What would cause the baseline to be curved?

  • 1
  • Question
  • Updated 3 years ago
  • Answered
As Tim once said...

The cool thing about a panadapter is you can see things.  It is also the curse.

I think for years I tuned across what I thought were "birdies" and I figured they were artifacts of my radio.  Now I get to see what they look like and I have no idea what they are often.

I'm curious what would cause the effect below and if it is to be expected.  I am using an end-fed wire antenna.  I have no tuner engaged, I'm sure my ground is less than optimal.

Photo of Mark WS7M

Mark WS7M

  • 505 Posts
  • 152 Reply Likes

Posted 3 years ago

  • 1
Photo of Simon Lewis

Simon Lewis

  • 495 Posts
  • 118 Reply Likes
will be a different resonances point on the antenna vs frequency ... You can see it on my Hex beam if you look wide enough ..
Photo of Norm - W7CK

Norm - W7CK

  • 757 Posts
  • 163 Reply Likes
I'll agree with Simon.  The peaks or humps is where the antenna is resonant, the valleys are areas where the antenna is less resonant.

I have a ZS6BKW that shows these waves really well.  When you zoom out and take a look at several MHz, it is evident where the antenna is resonant.  Pretty cool.

Its great being able to SEE the spectrum now.  It gives us a whole new perspective as to what is going on. 
Photo of Sergey, R5AU

Sergey, R5AU

  • 860 Posts
  • 117 Reply Likes
Mark, you can use any RF network analyser and can see the same base line curve that reflecting you antenna system :  Antenna+Cable+"if any exist" 
Photo of mikeatthebeach .

mikeatthebeach .

  • 480 Posts
  • 82 Reply Likes
Maybe you tried a Center-Fed Antenna with a Balun might give you a flat noise floor.

I use a Palomar Balun Cube in shack as well as one at Antenna feed to reduce Currents 
on Outside braid grounds 

Ground Rods with Shack, Antenna, Electrical at Junction Box all bonded together directly 

Really helps a lot here 

Also hope your RF pre-amp gain is not set too high. See some possible mix products on Spectrum plot
Photo of Tim - W4TME

Tim - W4TME, Customer Experience Manager

  • 9186 Posts
  • 3548 Reply Likes
I too agree with Simon.  It is the resonance characteristics of your antenna.  This does not indicate anything is wrong with the antenna, it just has a low resonance point.
Photo of Mark WS7M

Mark WS7M

  • 505 Posts
  • 152 Reply Likes
Thanks all.   Duh... I should have considered resonance.    The EF wire does have a small matching network at the feed point.

I do plan to get a full ATU but that will have to wait a while.

Thanks for the links.   The RFI I am aware of and slowly when I have time trying to track it down.  RF is at 0.

The interesting thing about this baseline thing is that it is not always like that.  Some times it is flatter which makes me think something is changing.

This is the only antenna I can put up and keep hidden until the parity act goes through then I'll put up a 2000' tower with stacked beams!  (just kidding).

Thanks all!