Seems to be on multiple frequencies across the band but it’s the same broadcast. As stated above, it could be false images due to the sheer strength of the signal.
Does such a thing exist?
- Thread starter RDowson
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The above (by Bob) was what I was going to post. It consists of a 1/4w electrical (not physical) length of coaxial cable, spliced into the feedline via a T connector. You'll need a piece of the same type (impedance) as your feedline and you'll need to know its velocity factor so you can calculate the physical length for the frequency of the offending station. The end opposite the T is trimmed to expose center conductor and shield; these are shunted by an air-variable capacitor. I'd start with a 2-15pF unit for this application and tune it for minimum interference.
Another option is to get a tunable duplexer set of the type used for 144-148MHz ham radio repeaters and connect one or more of its cans inline. Some of them can be adjusted a good bit below the 2M band. You'll want to connect the band-stop (reject) section and leave its band-pass counterpart out of line unless you want a narrow effective reception range. A nice thing about this approach is that if you have a 3-can setup and 3 interfering frequencies, you can tune one per offender with minimal effect on things you want to monitor.
Another option is to get a tunable duplexer set of the type used for 144-148MHz ham radio repeaters and connect one or more of its cans inline. Some of them can be adjusted a good bit below the 2M band. You'll want to connect the band-stop (reject) section and leave its band-pass counterpart out of line unless you want a narrow effective reception range. A nice thing about this approach is that if you have a 3-can setup and 3 interfering frequencies, you can tune one per offender with minimal effect on things you want to monitor.
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So looking at the above article, what would I need to do to attenuate by, say, 5db?
I’m using Webro WF100 with a velocity ratio of 0.81 and the frequency is 125.725 MHz.
Constructing a notch filter is easy. Tuning it correctly, not so much. Your best bet is to use a VNA (such as the NANO VNA) with the T connected between ports 1 and 2, with the freq range set so you can see the filter tuning--say the desired notch freq +-5MHz. Then you can snip a bit off the end of the coax stub or adjust the duplexer can and see what effect you've had in real time by watching the S21 trace.
majoco
Stirrer
If I understand correctly, you want to reduce the aircraft band sensitivity but still retain everything else, but you would like the ability to bring back the aircraft band occasionally. You get an aircraft bandpass filter, put in a Tee piece in your antenna feed line, then connect the input of bandpass filter from one leg of the Tee and the output to ground. This should shunt all the aircraft band signals to ground but leave everything else. If this works satisfactorily, install a variable attenuator in the feed from the Tee to the filter. It can be an expensive HP 355D attenuator or a simple 10 or 20dB switched attenuator you can make yourself.
Here's the gotcha:
The lower depth the notch is, the wider the skirts (by virtue of circuit "Q") and more of the desired signals either side of the undesirable will be attenuated. A minimum figure of 27-30dB single-signal rejection is attainable with good-quality coax and connectors; more with high-grade line or with a tuned cavity. Remember the higher the Q, the narrower the response curve and the deeper the notch.
I'd park said notch right on the offender and make it as deep and narrow as possible. If your math is good, you should be able to calculate, cut and tune without the use of a network analyzer but if you have ready access to one by all means use it.
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I have an SDR dongle so I assume that would allow me to test it?
What length do I need to cut the coax though? That’s what I’m not sure about
This calculator should make it easy - just plug in the frequency of the station and use the 0.80 velocity factor in the drop-down. That'll give you the 1/4w stub length in both feet and CM: Online Wavelength Calculator
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Thanks!
Looks like 47.7cm then. Is that the length of the cable before the connector is added to one and and the other end is stripped back?
Do I just short the connection between the core and the shielding?
Finally, do you need to cut it to the 1/4w length or can you use any of the listed lengths?
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Well spotted. My mistake. Should be 125.725MHz
Edited the post now. The questions about length and stuff still apply.
majoco
Stirrer
Any odd multiple of a quarter wavelength but there is no advantage of anything other than a quarter -and leave the end open circuit. Make it too long, say 50cm and then trim off 0.5cm at a time. Unfortunately you will go past the 'sweet spot' by one snip, but you could always make another one now that you have the proper length.
I thought it should be shorted end? What’s the difference?
If it’s open end how do I strip the cable? Is there a certain way it should be done?
That's why I indicated the use of a variable capacitor earlier in the thread - you can tune the notch point a bit.
Let me ask what may be a bad question: Wouldn't it be easier/cheaper and possibly all around "more better" to get a second scanner? Let the first one do everything but the offending "something" and the second do the offending "something" which can be dealt with by antenna placement/modification?
I have a second scanner. One does UHF and the other does VHF with an attenuator connected to it. I was trying to reduce the need for 2 antennas and 2 scanners by finding a solution that would work with just one scanner.
Ubbe
Member
You can try a stub filter but I would suggest using it with a $20 diplexer so it wont affect UHF.
Connect the diplexer to the antenna coax and then put the stub filter only on the VHF out and then connect the VHF signal to the UHF to get one coax to the scanner. Monitor the signal strenght from that VOR frequency and make the stub a bit longer than calculated and cut small peices off it until you get max attenuation. If you use cheap RG6 and twist on connectors it will be easy to start over with a new coax if you cut off too much.
Stub filter
Diplexer
/Ubbe
paulears
Member
It might be worth seeing if you can find (or make) a VHF notch filter similar to those in repeaters - you could use this to remove the transmission from that close source, and they have little attenuation just a channel or two away? Three cavity ones do need some kit to assist tuning but are great for exactly what you need -- second hand ones appear from time to time for the old 136MHZ mid band system, and could be modded to go a bit lower.
majoco
Stirrer
An open circuit at the end of a quarter-wave stub puts a short-circuit at the other end - you have effectively joined the centre conductor to the braid at only one frequency. Not quite true - there will be another higher frequency where the stub is 3/4 of a wavelength and 5/8 and 7/8 and so on ad infinitum.
Why do you want to strip the cable? If you want to seal the end just get a couple of inches of heatshrink and put the cut end one inch into the heatshrink - shrink it, then fold over the empty bit - slip another one inch over the folded end and the cable and shrink that on to seal it.
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