[1000mp] On-Air test for Roofing Filter check

[Bill Tippett]

  	A simple check is to tune around on 40 meters
where strong BC stations are (7100-7150 is a good
area).  Using the Sub-RX and you may hear weak IMD
products every even 5.0 kHz (i.e. 7105.000, 7110.000,
etc.)  If you can find two strong carriers 5 kHz
apart (e.g. 7120 & 7125), listen at 7130 and 7115,
assuming they are not also occupied by carriers.
You should hear a weak tone which is the 3rd order
IMD product 2f1 - f2 or 2f2 - f1.  Now try switching
to the Main RX and the tones should disappear.
You have just checked IMD for 5 kHz spacings
which should be very good using the Inrad filter.

			73,  Bill  W4ZV

P.S.  You can verify that the weak tones are IMD
products by switching in attenuation.  It will likely
make IMD completely vanish but will simply reduce
"real" carriers by the selected level of attenuation.
Attenuation on the front end has a 3X reduction
effect on IMD products.