TopBand: LPL Filters for AM BC QRM & Beverage Multiplexing

[Frank Donovan]

Mike,

Have u tried W3LPL receiving bandpass filters?  The 160M filter should
do the trick if KOA is below 1400 kHz.  The 80M filter will suppress any
AM broadcast station by over 80 dB.

W3LPL bandpass filters also make great duplexers or triplexers, for
receiving only obviously!  Just connect the inputs of any two or three
filters together (for adjacent bands: 160-80-40  80-40-20  40-20-15 or
20-15-10) and you can share a Beverage or other antenna between two or
three radios with about one dB of loss.  Use short cables to join the
inputs of the two or three filters to avoid capacitive loading which would
deteriorate the performance of the filters.  Details of each filter follow
the text of this e-mail.

73
Frank
W3LPL
donovanf@sgate.com

           W3LPL RECEIVING FILTERS and BEVERAGE MULTIPLEXERS

Receive only filters optimized for low cost, ease of construction, minimal
loss and excellent rejection of frequencies below 75% of the filter
center frequency.  W3LPL receiving filters can also be used to build low
loss receiving duplexers or triplexers.

W3LPL receiving 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 input and
output resonators to 50 ohms.

Be sure to use good quality RF connectors (UHF, BNC or N; not phono!).
Internal shielding between the resonator sections is not required.  All
ground connections must be interconnected via low loss construction
(use good solder joints and excellent bonding to the shielded enclosure).
Poor grounding practices will definitely degrade the performance of the
filter!

               
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

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

> On Sun, 3 Aug 1997, Mike Fatchett wrote:

> Looking for decent bandpass filters or other solutions to remove broadcast 
> interference on 160/80.
> 
> I don't recall having this problem a few years ago.  It appears on 2 
> different radios on different antennas.
> 
> Seems to be sneaking in on the beverage as well.  I have built a 9:1 
> matchbox for the beverage that I thought was to eliminate this.  I live 
> 5.5 miles from KOA and her 50KW!
> 
> Any thoughts or suggestions would be appreciated.
> 
> Mike
> W0MU


--
FAQ on WWW:               http://www.contesting.com/topband.html
Submissions:              topband@contesting.com
Administrative requests:  topband-REQUEST@contesting.com
Problems:                 owner-topband@contesting.com