FM broadcast filters: an analysis (of sorts)

[smokeyjones666]

(This thread was inspired by a thread on the Radio Shack Scanner forum.)

I have two filters. One of them is a PAR Electronics VHF-FM filter that, while a bit on the pricey side, turned out to be well worth every penny. The other one, a discontinued Radio Shack FM Trap (15-577) that I bought for $5 on eBay.

I have two 'instruments'. The first one, a Yaesu VX-7R which I'll be using for the spectrum analyzer. The second, a Sencore SLM 1453 RF Level and Leakage Level Meter - a relic from my days as a cable installer. I'll use these to look at the FM broadcast band.

The Sencore is configured to measure signal in dBµV, decibels above 1µV across a 75ohm load (0 dBµV = 1 µV, +60 dBµV = 0 dBmV according to the Sencore manual.) My signal source is a set of towers just a few miles to the north of me, blasting out what seems like a zillion-jillion megawatts of front-end overloading classic rock, adult contemporary, country and countless other demographically-adjusted arbitron-inspired powerhouse FM radio stations. My antenna is an Antennacraft ST-2, mounted approx. 50' from the ground and fed with roughly 100'-150' of 75ohm coax (half of this length is double-shield Commscope RG11 double-shielded, the other half tri-shield Times Microwave RG6.)

I took a set of readings from the Sencore at various points in the FM broadcast band. Something I find a little irritating about the Sencore is that I can't directly tune some frequencies, like 88.1 - instead, I have to hit something close like 88.125. I don't know if this is because the frequency is really 88.125 MHz but our radios lie and tell us something comforting like 88.1, or if the meter is just stupid and that's why it can't tune 88.1.

A word of warning, I'm no expert - not even close. I'm just using the tools I have at hand to measure the relative strengths and weaknesses of a couple of filters and compare them to no filter at all. The measurements are what I read from the tools I have at hand, anything else I'm probably making up as I go along.

[u][b]Sencore SLM 1453 Measurements[/b][/u]
-----------------------------

[u][b]No Filter[/b][/u]
88.125 MHz*   88.5 MHz   92.5 MHz   98.0 MHz*   104.125 MHz   107.937 MHz
48.0 dBµV     76.5 dBµV  82.7 dBµV  68.6 dBµV   83.0 dBµV     69.3 dBµV

[u][b]Radio Shack FM Trap (15-577)[/b][/u]
88.125 MHz*   88.5 MHz   92.5 MHz   98.0 MHz*   104.125 MHz   107.937 MHz
43.5 dBµV     70.8 dBµV  70.7 dBµV  37.1 dBµV   39.6 dBµV     40.3 dBµV

[u][b]PAR Electronics VHF-FM filter[/b][/u]
88.125 MHz*   88.5 MHz   92.5 MHz   98.0 MHz*   104.125 MHz   107.937 MHz
33.7 dBµV     47.8 dBµV  58.3 dBµV  22.5 dBµV   51.6 dBµV     42.3 dBµV

* [i]No stations broadcasting on or near 88.125 MHz or 98.0 MHz.[/i]

 

[pro92b]

The FM band begins at 88.1 and there are channels every 0.2 MHz up to 107.9. 88.125 is off channel a bit.

If you have a scanner with an S-meter, I'd be interested to hear how much the filter affects the signal at 155, 460, and 855 MHz. The ones I have do seem to cause quite a bit of loss, although I don't have a PAR filter like you do.

 

[smokeyjones666]

The only scanner I have with an s-meter is my PSR-500, which as we all know can prove frustratingly sensitive. The s-meter goes from 0 to 5 bars, nothing fancy but here goes:

                [b][u]155 MHz[/u][/b]     [b][u]460 MHz[/u][/b]   [b][u]855 MHz[/u][/b]
[b][u]No filter[/u][/b]       5 bars      0 bars    0 bars**
[b][u]RS FM Trap[/u][/b]      5 bars*     0 bars    0 bars**
[b][u]PAR VHF-FM[/u][/b]      5 bars***   0 bars    0 bars**

* [i]Last bar flickers off ever so slightly, but is mostly solid on.[/i]
** [i]First bar flickers on occasionally, but is mostly off.[/i]
*** [i]Last bar flickers off about 50% of the time.[/i]

I don't think this is terribly indicative of anything regarding FM broadcast interference in my situation, otherwise I think there would be more of a difference in the levels. There's an ever-so-slight difference in my readings at 155 MHz, but that's it. I believe my filtering could be better, I know there's a powerful weather radio transmitter (162.550 MHz) on the tower behind KSTP TV a few miles to the south of me. I also know from experience that without the PAR filter I have problems with audio cutting out when I listen to the local P25 system, while with the filter it is clear. Also, without filtering on my Pro-2006 I can only hear aircraft as they pass overhead, but with the PAR filter I can hear the tower at KMSP approx. 10 miles to my south as well as airplanes on the ground.

One things for certain, after looking at the spectrum analyzer of the FM broadcast band I'd say in my situation I could probably stand to reduce the levels in that band a little further. (Is this what you'd call a 'hostile RF environment'?)

Edit: Wow, I just looked at 155 MHz with the Yaesu and the spectrum analyzer is pegged solid without the filter. I'll see if I can find the time to take some more pictures of the spectrum analyzer screen later this evening after I put my kid to bed.

 

[jeffkraussws]

I took a set of readings from the Sencore at various points in the FM broadcast band. Something I find a little irritating about the Sencore is that I can't directly tune some frequencies, like 88.1 - instead, I have to hit something close like 88.125. I don't know if this is because the frequency is really 88.125 MHz but our radios lie and tell us something comforting like 88.1, or if the meter is just stupid and that's why it can't tune 88.1.

A word of warning, I'm no expert - not even close.

I'm not an expert either. But the .125 suggests the Sencore is set up to measure NTSC video.
If so, it has a measurement bandwidth of 6 MHz.
If so, that covers a lot of FM stations.

 

[smokeyjones666]

I'm not an expert either. But the .125 suggests the Sencore is set up to measure NTSC video.
If so, it has a measurement bandwidth of 6 MHz.
If so, that covers a lot of FM stations.

I took a look in the Sencore manual, and it says "The SLM 1453 tunes in the frequency mode by 62.5kHz steps.", so that makes sense. Probably not the best tool in the world for this purpose, it's feature set makes it obvious that it's intended purpose is for cable and tv antenna installation, but it's a tool that I have; kinda like having vice grips when you'd rather have a wrench - both get the job done, one happens to be better suited to the task at hand but the other is the one that's in your kit.

Anyway, here's some more images from the Yaesu's spectrum analyzer. On thing I noticed as I was toying around 155 MHz was that the static sounded a tad orderly and repetitive, I tuned down to a peak at 154.915 MHz and heard a weather radio broadcast combined with a hint of two-way radio traffic. (The peak just to the right of 155 MHz is two-way radio, but I didn't have the presence of mind to determine if it was the same as the one to the left.)

I think these measurements only speak for the Yaesu. My PSR-500, Pro-97 and Pro-2006 obviously suffer from interference without FM broadcast band filtering. I think in this respect, it's more of a testament to the ability of the Yaesu to reject interference (in UHF, of course ).

Of course, as I stated in a previous post I'm probably making this stuff up.

 

[gmclam]

If the filters are designed to filter from 88-108 MHz, then I'd like to see data for frequencies outside of the filter range. What happens to signals at 80.0 MHz or 112.0 MHz for example? You should show enough points to see where there is (virtually) no reduction of a frequency with the filter installed.

 

[pro92b]

If the filters are designed to filter from 88-108 MHz, then I'd like to see data for frequencies outside of the filter range. What happens to signals at 80.0 MHz or 112.0 MHz for example? You should show enough points to see where there is (virtually) no reduction of a frequency with the filter installed.

That's what I was getting at but didn't communicate clearly. With all the interference at smokeyjones666's location he is going to have a hard time doing that test reliably. The junky filters I have cause considerable attenuation outside the FM band. I am fortunate to not have many FM broadcast stations nearby and I receive better without the filters that I have on hand.

 

[smokeyjones666]

If the filters are designed to filter from 88-108 MHz, then I'd like to see data for frequencies outside of the filter range. What happens to signals at 80.0 MHz or 112.0 MHz for example? You should show enough points to see where there is (virtually) no reduction of a frequency with the filter installed.

I kinda figured the images from the spectrum analyzer would give a decent idea of what was happening to the interference just below and above the band, you can see a little of this with the images of the analyzer centered at 88.1 MHz and 108.0 MHz in the first post.

That's not the answer you're looking for now though. Also, now that you mention it, pro92b, I think I completely misunderstood your first reply to the thread. I had my head all stuck on reduction of interference, but now I believe you were asking me about attenuation on those frequencies instead.

I could make an attempt to measure the attenuation outside of the filter specifications, I'm sure I can find frequencies in the database to listen for traffic on, but I think any measurements I took of those would be sporadic and unreliable. I might have a ghetto-style solution though: I have an rf modulator that can output channels from 2 to 125, I could see if I can rig that up with a short length of coax and look for attenuation on a small sampling of cable channels near the frequencies of interest. It may not be as good as a dedicated lab-quality signal generator, but it's all I've got.

 

[obijohn]

Your VX-7R is not a "calibrated" piece of test equipment. Actually, it will give quite misleading results.

Stick to "real" test equipment for reliable results that you can reproduce.

 

[smokeyjones666]

Your VX-7R is not a "calibrated" piece of test equipment. Actually, it will give quite misleading results.

Stick to "real" test equipment for reliable results that you can reproduce.

Yeah ok, thanks for the tip.

If you hadn't figured out from the previous posts, I was merely attempting to illustrate the effect these filters have on signal reception with the tools I have available. I tell ya what though, if a rich relative ever dies and leaves me a bunch of cash - I'll make sure to run right out and buy myself a full featured network analyzer.

 

[mtindor]

Yeah ok, thanks for the tip.

If you hadn't figured out from the previous posts, I was merely attempting to illustrate the effect these filters have on signal reception with the tools I have available. I tell ya what though, if a rich relative ever dies and leaves me a bunch of cash - I'll make sure to run right out and buy myself a full featured network analyzer.

Don't pay attention to guys like Obi. I felt your little experiment was pretty enlightening. It may not be 100% accurate sccientific data, but it's more data than 99% of the people (including Obi or myself) have produced for the rest of RR to see. So, I say great job!

Mike

 

[smokeyjones666]

Don't pay attention to guys like Obi. I felt your little experiment was pretty enlightening. It may not be 100% accurate sccientific data, but it's more data than 99% of the people (including Obi or myself) have produced for the rest of RR to see. So, I say great job!

Mike

Thanks for the comment, it makes me feel like some of this effort is at least a little worthwhile. After all, it's not like I'm trying to make an absolute statement about the performance of these filters, I'm just trying to get some idea of what all of the pieces are doing relative to each other - with the janky equipment that I have.

I'm not completely through yet, I was thinking about taking another set of measurements with my Sencore and my rf modulator. I'll probably skip the Yaesu for this one, since I'm trying to get relative dB levels from a local signal source and not a snapshot of a bunch of crap being spewed by some Clearchannel affiliates. I'm not sure if it'll make a difference, but I'll probably try to use terminating resistors on the modulator inputs so the input doesn't vary - doubt it can hurt, anyway.

I know my modulator has certain restrictions. Channels 14-69 in the UHF broadcast band, 67-94 and 100-125 HRC and IRC. Channel 20 UHF would be 157.25 MHz, channel 64 is 462.0231 MHz HRC and channel 125 is 799.25 IRC. So I think I can get a look at what it's doing near 155 MHz and 460 MHz, but I can't quite get close to 855 MHz - which is kind of a bummer, because lately most of my monitoring has taken place between 850 and 860 MHz.

 

[Mike_G_D]

tests

Well, my background is RF - RF in an engineering lab with $100K and $200K and such pieces of test equipment. Given that, I don't have access to that kind of equipment now, in fact, your equipment is better than what I currently have! Heck, I've been sniffing the LO signals from my PRO-197 in an effort to deduce the internal frequency plan (there's a lot more to it than the skimpy specs from the manual, BTW!) by using leakage from the antenna terminals and using my ICOM R7000 and SONY ICF-2010 to do the sniffing!

I think you have done a fine job with what you have! You gave the proper caveat's about your knowledge level and equipment compromises and stuck to going with relative measurements with clear before and after data. That's more than I have seen many others do!

If you know what you are doing and you know your limitations and those of your equipment at hand then you can still provide decent data especially by posting those limitations along with the data and sticking with relative levels (not trying to do absolutes).

As far as I can see, this is what you have done and I applaud you for doing so!

I, too, wish I could get that $150K network analyzer and the $60K spectrum analyzer with all of the active and passive probes and calibration kits, lab quality attenuators, splitter/combiners, directional couplers, and calibrated low loss test cables, etc., etc. all of which I have worked with in the past and have much experience with and knowledge about. But I can't.

Anyway, I say good job!

-Mike