Expectation of FM Broadcast Filter?

[eorange]

I have the Mini-Circuits ZBSF-95-N+ band stop filter, 88-108 MHz. Using my outside discone without the filter, a powerhouse FM station is 5/5 bars on a BCT15X. With the filter it's 3/5 bars. To my ears it sounds exactly the same. Same results for a lower power FM station. At face value you'd think this filter is doing nothing at all.

Is this expected? I know this filter is supposed to be no slouch, so I can only guess the filtered signal - despite what my ears are hearing - on other bands is enough to allow the reception of lower power non-FM broadcast signals (i.e. AM airband) that would normally get blasted over. Am I thinking about this correctly?

 

[KevinC]

I don't know how strong the station is hitting your filter, but let's say it's -40 dBm. If that filter has 45 dB insertion loss at the station frequency it's still a -85 dBm signal (which would still be full quieting). But what it's doing is preventing your scanner from overloading due to that -40 signal (and maybe more composite power if you have several hitting you strong enough).

 

[dlwtrunked]

Keep in mind that 5/5 might really be 10/10--we really do not know what that strength means other that it is some designers full scale. The Mini-circuits out do all other filters I have tried (Par, Radio Shack, others) but I use two with an FM broadcast antenna visible 0.8 miles out my front door--those knock it down yo S-4 on an ICOM R9500. Also what cox is between the filter and receiver?

 

[eorange]

Thanks, that helps put it in empirical terms. I have 90 feet of rg8x mini.

I know using the filter is a good thing; it's primarily for when I listen to VHF/UHF airband. But I do wonder about the actual difference it's making. If I heard some transmission that the filter now makes possible...how would I know? As for a fixed reference point... I'm barely able to hear KCLE ATIS on some days, and the filter makes no difference there.

I'm not in a terrible RF environment. What made me want to experiment with that filter: I was getting blasted with FMB while listening to airband with my R30 on my discone. That was the big realization.

 

[prcguy]

The unknown here is how many dBm it takes to reach 5 out of 5 bars on your radio and if its actually way over 5 bars and the radio just stops displaying any more. If you have a dongle or other SDR with spectrum display I think it will give you a better idea of what's actually going on and my experience with the MiniCircuits filter is very positive, the best I've tested so far.

As others have pointed out you could be hearing a weaker signal by 40 or 50dB with the filter inline but it could still be full quieting on your receiver and low enough in level to never cause any problems.

I have the Mini-Circuits ZBSF-95-N+ band stop filter, 88-108 MHz. Using my outside discone without the filter, a powerhouse FM station is 5/5 bars on a BCT15X. With the filter it's 3/5 bars. To my ears it sounds exactly the same. Same results for a lower power FM station. At face value you'd think this filter is doing nothing at all.

Is this expected? I know this filter is supposed to be no slouch, so I can only guess the filtered signal - despite what my ears are hearing - on other bands is enough to allow the reception of lower power non-FM broadcast signals (i.e. AM airband) that would normally get blasted over. Am I thinking about this correctly?

 

[AM909]

To add some perspective about S-units, by the 50 5 μV S-9 and 6 dB per S-unit standard, 20dB quieting should happen below S2 (0.4 μV @ 50 Ω) S5 (0.32 μV @ 50 Ω) in even a mediocre VHF/UHF receiver.

As far as how those strengths equate to a measurement with only 5 possible values, perhaps someone with a calibrated service monitor or other generator can give us an idea how some scanners (if not the 15X in question) are calibrated? I haven't found anything with a little searching.

If marketeers were asked during the design, they might want to make weaker signals register more bars, which, oddly enough, is how I think I'd want it, say S2 S5 = 4 bars (of 5) so you have more resolution in the range that matters where you're trying to pull a weak signal out of the mud by changing antennas/cable/location. Of course, I'd prefer a calibrated digital dBm (or dbμV) readout and a variable frequency tone or clicker (like a Geiger counter).

[Added: and original struck out]
Wikipedia says that IARU Region 1 Technical Recommendation R.1 specs S9 = 5 μV at VHF/UHF (20 dB less than HF).

 

[pro92b]

BCT15X S-meter threshold vs signal input
Frequency           Number of S-meter bars
  MHz          1       2       3       4       5
155.175     0.28uV  0.53uV  0.93uV  1.50uV  1.91uV
463.000     0.32uV  0.59uV  1.05uV  1.68uV  2.20uV
856.1125    0.36uV  0.68uV  1.18uV  1.86uV  2.40uV

Fluke 6060B signal generator

 

[prcguy]

That's a sliding scale reading with about 6dB per light at the low signal end and less than 3dB at the upper end and only 5 bars to cover all levels. Not very useful.

BCT15X S-meter threshold vs signal input
Frequency           Number of S-meter bars
  MHz          1       2       3       4       5
155.175     0.28uV  0.53uV  0.93uV  1.50uV  1.91uV
463.000     0.32uV  0.59uV  1.05uV  1.68uV  2.20uV
856.1125    0.36uV  0.68uV  1.18uV  1.86uV  2.40uV

Fluke 6060B signal generator

 

[majoco]

Judging the signal level of an FM transmission by the resultant audio volume is not a valid test - the audio is derived from the frequency shift, not the level. In most FM receivers the final stage of the intermediate amplifier is a 'limiter' which clips all the peaks off the signal at quite a low RF level. The audio volume won't get any louder after the 'limiter' regardless of the RF level - but what happens in the earlier stages of the radio is that they can get overloaded with a very strong signal and an even stronger off-frequency signal. If your filter is managing to bring the signals down to a level shown on the bars it's doing a reasonable job. Daisy-chaining another filter may cause you to lose the signal you want to hear as they are not perfect!

Although we often say that using a 75ohm coax cable won't make much difference to your reception, it may make have an adverse effect on a filter. Filters are fussy things and they designed with a specific in-and-out impedance to get the acceptance/rejection curves required. Feeding them with a different impedance cable can make your filter change it's curves dramatically and not for the better.

You may like to compare the signal levels shown in 'pro92b's' scale shown above compared to what we see on a calibrated HF communications receiver - the "S" meter scale starts at "S0" of course which is supposedly about 0.1uV in 50ohms and goes up to "S9" which is usually taken to be 50uV but the scale then continues to 40dB over "S9" which is 5000uV! I've never seen a signal level this high but 20dB over "S9"/500uV is quite common at night for powerful broadcast station, 500kWatts power is not uncommon and even more for a direction antenna.

 

[bearcatrp]

I need one of these but sucks I need adapters for PL 259.