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Overloading problem solution
http://www.interceptradio.com/bbs/viewtopic.php?f=1&t=7773		 Page 1 of 1
Author:  Jim [ Sun Sep 28, 2014 12:11 pm ]
Post subject:  Overloading problem solution
About two weeks ago I connected my discone antenna up to my RS PRO-197 scanner.

I noticed a fair improvement on UHF but on VHF the s-meter was always pegged and I found after a frequency on VHF was received that the squelch would stay open and could not be adjusted to have any effect.

By adding attenuation on each VHF frequency that problem was somewhat less. But what a pain. I could only receive one NOAA weather channel.

Did some research and found that adding a FM trap in line with the antenna coax that now the s-meter was no longer showing an indication when no signal was present. Now I can run all VHF frequencies with no attenuation switched on. I can now receive 4 NOAA weather channels. There is very little degrading of the signal strength on AIR/UHF/800 bands with the trap in line.

If you are having problems with overloading it is worth trying the FM trap. It was very cheap and sure works great for me.

http://www.radioshack.com/product/index ... d=10939219

Attachments:
RS FM TRAP.jpg
RS FM TRAP.jpg [ 5.86 KiB | Viewed 2626 times ]
Author:  nickcarr [ Sun Sep 28, 2014 6:47 pm ]
Post subject:  Re: Overloading problem solution
Jim, it's a cheap solution but not sure how effective it will be in the long run. The idea of how this works is that inductor coils notch out a portion of the FM broadcast band. The generated insertion loss is an attenuation of RF signal. So in effect you really did just add an attenuator to your antenna.

Like me, you're probably picking up a lot of paging interference which is creating the intermod you hear. (It doesn't help that most handheld scanners were meant for use on rubber duck antennas either!)

My location in SE Bellevue is a magnet for RF. All of my external antennas have to use proper filters or I'd be in a world of hurt. The paging interference is incredibly high here. I use a PAR filter VHF TN152-158... it's pricey but well worth the cost.

http://www.parelectronics.com/amateur.php

http://www.parelectronics.com/vhf-tn152-158-specs.php

Image
Author:  Jim [ Sun Sep 28, 2014 7:45 pm ]
Post subject:  Re: Overloading problem solution
For $4.99 it works pretty well. I can receive WSP from Bremerton and Island County Sheriff here in Bellingham.
There are a couple of FM transmitters located about 1/2 mile away from me on my hill so that is probably why it works so well for me.

Attachments:
RS FM TRAP GRAPH.jpg
RS FM TRAP GRAPH.jpg [ 56.37 KiB | Viewed 2605 times ]
Author:  Jim [ Sun Sep 28, 2014 8:39 pm ]
Post subject:  Re: Overloading problem solution
That PAR pager trap looks like it attenuates over 10dB from 150 to 160 MHz! Most of the VHF High band less 2 meters.
The RS FM trap attenuates about 4.5dB above and below the FM band.
Author:  nickcarr [ Mon Sep 29, 2014 4:32 pm ]
Post subject:  Re: Overloading problem solution
Jim wrote:
That PAR pager trap looks like it attenuates over 10dB from 150 to 160 MHz! Most of the VHF High band less 2 meters. The RS FM trap attenuates about 4.5dB above and below the FM band.


Yes, it's doing its job. It's knocking down those pager frequencies.

10dB isn't exactly a lot of signal. When you're getting interference, this is about 1.5 s-units. For an external antenna, this is negligible.

The FM trap is OK but you're not even getting a s-unit of reduction. I'm surprised it's making that much of a difference. If you have heavy WFM interference then a s-unit isn't that much of a reduction...
Author:  Jim [ Mon Sep 29, 2014 5:08 pm ]
Post subject:  Re: Overloading problem solution
The FM trap rejection is -6dB @ 89MHz & -20dB between 100 & 108 MHz.
Loss between 5 & 87 MHz is -5dB and between 125 & 890 MHz is -4.5 dB.
As per the back of the pack it came in.
Author:  nickcarr [ Mon Sep 29, 2014 5:29 pm ]
Post subject:  Re: Overloading problem solution
Jim wrote:
The FM trap rejection is -6dB @ 89MHz & -20dB between 100 & 108 MHz.
Loss between 5 & 87 MHz is -5dB and between 125 & 890 MHz is -4.5 dB.
As per the back of the pack it came in.


Ah okay sorry I misread that then... -20db should be respectable.
Author:  icom1020 [ Mon Sep 29, 2014 6:16 pm ]
Post subject:  Re: Overloading problem solution
I thought it overloaded on 800 mhz as well, driving anywhere near a cell tower would cause desense. I just got used to pressing the ATT button and still hear any system within -- miles of a TRS tower. I think the GRE people knew they had a hot front end receiver.
Author:  Jim [ Sat Oct 11, 2014 11:06 am ]
Post subject:  Re: Overloading problem solution
I have installed a bypass coax switch so I can bypass the FM trap if necessary.

From Bellingham I can receive WSP in Whatcom, Skagit, Snohomish, Jefferson, & N. King Counties & Bremerton with the discone and FM trap in line.
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