w6act
Member
Thanks for the tip. One more question, I read that the low noise amplifier should be mounted as close to the antenna as possible. Is that true?
Alex
Alex
LMR 400 OR RG6 coax
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That is best practice, but not mandatory as sometimes it is not practical due to the amp design. Depending on what you're trying to monitor, it may not be needed either. For example, local PD/Fire etc. using a repeater that booms in to your location obviously does not require an amplifier. If you are trying to monitor distant or weak signals, whether they use a repeater or not, an amplifier would be helpful.
* Note: An amplifier can also cause problems as it can increase interference from powerful local FM broadcast stations, pager transmissions, NOAA, local PD/Fire or whatever. As everyone's location is different, as well as their hobby needs, solutions can be different. For example, I listen to military aircraft which can be quite distant. I use an inline filter on the coax that mostly passes 225 - 400 MHz, which is the range I want to hear. This filter works well enough that when I TX on a nearby antenna, it does not desense reasonably strong enough military air signals. It also filters out (attenuates) FM broadcast, NOAA, etc. as those transmissions are not in the 225-400 MHz range. Think of a filter as a earplug when two people are talking and you want to hear the person on the right. The person on the left is still there, but it allows you to focus on the person to the right.
First things first, regardless of amplifier or not. If you live anywhere near an FM broadcast station, I recommend you put an FM filter inline. They are not equal either. There's a thread on here somewhere, but the best of the bunch is a MiniCircuits FM filter for around $100. There is a less expensive one for around $20, but it may drastically attenuate signals on frequencies you want to receive, or it may not.
* Note: An amplifier can also cause problems as it can increase interference from powerful local FM broadcast stations, pager transmissions, NOAA, local PD/Fire or whatever. As everyone's location is different, as well as their hobby needs, solutions can be different. For example, I listen to military aircraft which can be quite distant. I use an inline filter on the coax that mostly passes 225 - 400 MHz, which is the range I want to hear. This filter works well enough that when I TX on a nearby antenna, it does not desense reasonably strong enough military air signals. It also filters out (attenuates) FM broadcast, NOAA, etc. as those transmissions are not in the 225-400 MHz range. Think of a filter as a earplug when two people are talking and you want to hear the person on the right. The person on the left is still there, but it allows you to focus on the person to the right.
First things first, regardless of amplifier or not. If you live anywhere near an FM broadcast station, I recommend you put an FM filter inline. They are not equal either. There's a thread on here somewhere, but the best of the bunch is a MiniCircuits FM filter for around $100. There is a less expensive one for around $20, but it may drastically attenuate signals on frequencies you want to receive, or it may not.
I run RG11 for all my HF runs, and on my 978 and 1090 recievers. The difference between 50 ohm and 75 ohm for me is lmr-400 1.1:1 and the RG11 is 1.5:1 and quad shield , PPC rg6 from a scantenna. No issues for the past 10 years using the same feedlines
Ubbe
Member
A receiver with internal noise of 6dB will add that noise to the signal and at lower signal levels it will begin to be more noise than signal. Amplifying the signal to let it be 6dB stronger will raise the weak signal above the noise level and the 6dB noise in the receiver will then have much less impact on the systems total noise figure.
Of course the measurement are done on one single filter, which in this case are the 760-1300MHz filter in the scanner and the diagram shows what the coax will see from the scanners impedance at different frequencies. The band filter are fixed and does not change depending of the frequency tuned to in that specific frequency band and there's a preamplifier and a LNA in the scanner that isolate the mixer from the band filters. The only scanner I know of that change it's filter to the tuned frequency are the BC780 and then only in the VHF band 118-175Mhz.
If you only monitor frequencies that are noise free you will have no use of an amplifier. It's only if the signal has some noise in it at lower signal levels that an amplifier should be concidered, or if you want to split up the signal to several receivers and the signal strenght isn't enough to overcome the splitters attenuation without adding noise.
/Ubbe
To add to the discussion.... I’ve got 75’ of LMR-400 on the way but I can’t find anywhere where it says what the impedance of the cable is. One of my antennas is a folded dipole with a 300 to 75 ohm transformer. Can I use this regardless of the cable?
mmckenna
I ♥ Ø
Standard LMR-400 is 50Ω cable.
Yes, there will be a slight mismatch in impedance, but for scanner use, that will not be noticeable.
They do make an LMR-400-75, that is 75Ω, but don't change your order, you won't have a problem with standard LMR-400.
LMR®-75 Ohm - Times Microwave
www.timesmicrowave.com
Standard LMR-400 is 50Ω cable.
Yes, there will be a slight mismatch in impedance, but for scanner use, that will not be noticeable.
They do make an LMR-400-75, that is 75Ω, but don't change your order, you won't have a problem with standard LMR-400.
LMR®-75 Ohm - Times Microwave
Thanks, I didn’t even think of this being an issue before ordering. I can always get a 50 ohm transformer so it all matches up.
Standard LMR-400 is 50Ω cable.
Yes, there will be a slight mismatch in impedance, but for scanner use, that will not be noticeable.
They do make an LMR-400-75, that is 75Ω, but don't change your order, you won't have a problem with standard LMR-400.
LMR®-75 Ohm - Times Microwave
Well scratch that, it seems that might not exist. 300 to 75 seems to be the standard. Hopefully the mismatch won’t be noticeable.
It won't!!!
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Most of those 75-300 transformers accept an "F" connector, typically used for TV antenna and cable signals, and I don't believe that F connectors for LMR400 cable are common. If this is your situation, you'll need an adapter between the LMR400 connector and the F connector of the transformer.
mmckenna
I ♥ Ø
I concur.
LMR-400 is a bit stiff, so connecting it directly to a 75Ω to 300Ω transformer is probably not your best plan anyway. Use a short jumper of RG-6 with an F connector on one end and a BNC on the other. Fairly common practice to use a short flexible jumper at the antenna and the radio end of stiffer coax.
Thanks guys.
The impedance isn't that important but the length of the run can be. Which of the two has the least loss for the band(s) you'll be using? Then decide if that amount is loss is worth the cost. I doubt if there will be that much difference anyway.
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