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VK2TV > TECH 29.06.07 06:53l 66 Lines 3189 Bytes #999 (0) @ WW
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# On : 6/29/2007 15:42:27
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Oops!
In my previous posting I managed to shift from apples to oranges by turning an 87dB
difference into -87dBm, which is something quite different.
I said this ....
The other factor is the receiver's sensitivity and the amount of sideband
noise a transmitter produces at the receivers frequency. A typical receiver
might have a sensitivity of 0.3uV or -117dBm. A typical transmitter might
have a sideband noise level at the receiver's frequency of say, -70dBc.
That is 70dB below its carrier power. For our ten watt transmitter with a
power level of 40dBm, sideband noise falling on our receiver frequency is
-30dBm or 1mW. That's 87dB above the sensitivity of our receiver and that
equates to a signal level of 10uV being needed to overcome transmitter
noise. It surprising how few received signals are of this magnitude, unless
they are quite close.
Replace the last two sentences with this .....
That's 87dB above the sensitivity of our receiver and that equates to a signal
level of 7.1mV or 7100uV. It would take a very local wanted signal to exceed that
level!
Just to add a little more. The quality of both the receiver and transmitter
play an important part in reducing offending levels. Whilst the vast
majority of modern solid-state transmitters could be regarded as "clean",
the same can't be said about some modern receivers. If your 2m receiver
front-end covers the entire band, or even much more in some cases, and it
does this without retuning, it will provide very little, if any,
attenuation at the transmitter frequency in question. On the other hand, an
old crystal locked commercial rig with a reasonably selective front-end
could provide significant protection against the transmitter.
I run two, 2m transmitters here, one on 144.900 and the other on 147.550
(our band is 144-148MHz). Both radios are 25W crystal controlled commercial
rigs - the Philips FM828, a 25 year old model that is still being installed
to this day, mainly for solar powered sites. The antennas are reasonably
close (50' horizontally), and without additional filtering I have about 1dB
of desense. I could eliminate this by adding a single 4" cavity to each
antenna feedline but the 1dB insertion loss of the cavity would make the rx
just as "deaf" as it is now so there's no advantage. I can live with 1dB of
desense.
Andy made comment about tv transmitters sharing a common antenna. This also
applies to multi-transmitter FM radio sites, and multicoupling or channel
combining is very widely used in the commercial two-way radio arena,
especially when antenna real estate is at a premium, if it can be obtained
at all. When I looked after radio base sites for an electricity supplier,
all of out UHF sites shared common antennas, one for the receivers, and one
for the transmitters. One site in Sydney had 9 transmitters feeding the
antenna. Each transmitter was routed through a dual circulator and a 10"
cavity. There are sites with dozens of transmitters to a single antenna but
it all comes at a cost.
Cheers ... Ray
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