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G0FTD > TECH 02.03.08 14:30l 79 Lines 3180 Bytes #999 (0) @ WW
BID : 000278061PZT
Read: GUEST
Subj: Re: HF Rx Suckout Trap
Path: IZ3LSV<IW2OHX<I0TVL<ON0AR<GB7CIP<GB7YFS<GB7PZT
Sent: 080302/1156Z @:GB7PZT.#24.GBR.EU [Kidderminster] #:27800 XSERV410h
Date: Sun, 02 Mar 2008 11:56:37 +0000
From: g0ftd@gb7pzt.#24.gbr.eu (Andy)
To: tech@ww
Subject: Re: HF Rx Suckout Trap
I downloaded John's bulletin on this subject as I was completing the
same project. In my case I find that operating from the beach into an
efficient aerial, combined with the improvement in signal strength
accordingly means that those pesky broadcasters tend to give my front
end a bit of a headache.
Switching the preamp off or inserting an attenuator cures the intermod
but then degrades receiver sensitivity too much to work the weak signals!
So I decided that I would build a tuneable notch filter. The idea is that
if I can reduce one of the broadcast bands where the strong signal are
then they wont be mixed in the front end and thus no intermod.
In my tests I find that it is typical to acheive a 15-20db maximum of notch
without the bandwidth of the notch being too wide and reducing senstivity.
Deeper notches can be acheived down to 32db by using maximum capacitance
and only a small amount of inductance. However it's at the expense of
losing 6db or less on other bands. This *may* be OK or not depending upon
what the operator is happy with. It pays to have alternative inductor taps
so that you can play a little with the Q vs notch depth. I have not managed
to acheive notches greater than 32db (so far).
For example a 0.15uH with about 2500pF will form an effective 7Mhz
band notch typically about 1Mhz wide with 30db,however 6db is lost at
other higher bands. All measurments were taken using a mini vector network
analyser.
I used a 12 way rotary switch and an inductance of 17 turns wound on
a FT240-61 Amidon toroid (expensive, but it's what I had available at the
time,and tuned with a 750pF dilecon capacitor which has a very small
physical size. One of the positions on the switch is left open circuit,
thuis providing an "off" switch for free, whilst I had two positions
tunes for the 7Mhz and 17Mhz broadcast bands with 30db notches but
have almost 6db attenuation above their operating frequencies.
The 7Mhz/17Mhz bands I find are the most troublesome in my usual
operations. They are built with NO inductances, just using approx 8cm
of "freespace" wire (0.15uH) and a fixed capacitor inserted in it ;-)
The other switch positions are used for generically changing the resonant
freq of the tuned circuit.
Like John, I've made a simple relay board which is switched via the rig's
12v acc socket and PTT line.
+12V>ÄÄÄÄÄÄÄÄÄÄÄÄ¿
ÚÄÄÁÄÄ¿
³relay³
ÀÄÄÂÄÄÙ
PTT>Ä´<ÃÄÄ/\/\/ÄÄÙ
LED 33R
The LED gives me an indication of operation, and as a fuse, because
they burn out at 150mA or more.
WARNING:
NEVER short out the 12v acc socket output on the back of IC706's or
FT857's, because they pop the internal SMD fuses, which are a real
pain in the butt to get to and they also affect the operation of
other circuits in these rigs.
I suspect that other rigs would have the same problem, I'm only quoting
my experiences with rigs I've affected in the past - so be careful!
The relay circuit draws 22mA in my case.
- Andy -
G0FTD @ GB7PZT
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