[amsat-bb] Re: 145 MHz IF on V/s?

i8cvs domenico.i8cvs at tin.it
Mon May 19 10:51:57 PDT 2008

----- Original Message -----
From: "Edward Cole" <kl7uw at acsalaska.net>
To: "i8cvs" <domenico.i8cvs at tin.it>; "K & R Yoksh" <yokshs at sbcglobal.net>;
"Amsat BB" <amsat-bb at amsat.org>
Sent: Monday, May 19, 2008 4:29 PM
Subject: Re: [amsat-bb] Re: 145 MHz IF on V/s?

> I agree with Dom's analysis.  What you are facing is almost the
> classic problem of running in-band duplex (that is the problem every
> repeater faces).

Hi Ed, KL7UW
Receiving the satellite subband from 145.800 to 146.000 MHz here in the
gulf of Naples is like the classic problem you mentioned of running in-band
duplex because I am receiving the satellite band while my ham-radio fellows
are transmitting locally FM with high power from 145.450 to 145.475 MHz
desensing my receiving system and this problem started with OSCAR-13 early
1994 when high power 2 meters amplifiers where readily available in the

> One solution would be to use a repeater duplexer
> tuned for your 720-KHz split.  However there is a catch:  duplexers
> normally operate at a single channel frequency and do not allow
> tuning a subband.  If you are running low power you might be OK with
> bandpass filters tuned for a wider band acceptance on the receive
> frequency.

You are reading in my brain Ed because to overcome the problem of desensity
I had to build a Norton type preamplifier with an IP3 of +14 dBm relative to
the input and associate NF= 1.3 dB and G= 18.5 dB
This preamplifier have a 1 dB compression point at -1 dBm and is antenna
mounted but it was not in condition to complitely solve the problem of gain
reduction and gain compression of my receiving system while my near by
friends were locally transmitting FM about 500 KHz belove the satellite
For the above reason I added in the shak a band-pass cavity filter centered
to 145.950 MHz tuned with a flatness of +/- 1 dB in to the satellite band
and rejecting the unwanted 145.450 MHz signals by only -12 dB

After the above BPF I added in series a notch cavity filter of the type used
for the 2 meters FM repeaters producing a hole -38 dB deep at 145.450 MHz
and only an attenuation of -1dB into the satellite band.
The output of the above notch is connected to the input of a homebrewed
144/28 MHz high dinamic range downconverter that make use of two Norton
input stages and a very high IP3 intercept point mixer type VAY-1 requiring
a power of +30 dBm for the LO
With this setup that was published in Radio Rivista's  3-94, 4-94, 5-94 and
6-94 I am actually safe of any station using high power in the gulf.
I remember that I was accustomed to locally show how OSCAR-13 was nice
receiving the bird at 145.950 MHz and translating it on 145.450 MHz only
500 kHz belove using 10 watt over another antenna.
Obviously I cannot move this receiver from the 2 meters satellite band for
wich it is completely tuned and dedicated but no way do do different because
the 2 meters life is very hard here in the gulf.
> So if you can receive on 123-MHz this is infinitely
> easier/cheaper.  I run 123-MHz on one of my Drake converters with no
> problems from 145 MHz transmissions.  MY other Drake is modified for
> IF of 435 MHz which makes mode-LS simple as I nver have to face your
> dilemma.
I agree

> 73 Ed - KL7UW

Best 73" de

i8CVS Domenico

> At 03:37 AM 5/19/2008, i8cvs wrote:
> >----- Original Message -----
> >From: "K & R Yoksh" <yokshs at sbcglobal.net>
> >To: "Amsat BB" <amsat-bb at amsat.org>
> >Sent: Monday, May 19, 2008 12:56 AM
> >Subject: [amsat-bb] 145 MHz IF on V/s?
> > >
> > > Hi all,
> > >
> > > Thanks for the replies, however, I've still not had my question
> > > so I'll ask again.
> > >
> > > I'd like to hear from anyone using a 145 MHz IF when working mode V/s.
> > > you experience desense when operating V/s, but not other uplink modes?
> > >
> > > I'm planning to use my AIDC 3731 with 145 MHz IF when I install my
> > > this summer. I really want to use it for HEO operation, but it'd be a
> > > bonus if I could also work the LEOs.
> > >
> > > I could go with a 123 MHz IF unit if that'd work better.
> > >
> > > Thanks and 73,
> > >
> > > Kyle Yoksh
> > > K0KN
> > > Olathe, Kansas
> > >
> >Hi Kyle, K0KN
> >
> >Don't warry about to use 145 MHz IF for HEO operation because P3-E
> >will use the V band only for downlink.
> >The builders of P3-E are aware of the problem and they don't want to
> >complicate the life to the users.
> >
> >For the LEOs S band using downconverters 2400-2402 MHz with an IF
> >144-146 MHz :
> >Receiving AO-51 at 2401.200 MHz the IF fall into 145.200 MHz and
> >transmitting at 145.920 MHz FM the difference between the IF and TX
> >frequency is 720 kHz
> >In this condition there are three possible outcomes:
> >The 145.200 MHz uplink signal is picked up directly by the cable after
> >the mixer of the S band converter and sent down to the shak into your IF
> >receiver flowing in general through the outside surface of the braid of
> >the coax cable. (common currents)
> >The 145.200 MHz uplink signal can pass directly through the converter
> >first stage up to the IF receiver in the shak.
> >The 145.200 MHz uplink signal is picked up directly by your IF receiver
> >because your power is very high and TX/RX are very close.
> >Since there are many variables involved mostly depending on your TX power
> >it is very difficult to say yes or not and so a test to see if you will
> >suffer or not a desensing  is mandatory.
> >If  you have a desensing there are many possible cures for it but if you
> >don't have the time to work on this problem it is best to use your 123
> >IF unit.
> >I remember that receiving AO40 I got interference over the 2401.323 MHz
> >bpsk beacon when an amateur friend living few miles away of me was
> >transmitting FM using high power at 145.325 MHz
> >145.325 MHz was at the same time his own frequency and my IF frequency.
> >My investigation demonstrated that the interfering signal was piched up
> >directly by the coax cable running from the S band downconverter to the
> >IF receiver in the shak.
> >Unfortunately i don't had on hand a S band downconverter with a 123 MHz
> >
> >Best 73" de
> >
> >i8CVS Domenico
> >

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