crystal filter and preselector insertion losses

[adlertom]

I'm considering adding a Mtron 4 pole crystal filter to a VHF AstroTac receiver, in order to mitigate some interference from a transmitter that is a few miles away, and only 97.5 KHz up from my receive frequency.

The crystal filter has a published insertion loss of 7dB (although the Mtron engineers say that they're more typically around 5.5 dB).

My questions have to do with the preselector on the AstroTac. Since the bandpass of the crystal filter is so tight, would this make the preselector redundant/unnecessary? Could I bypass the preselector and install the crystal filter directly before the receiver antenna input, in order to compensate for some of the loss of the crystal filter?

What is the typical insertion loss of an AstroTac VHF preselector?

Mtron doesn't recommend using a preamp. It also makes more sense to me to eliminate unnecessary losses before adding amplification.

[Bill_G]

Correct - If you use a crystal filter, you can remove the standard presel, but you won't recover much in losses. A properly tuned presel is a broad window filter with less than a db of insertion loss. Compared to the xtal filter, it's nothing. You could leave it in, and not notice the difference.

[adlertom]

Thank you Bill. I suspected that the preselector would be pretty low loss.

Tom

[DarkHelmet]

A few miles away should be plenty of spacing for the receiver to be able to reject this signal on it's own. How much power is this interfering transmitter running?

You might consider building/buying a notch stub. Lower insertion loss, and it would be tunable to notch out the particular frequency.

[Bill_G]

I've had this problem. Tight skirt filtering helped, but didn't eliminate it. They were 25khz away. Both of our antennas were at the same elevation, and despite being miles apart on different hilltops, we felt the full gain of their xmit antenna on our rcvr input. We didn't affect them. Our xmit was 500khz away from their input. We ultimately had to recoordinate for a different input. Railroad channels can be a challenge.