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On Friday 23rd, I travelled up to Mt.Wellington to do a power upgrade in the form of a battery install. A switch mode 40A power supply had been used since day one of the repeater being installed, and on occasion a small power outage occurs when the generator starts up if they power supply to the building is interrupted, causing a repeater controller reset. This can be annoying if it is in the middle of a broadcast, conversation, or if some other setting is reset on initial startup by the controller. Thus a battery was installed so the power never goes off to the repeater.
I had intended to neaten everything up with fancy Anderson power pole connectors, but it was cold and I had other things to do in town, so it is a little messy but works.
The rack now presently consists of from top to bottom:
- KL450 70cm UHF
- Arcom RC-210 three port controller
- PRM80 Link to VK7RAA
- PRM80 APRS Digi with TNC-X
Top dipole is for the UHF repeater. Next dipole down is link to VK7RAA.
As you can see, the antenna setup is rather modest. A single UHF SMD exhibits zero gain over a dipole (obviously), or about 2.2dBi for those who like that measurement.
Working out 7RTC's system losses:
Total Loss: 3dB not including jumper cables etc which is probably negligible.
The power output is about 45 Watts out of the transmitter. What's the EIRP without cable loss? I calculate 74.7 Watts? Is that correct? If it is we have negative gain in the antenna system as there is more loss than antenna gain.
So take 3dB or half from that. 37.35 Watts?
So the Equivalent Isotropically Radiated Power (EIRP) is 37.35 Watts, in an apparently omni directional fashion. The UHF SMD as pictured above is actually probably spaced 1/4 from the mast, so there is a little bit more gain in the direction it is facing, but mostly irrelevant.
So... a well known "blackspot" is the Southern Outlet. Here is a Splat! plot of the path loss:
Yellow is a lower path loss than purple which is quite high. Radio Mobile seems to agree with Splat!
Splat also puts out a HAAT calculation:
Average terrain at 0 degrees azimuth: 180.09 meters AMSL
Average terrain at 45 degrees azimuth: 105.52 meters AMSL
Average terrain at 90 degrees azimuth: 40.71 meters AMSL - Out to sea... not much out there
Average terrain at 135 degrees azimuth: 229.09 meters AMSL
Average terrain at 180 degrees azimuth: 280.54 meters AMSL
Average terrain at 225 degrees azimuth: 473.28 meters AMSL
Average terrain at 270 degrees azimuth: 840.10 meters AMSL
Average terrain at 315 degrees azimuth: 399.66 meters AMSL
So terrain is pretty good to the North/North East overlooking Hobart. But other areas it is pretty poor.
Next trip, I will try a Polar vertical with an apparent 6dBd gain and see if we get an improvement over the SMD until I build a SMD array. Problem we may encounter is most of the gain of the antenna overshooting users in Hobart and down in the valleys as most likely the collinear does not have any vertical downtilt.