3VHF is fairly weak with me (Nr Reading). Recently it has been changing a lot over the day, especially in the late afternoon onwards. I put this down to tropospheric propagation caused by the fairly static high pressure area that has been with us for the past couple of weeks, most of that propagation I have put down to the stable air and temperature inversions that promote ducting of VHF signals. Of course where we are (which is a fair distance from the beacon) we get both the ground wave, and the sky wave. The two combined will produce a varying signal which sometimes will be accompanied with distortion.



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KE3WD
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« Reply #5 on: May 07, 2011, 06:36:14 AM »

Might just be component failure in the beacon transmitter itself...


73
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M5AEO
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« Reply #6 on: May 07, 2011, 11:50:48 AM »

KE3WD. Nope, checked with the beacon keeper!  At the moment (early evening) the signal is a steady S7, and the London City Airport is closed, so perhaps that is the problem.  Lots more monitoring required!

M5AEO


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AA4HA
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« Reply #7 on: May 07, 2011, 02:59:23 PM »

By changing the antenna height above ground you have also effectively modified the antenna radiation pattern. If you ever have looked in detail at +VHF antennas they are usually shown without the effects of an interplay with a nearby ground plane.

By being low the takeoff angle for the antenna may have been much greater (or less). It is very difficult to determine what way it will go as it is a factor of a great many things.

Tisha, AA4HA
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KE3WD
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« Reply #8 on: May 07, 2011, 04:37:33 PM »

Tisha brings up a good point, as usual.  Raising the antenna without knowing how much downtilt it may or may not have, could be the problem.  Do you have a way to listen to the signal on a lowered antenna setup at the same time? 

I recall one situation with VHF repeater link where multipath was incredibly severe, resulting in an almost 180 degree phase shift, which resulted in complete cancellation, so don't rule out the multipath theory here as well.

What polarization is the beacon? 


73
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K4JSR
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« Reply #9 on: May 07, 2011, 07:43:43 PM »

Jonathon, there is nothing strange about what you are observing.
VHF through microwaves all display the tendency to to shift their paths
of propagation especially in warm months and over bodies of water.
(Or marshy areas.)
AT&T Long Lines noted the same problem a long time ago regarding path
shifting of their microwave long distance relay links.  Most especially in
in the southern part of the USA.
They developed a method of Space Diversity Antennas and switching systems that cleverly fixed the problem.   You can read about the early
research here:  http://www.micropath.com/downloads/Docs/RF%20Engineering%20-%20Misc/BSTJ%20Jan%201975%20%20Space-Diversity%20Engineering.pdf

Oddly enough, in most applications the diversity antenna was lower than the main antenna.    This was due primarily to refraction of the signal and
not multipath.  Remember that these were point-to-point highly directional
antennas they used and that pretty much deleted multipath IMD from the
equation.

Enjoy the read.  Your brought up an excellent question that many folks
in the electronics world are not aware exists.  You just don't run into the
situation unless you get involved in point-to-point communication.

73,
Cal  K4JSR

 
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AC5UP
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« Reply #10 on: May 07, 2011, 09:03:36 PM »

AT&T Long Lines noted the same problem a long time ago regarding path shifting of their microwave long distance relay links.

I haven't thought of this in many years but you reminded me... Allegedly when AT&T decided their future was in microwave and not copper for long-haul links, and the TV networks were building distribution, one of the more challenging 'hops' was across the Great Salt Lake / Bonneville Flats.

On paper it looked relatively easy. Approximately 90 miles line of sight with mountains on both sides and a flat valley plane. At night a commercial spotlight (think Bat Signal) could easily be seen across the link. Even with 50's technology a 10 watt signal was more than adequate. Except... The lake is in a desert valley and completely out of place in terms of climate. The lake has its own micro weather patterns and can see intense pop-up storms over water that dissipate almost instantly when they mix with the dry air over land.

In 1974 yours truly worked nights doing tape delay at a TV station in Boise. The raw feed from NYC was daisy chained up through Denver to SLC then split to BOI where I recorded three hours of prime time and played it back one hour delayed. There were times in the late spring & summer when I'd show up around 4:00 in the afternoon for the newscast and the SLC link would be down due to weather. No prob as that was local origination (old syndicated shows on film) but all I could do was wait to see if it cleared by 6:00pm when I started recording. Usually it did, but there were times when the first half hour or so of prime time was loaded with sparklies...

Local legend had it this problem had been with the SLC link since Day 1 and folks much smarter than the average bear never could figure a reliable way to beat the weather. Apparently it doesn't matter how much power you throw at it... When you lose the path by absorption or refraction, you lose the path.

Huh
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G8JNJ
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« Reply #11 on: May 08, 2011, 06:39:16 AM »

Hi Jonathan,

I think the fact that you are using a vertical antenna to receive a horizontally polarised transmission (which is not that far away) is resulting in a lot of multipath reception.

The cross polarised direct signal will be about 20dB down WRT to a correctly polarised signal. The signals reflected from nearby objects will be of mixed polarisation and so are likely to be just as strong (if not stronger) than the direct (cross polarised) signal. As a result of this it only takes a small phase shift in the reflected signals to cause large variations in received signal strength. As the phase shifted signal path varies slightly it becomes in phase or out of phase with the direct signal. This could be due to changes in air temperature, or almost any other activity which occurs more during the day but not at night.

Try using a horizontally polarised RX antenna and see if that improves the situation.

Regards,

Martin - G8JNJ

www.g8jnj.webs.com
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AA5TB
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« Reply #12 on: May 08, 2011, 11:15:34 AM »

Jonathan,

Just to add to Martin's and others excellent comments, it is also probable that when you moved you antenna you placed it in a location where the direct and multi-path waves subtract due to destructive interference.  If that is the case then only a very small change in the path characteristics, ie., aircraft bounce, tropo enhancements, etc., can cause a big difference in observed signal strength.

73,
Steve - AA5TB
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K0ZN
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« Reply #13 on: May 08, 2011, 07:47:41 PM »

Hi.

It is not at all unusual to hear VHF and UHF signals vary in strength even on short paths. There are just all kinds of variables that come into play
with shorter wave lengths. I worked in military Tropo Scatter and we had recorders on the signals; it was amazing how much a signal would change
with weather, humidity, fronts, temp inversions, etc. To some degree this is all possible on "local" signals too.... issues
like variations in foliage moisture, surface humidity, surface temp inversions, effects by bodies of water, etc, etc. reflections off vehicles, and who knows.

FYI: airplane flutter multipath is usually pretty obvious, often starts slowly, builds up then slows down all in the matter of seconds to a minute.
 
Seasonal and daily foliage moisture changes can be a fairly big deal on UHF on shorter paths. We see that in the commercial two-way radios here. A good, wet,
heavy snow or a wet morning dew can really elevate signal attenuation, especially for vehicles operating in low areas.

How about this!!...  How boring would this hobby be if signals NEVER varied !?

73,  K0ZN
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M5AEO
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« Reply #14 on: May 09, 2011, 10:01:37 AM »

Thank you everyone for your fascinating contributions.  I do find propagation very interesting and baffling!  I will continue to monitor the situation and see what pattern forms.  So far it seems releted to the City Airport flight path and the change in temperature between day and night time.  I am still pleased with the new antenna position as all the repeaters are now stronger (vertical polarization), but it is interesting to see what's happened to the beacon signal.  My next move is to put up a horizontal antenna and see what the difference is.

M5AEO


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