[amsat-bb] Re: searching advice on low hpass filter for 146 MHz

i8cvs domenico.i8cvs at tin.it
Wed Apr 27 15:07:37 PDT 2011

----- Original Message ----- 
From: "Werner Kullmann, HB9BNK" <hb9bnk at uska.ch>
To: <amsat-bb at amsat.org>
Sent: Wednesday, April 27, 2011 3:41 PM
Subject: [amsat-bb] searching advice on low hpass filter for 146 MHz

> To improve the reception of  the sats on 145.9xx MHz with my 
> little yagi, I recently acquired a SSB-preamplifier SP-2000. To my 
> dismay, the reception on my IC910H was intermittend heavily 
> distorted.
> After a while I found, that a nearby located commercial 
> transmitter (POCSAG) emitting bursts on 147.300 is so strong, that 
> the frontend of the IC910H (or its AGC) practically quites the 
> receiver on 145.900.  When the bursts stop, reception resumes. 
> Difficult to qso.
> I have experimented with a large cavity filter and found, that the 
> impact of the bursts can be reduced, so that normal satellite work 
> is possible again.
> Now I am looking for advice on how to build a steep low pass 
> filter, eliminating everything above 146 MHz, to be mounted before 
> the preamp on the mast. The filter must however be capable to 
> accept the rf power, if I work in V/U mode.
> Thank you for any ideas !
> Werner, HB9BNK

Hi Werner , HB9BNK

Few years ago I was suffering the same problem and I fixed it using
cavity filters as passband filters and notch filters normally used in the
FM repeaters.

In addition I built high dinamic range Norton type preamplifiers and 
high intercept point 2 meters converters using double balanced mixers.

At the end of the above succesfull experimentation I writes a few
technical articles covering the above matter published into the italian
Radio Rivista magazine of ARI that you can request as photocopy 
at no cost to:

ARI Via Domenico Scarlatti, 31     20124 Milano Italy. 
TEL 02 6692894   and  FAX 02 67078923  

Obviously the text is in italian but you will understand the drawings
the pictures and the schematic diagrams.

Radio Rivista march  1994 pagg  30 to 33
   "            "      april       "        "      38 to 41
   "            "      may        "        "      27 to 32
   "            "      june        "        "     37 to 41
   "            "      august    "        "     29 to 33

If you are looking on how to build a steep low pass filter, eliminating
everything above 146 MHz, to be mounted before the preamp on the
mast and the filter must be capable to accept the rf power because you
work in V/U mode I believe that a low pass filter with the capability to 
pass 145.900 MHz and reject with high attenuation 147.300 MHz is
impossible to be built.

I have in my hand one of the best professional tubular filter capable of
500 watt built by the French manufacturer STTA model FL.24.A .
it is 21 cm long and 22 mm in diameter including N connectors.

I have measured it with my spectrum analyser HP 8555A and tracking
generator HP 8444A and the results are the following:

Frequency (MHz)              Insertion Loss (dB)

   144                                           0.3 
   160                                           0.4
   180                                        > 50 

As you can realize even if the tubular filter rejects by more than 50 dB a 
180 MHz signal from a 144 MHz signal it reject nothing between
144 MHz to 160 MHz  and so in your case only a big passband cavity
filter tuned to 145.900 MHz plus another cavity filter tuned as a notch
to 147.200 MHz  can do the job othervise you must remove the SP-2000
preamplifier wich in your situation is responsible to generate gain
compression and intermodulation distorsion.

Best 73" de

i8CVS Domenico

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