#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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