A little Confused

[N1SQB]

Ok, I took the plunge and ordered two PAR filters, the Civil Air ( VHFBP 117-136 ) and the Milair ( UHFBP 225-400) to put inline on two scanners dedicated to those bands. I understood that with band pass filters everything below a certain frequency and above a certain frequency would be cut off, therefore allowing only what is meant to be in that particular band. I haven't tested the UHF Milair one yet but the civil air filter which should cut off everything above 137 mhz or so, is letting in about 3 different channels of the 162 mhz NOAA weather service.
I thought I had the filter installed wrong but I put it inline different ways and still nothing changed. I have a single antenna ( D130J discone ) feeding 4 scanners via a 4 port Stridsberg 204M Active multicoupler.While I do get great signal on all other bands, I see no change at all on the civil airband. I'm kind of confused at this point.

Manny

 

[ka3jjz]

Here's an obvious test - take one scanner feedline out of the multicoupler, connect the antenna to one of the filters, and connect the filter to the scanner with a VERY SHORT piece of coax. What happens?

How long is the coax that goes from the multicoupler to the scanners you're trying to filter? If it happens to be around 2 foot, that's close enough for that to work as a crude VHF antenna- and with the amp from the multicoupler at the top of the coax connection, it's just making things that much worse.

Where are you putting the filters? To me they should be after, not before, the multicoupler and connected to the scanner with a VERY SHORT piece of coax (better if you can use a male/female shielded plug).

Mike

 

[prcguy]

A filter with "brick wall" response across one of the air bands and skirts that drop 30dB within a MHz of the corner frequency and have 40-60dB rejection everywhere but the passband is a $600+ filter in single qty. A filter like this would probably need upwards of 11 or more sections and they usually design in notch filters at the band edges to get seriously good skirts.

How much did the PAR filters cost you? I suspect the PAR filters are designed for low insertion loss within the specified bands and roll off at least 20 or 30dB after 5 or 10MHz of the band edges because that's what you are paying for.

Another thing to consider is if your local 162MHz weather channel has a high signal level at your antenna to provide say -50dBm at your receiver and your PAR filter has 40dB rejection at that frequency, you would still have -90dBm or about 7uV left over, which is nearly full quieting on most receivers.

The bottom line is the PAR filter is probably keeping FM broadcast, paging and other out of band high level signals from creating IMD in your receiver and your aircraft reception will be much better with the filter in line.
prcguy

Ok, I took the plunge and ordered two PAR filters, the Civil Air ( VHFBP 117-136 ) and the Milair ( UHFBP 225-400) to put inline on two scanners dedicated to those bands. I understood that with band pass filters everything below a certain frequency and above a certain frequency would be cut off, therefore allowing only what is meant to be in that particular band. I haven't tested the UHF Milair one yet but the civil air filter which should cut off everything above 137 mhz or so, is letting in about 3 different channels of the 162 mhz NOAA weather service.
I thought I had the filter installed wrong but I put it inline different ways and still nothing changed. I have a single antenna ( D130J discone ) feeding 4 scanners via a 4 port Stridsberg 204M Active multicoupler.While I do get great signal on all other bands, I see no change at all on the civil airband. I'm kind of confused at this point.

Manny

 

[cmdrwill]

May be a bad combination of the poor D130J antenna and the 4 port ACTIVE multi coupler, You may need a dedicated VHR 'airband ' antenna.

 

[KevinC]

I'm guessing you're talking about this one...

Scanner Bandpass Filters | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

It provides a little over 25 dB rejection at 160 MHz. So if your NWS is coming in anything stronger than about -95 it will get through.

 

[N1SQB]

Here's an obvious test - take one scanner feedline out of the multicoupler, connect the antenna to one of the filters, and connect the filter to the scanner with a VERY SHORT piece of coax. What happens?

How long is the coax that goes from the multicoupler to the scanners you're trying to filter? If it happens to be around 2 foot, that's close enough for that to work as a crude VHF antenna- and with the amp from the multicoupler at the top of the coax connection, it's just making things that much worse.

Where are you putting the filters? To me they should be after, not before, the multicoupler and connected to the scanner with a VERY SHORT piece of coax (better if you can use a male/female shielded plug).

Mike

Mike, the coax jumper is about 3 feet long. no connectors on top of connectors. Direct BNC on both ends of the jumper.

A filter with "brick wall" response across one of the air bands and skirts that drop 30dB within a MHz of the corner frequency and have 40-60dB rejection everywghthere but the passband is a $600+ filter in single qty. A filter like this would probably need upwards of 11 or more sections and they usually design in notch filters at the band edges to get seriously good skirts.

How much did the PAR filters cost you? I suspect the PAR filters are designed for low insertion loss within the specified bands and roll off at least 20 or 30dB after 5 or 10MHz of the band edges because that's what you are paying for.

Another thing to consider is if your local 162MHz weather channel has a high signal level at your antenna to provide say -50dBm at your receiver and your PAR filter has 40dB rejection at that frequency, you would still have -90dBm or about 7uV left over, which is nearly full quieting on most receivers.

The bottom line is the PAR filter is probably keeping FM broadcast, paging and other out of band high level signals from creating IMD in your receiver and your aircraft reception will be much better with the filter in line.
prcguy

As it turns out, in my case, the best way to get the best results is to bypass the multicoupler altogether. With my external antenna height and the signal level it brings in, Im better off bypassing the multicoupler because of its internal amp. Once I bypassed it, the weather service was no longer an issue and my airband signals were cleaner.


Manny

 

[prcguy]

The filters should go in front of any active device like an amplified multicoupler. If your antenna is connected to the multicoupler first then its internal amplifier is probably getting hit by some strong signals and creating IMD which is degrading your reception.

One way around this might be to run your single antenna into a diplexer to split between the VHF and UHF air bands, then run through your filters at the output of the diplexer, then to another diplexer to combine them and off to the amplified multicoupler. A diplexer has very minimal loss and the end result would be keeping some strong out of band signals from overloading the amplifier in your multicoupler and creating IMD. Two diplexers and your filters in line would probably add up to about 1dB or so of loss before the amp in the multicoupler, which is not so bad considering the problems it could fix.

The only other way would be to find the holy grail super high level broad band amp that does not get overloaded from your antenna, then use a passive divider after that. Problem is finding an amp with low noise figure, low to moderate gain around 10-15dB and a 1dB compression point of maybe 30dBm is very hard to find and expensive when you do.
prcguy

Mike, the coax jumper is about 3 feet long. no connectors on top of connectors. Direct BNC on both ends of the jumper.




As it turns out, in my case, the best way to get the best results is to bypass the multicoupler altogether. With my external antenna height and the signal level it brings in, Im better off bypassing the multicoupler because of its internal amp. Once I bypassed it, the weather service was no longer an issue and my airband signals were cleaner.


Manny

 

[N1SQB]

May be a bad combination of the poor D130J antenna and the 4 port ACTIVE multi coupler, You may need a dedicated VHR 'airband ' antenna.

I have to disagree with you here. While there is no substitute for a dedicated airband antenna, the discone is not the problem child here. I live at an altitude of 450 feet above sea level at ground level. I have my two antennas about 25 feet further up. Both the Diamond D130J discone and the 2m / 440 vertical antenna are bringing in very good signals. ( only 40 foot run of low loss coax ) Outside of the am aircraft bands, I have NO issues anywhere else. The built in amp on the multicoupler is the problem from what I have discovered. I bypassed the coupler and split the signal from the vertical 2m / 440 antenna in two directions. One directly into the VHF filter then right to the radio, the other into the UHF filter into the other radio. The discone goes into the input of the multicoupler for all other bands. Everything is stable now and there are no more issues with the weather service. Both antennas give me good signals but I like the vertical a little more because it is higher up on the pole.

Manny