Re: roofing filters - when to use them

[Bill W4ZV]

Hi Wayne,

N6KR:
 >There is a third, equally important issue, and this is often the
dominant one during contests or in pile-ups: Very strong interfering
signals within the passband of the roofing filter can de-sense the
reciever


         I respectfully disagree.  What you are describing
is measured by Blocking Dynamic Range (BDR).  BDR is
measured by determining the point at which a single
strong signal will cause RX desensing.  Today's best
rigs have BDR in the range of 130 dB at 2 kHz spacing.
That is a lot of range before you would have any desense
issues.  The only time you would likely hear this would be
when another station is operating very close such as in a
multi-transmitter environment (e.g Field Day or Multi-Multi
category in contests with 2 stations on the same band) or if
you had a very close neighbor running high power with a gain
antenna pointed at you.  For example if the sensitivity of
a receiver is -127 dBm, and you had 130 dB of BDR, it would
take a +3 dBm (-127 +130) signal before you would hear any
desensing.  A +3 dBm signal is equivalent to S9+76 dB using
the normal definition of S9 being -73 dBm (50 uV).

         By contrast, Intermodulation Dynamic Range (IMD)
happens when two strong signals cause unwanted mixing
products in the receiver.  This is the case in a contest or
pileup with many strong signals spaced close together.
Today's better rigs will typically have ~85 dB IMD at 2 kHz.
Using the above example of -127 dBm sensitivity in the RX,
it would take signals of -42 dBm (-127 + 85) to create IMD
products.  A -42 dBm signal is equivalent to S9+29 dB
using the same definition as above.  S9+29 dB is easily
within the range we typically see in contests or pileups.

         Thus we are much more likely to have IMD issues
well before we have BDR issues, simply because most rigs
have ~45 dB (130 vs 85) better BDR performance than IMD
performance.  An exception to this is SDR rigs.  Since
their BDR and IMD are both largely determined by the sound
card, there is not as much difference in BDR and IMD.  ARRL
measured the SDR-1000 with 98 dB IMD but only 110 dB BDR.
In that case we would expect to see BDR issues much sooner
than with today's best superhet rigs.  W8JI has recently
verified this in a series of posts on the Topband reflector.

         G3RZP wrote an article in May/June 2002 QEX titled
"HF Receiver Dynamic Range:  How Much Do We Really Need?".
In that article, based on actual measurements on 40 meters,
he concluded we need in the range of mid-90's IMD (he
calls it Intermodulation Limited Dynamic Range (ILDR).
While this was based on the effect of very strong BC
stations at much wider spacings than we have in contests
or pileups, it does give another idea of what is needed.
W8JI states on his website that 85 dB is sufficient for
most cases he sees, although there are exceptions that
may require 95 dB.  G3RZP also goes into the effects of
phase noise in his article but that would get us into a
whole 'nother discussion.  I am also very happy to hear
that K3's phase noise will be much better than the K2's.

                                         73,  Bill  W4ZV

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