Spectrum Analyzer to View what the MSF is seeing

[krazybob]

I have been dealing with some pretty crazy FM broadcast stuff and after much fuss my "Elmer" realized that maybe we should try retuning the repeater. Ahmmm. No comment. He referred me to the 48 page manual and offered me the chance to teach myself. OK. Trial by fire.

I don't think that the interference is cleaned by the use of DCI 4-pole band pass cavity with a 4MHz windows followed by an Angle Linear dual band pass cavity with a 1MHz window. Both have insertion loss that if adding a preamp just adds to the noise figure. I have used the service monitor off of the antenna to view all of the grunge coming down our antenna. But after tuning the MSF by reading the manual and using th4e Motorola Test set can I plug in after the preselector to see what is coming through? Or is there another spot? If the signal is clean I see no reason to run all of the extraneous hardware, although the Angle Linear is worth placing inline.

Suggestions?

Thank you in advance.

[kcbooboo]

You can insert the tuning probe into any of the holes in the receiver's front end, the same ones you used for alignment. Connect the BNC connector to a spectrum analyzer. There will be about 30-40dB of loss with the probe, as compared to a direct connection, at least on a UHF station. It may be different on a VHF station.

The way to determine how much loss you get is to feed a signal directly into the input of the receiver from a signal generator and adjust the level for some convenient M2 meter reading, such as 14 or 16. Note the level of the signal you needed. Then do the same with the tuning probe. You'll need more signal and it will be slightly different for each hole. The difference in level is roughly the attenuation the probe will give you when you connect it to a spectrum analyzer. However analyzers that have a 50 ohm input impedance may give you a lower signal level than ones with an "antenna" input that's high impedance.

If your interference is coming in on the frequency the station is trying to receive, no filter in the world will get rid of it.

Bob M.

[krazybob]

Thank you, Bob.

We have made significant progress. We added a DCI filter in front of the Angle Linear. Chip Angle, what a guy! I sent him an extra $10 to ship the rebuilt preamp back Priority, which he did, but with a $10 bill enclosed. It is a shame that he won't be dong ham stuff anymore.

I'll take your advice and check the signal after I do a tune-up. We are way down after only moving the repeater. What was once a -125dBm before the duplexer is down at -112dBm and -108dBm after all filtration.