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On 4/15/2011 11:22 AM, Bob Bruninga wrote:
<blockquote cite="mid:03c601cbfb9a$0fa35990$2eea0cb0$@edu"
type="cite">
<pre wrap="">The key to this is simple SPECTRUM PLANNING!
DUH! It is not the receiver's fault. That is why we have SPECTRUM
PLANNING, so that one does not place high power transmitters in bands
ADJACENT to weak signal receivers.
Aren't there any RF engineers left at the FCC?
Bob, WB4APR
</pre>
</blockquote>
<br>
------- NO!!! ------------- THEY ARE TERMINALLY CLUELESS <br>
<br>
<br>
Two issues of this type nearly identical (in that they involve
disrupting existing networks with high-power intruders) I have dealt
with in the past two decades:<br>
<br>
1) "Coordinating" half-KW ERP or higher one-way paging
transmitters on frequencies directly adjacent to the 9 medical
telemetry channels in the UHF band. <br>
<br>
These "med" channels are used for EKG telemetry by paramedics at
accident scenes to send data directly to hospitals. The
in-the-field end of these systems were devices such as the Motorola
Apcor radio, which are 5 watt "porta-luggies" based on handhelds
built into a box with the EKG gear. This box with a so-so antenna
typically sits on the ground next to victims. These systems operate
duplex like a classic "car phone" (the box actually has a
traditional telephone handset on it), but no repeaters are used.
In other words, rather low TX ERP on a direct path back to the base
station at a hospital. <br>
<br>
The UHF channels for classic IMTS "car phones", located directly
above the 9 med channels, were repurposed for paging in the late
1980s, after cellular took off. The 50-100 watt ERP base stations
on distant mountain tops or buildings used for IMTS phones, were
replaced with dozens of 500-1KW ERP paging transmitters on every
tall building in the metro area, in order to reach cheap pagers with
no real antenna inside buildings -- in other words total RF
overload. When these paging systems started turning up, I
received numerous complaints from the county health system and
paramedics in Los Angeles about interference and communications
failures in the UHF "med band". The usable range had dropped from
5-10 miles from the base to as little as 1-2 miles. <br>
<br>
I ran RF coverage surveys and signal level measurements with a
spectrum analyzer in my car, and discovered these paging systems
were laying down minimum RF levels of -70 dBm and typicall far
higher virtually everywhere in the greater Los Angeles area. Some of
these signals were as close as 50 KHz to MED 9 where the typical RX
level at the hospital from a field unit was running at -100 to -110
dBm. The modest med radio front ends were being totally overloaded
by the massive RF levels of the paging networks. <br>
<br>
I improvised some front end selectivity at some installations with
strings of repeater-style cavities, but of course this limited the
base to a single channel, and of course did nothing for the so-so
handheld receivers in the field. <br>
<br>
The problem has now sort of "gone away" now as paging has declined
in the face of universal cellular usage, but for nearly two decades
this was a major problem in L.A.<br>
<br>
<br>
2) The similar fiasco in the 800 MHz spectrum where NexTel was
allowed to saturate urban areas with hundreds of essentially
cellular transmitters on channels interleaved with conventional and
trunked analog public safety systems. <br>
<br>
The problem was that public safety systems are relatively thin
networks with a handful of base stations at remote locations
communicating with hand-helds in town; i.e. a rather low RF level
delivered to users on the ground. The Nextel systems involved
hundreds of relatively high-power base stations "close-in" every
half mile or a mile or so, delivering several orders of magnitude
more RF on the ground "in-town". Public safety hand-helds were
desensed by the RF onslaught on adjacent channels. Since the Nextel
channels were mainly digital, users didn't perceive interference in
the sense of "strange voices" on their channels. The
white-noise-like unwanted signals just held the noise squelch of the
analog radios closed, causing large areas of their previous coverage
area to just disappear and "not work". <br>
<br>
In a frantic effort at damage control, the FCC essentially bribed
Nextel by offering them additional spectrum in the 900 MHz band, if
they would spend tons of money "rebanding" 800 MHz; i.e.
segregating existing Nextel systems and public safety systems into
separate parts of the 800 band. Tens of thousands of 800 MHz public
safety radios had to be replaced in order to make the move. This
mess has been going on now for over a decade, with hundreds of
millions of dollars spent on studies with no end in sight, as Nextel
and it's public safety victims haggle endlessly on how much Nextel
needs to pay for "equivalent systems". <br>
<br>
Somehow the FCC has managed to overlook the fact that Nextel had
massively overreached and violated the original charter of their
license in the first place. Nextel was supposed to be a wide-area
shared trunked community repeater system for business users, not a
wanna-be cellular network. Wireline interconnect was supposed to be
a minor part of a system primarily for land-mobile communications
for fleets of trucks. Such a network, targeted at mobiles rather
than handsets, would normally use a handful of relatively
remotely-located base stations, instead of the massive saturation
build-out of close-in cellular-style transmitters required to reach
wimpy handsets indoors in town. <br>
<br>
About the only justice to come out of this mess is that Nextel has
been nearly bankrupted by the rebanding effort, and is about to take
it's currrent owner, Sprint, down with it.......<br>
<br>
<br>
<hr size="2" width="100%"><br>
--<br>
<br>
Stephen H. Smith wa8lmf (at) aol.com <br>
EchoLink Node: WA8LMF or 14400 [Think bottom of the 2M
band]<br>
Skype: WA8LMF<br>
Home Page: <a class="moz-txt-link-freetext" href="http://wa8lmf.net">http://wa8lmf.net</a><br>
<br>
===== Vista & Win7 Install Issues for UI-View and Precision
Mapping =====<br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/aprs/UIview_Notes.htm#VistaWin7">http://wa8lmf.net/aprs/UIview_Notes.htm#VistaWin7</a><br>
<br>
*** HF APRS over PSK63 ***<br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/APRS_PSK63/index.htm">http://wa8lmf.net/APRS_PSK63/index.htm</a><br>
<br>
"APRS 101" Explanation of APRS Path Selection & Digipeating <br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/DigiPaths">http://wa8lmf.net/DigiPaths</a> <br>
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