[amsat-bb] 70cm high-pass filter designs

Daniel Estévez daniel at destevez.net
Thu Apr 16 17:41:48 UTC 2015

Many thanks to everybody for all the suggestions. I'll have a look at
the filter designs.

There are some people that have suggested that the problem might be the
3rd harmonic from the 2m TX, instead of being overload by the 2m signal
somewhere in the receiver. I'm also suspicious that this could be the
case, but I did yesterday some tests that seem to indicate that the
level of the 3rd harmonic is fine.

I tested with different antennas for TX and RX, with about 2 or 3 meter
separation between them (my normal antenna is an arrow type antenna,
with on coax for the 2m yagi and other coax for the 70cm yagi). Fist I
connected the dongle to the 70cm port on the diplexer (and the antenna
to the common port). With this setup I got a bit of desense as usual.
Then, I connected the dongle to the 2m port on the diplexer. With this
setup the desense is very big.

This makes me think that the problem is with the 2m frequency and not
the 3rd harmonic, because the distance between the antennas and the 2m
port of the diplexer should be more than enough to kill any 3rd harmonic.

However, it's possible that I am seeing two different effects with this:
the 3rd harmonic when the dongle is on the 70cm port and overload from
the 2m signal when the dongle is on the 2m port.

What do you guys think?


Dani EA4GPZ.

El 16/04/15 a las 00:07, Daniel Estévez escribió:
> Hi all,
> My portable satellite station consists of a Funcube Dongle Pro+ for RX
> and a FT-817ND for TX. I use a diplexer in front of the dongle to avoid
> desensing, but still some of the 2m signal is getting in and causing
> desensing when working V/U birds. It seems to me that a 70cm high-pass
> filter in the receive chain would help.
> Can anyone point me to some easy to build reference designs for this
> kind of filters? Power handling is not an issue, since this will only be
> used for RX. I'm mainly looking to something that I could build from
> things in my junk box, using air core inductors and ceramic caps (if
> this a sensible thing to do).
> 73,
> Dani EA4GPZ.
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