When someone asked Kitz support about FM traps, Jeff Kitz recommended a specific FM trap from a US store. A dealer in Ottawa decided to carry those based on that recommandation. I could look up details if anyone thought it was useful for the thread, i think i got some details in an email since i got that same trap now in case I decide on using my KT200.

If that is the store where we ran into each other, then I suspect it is the FT-7500 as it is the only FM trap he has in stock. As I said before, this one is not appropriate for use in Ottawa as it will interfere with Channel 6.

The options for band shifted FM traps are limited. You often get old Radio Shack FM traps on a popular online auction site (I bought a batch of them and was selling them at cost, but I don't have any spares left).

Tinlee will make one for you, but they are expensive any you may be better off buying a CM-7777 that has one built in.

Other than that, I don't know of any other band shifted FM traps available.

Yes that is the store where we met... which was quite a coincidence...
I don't think there was any Winegard or FT-7500 marking on it, I do think it's another kind but I have no clue who makes it because when I got mine I could not recognize who manufactured it. It looks very generic, and if Mr Kitz himself did not recommend it, I doubt anyone would have paid any attention to it.
I could take pictures I guess ?

After some more checking and upon seeing actual reviews of that trap (called FM-88, a different one than the 7500 but with same effects) it looks like it will kill channel 6, even maybe channel 5 as well. So we're back to square one.

I think Mr Kitz's recommendation was in the context of an overloading KT200 because of FM stations, without taking into account channel 6 reception.

When I bought my KT100VG Kitztech amplifier, it came with an FM trap included in the package. The only marking on it was "FM 88."

I'm using it now and my analog CBC on rf5 still comes in very strong (I'm four miles from the CN tower). I'm using the wrong antenna for it though.

Any effect it would have on channel 5 (especially analog) would likely be marginal, but channel 6 would be a different story. If it was DTV on channel 5 it might be different though since a slight roll-off at the upper end could confuse the decoder. I don't think channel 5 will be used for DTV anywhere in Canada though (AFAIK only channels 2, 4 and 6 for VHF-LO). It is used by a few stations in the US.

The "new" Radio Shack FM Trap, 15-0024, is a fair compromise between the need to preserve channel 6 while attenuating as much FM as possible. I tested one a while back, but I can't find my plots at the moment.

In comparison, our FM trap absolutely will affect Channel 6 while the "old" Radio Shack Trap cuts in at a much higher frequency as shown in this plot:

http://i1203.photobucket.com/albums/bb390/AntDirect/FMFILTERVSRS15-377.gif

Hi guys , first thanks for all the great info!

I have to let it seep into my thought process to let it all make complete sense.
Then I'll try some re-combinations ...

After some more checking and upon seeing actual reviews of that trap (called FM-88, a different one than the 7500 but with same effects) it looks like it will kill channel 6, even maybe channel 5 as well. So we're back to square one.

I think Mr Kitz's recommendation was in the context of an overloading KT200 because of FM stations, without taking into account channel 6 reception.

I send Kitz Tech. an e-mail about band shifted FM traps for receiving channel 6 and his first response was:
FM-88 is a pretty good trap and it does not go too much into channel 6.
After questioning this and asking for measurements, he said his results were:

80 mhz = -3.5 db
88 mhz = -28 db
Not what I would call "not going too much into channel 6." :o He did say you could change the response by bending the coils, but that is tough to do without a spectrum analyzer.

Apparently there used to be a sister to the FM-88 (at least the one made by PicoMacom/Tru-Spec) called the FMT and it was band shifted to reject 95-108 MHz, but I haven't been able to find one anywhere (though I did find a catalog page here (http://maiteymario.net/mario/work/PSG/catalog/PICO/catv23.pdf)).

The good news is I discovered that Radio Shack apparently does still sell the 15-0024, so that may be a good option if you are traveling down to the US.

Other than that, I don't know of any other band shifted FM traps available.
The Winegard FT7600 Two Stage Variable FM Trap still seems to be available.
Two stage variable FM trap attenuates stations in the FM band (88 to 108 MHz). When both stages are tuned to a single frequency, it will be attenuated 26 dB or about 1/20 of its previous level. When each stage is tuned to a different frequency, it will be reduced from 7 to 10 dB or 1/2 to 1/3 of the original level. Used to reduce strong signals from local FM stations and prevent overload of amplifier and TV sets. Unit is enclosed in weatherproof housing for mounting on antenna boom, mast or wall. Will pass power to antenna-mounted preamplifier. Mounting hardware, F-connectors included. (Tunable traps require the use of a scope, analyzer, or signal level meter to tune accurately) But there is still the problem of tuning it properly.

The Winegard FT7600 is a sharp notch (actually two of them) not a band shifted FM trap. A sharp notch will let you take out one specific FM station, where as an FM trap will filter out all (or most if is band shifted) FM stations.

As you said, tuning a sharp notch is always fun, but it can be done by connecting it to an FM receiver tuned to the station you want to notch out and find the place where reception is worst. Not a accurate as a spectrum analyzer, but it can work.

The top end of CH6 is 88 MHz. Even if you shift the FM filter attenuation curve higher to keep from harming CH6, you will expose any strong FM signals at the low end. This is also true about the no longer available FMT. (http://www.kyes.com/antenna/interference/fm/fmfilter.html) I don't know of any sharp "brick wall" filters that would have a steep enough curve at the low end. Sounds like a custom filter job to me.

Using the opposite approach, maybe a CH6 bandpass filter, but I haven't seen the list of VHF-lo channels needed, only CH6?
.....but it can be done by connecting it to an FM receiver tuned to the station you want to notch out and find the place where reception is worst.That might work.

a usable filter may be doable if someone wanted to try building their own,
for the fun of it.

http://www.rfdh.com/rfdb/bsf.htm

select chebyschev,
plug in order=7, fc=98 MHZ, BW = 20 Mhz, Zo=75 ohms, and ripple = .2 dB

Similar component values using qucs, an rf simulation program for linux (might be a win32 version also?).

synthesised schematic from qucs
http://www.wuala.com/whosurbud/Documents/Pictures/chebyschev_7thorder_fmbandstop_schem.pdf

simulated in qucs
http://www.wuala.com/whosurbud/Documents/Pictures/chebyschev_7thorder_fmbandstop_plot.pdf

ADTech, your plot in post #1267 (http://www.digitalhome.ca/forum/showpost.php?p=1255180&postcount=1267) says you are analyzing a Radio Shack 15-377. I can't find any reference to this part, but I have a 15-577 (I found the frequency response specs for it here (http://support.radioshack.com/support_tutorials/video/faq-1500577.htm) BTW), is this a typo?

Also, from your memory, which Radio Shack FM trap is better at protecting channel 6 (old or new)?

rabbit73, I am fully aware that channel 6 is adjacent to the FM band, so band shifted FM traps won't significantly affect FM stations close to 88MHz. Fortunately the regulators know about this problem and regulate the power of low frequency FM stations within the protected contours of a channel 6 station so they won't interfere with it. Unfortunately with crowding on the FM band they have been pushing the limits recently, so there are cases where a sharp notch or two are required.

As for using a channel 6 band pass filter, it should have similar characteristics as a band shifted FM trap on the FM band. It will block all other channels (not just VHF-LO) though, so it would only be useful if you have a separate antenna for channel 6. That is what I will be doing, but many others won't

For those who don't need VHF-LO at all, I would think a high pass filter tuned to pass everything above 174 MHz would be better than an FM trap, since not only will it block FM, but it will block amateur and aviation radio signals.

GeoStar:
The pic below is how I think it should work with a join - a -tenna . Is this the way to go , or is there something better ? everything works great right now , just am greedy for 6 ....A CH 6 JoinTenna contains a CH 6 bandpass filter, but it is a simple one that was never meant to reject adjacent channel interference. So also with a broadband FM trap, shifted or not. A dedicated single-channel CH 6 bandpass filter is better able to reject adjacent channel interference because it is a more complicated design with many poles giving steeper skirts.
For those who don't need VHF-LO at all, I would think a high pass filter tuned to pass everything above 174 MHz would be better than an FM trap, since not only will it block FM, but it will block amateur and aviation radio signals. That's what the "H" of a HLSJ is designed to do.

rabbit73, to get a CH 6 bandpass filter with many poles giving steeper skirts you would likely have to get a commercial filter (though this is likely true anyway since I have never seen a consumer grade CH 6 bandpass filter). These are not cheap, but may be a good alternative to sharp notches for specific low frequency problem FM stations.

I would say try using a Join-Tenna or band shifted FM trap as they are much cheaper and if you are still having problems, think about getting a CH.6 bandpass filter (such as the Tin Lee CFAL-ch.6).

Another compromise would be a commercial grade band shifted FM trap (such as the Tin Lee CR7 CH.FMU/6) that has tighter skirts than a consumer grade one (it is speced to attenuate 88MHz at 2dB and 92MHz at 35dB). It should be significantly cheaper than a CH.6 bandpass filter.

I would say try using a Join-Tenna or band shifted FM trap as they are much cheaper and if you are still having problems, think about getting a CH.6 bandpass filter (such as the Tin Lee CFAL-ch.6). I agree; GeoStar should try the least expensive ways first. A lot depends upon what his fmfool report looks like. I haven't seen it or his tvfool report. If his desire for CH6 was strong enough for him to call Tin Lee, then the engineer that he talked to would want to know not only what TV channels he wanted, but also what signals he needed to reject, for a cost-effective solution.

He should try the Join-Tenna first, then the Winegard FT7600, and then both if needed. He could use the Join-Tenna CH6 input bandpass section with a HLSJ for 6 and VHF-Hi.

AFAIK, FMFool has not been updated to use Industry Canada data, so its results are not very good for Canadian stations, so I wouldn't rely on it.

Roger,

Yes, that is a typo. It should indeed be 15-577. Thanks for pointing it out.

The older filter cuts in much later (at a higher frequency) than does the newset version.