Tri-County Repeater Coverage Maps available on OCRA-DFMA groups.io

Link

The repeater reach-ability testing during several recent OCRA Auxcomm nets on the 442.150 repeater and the demo presentation at the April 10 OCRA meeting (see Youtube video) illustrates many examples of how the topography from your location to a repeater site greatly affects the ability to communicate. 

Without quantitative tools to analyze the issue, it can be a difficult decision on how to fix the problem. What is the problem? What will be the most effective fix?

Repeater topographic coverage maps can help answer questions specific to your location.

To take advantage of these maps you will need to do 2 things:

  1. Download and install the Google Maps application from this URL: https://www.google.com/earth/versions/#earth-pro  
  2. Download the Tri-County Repeater Coverage maps  located on the OCRA -DFMA groups.io file storage area.

The URL link below contains repeater coverage topographic mapping for the 442.150, 145.450, 147.225 and 145.230 repeaters. More repeaters will be added in the future.

Three maps are available for each repeater to show coverage that can be expected with the typical HT, Mobile or Fixed base (home) installations.

Downloading the Repeater Coverage Maps:

You need to use your OCRA-DFMA groups.io login ID and password to access the files area shown below.

Click on this URL https://groups.io/g/OCRA-DFMA/files/Repeater%20Coverage  to go to the file storage area. 

When you click on the “Tri-County Repeater Coverage_V3C.kmz” file link as shown below, the file will download onto your computer, per your browser’s download settings.

Using the Google Earth Pro Application:

Click on the Tri-County Repeater Coverage_V3C.kmz file to open it with the Google Earth Pro application.

Several Google Earth features need to be active so you can select the coverage maps available for the repeater of interest.

The sidebar on the left will be needed to navigate the maps. If not visible, click on the command line “View, Toolbar” to turn it on.

Click the caret symbol to expand the settings and the sub-directories within the .kmz file.

You will want to turn on certain overlays by placing check marks for the applicable blocks as indicated below.

for the “Color Legend” (indicating signal strength into the repeater; more on this later.

“Key Locations” and “Other Locations” will show the repeaters and QTH locations for over 100 hams in the area.

Place a check mark next to the repeater map and station type of interest. In this example, the check mark is for the  442.150 repeater being used by a portable/home station.

Only 1 repeater map/station type should be turned on at a time.

Note:

  • A portable station is one that could be set up for emergency communications and is equivalent to a home station using a 30-50 watt mobile radio,  and a small tower or building mounted fixed base antenna up 20 feet off the ground.
  • The 147.250 file name below is a typo. It is the 147.225 DFMA repeater.

The maps for our region, created with Radio Mobile – RF propagation simulation software , shows the significant role terrain blocking plays in repeater coverage in our in our area. The color indicated at the selected station’s location indicates the expected signal strength back into the repeater. 

  • Red (-77dBm strength back to the repeater)  is solid copy, no noise or fading communications)
  • Yellow (-87dBm), Green (-97dBm) Good communications, may have some occasional noise down in the green areas
  • Turquiose (-107dBm), Blue (-117dBm) Will be unpredictable. May work at times reasonably OK with noise & dropouts or not at all.
  • White (on the color legend) (-117dBm)  No communications. These areas will simply the Google Earth imaging without any coloring.

Common radio setups used for repeater communications are the basis for the 3 topographic map runs made for each repeater.

  • 5 watt HT with the standard rubber duck antenna
  • Vehicle with a mobile radio with 30 UHF, 50 watts UHF output and vehicle antenna
  • Fixed based installation using a mobile radio and a base station antenna higher off the ground.

Additional Information:

Knowing the Effective Radiated Power (ERP) that your radio setup is providing back into the repeater can provide a more tangible perspective. 

  • The ERP Calculator  results below reveals a little better the actual radiated power levels than just considering the antenna gain expressed in decibels. 
  • HT 5 Watts with standard rubber duck antenna loss (VHF -6dBi, UHF -3dBi)

    •  VHF 5W ERP 0.766 watts
    •  UHF 5W ERP 1.53 watts 
  • Vehicle radio and external mounted antenna (e.g. Comet SBB14 VHF 3.5dBi UHF 6.0dBi gain)

    • VHF 5W   ERP 6.83 watts
    • UHF 5W   ERP 12.14 watts
    • VHF 50W ERP 68.25 watts
    • UHF 30W ERP 72.82 watts
  • Fixed based installation again using a mobile radio and fixed base antenna (e.g. Comet GP-9N VHF 8.5dbi UHF 11.9dBi gain)

    • VHF 5W   ERP 21.58 watts
    • UHF 5W   ERP 47.22 watts
    • VHF 50W ERP 215.84watts,
    • UHF 30W ERP 283.32 watts 

Coax loss with also have to be considered for the vehicle and fixed based antennas.

I wish to express my thanks and gratitude to Steve, W3AHL for sharing his valuable knowledge and experience with the Radio Mobile RF propagation application.

Dan, KR4UB

Plan to use an Inverter Generator or run your Field Day station from a generator?

Posted on by 
Original post date April 2021 by KR4UB

Variable speed inverter based generators offer significant reduction in audible noise and fuel consumption. Most consumer quality, constant speed generators that typically run at 3600 rpm are noisy and consume much more fuel.

  • The small inverter generators are well suited for Field Day due to their lower acoustic noise profile, provided their AC output is filtered to eliminate RF noise typically occuring in the 80m band.
  • Using short AC cords with no filter on an inverter based generator still presents unacceptable noise on 80M.
  • With no filter, detectable noise on the 40m through 10m bands was not found with the Honda generator shown in the next article below.
  • The RF noise filter in the next article using 3 large 4″ OD Fair-rite mix 31 toroid cores has been proven in a series of EMI test configurations to be effective in eliminating inverter generator noise. The complete, enclosed filter including a GFCI outlet with in-use rated cover for outdoor use, costs around $150 to build.

The degree of interferenence to an 80m station from an inverter based generator & attached cord radiated RF noise coupling into the Field Day antenna will vary somewhat based on the station physical arrangement. The graphs below are the received signal into an 80M station antenna located up 55 feet in the air, with a 50 foot drop cord lying on the ground underneath a portion of the antenna that connects the generator to the load.

noise profile

photo by KR4UB, © OCRA Inc

Thinking about running your Field Day station directly off a generator and not using batteries?

There has been discussion of eliminating batteries all together (as the Field Day rules permit) and run the station on a small generator. Tests should be done before Field Day with the specific AC to DC power supply and generator to be used to determine if the fluctuating loads of a CW/SSB transmitter is going to cause deep fluctuations in generator engine speed.

Why would one care?  Most hams know that with synchronous (aka constant speed) generators the frequency output is determined by the speed the armature is rotating, and the number of poles in the armature.  What may not be known is that the raw voltage output of the generator winding varies proportionally with speed at which it passes through the magnetic field (aka armature rpm). That’s why all synchronous generators have a voltage regulator circuit that adjusts the other variable affecting voltage output, the strength of the magnetic field.  The question is how well in a consumer quality generator does the voltage regulator and, the mechanical engine speed governor handle wild fluctuations in electrical load.

Rather than using mechnical speed governors, commercial large scale generators use electronic throttle control systems for more precise control of engine speed by comparing generator output frequency to the throttle controller’s internal 60Hz frequency standard.

Rather than expose expensive radios and DC power supplies to such effects,  one fix could be to still use a battery or two in the 12VDC circuit to buffer the generator from the wildly fluctuating loads such as a CW transmitter.