Message-ID: <3767551C.262@xtra.co.nz>
Date: Wed, 16 Jun 1999 19:41:16 +1200
From: "vernall" <vernall@xtra.co.nz>
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Antennas, bandwidth, etc
References: <199906160131_MC2-7996-56AF@compuserve.com>
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Comments on bandwidth and noise blankers:
 
> Mike is probably right.  Unfortunately most of us see the transmit
> bandwidth of the antenna as being that for which the SWR is acceptable.
> With my present loop antenna the usable bandwidth, ie that within which the
> SWR is less than 2:1, is about 200Hz.  Having not yet built my remote loop
> tuner this is embarrasingly narrow! However the bandwidth the receiver sees
> is probably rather a lot more than this.  In any case I have a couple of
> fixed tuned circuits in the receive path which limits the bandwidth to a
> few kHz.
> 
> It is true to say that I can detect no slurring of the edges of dots on any
> signals I copy. 

It has been noted in ZL that noise blankers do not work so well on
narrowband loop receiving antennas.  In the usual type of receiver, the
noise blanker runs off a separate IF, and has wider bandwidth than the
"tail end" IF.  The wider bandwidth is to allow for good rise/fall times
and for noise gating to occur before much of the noise spike passes the
gating circuitry.  If the antenna itself is very narrowband, then the
"noise spikes" of field strength reach the receiver input as "noise
blobs" and the noise blanker IF can do nothing to sort out any high rise
time pulses to blank out, as there are none appearing!  So if noise
spikes are an issue, and noise blankers generally do quite a good job on
minimising the impact, then there is a case to receive via an antenna
system with a response bandwidth of not less than the noise blanker IF
(typically the 1st IF roofing filter bandwidth of about 15 kHz).

Also note that it is ineffective to use a separate noise receiver to
derive noise blanking drive, as it does not "reverse" the situation that
once a noise spike has become a noise blob (due to band limiting) then
the process can not be reversed, and the noise blob passes through the
receiver to the demodulator.

Transmitting antennas are another matter, where efficiency is perhaps
the main factor, but I will force myself to not divert from the point
made about receiving :)

Regards,

Bob ZL2CA