How to count and identify carrier frequencies?

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snn47
Posts: 213
Joined: Tue Dec 27, 2016 11:00 pm

Re: How to count and identify carrier frequencies?

Post by snn47 » Wed Mar 15, 2017 2:45 pm

For your use it it is not important to have a precise value of attenuation to the tenth of a dB like it is necessary for measurements, but that the Pi or T Attenuator used, which consists of 3 resistors, matches the impedance of your cable/receiver/antenna.


An ATT overview and explanations can be found here http://www.radio-electronics.com/info/r ... values.php

A DIY design proposal for a multiple stage ATT from QST here https://www.arrl.org/files/file/Technol ... nuator.pdf which should work up to VHF/UHF, since it makes more sense to be able to switch the attenuation on/off than having to unsrew the attenuator.

More guidance and resistor tables for 50 and 75 Ohm here https://translate.google.co.nz/translat ... edit-text=

An ATT calculator to calculate the required resistor values and optimize them to available resistor values and tolerances is here http://chemandy.com/calculators/matchin ... ulator.htm

There are also costly variable Attenuators available which cannot be built by yourself.

However for satellite IF reception purposes cheap variable 0-20 dB are available, which are unlike the high quality ones for labarotory use, not designed to be adjusted to often e.g. https://www.amazon.de/SAT-D%C3%A4mpfung ... B0020JC69C.

hotpaw2
Posts: 127
Joined: Sat Jan 14, 2017 11:07 pm
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Re: How to count and identify carrier frequencies?

Post by hotpaw2 » Wed Mar 15, 2017 4:51 pm

lsmod wrote:The source of the interference would be interesting.
Any idea how to locate them with simple tools?
If you are using a USB RTL-SDR, the simplest tools to start with to find the source(s) might be 2 cables and some ferrites.

Use a USB extension cable to move the RTL-SDR as far away from any computers, power supplies, cables, and other electronics as possible. Then use a coax to move the antenna even farther away from everything (even to out the window and up high if possible). Put some snap-on ferrites around both cables to see if that helps.

If the interference is reduced, then move the antenna closer to things in the room one at a time until you see your familiar looking noise spikes re-appear in the spectrum. When you do, you've located the interference.

If you do find it, let us know what it was.

lsmod
Posts: 7
Joined: Sun Mar 05, 2017 3:32 pm

Re: How to count and identify carrier frequencies?

Post by lsmod » Thu Mar 16, 2017 9:25 am

Thank you for the mass of tips!
Now i must see to use them ...

1. I am using an RTL-SDR with additional antenna amplifier and an up-converter for frequencies below 70 MHz.
I built this up here: http://www.darc-husum.de/sdr-konverter.html

2. So far the antenna is simply an wire of about 1,50m
Up to now i did not find the time to build up an balun for it. http://www.darc-husum.de/balun.html
It is planned to built this broadband antenna too http://superkuh.com/spiralantenna.html

3. The measured "effects" seems not to be local because
3.1 Some weeks ago they where not existant.
3.2 The effects are independent from the PC, because i can measure them the same way with another PC system.
3.3 I can measure these effects outside mobil, driving to different places miles away far outside of a city. The USB-cable did have an ferrite.

Any ideas how this signals of the screenshots i posted first can be explained?


I am not familiar with radio engineering - so i must build up my knowledge. ;)
For the low frequency below 10 MHz i must use a long wire antenna or a magnetic loop correct?
I should use a balun for impedance correction?

What's about the measuring of high frequencies at 60, 120, 240, 480 MHz so far?

The antenna amplifier is a MAR-6SM now with about 20 dB gain.
So no gain within the stick is needed so far.


P.S. What must be done that the notify for new posts will work - everything is activated but i don't get any message?

lsmod
Posts: 7
Joined: Sun Mar 05, 2017 3:32 pm

Re: How to count and identify carrier frequencies?

Post by lsmod » Thu Mar 16, 2017 10:15 am

snn47 wrote:However for satellite IF reception purposes cheap variable 0-20 dB are available, which are unlike the high quality ones for labarotory use, not designed to be adjusted to often e.g. https://www.amazon.de/SAT-D%C3%A4mpfung ... B0020JC69C.
Good and simple idea.
I buyed one http://www.ebay.de/itm/201116947429

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