From: Brian Winter (brian@dbstart.co.uk)
Date: Fri Feb 28 2003 - 15:21:45 EST
From: "Al Simcoe"
Sent: Friday, February 28, 2003 5:04 PM
> I am not sure if where the placement of the rf amp in the antenna line
> is crucial or critical (near the antenna head or near the receiver) or
> not. RTFM I guess !! :)
>
> There is something else to keep in mind. All "rf amps" will not only
> amplify the intended signal but will also amplify the noise that comes
> along with that signal.
... but thats not an RFI dicussion, rather one on noise.
I know this is granny/eggs (no offence intended), but for others benefit:
As you have said, first get a good noise floor ie get rid of local RFI
caused by computers, flourescent lights etc often by good RF grounding,
shielding, ferrite supression, location as discussed already. All that
stuff tends to make a great deal of difference at low freqs - unsurprisingly
cos thats where all that man made cr*p and its strongest harmonics is/are -
at low freq.
Once this is done properly, the overall system sensitivity (the lowest
signal you can hear) should be dependant on the noise generated in the
"front end", normally the first stage of amplification in your receiver.
You can't hear any RF signal below the noise level at the first amplifier.
Thats why SHF receivers (eg satellite LNA) quote performance in terms of
"noise figure". Various techniques might be used to lower this (eg earth
stations have their LNAs cooled with liquid nitrogen (or CO2 ?) 'cos noise
figure is temp dependant) but essentially your stuck with it - your only as
good as your receiver.
I rather lost the thread (connection) somewhere but you're now talking about
"rf amp in the antenna line". You could put your amp near the rx but this
would defeat the purpose. the signal arriving at the antenna will have been
reduced by the feeder loss by the time it gets there and you will simply be
able to see that few dBs worse siganl before hitting your front end (in this
new amp) noise. If you put the amp at the antenna end (eg TV masthead amps,
satellite LNA on the dish) you will amplify everything you see at the
antenna down to the noise generated in the amplifier, thus gaining a few dBs
headroom. The only way to improve upon this is to increase the gain of your
antenna, or get the other end to turn the wick up. Don't be fooled into
believing that a 10dB amplifer will give you a further 10dB of signal to
listen to. Assuming the noise figure of the masthead amp is similar to the
noise figure of your receiver (and its often better/more expensive for this
very reason) then you will only improve things by the amount lost in the
feeder.
But even after all that (wot more !!!), I thought the originator described
co- or adjacent channel interference - not RFI or noise/sensitivity issues.
At 137.5, as stated in another response there are other out of control
satellites on the same freq which you can do very little about. Thats
interference of the worst kind. You can't easily reduce it without reducing
your "wanted" signal. You could gain something by tracking with a
directional antenna but thats silly for this application :) - common for
HRPT at 1.7Ghz from the same satellites though for gain rather than
interference reasons.
There are other unwanted signals that you can help remove though. By its
very nature the front end of the PCR 1000 is VERY wide (in freq) - hey its
designed to do almost DC to blue light :). Sure, you tune the thing to a
particular freq, but thats after every other signal in the area has swamped
the first stages. In the UK there are a number of paging freqs nearby and
folks that live near high power paging transmitters have a massive problem
with satellite APT decoding. And its a "now you see me, now you don't"
problem rather than a continous broadcast issue. In any event, whether your
rx is swamped by local high power transmissions or products of a number of
different RF transmit sources, the only thing you can do is put a narrow
filter before the first stage (that may be at the masthead preamp if one
exists). Hams talk of "pre-selection" at HF I believe. This needs to be
sharp enough so that when tuned to your wanted freq, the unwanted signals,
products etc are attenuated sufficiently so as not to make a mess of your
front end. I guess this is what you had in mind when you talk of "the noise
that comes along with that signal" - not really noise though.
In short you will get more mileage out of reducing unwanted RF signals than
trying to "amplify your way out of it"
Sorry if thats a bit long winded. Any errors are of course intentional
jests.
If you spot any real unintended howlers, then I don't want you worrying
about me - I'll be holding on to my day job for the time being :)
bw
_________________________________________________________________
PCR-1000 Mailing List
Post Address: PCR-1000@mailman.qth.net
Subscribe / Unsubscribe:
http://mailman.qth.net/mailman/listinfo/pcr-1000
Mailman FAQ and Users Guide
http://mailman.qth.net/faq.htm
This archive was generated by hypermail 2.1.7 : Sun Apr 13 2003 - 01:00:09 EDT