[Premium-Rx] RF-590 VLF sensitivity improvement

[ToddRoberts2001 at aol.com]

In a message dated 2/24/2006 10:44:15 AM Eastern Standard Time, 
ka5qep at sbcglobal.net writes:
I think all the 590s use a MiniCircuits SAY-1 mixer.  It's rated at +20 dBm 
RF level and coverage from .1-500 MHz.  That may have something to do with 
the poor performance below 30 KHz.

John Reed

The RF-590 also seems to have a very wide roofing filter arrangement. In the 
16KHz selectivivty position on AM the receiver is actually using the roofing 
filter as the determining bandwidth. Mine reads more like 22-25KHz wide and 
this will limit the lower frequency limit the receiver will tune to. I notice LO 
bleedthru in mine as high as 12KHz  and depending on how narrow an I.F.  
filter you choose it can degrade reception up to 15KHz or higher. The wide roofing 
filter will degrade VLF reception using an upconverter also. A good solution 
would be to install something like a 10KHz roofing filter from the Cubic 
R3030/R2411 or similar receiver.  Sometimes the narrower 8KHz roofing filters  that 
were used in WJ radios can be obtained. Perhaps an upgrade to a newer 
Mini-circuits mixer would help also.  Also tuning the receive antenna can help 
dramatically to increase sensitivity at VLF. I use a 160 meter dipole for receiving 
LF/VLF. I ground the outer coax to use as a shielded downlead and I tune 1/2 
the dipole using a series adjustable inductor in series with the center lead. 
For VLF I use an old J.W.Miller slug-tuned inductor with a range of about 8-52 
mH (milliHenries). This will resonate 1/2 the dipole and downlead from about 
25-10KHz. An 18.6KHz MSK RTTY signal that was barely detectable with straight 
input from the dipole increased to about 70-80 uV on the S-meter and was plainly 
audible after resonating the antenna.  A typical wire antenna 50-150 feet 
long has a tremendous mismatch to a 50 ohm receiver input at VLF frequencies. 73 
Todd WD4NGG