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Re: 2 meter uplink interference on 70 cm downlink

Erik Smith - ON1DAG wrote:

> Hello,
> When I am using FO20 or FO29 in JA mode I cannot copy myself on the 70 cm
> downlink because of havy interference. When using CW I can copy my signal,
> so I know that I have downlink signal. For SSB I can not copy my downlink
> signal. Looking at some documents about amateur satellite they are talking
> about a mode J filter.
> But what is the best way? Making a filter for my 70 cm pre amp or making a
> filter for my 2 meter up-link?
> Or is my uplink signal too much for my first stage of my 70 cm receiver? Or
> is a 3rd harmonic of my 2 mtr spoiling my 70 cm receiver?
> Is a part of a duplexer a good way to filter and has some one an easy to
> build filter for qrp power (like something which can be put in serie with
> the arrow antenna)?

A couple of thoughts.

The third harmonic of the 2M uplink is very close to the downlink.

A low pass filter will attenuate the 3rd harmonic energy from the 2M
transmitter - the 2M port of a 2M / 440 diplexer will work quite well -
terminate the 440 port with a low power dummy load.

A cavity type filter on the 2M transmit wont help at all as the third harmonic
energy will pass through the cavity just as easier as the fundamental signal.

A cavity type filter on the 435 MHz receive WILL help reduce the 2M signal.  If
you have a preamp on the 435 MHz receive, the cavity should be between the
antenna and the preamp to prevent having the strong 3rd harmonic signal from
overloading the preamp.

Jim Walls - K6CCC
626-302-8515    FAX  626-302-9999
AMSAT member 32537 / WSWSS member 395

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