Sit. Rep. #80: Phasing and Propagation

1- I am going to break my usual system of touching on several topics in one Sit. Rep.  This Sit. Rep. is devoted to radio propagation.  I think that having everything related to this topic of Ham Radio in one place is easier than trying to digest it a little at a time.  (Thanks to KB6NU Ham Radio Study Guides)

1A- The word “Phasing” means time or timing.  Propagation is how the radio signal travels between the transmitter’s antenna and the receiver’s antenna.

2-  Properties of radio waves and propagation modes:  To do this, we need to know how radio signals travel from one point to another and what effect frequency, our antennas and even our location have on signal propagation.

     Communications at VHF and UHF frequencies are generally “line-of-sight” communications.  This means they normally travel in a straight line from the transmitter to the receiver. For this reason, they are normally used for local communications.

Why are direct UHF signals rarely heard from stations outside your local coverage area?
      UHF signals are usually not reflected by the ionosphere

   Because VHF and UHF signals are line-of-sight, at some distance, the signals will be blocked by the curvature of the earth. The maximum distance for line-of-sight communications is called the radio horizon. The radio horizon extends somewhat farther than the visual horizon.

Why do VHF and UHF radio signals usually travel somewhat farther than the visual line of sight distance between two stations? 
     The Earth seems less curved to radio waves than to light

3-  One problem often encountered when using VHF and UHF frequencies is multi-path distortion.  Multi-path distortion occurs when your signals arrive at a receiving station via two or more paths.  Since the signal paths may be different lengths, the signals may arrive out of phase and cancel one another.

What should you do if another operator reports that your station’s 2 meter signals were strong just a moment ago, but now they are weak or distorted?
     Try moving a few feet or changing the direction of your antenna, if possible, as reflections may be causing      multi-path distortion

 3B- Multi-path distortion affects both voice and digital transmissions.

What may occur if data signals arrive via multiple paths?
     Error rates are likely to increase

4-   Knowing how VHF and UHF signals propagate can help you communicate even in adverse conditions.  When trying to use a repeater, for example, you may find yourself in a place where a direct path to the repeater is not possible. If you find yourself in this situation, you could try using a directional antenna and bounce your signal of buildings or other obstructions.

When using a directional antenna, how might your station be able to access a distant repeater if buildings or obstructions are blocking the direct line-of-sight path?
     Try to find a path that reflects signals to the repeater

5-  Another phenomenon you might use when a direct path to a repeater is not possible is “knife-edge” diffraction. You might be able to use this phenomenon to get your signal around a building in an urban setting.

Which of the following effects might cause radio signals to be heard despite obstructions between the transmitting and receiving stations?
     Knife-edge diffraction

6-  Mobile operation has its own unique challenges as your transmitter location is constantly changing.  This means that the signal at the receiving station constantly changes as well.

What term is commonly used to describe the rapid fluttering sound sometimes heard from mobile stations that are moving while transmitting? 
     Picket fencing

7-  Another condition that could impede the transmission of VHF and UHF signals is vegetation. So, keep your antennas out of trees or above trees.

Why might the range of VHF and UHF signals be greater in the winter?
      Less absorption by vegetation

8-  Antenna polarization is important at VHF and UHF frequencies. How you mount an antenna directly affects its polarization.

What property of a radio wave is used to describe its polarization.
     The orientation of the electric field

   What can happen if the antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization? 
     Signals could be significantly weaker

8B-  When using a repeater, vertical polarization is most often used. So, when using a handheld transceiver, make sure to hold it so that your antenna is vertically oriented.  Different activities use different antenna polarizations.

What antenna polarization is normally used for long-distance weak-signal CW and SSB contacts using the VHF and UHF bands?  Horizontal

9-  Operators are often using beam antennas, and it’s much easier to mount and operate beam antennas horizontally than it is to mount them vertically.  Even though VHF communications are most often line-of-sight, there are times when it’s possible to
communicate over long distances. Sometimes, VHF signals will bounce off the x layer of the ionosphere. This phenomenon is called “sporadic E” because it happens only sporadically.

Which of the following propagation types is most commonly associated with occasional strong over-the-horizon signals on the 10, 6, and 2 meter bands?
     Sporadic E

   Other interesting propagation phenomena at VHF frequencies include aurora reflection, meteor scatter, troposphere scatter, and tropospheric ducting.  Bouncing signals off the earth’s aurora is very interesting.  Some hams also bounce signals of meteor showers.

What is a characteristic of VHF signals received via aurora refection?
    The signals exhibit rapid fluctuations of strength and often sound distorted

What band is best suited to communicating via meteor scatter?
    6-meter band

10-   Does the weather affects radio wave propagation. The short answer is no, but the exception to the rule is tropospheric ducting or tropospheric scatter. The troposphere is the lowest region of the atmosphere, extending from the earth’s surface to a height of about 6–10 km.

What mode is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis?     Tropospheric scatter

What causes tropospheric ducting?   Temperature inversions in the atmosphere

10B-  Tropospheric ducting can also propagate VHF signals for many hundreds of miles.  Another exception to the rule occurs at microwave frequencies. Precipitation, including rain, snow, or ice can absorb microwave signals, especially at frequencies above 11 MHz. This phenomenon is called rain fade.  At lower frequencies, precipitation has little or no effect.

What weather conditions would decrease range at microwave frequencies?   Precipitation

How might fog and light rain affect radio range on the 10 meters and 6-meter bands?
Fog and light rain will have little effect on these bands


 R- Two old Scout Masters were having a cup of coffee together: The first one says ” I am sorry I just can not remember your name.  Please share it with me.”
S- The second old Scout Master took a few moments to give his reply:  “How soon do you need to know?”

Keep smiling,

T- Two antennas met on the roof, fell in love, and got married.
U- The wedding wasn’t much but the reception was wonderful.

Run-Ons and Even More Scout Skits by Thomas Mercaldo

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