What would cause the baseline to be curved?

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  • Updated 2 years ago
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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.

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Mark WS7M

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Posted 2 years ago

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Simon Lewis

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will be a different resonances point on the antenna vs frequency ... You can see it on my Hex beam if you look wide enough ..
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Norm - W7CK

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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. 
(Edited)
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Sergey, R5AU

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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" 
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Paul

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Hi, since a random wire presents a high impedance at resonance, I would suggest that the minimum in the display coincides with resonance and not the maxima as stated earlier in this thread.

The display is a plot of the frequency response of your antenna combined with its feeder. The dip in the centre occurs where the impedance presented to the rig is not a good match. The maxima occur where the system is a better match to the rig, so coupling more energy into the receiver.


Ideally the response would be flat over the band of interest. If you use an ATU this will not change the point of resonance but it should enable you to achieve a better match where you wish to operate. It will also help the SWR.


A manual ATU will allow you to see the effect explicitly on the display. An auto ATU will carry out the necessary matching but you will only see the end result, not the transition as the match improves.


The spikes look to me to be some form of RFI (which will not be removed by an ATU)
(Edited)
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mikeatthebeach .

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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
(Edited)
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Tim - W4TME, Customer Experience Manager

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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.
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Paul

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Agreed. Although there are much better wire antennas, there is definitely nothing intrinsically wrong with an end fed random wire. However, the absence af an ATU will severely attenuate signals at the point of resonance; in this case by 10dB. This behaviour will be periodic throughout the spectrum for a given length of wire, as discussed at:

http://www.hamuniverse.com/randomwire...

I would certainly encourage Mark to consult this site and prune his wire if necessary *and* to add a simple manual ATU (many auto ATU's will be unable to find a match if the mismatch causes an SWR above 3:1). A decent earth or counterpoise might also help reduce RFI.

Good luck with it Mark.
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Mark WS7M

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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!