Forgot to say that bandpass filters at VHF and UHF normally have an insertion loss of a fraction of a dB (around <0.2 dB) thus the loss putting it in front of your preamp is negligible loss compared with the noise created by the 3rd harmonic of the 2m Tx.

Hope this also helps.


Jean Marc (3B8DU)

On Feb 8, 2022, at 9:31 PM, Jean Marc Momple <[email protected]> wrote:


As Matthias suggested I highly recommend filters be added before the preamp, I mean between antenna and preamp..

I made many experiment with same. The reason is actually quite logical, your preamp will amplify both the wanted signal and also the 3rd harmonic from your Tx 2m transmission (which falls within the 70cm band) and possibly overload it) thus the wanted amplification at 70cm will be well below the noise created by your 3rd Tx harmonics (worse offender normally).

Just a suggestion and hope it work for your setup.


Jean Marc (3b8DU)

On Feb 8, 2022, at 9:06 PM, Matthias Bopp <[email protected]> wrote:

Hello Dave,
you can do 2 things:
  1. Add a highpass filter or bandpass filter in your receive path in order to suppress the 2m signal as good as possible.
    You can try first to insert the filter in front of your receiver (between the preamp and your receiver) and if that is not 
    helping (enough) insert it in front of the preamplifier. 
    Here is a link how to build such a filter: http://www.dd1us.de/Downloads/Mode-J%20Filter%20rev1_1.pdf

  2. Add a lowpass filter to your 2m transmitter in order to reduce the harmonics emitted in the 70cm band as good as possible.
Kind regards
Von: Dave Julian via AMSAT-BB <[email protected]> 
Gesendet: Dienstag, 8. Februar 2022 17:41
An: [email protected]
Betreff: [AMSAT-BB] IC-9700 and PS 70 not playing well together
Just got a new SSB SP 70 preamplifier for my IC-9700 and having an issue with working 435 RX satellites like RS-44.
The amplifier works fine but I am getting terrible overloaad from the 2M TX signal. It causes the OVF light to come on and
very distorted audio. I have lowered the RX gain and TX power on the IC-9700 but by that time it kind of defeats the purpose
of a preamplifier. My set-up is an M2 2MCP22 with fixed polarity on 2m and a 436CP30 with the PS-70Cm polarity switch for UHF.
I have about a 200' run on LMR-400 to the base of the tower with lightning arrestors then about 40' of RG-213 to the antennas.
For testing I have the Sp 70 preamplifier temporarly installed at the base of the tower. No preamplifier for 2m.
Power to the preamplifier is over the coax from the IC-9700. I am looking any any suggestions on sorting this out.
I have removed the PS 70 at this point to avoid damage the radio or preamplifier.
Dave WB9YIG 


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