Hello gang,
after my extreme successful experience with the SIERRA, i tried the NorCal40A. I
was surprised about the fantastic signal to noise over the day, but had some
trouble at night with overloading the frontend. To explain, what type of
problems i am speaking of: My Antenna is a 73meter long off center feed dipole.
I use a high level symmetrical matchbox (no balun, real symmetric). At daylight
hours i measure abt 5mV at 50 Ohm BEHIND the matcher, after 800 pm utc the sum
voltage at 50 Ohm is > 150 mV (300mV without the matcher)
After some discussions with Wayne, I tried a Cohn filter frontend modification.
The result is great. I use the 40A without any attenuator now, and the signal to
noise is as good as in the daylight. To be honest, it is as good as with my
expensive YAESU FT1000. Thats unbelievable, in fact I have to use the 250 Hz
cascaded xtal filters to have a better s/n with the FT1000.
Pat Hawker, G3VA, wrote in his "Technical topics" in RadCom some stuff about
different minimum loss filters. In January RadCom there was a very good
description of a PA0SE designed Cohn type frontend. I tried it with some slight
modifications and it is working great. I loose only abt 4dB gain - in practice
on 40meter nearly impossible to recognize without test equipment- an it is
possible to work in the evening without any attenuator.
What I did:
removed C1, L1, T2, C2,C5
coupled input of new filter by 4.7 pF at point RX ANT (former C1 in)
coupled output of new filter by 4.7pF at R2 (former connection between L1 and
R2)
47nF between R2 and NE602 input
The Cohn type filter:
L1 1p5 L2 1p5 L3 1p5 L4
>>----||-----+-+-+--||--+-+-+--||--+-+-+--||--+-+-+------||---->>
4p7 / | | / | | / | | / | | 4p7
/ = = / = = / = | / = =
| | | | | | | | | | | |
ground-------+-+-+------+-+-+------+-+-+------+-+-+-------ground
5.7 C1 C2 C3 C4
L1 to L4 are all 5.72 uF. I used T50-6 by Amidon, 35 turns of 0.5mm coated wire
(the same type you send with the kit, but a little bit thicker, because I could
not found No26 in my junk box :-)
C1 / C4 87p5 + small trim cap
C2 / C3 86p + small trim cap
I used 80pF (33p+47p ceramic) with a little trimmer cap parallel. All ugly on a
piece of copper/epoxy board, the two 4p7 with legs as short as possible between
pcb and new board. I mounted the new board vertical at the left side wall near
the place where c1 has been before.
alignment is very easy if you try the minimum maximum method as described in Rad
Com too:
RF probe at hot end of L1
ground after 1st coupling c and trim C1 to maximum RF
ground after 2nd coupling C and trim C2 to minimum RF
ground after 3rd coupling C and trim C3 to maximum RF
trim C3 to minimum RF
If your probe has noticebel C then realign C1 with a very small signal.
72 de Peter, DLFI
!^NavFont02F08B70007IGHHOB9186C
E-mail von: Peter Zenker, 23-Feb-1997
[
QRP-L Archive |
]
[
1993 |
1994 |
1995 |
1996 |
1997 |
1998 |
1999 |
2000
]