TopBand: Preamplifiers
Frank Donovan
donovanf@sgate.com
Wed, 24 Sep 1997 20:33:12 -0400 (EDT)
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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