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.
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
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
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.
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