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[TowerTalk] Passive Preselector Circuits for Receivers.

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Subject: [TowerTalk] Passive Preselector Circuits for Receivers.
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Date: Thu, 2 Dec 1999 10:27:36 EST
#20 In regard to the need for a tunable low loss narrow filter for untuned 
mixer receivers, I mentioned a passive preselector circuit that was in CQ.  
In QST's Compendium 6, there is an article about this very problem.  On p170 
lower right, there is a similar circuit.  It uses a capacitive divider for 
input and output.  It has the coupling capacitor between the tank circuits 
that allows one to over couple and under couple for max selectivity.  I 
believe the input and output variables would have to be differential Xc's and 
ganged together for speed tuning. The original circuit used link input and 
output instead.  That simplifies the ganged single variables across the tank. 
    The author used this tuner on transmit also.  I'd use this in the by-pass 
receiver circuit only that is in many receivers and the ability to switch 
around it.  When I get to SD I will build both of these. The article had 
other useful info pertaining to transmitting.
     In the old days with link coupling hams could match a fairly wide range 
of Z's up to about 1000 ohms with no tuner and using open wire feed by 
changing the number of turns in the link and using a variable Xc--in the 
link. Tuners matched the higher Z's but with another 1/4 WL of open wire 
feedline you could have a low easy to match Z again with great bandwidth 
thanks to the variable Xc in the link.  By using the right multiple of 1/4 WL 
open wire feedlines, I could control the Z at the end.  The link circuit also 
would handle unbalanced loads like coax and LW's just by grounding one side 
of the link through the variable Xc.  Faraday shields on the link would 
reduce harmonic transfer very effectively. 
    Then the pie network was used for coax and would match a fair range of 
SWR's but at less efficiency of the final and some SWR loss in the feedline 
starting to be a factor at say 1.5:1.  Tubes could handle some extra heat 
dissipation. It worked and with only 2 knobs fairly well.  Collins had the 
Pie-L for better harmonic reduction.  
    Then the "No-Knober No-Brainer CB Final" came out and worked well with 
low SWR's.  In 100W rigs, cut off circuits were added to protect the finals 
from excess dissipation and final damage caused by high SWR mis-matches.  So 
to get the power back out, tuners with "Brainy Knobs" came back. You no 
longer had a "No-Knober No Brainer" rig. The Brainy Z matching knobs are on 
the outside now--if you need them.  I Iike this arrangement as with low SWR, 
a true no-knober exists.  The no-knobers have built in automatic tuners now 
that work fairly well and some tune quickly--just by pushing a button.  Some 
external tuners seem to have larger wire in them as their insertion loss 
seems to be less.  Some of them could be mounted external to the transceiver 
a fair distance but controlled by it and I like that also.  There are ways to 
match many antennas just with a variable Xc (often less than 150 degrees of 
rotation) or inductor just as we did in the 30's and 20's before me.  To 
solve many of the modern problems plaguing the world today I will be showing 
how to do this with "old hi-tech advanced stuff" in some articles and help 
you all make the full swing back to the 20&30's and with bandwidth--like 200 
KHz on 160M--WOW!.  That is what I call real progress!  Fortunately the Flea 
Markets still have the parts. Yet there are those who have to dragged kicking 
and screaming into the next century.  
     I still have my old final with a Pr-813's (may add 2 more) using plug in 
coils and link coupling for balanced or unbalanced feedlines. You new 
transistor only hams don''t know the beauty of slightly red carbon plates, 
peaking the finals, open wire line and beating your buddies--with all the 
modern equipment.  Balanced pie networks exist and could match the L0&Hi-Z's 
of open wire line--directly.  I may build one of those finals.     
k7gco Ken Open Wire Glanzer

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