BC785D - Need fix for Intermod?

[WD4CQ]

I have owned (2) Bearcat BC785D Radio Scanners for over 6 months now that I really enjoy, However I get lots of intermod. I am using the scanner as a base with an outside discone antenna.

Question? Does anyone have any experience or success using these filters
below on the BC785D scanner and if so, which one? If I had a handfull of filters in my possesion, FM band, Business/Paging band and different brands, I could try them out and simply know which ones worked and which band to filter out. I do have a powerful FM station in the area on the low end of the FM band, also a powerful NOOA station on 162.4 Mhz and the common 152 (somewhere) Mhz paging freqs in my area. I love the BC785D but the intermod is driving me nutz! :? Any experience from anyone with a BC785D?

Filters for Radio Scanners:
http://www.radiofilters.com/
http://www.stridsberg.com/prod02.htm
http://www.grove-ent.com/filters.html

 

[WD4CQ]

After doing careful monitoring, these are freqs causing my intermod

152.480 very strong paging
158.700 very strong paging
162.400 powerful NOOA
88.5 FM strong
89.3 FM very strong

also I have found the following info after much research, looks promising: :lol:

http://www.eham.net/reviews/detail/3086
http://www.parelectronics.com/scanner.htm

any comments helpful??

 

[LarrySC]

I live in RF Hell. Almost under Paris Mtn N of Greenville where most radio towers are located. I use a 10" all band channel master antenna. My two [2] 780's and one [1] 796 are not bothered by intermod because I use PL/DPL on almost every frequency. Those freqs without PL some of which are RailRoad, GMRS, etc are ATT'ed. Any freq that is bothered by intermod should be PL'ed. If you dont have all the PL tones in your area, post on your STATE forum with a request for that info. Good Luck

 

[ReceiverBeaver]

And if the pl or filter option isn't good for you, then consider upgrading to a scanner or receiver that is Triple Conversion in the receiver scheme. Most scanners are only double conversion or worst yet, single...and they will be the most prone to intermod.
Good luck

 

[INDY72]

Umm Beaver..... Almost all modern scanners are triple conversion, and some are in the works with quad conversion.... he is using a Digital Capable scanner.. thus it IS a triple conversion scanner. Also there is NOT a scanner made that has CTCSS/CDCSS, and NOT triple conversion. The only scanners that run single conversion are the true antiques. There are still a few that run double.

 

[Tweekerbob]

Intermod FIX

dgarris,

Recently a member, HowD, had a similar problem and posted this: http://www.radioreference.com/modul...amp;postorder=asc&highlight=coaxial+stub

As you will see, he used a FM notch filter from radio shack, similar to the one you found at radiofilter.com. In addition, he created 1/4 wavelength open coaxial stubs to notch out the offending signal that fell outside of the FM broadcast band.

These are VERY simple and SUPER CHEAP to make. All said and done, you might be out 10-15 bucks for the stubs and another 20-30 bucks for the fm notch filter. Compare this to $70 per INTERMOD filter (and I bet you will need three of them $210).

A few points I will add to that post to help you out. First, try and use the best coax you can find. By best, I mean the one that has the lowest amount of loss (attenuation). This will provide you with the deepest notch, meaning that it will do a better job of filtering out the offending frequencies. At the very least, use RG-8/X. Although, I have used RG-59 with success in the past, it sounds as if you might need the most amount of reduction of the offending signal as possible. SAT RG-6 (quad shield) might work well too. However, if you are not too handy with a soldering iron/gun and not too familiar with attaching RF connectors to coax, another idea would be to buy a few 3ft. BNC patch cables and cutting them to the approriate lengths. In all cases, please make a note of the velocity factor of whatever coax you end up using, as this is required for determining the proper length of the stub.

I am not familiar with your scanner. If it has a signal meter, this will be helpful in fine tuning the stub as discussed in the aforementioned post.

Note that one stub is required per offending frequency (at least in your case because they are several MHz apart). I would suggest, that you use the FM trap (notch) filter in conjunction with one or more of the coaxial stubs to reduce or eliminate your intermod problem.

If you use good quality coax and you follow the directions for fine tuning to a T, you should be able to rival, if not exceed, the performance of the the $70 filter and save yourself some cash.

Although there are a few different type of stubs out there, this one is fairly simple and straightforward. I have used one to notch out a paging frequency that was plaguing my scanner with intermod.

Hope this helps.
Ryan

 

[pro92b]

Also there is NOT a scanner made that has CTCSS/CDCSS, and NOT triple conversion.

There are dual conversion scanners with CTCSS: PRO-2036, HS200, MS200, BC760XLT, and BC890XLT. Some of these use optional CTCSS boards and only the HS200 and MS200 have DCS too.

The BC785 is triple conversion as you noted.

Stub filters will generally be lower Q than the PAR filters and will create more loss at frequencies adjacent to the notch point. The referenced thread detailed a stub filter for TV channel 12 which is at 209 MHz. This is well outside the usual scanner bands and the wide notch of a stub filter is not a problem. Using stubs at 152, 158, and 162.4 MHz simultaneously would probably reduce reception across much of the VHF HI band.

 

[Tweekerbob]

The theory behind the 1/4 wave coaxial stub predicates it as being narrowbanded. Typical results will yield a notch close to 500KHz but no more than 1 MHz.

In practicality, the stub I used to filter out 152.840 had no measurable amount of attenuation (in dBm) to any of the frequencies I listen to (over 40 different channels) in the 151 and 154 MHz range. What is significant is that if properly done, the notch will be DEEPER (certainly not wider, in fact narrower) than the PAR units by as much as 10-15 dB.

The fact is, both the coaxial stub and the PAR units will yield relatively similar results in both the width and the depth of the notch.

As long as the stubs are properly tuned, you should have little to no attentuation of desired signals. Certainly no more than the PAR units.

 

[pro92b]

Your results are far better than what is reported by others. See the articles below. What did you do differently?

www.radagast.org/~dplatt/hamradio/K6OIK-filters.pdf
http://www.qsl.net/w1gmf/tip11.htm
www.qth.com/ka9fox/coax_stubs.txt

Graphs for the PAR filters are at http://www.grove-ent.com/pargraphs.html