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[TowerTalk] Do Log-Periodics Stress the Selectivity oftheReceiver?

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Subject: [TowerTalk] Do Log-Periodics Stress the Selectivity oftheReceiver?
From: (
Date: Tue, 30 Nov 1999 01:18:10 EST
In a message dated 29.11.99 18:19:55 Pacific Standard Time, writes:

<< There are advanced AVC circuit requirements for DXers that allow working
 300/hr on CW in contests.  The 1000MP has a modification just for that and
 it works.
 I use a 1000MP, can you tell me where I can locate the info regarding the
 mod you mentioned in your post?
 Thanks, Paul
 VE3ZT >>
    I've had several requests for this information.  I got it from Dennis 
K7BV. He's the editor of NCJ and a contester on CW.  He mentioned that the 
1000MP was a contest CW receiver with an AVC mod.  I asked if Yeasu knew 
about it and he said they did.   It would seem logical that if you want to 
know if the factory knows and who should want to know--ask them.  I asked 
Dennis if the mod would help in SSB piles ups and he thought it would.  His E 
mail is
    There are many variations of AVC action other than fast, medium & slow.  
W7wjp was telling me that with on receive if you switched to an off band 
antenna the voice volume stayed the same.  With another receiver the voice 
volume changed.
    One DXer was monitoring another and noticed he was missing certain 
letters on SSB.  They switched receivers and problem went with the receiver.  
This would not have been known without a side by side check by top operators. 
    Years ago a W6 DXer pulled a prank on his friend by inserting a 10 dB 
attenuator in the antenna lead to his 75A4.  In LA during a contest there is 
enough RF floating around to burn out the antenna coils in the receiver.  
Even off band signals raised the AVC voltage so that weak ones can't be 
heard.  The 10 dB loss was just enough to keep the average AVC voltage down 
enough so that he heard more weak ones than his prankster friend.  He removed 
it from the receiver coax later and didn't tell him about it.  He would have 
never heard the end of it.
    Reducing the RF gain a bit can be helpful at times.
    RF Preamps that have gain controls could be very useful.  They some times 
have too much gain.  Using one properly can have the affect to further filter 
of off frequency signals.
     In the 40's Millen came out with a RF Pre-amp with plug in coils for 20, 
15, 10 & 6M.  It had input and output variable Xc's for tuning each tank 
circuit.  It had a screen voltage control to vary the gain and an in&out 
switch.  It worked but the plug in coils had tuning slugs in them which 
lowered the Q.  I intend to increase the size of the wire and see what the 
difference is.  I plan to make an adapter that allows me to plug in KW tank 
     In those AM transistor pocket radios I noticed one had an in phase 
feedback capacitor (regenerative style) of such value in a single IF stage 
that it gave more gain and selectivity thereby requiring only 1 IF stage.  I 
thought that would be a great idea for a RF pre-amp.  They all used 
individual tank circuits for each band.  I have a couple of these R9er's and 
other RF tube preamps to work over.  R9er's are available at flea markets for 
usually $15 with coils.
     Then much to my embarrassment a CBer told me of such a circuit in QST 
about 25 years ago that he built.  It works.  The circuit is so efficient 
that it used only 1 tank circuit for 20, 15 & 10.  This variable in phase 
feedback increases the stage gain and selectivity until it breaks into 
oscillation. I would use an attenuator between it and the receiver so that 
the reveiver works with a S9 or weaker signal no matter how strong the 
initial signal is.
     For some great reading, read the QST's and Radio Mags from the 20's & 
30's.  A 39 QST had an article on a 5M RF Pre-amp with Hi-Q tank circuits.  
It used a 1/4 WL length of 4" copper tubing and copper center rod as the tank 
circuit for the grid and plate circuit.  This should be very Hi-Q and 
selective.  I picked up on this idea real fast and immediately zipped to the 
conclusion page to see what they would claim for performance.  It claimed 7 
S-Units gain where 2 were noise and 5 were signal.  If true or close to it 
this needs to be made up which I will do when I get back to SD.  I will make 
these plug in copper tube 1/4 WL tank circuit so I can plug them into the 
Millen R9er.  If it works on 6M I'll make it for 10 and lower next.  This 
circuit should also reject off frequency signals for the over loadable so 
called modern receivers with untuned mixers.  This should help reduce 
overloading on any receiver and suck those weak ones out.  
    I have those Converters that RME made for 10-2M.  They have a tuneable 
tank circuit in the 1st RF stage and by the time it comes through these 
circuits and being beat down to say 10M it should be real clean.  I've heard 
that 6M transceivers about 8 years ago would overload in the F layer 
openings.  One could work 100 J's in an hour with 10W from the NW.  There is 
a RME tunable RF pre-amp for .5 to 40 MHZ that I have.  I'm just waiting for 
the piles ups on 6M this cycle.  There are now more on 6M with bigger 
antennas and Linears.  Hams will be creating their version of HARP
    I have a SX-88, 75A1 with 3 kc filter and a KP-81 I will be comparing 
front end selectivity's to the modern ones.  The passive preselector that was 
in CQ is the best circuit I've seen and will try and find it.

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