[MLB-WIRELESS] Power output of 100mw card and helical

[Hamish Moffatt]

On Wed, Jan 16, 2002 at 10:08:28PM +1100, Matthew Boyd wrote:
> Short question for the techy guys.
> I've got a 100Mw card, planning on using an 18db (gain) antenna 
> (helical) probably 10-15m of cable to the antenna. I'm not very good 
> with the loss calculations but I'm a little concerned I'm going to be 
> over the output limit with that setup (based on a message along these 
> lines yesterday..ish)
> 
> Anyone out there got more of a clue than me on this?

OK, you have 100 mW of transmit power, which is
10 * log10(0.1) = -10 dB, or 30 + 10 * log10(0.1) = 20 dBm.
(I assume you don't really mean 100 megawatts!)

You have an 18 dBi antenna, giving an EIRP of 38 dBm,
ignoring the feedline loss. You are allowed 4W EIRP, 
which is 10 * log10(4) + 30 = 36 dBm.

You would easily lose 2 dB in a connector on each end plus
10-15m of cable. In LMR400 cable, according to the data sheet
at www.timesmicrowave.com, the attenuation at 2500 MHz is
22.2 dB per 100m, so you would expect to lose 2.2 - 3.3 dB
in your feed line. Expect some insertion loss from the
connectors too, and the pigtail from your LAN card.

The insertion loss in LMR1200 cable is only about 7 dB
per 100m at 2500 MHz - impressive! It's 3cm thick and
weights .67 kg per metre though.

I recently measured the attenuation of RG213 at 1.3 GHz
on an HP/Agilent network analyser. I can't remember the
numbers but they were pretty high - you wouldn't want
to use this stuff on 2.4 GHz!


regards
Hamish
-- 
Hamish Moffatt VK3SB <hamish at debian.org> <hamish at cloud.net.au>

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