[amsat-bb] Re: 70cm preamp

28 Sep 2006

      Hi Scott, NX7U
I agree with you about both bandpass filtering on the 70 cm side and
lowpass filtering on the 2 meter side.
1.  I use bandpass filtering on the 70 cm side with a K3PGP passband
     filter between the antenna and preamplifier input.
http://www.k3pgp.org/432filter.htm
The above filter has an insertion loss of only 0.062 dB at 435 MHz
and provides the bonus of adeguate selectivity to -3dB points at 412
and 466 MHz including a very interesting and original preamplifier
protection with easy mechanical fabrication.
2. I use lowpass filtering on the 2 meter side with "A High Power
    2-meter Filter" by Mike W. Cook AF9Y described in Proceeding
    of the 1990 Central States VHF Conference.
Unfortunately I was unable to find a description on internet
but if someone is interested on it I can scan a send a copy of the
article.
The above home brewed lowpass filter has insertion loss of
only -0.065 dB at 146 MHz with a return loss of -40 dB
The harmonic attenuation above 146 MHz as measured with
my spectrum analyser HP 8555A and tracking generator
HP8444A is the following:
2° hamonic 292 MHz   -20 dB
3°       "        438 MHz  - 45 dB
4°       "        584 MHz  - 55 dB
5°       "        730 MHz  - 60 dB
An additional advantage of a lowpass filter against a cavity bandpass
filter is that a lowpass filter don't need to be retuned moving between
144 to 146 MHz
I use an antenna mounted  70 cm preamplifier with NF=0.45 dB
and G=20 dB followed by a postamplifier with NF= 1.35 dB and
G= 15 dB
The 2 meter and 70 cm antennas separation is 2.3 meters and using
the above filtering no desensitization occurs even testing with
300 watt RF reaching the 2 meter antenna.
73" de
i8CVS Domenico
----- Original Message -----
From: "Scott Townley" nx7u@arrl.net
To: amsat-bb@amsat.org
Sent: Thursday, September 28, 2006 4:09 AM
Subject: [amsat-bb] Re: 70cm preamp
...
There are two potential issues with Mode J:

The 2m TX overloading the preamp on 70cm.  This problem can only be

addressed by reducing the amount of TX energy input to the preamp.  That
means
...
     a. bandpass/highpass filtering on the 70cm side
     b. increased separation of antennas
     c. reduction in TX power

A 70cm cavity bandpass filter is pretty easy to rig up.  I have
step-by-step pictures at http://members.cox.net/nx7u/mobile/4x3x5.htm
Now the SP7000 has bandpass filtering in it, but I don't know if it's
before or after the preamp.  If it's not before the preamp, then it won't
help the Mode J problem.

The 2m TX is transmitting a significant 3rd harmonic (145x3=435, a most

unfortunate combination).  Similarly, that means
         a. bandpass/lowpass filtering on the 2m side
         b. increased separation of antennas
         c. reduction in TX power
Even if the 3rd harmonic is -60dBc, with a 100W (+50dBm) TX that's still
-10dBm radiated--a big signal.
The most prudent (to me) solution is filtering on both ends.  When I ran
Mode J mobile--where the antennas had no directivity and necessarily were
very close together--using the 70cm cavity filter on RX and an ICE 2m BPF
filter on TX allowed for no-worries LEO operation.  Plus you have the
benefits of good amateur practice (filtering TX), and a more
intermod-resistant receive path on 70cm.  Nearby 460MHz paging/two-way
radio systems can really wreck your RX.
...
-----Original Message-----
From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On
Behalf Of Bill Bruno
Sent: Wednesday, September 27, 2006 7:04 PM
To: Amsat BB
Subject: [amsat-bb] 70cm preamp
On my V/U setup I have a SP7000 70cm preamp. Working FO29(JAS2) my 2
meter
...
...
signal overloads the preamp, my signal splattering like an adjacent
strong
...
...
station. With preamp off the problem is not there. In testing I found the
overload happens in any combination of 70cm 2m frequency pairs. I can
eliminate the problem by lowering the tx power, but by doing so I have a
hard time copying my own signal. How can I eliminate the 2m sig
overloading
...
...
the preamp while still maintaining  adequate power?
Antennas m2 436cp30,  Gulf Alpha 2m5el, both right hand polarization.
I could use some suggestions.
Thanks Bill

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[amsat-bb] Re: 70cm preamp

i8cvs