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Author Topic: Images in LW Band  (Read 2904 times)

Offline ihmeyers

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Images in LW Band
« on: December 19, 2013, 1941 UTC »
Need some help in troubleshooting some images I've got in the LW bands (primarily below 150khz).  The images are of very strong locals (typical s9+30 or 40).  The obvious answer is yeah, the locals are overloading the rx (it happens with all the radios in my shack).  The reason I am stumped is that I have inline a pre-selector I built for LW that has a Clifton Laboratories low pass filter and a PAR low pass filter in series.  The effect is to knock the two locals down to S1 (-100db on my SDR).  That's why I am confused; if the filters reduce BCB by 90db or more how is there enough signal strength getting through to create the images?  Should I buy a third filter to put in series with these two?

It's driving me crazy trying to troubleshoot it.

Thanks!!

Ian
Boynton Beach, Florida  :)

Offline ChrisSmolinski

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Re: Images in LW Band
« Reply #1 on: December 19, 2013, 2002 UTC »
Are you hearing an image, that is, one station at the incorrect LW frequency, or are you hearing a mixing product (two (or more) stations at the same time). If the latter, there is possibly some non-linear junction acting as a diode. It could be as simple as a rusty downspout. But if so, it is then radiating the mixing product at the lower frequency, so no amount of MW filtering is going to get rid of it.
Chris Smolinski
Westminster, MD
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Offline ihmeyers

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Re: Images in LW Band
« Reply #2 on: December 19, 2013, 2018 UTC »
Chris,

Yeah, it's a mixing product.  It happens primarily on my amplified vertical though I have had it appear on my Pixel loop as well.   Maybe I should check the connections outside from the antenna to a series of noise baluns to the coax to the shack.  The coax is pretty new (last 6 months) and is buried RG-6 quad shield to reduce noise.  I would love to be able to identify the culprit.  It's driving me crazy... :'(

Offline ihmeyers

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Re: Images in LW Band
« Reply #3 on: December 20, 2013, 1658 UTC »
Spent an hr this morning trying to troubleshoot it to no avail so would be most appreciate of any help!  Here's what I know:

1) Occurs primarily around 100-120 khz.  My strongest locals are 640 and 850.

2)  I hear it on all radios in the shack, Winradio G31, Icom R-75, Kenwood TS-940.

3)  Only audible on the active vertical; if it's there on the Pixel it's way down in noise.

4)  Taking the noise baluns out of the circuit didn't help at all.

5)  The vertical cox is RG-6 quad shield only a few month's old.

6) Under the right circumstances I get mixing products on the vertical around 6900khz, mostly at night.  I've got high pass filters in line with the vertical for HF that eliminates this most of the time though.

73,
Ian

Hoepfully I can find the culprit and rid my system of it.  I am afraid I might just have to live with it... :(

Offline Kilokat7

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Re: Images in LW Band
« Reply #4 on: December 20, 2013, 1824 UTC »
3)  Only audible on the active vertical; if it's there on the Pixel it's way down in noise.

I don't think your radios are overloading, I think the mixing products are occurring in the amplifier section of this active vertical.  What kind of antenna is this?  Make and model number?  Can the RF gain be backed off within the antenna itself, any adjustments?
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Offline ChrisSmolinski

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Re: Images in LW Band
« Reply #5 on: December 20, 2013, 1848 UTC »
I agree, it is most likely in the vertical's amp.
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 58 ft T2FD / 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop

Offline ihmeyers

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Re: Images in LW Band
« Reply #6 on: December 20, 2013, 1906 UTC »
Thank you both for the replies.

It is a DX Engineering DXE-ARAV3-1P (http://www.dxengineering.com/parts/dxe-arav3-1p),  I have replaced the 7 ft whip with a 28 foot stainless steel vertical (actually a metal cleaning pole I bought on ebay).  It brings in a good amount of signal but may  be overloading the amp.  I have to look to see if I can reduce the amp's gain.  I can add an attenuator before the coax but if the mixing is already in the whip I am guessing that won't do much good.

It would be more of a pain in the ass but because the vertical is a telescoping pole I could always dismount it from the tree and try reducing it's height a little and see if that helps.

Appreciate the response, I am pretty sure you guys have nailed it.  ;D

73,
Ian

Offline ihmeyers

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Re: Images in LW Band
« Reply #7 on: December 20, 2013, 2253 UTC »
It would be more of a pain in the ass but because the vertical is a telescoping pole I could always dismount it from the tree and try reducing it's height a little and see if that helps.

I reduced the length of the vertical by about 5 feet and that helped.  Unfortunately it seemed to have reduced sensitivity above 11mhz so maybe I will tweak it a little tomorrow...

Offline ChrisSmolinski

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Re: Images in LW Band
« Reply #8 on: December 21, 2013, 0034 UTC »
I wonder if you could put a high pass filter between the whip and pre-amp? Assuming you're not interested in MW.
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 58 ft T2FD / 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop

Offline ihmeyers

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Re: Images in LW Band
« Reply #9 on: December 21, 2013, 0159 UTC »
Chris,

I thought of that but had two issues:

1) I do DX BCB

2) I wasn't sure how to wire the coax to the whip since the connector is only a hot not a not/shield.  I guess maybe I would solder the hot onto the whip and just leave the shield unconnected in that case.

I actually thought of this is adding an inline attenuator (bnc connection) rather than a high pass or band pass filter but I still wasn't sure what to do with coax.

I might play around with that some tomorrow if I find time.

73,
Ian