Low Noise Amp and LMR400 Cable

[JCRR]

HI All,

Hopefully I have posted this in the correct chat
I have a discone antenna (25-2000Ghz) that will be going up soon with a 28ft length of LMR400 cable to the shack. I also have a low noise amp but wondering if I can get away with it on the receiver end due to the use of LMR400 cable (as will be an easier install) or does it have to go on the Ariel end?

Also not sure if the LMR400 N connectors are different to normal N ones? as the N connector on the Ariel seems to not fit the internal ring as per the pic?

Regards
Christopher

 

[bharvey2]

While there are times a low noise amp can be useful, I wouldn't automatically assume that one is needed. Depending upon what your intended monitoring "targets" are, you may not even need an amp and in many instances, the amp may just amplify unwanted signals and produce more problems than it solves. For example, if you want to monitor VHF frequencies, the local FM stations may overload your amp and overwhelm your receiver such that you can't receive any VHF. With only 28ft of LMR400, your loss, even up to around 900mhz, is only about 1-2dB. Apply the KISS principle and start with the antenna and cable (don't forget a properly installed grounding block where the coax enters your building) If you can receive what you intend then you've accomplished your mission. If not, you might consider the amp (by and large, the sooner in the antenna/coax chain the better - so, to answer your question, on the antenna end) ) but to be honest, a directional antenna may be a better solution.

As far as N connectors, the mating side should be the same as any other N connector gender not withstanding but the coax side will be different depending upon the coax it was designed to terminate. (LMR 400 will be larger than LMR240, etc.) I hope this helps.

 

[Ubbe]

It's something like a 1dB attenuation for 900MHz at 30ft for a LMR400 so no real need to have the amplifier at the antenna. Connectors looks fine and should fit together.

/Ubbe

 

[JCRR]

While there are times a low noise amp can be useful, I wouldn't automatically assume that one is needed. Depending upon what your intended monitoring "targets" are, you may not even need an amp and in many instances, the amp may just amplify unwanted signals and produce more problems than it solves. For example, if you want to monitor VHF frequencies, the local FM stations may overload your amp and overwhelm your receiver such that you can't receive any VHF. With only 28ft of LMR400, your loss, even up to around 900mhz, is only about 1-2dB. Apply the KISS principle and start with the antenna and cable (don't forget a properly installed grounding block where the coax enters your building) If you can receive what you intend then you've accomplished your mission. If not, you might consider the amp (by and large, the sooner in the antenna/coax chain the better - so, to answer your question, on the antenna end) ) but to be honest, a directional antenna may be a better solution.

As far as N connectors, the mating side should be the same as any other N connector gender not withstanding but the coax side will be different depending upon the coax it was designed to terminate. (LMR 400 will be larger than LMR240, etc.) I hope this helps.

Thanks for the info
I have a long-wire for HF stuff and am interested in the Aircraft as live near Man airport and also would like to listen to marine stuff etc. I thought it might be fun to compare the discone to the long-wire but not expecting it to be anywhere near as good .

It's something like a 1dB attenuation for 900MHz at 30ft for a LMR400 so no real need to have the amplifier at the antenna. Connectors looks fine and should fit together.

/Ubbe

Yes I realised that the connectors are fine as tested them. I was hoping for better than 1db but that's fine considering the length

 

[bharvey2]

Yes I realised that the connectors are fine as tested them. I was hoping for better than 1db but that's fine considering the length

At marine and air frequencies the loss on 30ft of LMR400 should be well under 1dB so you should be fine.

 

[AK9R]

There are several coax loss calculators on the web. Select your type of cable, input your frequency and your cable length, and get the exact loss value.

Here's one: Coax Loss Calculator | KV5R.COM

 

[paulears]

Ultimate signal strength is rarely worth chasing as you are after wide band and preamps will of course amplify the pagers and other crazy powerful signals get increased too and that causes issues worse than low signal!

 

[Pape]

I recommend you try without the LNA, unless you need for signal splitting and so on.
If you add the LNA you may also need to add filter and so on.

 

[Ubbe]

As the LNA can be placed at the receiver it can easily be tested if it will improve anything. Usually scanners has a worse internal noise figure than a LNA so adding LNA and then attenuate precisely to not get the receiver to desense will usually improve reception. I did that at a RF hell place without any additional filters and adjusting attenuation to get the best signal/noise ration did improve reception with a LNA that had less than 1dB NF.

/Ubbe