HFU HF Underground

Technical Topics => Equipment => Topic started by: Josh on January 04, 2019, 2351 UTC

Title: Improving scanner performance.
Post by: Josh on January 04, 2019, 2351 UTC
Without climbing onto the roof.

I recently installed a fm trap as well as a 50MHz high pass filter to the scanner, and didn't think much of it at first, as in no night and day difference in received sigs. This is with a BC9000XLT, and old analog scanner that covers mil air and the usual bands. Has a tuning knob, kinda neat.

Both the fm trap and high pass filters were old radio shack items I had laying around, rounded up enough "dapters" and coax bits to install them in the line.

What the fm trap does is greatly attenuate the fmbc band energy while passing the bands above and below the fmbc band unhindered. It's a tiny lump sum affair, with coils and caps inside a metal case, f connectors in and out, no moving parts. If you live in or near a large metro area or close to a fmbc tower, you are being illuminated with megawatts of fm and likely tv power. This will affect your scanner, likely desensing it at least to a degree and placing gobs of rf on every conductor in your domicile.

Another way to picture this is say you are the scanner, listening to every channel programmed into you, but there's several rock, country, and rap groups nearby, all with their amps at 11, and they never stop and you can't ignore them. The fm trap makes them stfu.

The 50MHz highpass filter is a bullet style inline affair with a f connector at one end and a length of rg6 coax out the other. What this does is excise every signal below 50MHz, keeping swbc and ambc powerhouses from saturating the scanner's delicate inners. This is ossum if you live near ambc or swbc transmitter sites and in such cases will likely make a noticeable difference in performance. It should also keep your hf or cb rig transmit power out of the scanner, potentially increasing the lifespan of the rf front end inners. A 50MHz highpass filter isn't likely to do as much for a scanner as a fm trap will, so you know. That being said, scanners typically have at least some highpass filtering built in.

So the proof in the pudding is I started to receive nearbyish HAM repeaters never noted before, at first I thought it was mere coincidence/skip, but no, there's a definite improvement in rx in my case. I also noted a much stronger/longer pass to HAM satellites as well as the usual Russian Cosmos birds, oddly enough.

These filters can be home built affairs, but it's a lot easier to buy them. Radio shack doesn't exist anymore that I know of, and their stuff wasn't that well designed or executed compared to other filters made today anyway. If you wanna role your own, just google filter design and get crazy.

So, if you have any of this stuff laying around, install it and see if it lets you hear a fly scratching his back on Nibi Nibi Island, or that new lunar lander the Chinese placed on the far side of the moon!

Anyway here's some links to places to get these thingys;
(my fm trap looks like the cheap one depicted in the above link)

For the highpass filter, ebay and the like are abounding with them and cheap, typically with 45MHz and up pass, just fine for our porpoises here.
Title: Re: Improving scanner performance.
Post by: kris on January 12, 2019, 0046 UTC
What you describe is important for receivers without input band filters, and especially for the SDR RTL type.
My first good experience began with CB radio, which, through a dedicated splitter, worked together with the AM / FM car receiver on a common antenna without mutual interference. As you wrote, in our listening installations it is worth to try splitters produced for the installation of Radio / TV and cable networks.
   My Icom has only one input socket. Although it has 3 band filters, I have a cable TV splitter installed on the antenna input with useful characteristics for me:
- input for HF antennas      5 -    65MHz          4.5 dB attenuation
                                      87 -  862MHz          8.5 dB
                                    862 -1000MHz             9dB
- input for V/UHF antennas 5 -   65MHz        > 40dB
                                      87-  862MHz             4dB
                                    862-1000MHz          5.6dB
This is not a revelation, are probably better for our needs, but it separates the antennas without switches and protects the receiver from overloading with unwanted signals.
   To split the signal from one antenna into two receivers, I also use TV cable 5 - 1000MHz/3,5dB splitters.
Radio signals from 0.1 to 5 MHz also pass without expressive attenuation.
Without measurements, I do not feel the weakening of signals.
     Look on the internet!