[CQ-Contest] Re: TopBand: Preamplifiers

[Frank Donovan]

Hi Gary!
Here's the info you requested

On Wed, 24 Sep 1997, Gary D Elliott wrote:
> Can you share the info re your LPL bandpass filter ahead of the preamp ?
> 73 Gary K7OX

                        W3LPL RECEIVING FILTERS

Receive-only filters optimized for minimal in-band loss and very high
rejection of frequencies below 75% of the filter center frequency.  

'LPL filters are also very effective as components of receive-only
diplexers, triplexers or quadraplexers.  Just connect one antenna to up to
four filters for adjacent ham bands, and run the filter outputs to
separate receivers.  This is especially useful for sharing Beverages in a
multi-operator station.  

'LPL filters use very high "Q" high impedance resonators consisting
of powdered iron core inductors and silver mica capacitors.  The resonators
are lightly top coupled with minimum value silver mica capacitors.  

C1/C2 and C6/C7 form capacitive voltage dividers to match the high
impedance input and output resonators to 50 ohms.

Be sure to use good quality RF connectors (UHF, BNC or N; not phono!).
Internal shielding in not necessary.

               
50 ohms --- C1 ------------ C3 ------------ C5 ------------- C7 --- 50 ohms
                  |    |           |    |          |    |
                  |    |           |    |          |    |
                  C2   L           C4   L          C6   L
                  |    |           |    |          |    |
                  |____|           |____|          |____|
                    |                |               |
                    |                |               |
                  Ground           Ground          Ground
                  
                  
                             TABLE OF VALUES 
                                         INDUCTOR  (all three identical)
Band   C1   C2   C3   C4   C5   C6   C7  turns  wire size  powdered iron core
 10    36   47    7   75    6   56   27    8       16      T50-10  BLACK
 15    51   62   10  110    8   75   36   10       18      T50-10  BLACK
 20    75   91   15  150   12  120   51   13       20      T50-10  BLACK
 40   150  180   27  300   24  240  100   18       20      T50-6   YELLOW
 80   300  360   51  620   47  470  200   23       22      T50-2   RED
160   600  720  100 1240  100 1000  400   28       24      T50-2   RED

Tune filters by squeezing/spreading turns and/or adding/subtracting turns
It's wise to start with a few extra turns on the inductors -- it's hard to
add turns!  Filters can be aligned with a signal generator and RF detector
(a voltmeter, well shielded receiver or oscilloscope) or ideally with a
spectrum analyzer and tracking generator.

Nominal 3 dB bandwidth: 10% of center freq

Nominal midband loss:  1 dB

Attenuation:  greater than 40 dB above 125% of center freq

Attenuation:  greater than 60 dB below 75% of center freq



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