FM Bandstop Filter

[SmartRFDesign]

Hi All,
I have just designed and made and tested an fm bandstop filter and was wondering if there is any demand out there to purchase something like this-please see some measurement details below:

1.00MHz -0.02dbc
10.00MHz -0.12dbc
20.00MHz -0.20dbc
30.00MHz -0.33dbc
68.00MHz -3.90dbc
88.00MHz -81.80dbc
98.00MHz -84.69dbc
108.00MHz -80.07dbc
132.00MHz -5.18dbc
135.00MHz -3.52dbc
150.00MHz -2.40dbc
188.00MHz -0.83dbc
200.00MHz -0.73dbc
300.00MHz -0.44dbc
500.00MHz -0.32dbc
1000.0MHz -0.47dbc

Best Regards,
SmartRFDesign

 

[vagrant]

What is the loss at 118 and 125 MHz?

I and probably others would prefer to take a loss on the lower end, versus up in the VHF air band, even if that means reduced attenuation at 108 MHz. There are plenty of sub $20 FM filters out there that do the job, but are unfriendly at VHF air due to their depth.

Take a look at this thread about FM filters if you have not already done so.

FM bandstop filter comparison

I rolled some numbers on the RTL-SDR FM bandstop filter and the HPN-30118 AM/FM filter using a VNA. The results are interesting compared to the ZBSF-95+, especially when you hit VHF Air voice at 118 - 137 MHz. The ZBSF-95+ numbers come from their spec PDF as I do not own that yet. There are...

forums.radioreference.com

 

[prcguy]

The ideal FM bandstop filter would have minimal loss starting at 118MHz on up. 5dB loss at 132Mhz will not work for scanner users.

 

[dlwtrunked]

What is the loss at 118 and 125 MHz?
...

You beat me to asking the same question. The loss at 132 MHz is large enough that I suspect it is too great at 118 MHz. Unless it less loss than a Mini-Circuits ZBSF-95+ (which is -1.15 dB at 119 MHz), I would not be a buyer. The filter you proposed is not even that good at farther away at 132 MHz. The filter you describe, as prcguy has indicated, is not acceptable by most.

 

[SmartRFDesign]

Thanks for your feedback.I will do a re-design and throw away a little bit of the rejection performance of the notch filter in the band 88-108 to improve the insertion loss at 118MHz (targetting -1.0dbc) and higher frequencies and then report back my findings.

 

[prcguy]

The scanner world needs a very good FM bandstop filter with at least 60dB rejection across 88 to 108MHz and minimal loss from 118MHz through about 900MHz like .5dB or less. If its 1dB loss at 118MHz and by 120MHz its down to .5dB that's ok. Loss below 88MHz is not critical until you get down to about 55MHz then it should be less than .5dB. And it has to be affordable. N female connectors preferable with BNC as a second choice. If you can met this criteria you will sell a lot of filters.

Thanks for your feedback.I will do a re-design and throw away a little bit of the rejection performance of the notch filter in the band 88-108 to improve the insertion loss at 118MHz (targetting -1.0dbc) and higher frequencies and then report back my findings.

 

[Ubbe]

It's difficult making cheap and good filters. Normal scanner people doesn't need more than a 30dB notch if they live a distance from the transmitter. Those who live next to it can connect two filters in series and accept a little more loss to all frequencies. But a standard scanner will then probably perform much better and compensate for the added loss and more, compared to using no filter. Remember that practically all scanners use bandpass filters that already attenuates FM broadcast some 30dB, so adding another 30dB would be enough. Antenna preamplifiers are usually more tolerant to strong signals than a scanner so a 30dB notch would normally be enough even for that use.

One of the better filters are MiniCircuits. Compare specs and price to that. I don't know about the need for N-connectors. No scanners has those. If anyone needs N they can use a BNC-N adapter.

If you focus solely on the US market it might not be much to listen to in the 55-87Mhz band but in some other countries there are. Two of the busiest channels I monitor are at 85Mhz. VHF-air goes down to 118Mhz in the whole world and needs to have as small loss as possible.

/Ubbe

 

[majoco]

Having to use a BNC-to-N adapter completely defeats the reason for going to N in the first place.

 

[prcguy]

I think the typical setup would be coax from the antenna has an N male which connects to the filter then a short cable with N male on the filter side and BNC male on the radio side, or whatever connector your radio uses. You would not want to connect a filter directly to a radio, especially if it has a BNC. N female is probably the best connector to specify on a filter because you can transition to any other connector or cable with basically no loss and its a very good mechanical connection.

Having to use a BNC-to-N adapter completely defeats the reason for going to N in the first place.

 

[Ubbe]

Having to use a BNC-to-N adapter completely defeats the reason for going to N in the first place.

Exactly. No need for N at all when using scanners.

/Ubbe

 

[Ubbe]

I think the typical setup would be coax from the antenna has an N male which connects to the filter then a short cable with N male on the filter side and BNC male on the radio side, or whatever connector your radio uses.

And an even more typical one are probably that you have a N-BNC adapter on the feed line and then a thin jumper coax with BNC at each end, making it possible to use splitters and Stridsberg devices all that use BNC and you can connect the filter anywhere in the chain if it has BNC. If you use antenna amplifier then most of them will use other connectors like SMA or F or BNC. Very few use N and those would be the professional expensive models not intended for amateur scanner use.

/Ubbe

 

[AM909]

Unless you have a physical space constraint, I wouldn't rule out N or SO-239. I also would support SMA, F, and 75-ohm impedance as options for maximum flexibility. I assume you're talking essentially custom, low-volume production anyway. Let 'em spec whatever they want.

 

[prcguy]

I think the way this could play out is you design and make some prototypes and submit to a few members here who have tried many different FM traps and know most don't work very well. If people here say its the best they have ever used or close to it and the price is right you might crank out 100 units and people on this forum will buy some. Word of mouth sells this type of item, so I think getting a few hard core users talking about it here and on other forums will be the ticket to selling out. But you have to do your part in producing a unit that can at least compete with the Minicircuits unit at an affordable price.

There are other types of filters and especially diplexers needed in the hobby world with specs geared towards public service and amateur bands with specs that exceed the typical cheap offshore made types and at affordable prices.

Do you feel like your up to the challenge? Well, do ya?

Unless you have a physical space constraint, I wouldn't rule out N or SO-239. I also would support SMA, F, and 75-ohm impedance as options for maximum flexibility. I assume you're talking essentially custom, low-volume production anyway. Let 'em spec whatever they want.