FM Trap For Airband

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ridgescan

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I just want to share a benefit with folks who like to monitor aero in the 108-138mHz band who may not know of this like I didn't.
I made good on advice from member "wise871" to get an FM trap from Radio Shack, and I am amazed at how dramatically the airband has opened up on my bc785d. I run it on a Diamond d130j discone on the roof. Oakland airport is 15 miles from me, San Francisco airport is 12.5 miles from me. Before the trap, I literally only got the approach/departure frequencies, and at that mostly the aircraft.
With the FM trap installed inline right short of the back of the scanner, I now get all including ground at both airports.
I even get an ATIS station out of Travis AFB at 49 miles-which to me is impressive for airband (especially my location in western SF)
Guys-$7 bucks and change for this. I think everyone should get one:)
 

benbenrf

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ridgescan

......... and depending on the quality of your coax and how long it is, you could experience even greater improvement moving the trap from behind the receiver to the base of the antenna.

Your experience emphasizes just what a big difference to reception, filtering before the receiver, can make to the quality of rf demodulated through many of todays receivers.
 

prcguy

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The placement of a passive filter in line makes no difference if its at the antenna or at the radio, the result is exactly the same.
prcguy

ridgescan

......... and depending on the quality of your coax and how long it is, you could experience even greater improvement moving the trap from behind the receiver to the base of the antenna.

Your experience emphasizes just what a big difference to reception, filtering before the receiver, can make to the quality of rf demodulated through many of todays receivers.
 

737mech

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Fm Trap

I agree with prcguy,should not make a difference at the antenna or at the scanner. I have read other posts about how long your coax is and maybe picking up noise along the run etc, I have never tried at the antenna but I have read enough about what the trap does to agree with prcguy. Certainly trap before any amp use. My experience with the RS trap lead me to a better trap from PAR electronics. The results were better for me because I live in a rf noise area. The whole idea as Dale Parfit will help you with is finding the actual problem source rf and filtering it out. Then the scanner will open for what you want and not the noise.
 

nmelfi

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I agree with prcguy,should not make a difference at the antenna or at the scanner. I have read other posts about how long your coax is and maybe picking up noise along the run etc, I have never tried at the antenna but I have read enough about what the trap does to agree with prcguy. Certainly trap before any amp use. My experience with the RS trap lead me to a better trap from PAR electronics. The results were better for me because I live in a rf noise area. The whole idea as Dale Parfit will help you with is finding the actual problem source rf and filtering it out. Then the scanner will open for what you want and not the noise.
+1 for Par and Dale. I used a RS FM trap on my BCT15 and it helped a little. Then I ordered a FM trap from Dale and wow did it open up the radio. I liked it so much I called back and ordered a NOAA weather filter that he tuned to my local frequency and put them both inline and now here at least double what I could lock before. Great products and great support, he is not afraid to call you and find out exatly what you are looking for when you email him.
 

nanZor

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I also used the RS FM-Trap to point me in the right direction to a PAR.

Note that most of the modern RS FM traps have notches well above the center of the FM band, and if you have a strong fm broadcaster in the lower half, about the only thing the rs filter will do is attenuate by just insertion loss.

Sometimes, that might be all you need.

I have a flamethrower down around 88.5 mhz, and the RS trap didn't even touch it, aside from a few db of insertion loss.

A tip from a review on Eham about these units got me to wind new input coils around a small drill bit coil-form to try and drop the notch more towards the center. It worked ok, and I even wound another coil to put the notch very low in the band, but it wasn't enough as the notch wasn't deep enough.

The RS trap is great if it solves your problem, just remember that they place the notch in the upper end of the fm band, so it might only serve as a pointer to get something with much better specs like a PAR if at first the RS unit doesn't seem to work.
 
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Qwerty7

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Ridgescan,
Dittos on the FM Trap results on VHF Airband. I am a stones throw from SFO and I could hear a/c clearly
but not the Nor-Cal Approach Controllers. With the FM Trap I now hear both.
I had no idea I was getting hammered by paging systems here but I was big time--huge improvement.
I strongly reccomend the FM Trap if you are running a Discone from your Scanner. The price is sure right.

Mary
 

ridgescan

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Glad to hear that Mary:) it definately did wonders here. Now the scanner that performed the worst of all my scanners on airband has become the best airband getter I ever owned lol!
 

Qwerty7

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Ridgescan,

I have a Band Pass Filter for Airband. Do you think I can use in in conjunction with the FM Trap or
not ?. If so, what order should they be in relative to the Scanner ?

Thanks,

Mary
 

ridgescan

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I don't know Mary-but you can give it a try and see what it does. I am surprized the antenna pros in here haven't answered you yet-
I would say it should be "antenna->FMtrap->bandpass filter->radio"
 

prcguy

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I bought a couple of these off Ebay when they were $15 and they are the best complete FM band elimination filters I've seen. I swept them and don't remember the exact specs but they have extremely sharp skirts around 87 and 109MHz and wipe out the entire FM band by about 30dB.

These occasionally come up from different sellers at much better prices and the only slight drawback is the TNC connectors.
prcguy

These seem like good quality fm band notch filters.

Anyone ever try these?

eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices
 

W6KRU

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The bandpass filter should block the FM band so using it with the trap is redundant and the insertion loss will cause some loss on the desired signal. Unless the bandpass filter isn't capable of completely removing the FM band for some reason, just use it alone.
 

Qwerty7

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I can report the Band Pass Filter used in conjunction with the FM Trap is a big success.
Where my reception of Nor Cal Approach was greatly improved using the Trap alone, it is even better
with the Filter. I kind of laugh now at how loud SFO Tower comes in as well as the controller on Woodside
Sector. Pretty amazing.
As for the downside, it does deafen reception on 800 Mhz badly as well as 400 Mhz but i expected that.
150-155 mhz where I listen to San Mateo County Fire is hissy but improved with Attenuation shut off.
I can easily remove the Filter and it's all good. A small price to pay to get completely revolutionized VHF
Airband reception. I bought the Filter at ScannerMaster.

Mary
 

fineshot1

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I bought a couple of these off Ebay when they were $15 and they are the best complete FM band elimination filters I've seen. I swept them and don't remember the exact specs but they have extremely sharp skirts around 87 and 109MHz and wipe out the entire FM band by about 30dB.

These occasionally come up from different sellers at much better prices and the only slight drawback is the TNC connectors.
prcguy
Thank you - i was thinking the specs and supplied sweep trace looked pretty good.

I may wait for a cheaper price as well as my need is not
immediate.
 

SCPD

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I know this is an old thread but i also just got the FM trap from radio shack and put it on my 996t and WOW big difference, Thanks ridge.

Mike
 

SCPD

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Maybe its just me but it seems the trap opened up most of the public safety band 400-480 as well as the airband.
 
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