<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:times new roman,new york,times,serif;font-size:12pt;color:#000000;"><div>Well, I got a bit goofed up on what I had sent and what had bounced so I am repeating my last 2 emails. If redundant, please accept my apologies.<br><br>73<br>Bob Groh, WA2CKY<br>------------------<br><div>Guess what? On my previous email, I'm wrong. Or at a minimum, not
entirely correct. We do NOT have a capacitive divider. My bad. <br><br>I
dug up a copy of the SB-610 manual and threw down some sketches on a
piece of paper. The SB-610 uses a tuned circuit on the vertical input
(for the 3 to 6 MHz range anyways). That inductor (L1 on the schematic)
is resonated against the tuning capacitor (330 pF according to the
manual), all the stray capacity AND the shunt capacity from the coax
cable. In other words, inductor L1 is tuned to absorb ALL the shunt
capacity from the coax cable (as well as stray capacity due to
connectors,etc).<br><br>With all the capacity 'zero'ed out' by L1, you
only have your series capacitor from the IF amp plate connection (e.g. 5
pF which is -9,500
ohms) working as a <span style="border-bottom: 2px dotted rgb(54, 99, 136); cursor: pointer; background: none repeat scroll 0% 0% transparent;" class="yshortcuts" id="lw_1331231044_0">voltage divider</span> against the Vertical Gain adjustment pot in the SB-610 (100K ohms). <br><br>Basically
the only limit on the choice of coax and the length of the coax is the
ability of L1 to tune out whatever shunt capacity the coax presents.
Loss, inductive effects, etc of the coax should be pretty much zip at
3395 kHz (e.g. a wavelength is 80 meters or about 240 ft!).<br><br>One
thing that could be happening is that L1 is running out of range (i.e.
max'ing out). This is fairly easy to see - you should get a 'double
peak' as you tune the inductor. If you don't, then you are at maximum
and your should reduce the shunt capacitor (e.g. from 330 pF to 270 pF).
<br><br>Now past this point, my previous post is correct. To get more
signal, you can increase the value of the series cap (e.g. from 5 pF to
10 pF). The limit is how much additional load you can tolerate on the
IF Amp.<br><br>Whew! I hope that is
correct and that it helps.<br><br>73<br>Bob, WA2CKY<br></div><br><font size="2" face="Tahoma"><hr size="1"><b><span style="font-weight:bold;">From:</span></b> Robert Groh <rgroh@swbell.net><br><b><span style="font-weight:bold;">To:</span></b> mdilli@nnwifi.com; Dick KF4NS <kf4nsradio@verizon.net>; ma.locksmith@juno.com<br><b><span style="font-weight:bold;">Cc:</span></b> Heath puck list <heath@puck.nether.net>; heathkit@mailman.qth.net<br><b><span style="font-weight:bold;">Sent:</span></b> Thu, March 8, 2012 8:36:54 AM<br><b><span style="font-weight:bold;">Subject:</span></b> Re: [Heath] Fw: Heathkit SB-610<br></font><br>
Morris has it dead on. If you think about it this way:<br><br>You
have a cap (5 pf in your example) connected from the source (the IF Amp
plate) to a load (the coax which drives the SB-610 and then the input
circuit of the 610 which has some (assumed) input impedance - lets say
the 610 input looks like 20K ohms in parallel with 10 pf. <br><br>For
all practical purposes, we can say the coax just looks like a shunt
capacitor (x ft at y pf/ft). This coax capacity is in parallel with the
610's input.<br><br>So we wind up with our source driving through a 5
pf cap to a parallel RC network - and the easiest way to think of this
is that the combination is a voltage divider (yeah, I am simplifying
it!). As an example, assume we have 5 pf in the series capacitor, 15
pf of
coax capacity, 10 pf of input C at the 610 and we ignore the R
component. We would have a voltage divider of 5 pf working against 25 pf
or a 5/30 voltage divider. <br><br>You can readily see that the longer
the cable or the higher the coax capacity per ft, the higher the
voltage division ratio. You can of course increase the series cap to
offset this (e.g. 10 pf --> 10/35 ratio) but the trade off is that
you are now affecting the tuning of the IF stage more. <br><br>A picture is worth a thousand words but hopefully this will help.<br><br>73<br>Bob Groh, WA2CKY<br></div>
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