[opendtv] Re: VHF vs UHF coverage
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 19 May 2009 16:24:04 -0400
Dale Kelly wrote:
> However, my concern in this post was an assigned UHF DTV
> channels ability to replicate the stations analog coverage.
> It is necessary, in this instance, to also compare UHF verses
> low band VHF propagation. Low band VHF's generally more robust
> signal and it's ability to provide some amount of over the
> horizon coverage, can't be replicated by UHF signals.
Right, you are coming at it from the opposite side as me. I'll give it a
shot, though, using the same techniques as before.
I'll use 67 MHz for low VHF, and 615 MHz for UHF. I'll use 100 Km range,
a 300 meter transmit antenna and a 10 meter receive antenna height.
RF path attenuation, again using the Hata model adjusted for long range,
is 140.4 dB for low VHF and 156.5 dB for UHF, or 16.1 dB advantage to
low VHF.
The low VHF analog power limit is (I think) 100 KW ERP. Using your 6 dB
difference for digital coverage parity, low VHF digital output power
would have to be just over 25 KW, to match the analog station's reach.
Increasing 25 KW by 16.1 dB results in 1.02 MW required for UHF. Not
half bad, so far. That's doable within the digital power limits.
But now try lowering the receive antenna to 2 meters and decreasing
range to 80 Km, to get something reasonable as limit of indoor
reception.
Path attenuation is now 145.6 dB for low VHF and 170.1 dB for UHF, so
that you would need 7.1 MW ERP for the UHF transmitter to equal the
coverage of the low VHF transmitter.
So if the FCC said 5 MW, sounds close enough to me.
A mitigating factor is, however, that analog low VHF was so hammered
with EM noise that long range reception was (in my experience) severely
limited anyway. In recent years.
Bert
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