Why does using the preamp on ten meters create so much wideband RFI with no antenna connected?

rusham

Attached is what my SSDR panafall looks like with the 10M preamp on and off.  Bad RFI with it on, no RFI with it off, and no antenna connected to the 6300.  Is this typical?

Lee - N2LEE

Are you familiar with how a preamp works and its purpose ?
This is an unterminated input which expects to see 50 ohms, have you tried this with a dummy load ?

An antenna amplifier boosts a radio signal considerably for devices that receive radio waves. Many devices have an RF amplifier stage in their circuitry, that amplifies the antenna signal, these include, but are not limited to; Radios, Televisions, Mobile Phones and Wi-Fi and Bluetooth devices. Amplifiers amplify everything, both the desired signal present at the antenna, and the noise. Typical signal noises include: ambient background noise (electric brush noise from electric motors, high voltage sources from, for example a gasoline engine ignition, or large dispersed currents in the vicinity of the desired reception electric fence). To add, consideration must be taken for the noise generated by the amplifier itself and all other electrical noise which may be generated by the device that is to receive a signal, for example a lot of consideration has to go into mobile phone circuitry design to eliminate as much noise from its own circuitry in order to not disturb the desired transmission signals from its own antenna(ae).

K3SF

hey lee

thank you for you response. i too have seen similar behaviour when antenna connection is grounded. Now makes sense knowing the preamp is expecting 50ohm load. I have seen it on a long run (50ft) of coax which is shorted and grounded at the remote antenna switch.

Paul K3SF

Lee - N2LEE

Paul, think of what happens when you turn up a stereo with no input. You get noise and hiss. This is essentially what happens when you turn on gain on the receiver by turning on the preamp with no antenna.

Do what ever you can to improve your antenna and you will find the need for the preamp will not be as important.

Lee

K3SF

hey lee

yep..understood...
AND
the neet thing about flex is now you can actually 'see' it happen over a much wider set frequencies rather than just hearing a little extra noise in narrow band receiver.

"seeing is believing"

paul k3sf

Doug Hall

It is certainly not that difficult to perform the same test as the original poster did and answer his question, "Is this typical?" Only takes a minute.

On my 6300 with no antenna connected (nothing connected to the SO-239 on the radio at all) and with the preamp on I don't see any signals in the waterfall. Nothing like what is posted above. Very little difference in preamp on and preamp off.

73,
Doug K4DSP

K3SF

hi doug

you are absolutely right...

i took your lead and totally made sure nothing connected

With NO connected antenna or coax at all on the so239...
the with and without preamp are the same....dead flat ... quiet....


Russ and myself have seen similar differences without and out preamp .  The error of our ways was something was indeed connected to the so239..even a very short piece of coax becomes an antenna.  In my case it was a 50ft long coax out to a remote antenna switch...

sometimes ...  just have DUH moments and do not realize it

thanks you all for your responses..
and
helping us see the errors of our ways...

Paul K3SF