antenna gain

EA4GLI

Antenna gain works as expected on 20m but when using the Flex 6700 in 630m to try to decode JT9 I ran into an odd behavior. +30dB Gain increases noise and artifacts on the band, then +40dB goes down again. I recorded it on a video, maybe someone can explain. Is this normal?


https://www.youtube.com/watch?v=24Ru5TGEVZ4

Tim - W4TME

RF gain is not effective on the lower frequencies where the noise is higher and you are noise limited at the antenna.  At those frequencies, you should never have to use the preamp. At +40 dB you are adversely affecting the ADC.

https://helpdesk.flexradio.com/hc/en-us/articles/204923669-How-to-determine-the-amount-of-RF-Preamp-...

It looks like unity gain (0 dB) is the correct setting for the video presented.

EA4GLI

I understand that Tim, but I am surprised by the behavior. At +20dB it goes up considerably to then go down at +30 and +40dB.
Also, while I will never use gain on those frequencies with a dipole or any antenna with any gain, I am testing different settings with a magnetic loop and I have seen some interesting improvements in 160m with +10dB vs 0dB.

Anyway, I just didn't expect the behavior... can you confirm it is the natural one? Maybe my 6700 is due a factory reset.

Feliz Navidad Tim and FRS  family.

Gerald-K5SDR

Tim is correct that you should not use gain at those low frequencies unless you have a negative gain antenna. The behavior you see is expected because the preamp that is enabled at 40 dB of gain has a low frequency roll off. The 20 dB preamp does not have the same roll off.

EA4GLI

Thanks for the explanation Gerald

Stan VA7NF

What I have found is  front end mixing products from AM broadcast band stations create a noise level.  Placing a BC filter in the ant out/in loop will reduce those levels resulting in better signals in VLF and also permits a modest pre-amp gain with my magnetic loop.

EA4GLI

I have one I built to stop AM bc stations. But it blocks all the way to 160m. 
Do you have a design for the one you use Stan? I love the Loop feature on the 6700, allows us to use home made low power filters to play around.

Stan VA7NF

Mine is a Clifton Laboratories Z10020B NDB/NAVTEX.  Reject 540 - 1700 KHz with BNC connectors

K3ZJ - Dave

FYI, the designs from Clifton Laboratories were bought by DX Engineering.  You'd have to query DXE about any plans to produce the AM filter line. See https://www.dxengineering.com/techarticles/dxepressreleases/dx-engineering-acquires-clifton-laboratories