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Author Topic: A interesting observation.  (Read 11646 times)
W8JX
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Posts: 6030




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« on: September 06, 2013, 09:20:12 AM »

I was doing some HF mobile on 17 today and I noticed something I never really noticed before. Whenever I would cross over a bridge on the interstate there was a increase in ignition like static noise. Did it every time and always sounded the same. I am curious if anyone else has noticed it.
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W9MMS
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Posts: 121




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« Reply #1 on: September 09, 2013, 05:26:13 PM »

Have you check the SWR on 17m lately?
How high of the road surface (terra firma) is your feed point?
Bear in mind, bridges has much more rebar plus Ibeam than the road surface.

((((73)))) Milverton.

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W8JX
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Posts: 6030




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« Reply #2 on: September 10, 2013, 06:09:21 AM »

Have you check the SWR on 17m lately?
How high of the road surface (terra firma) is your feed point?
Bear in mind, bridges has much more rebar plus Ibeam than the road surface.

((((73)))) Milverton.



Match is good. Feed point is about 30 inches above ground. I have not really listened on another band to see if it does it there. Mobile is only place I can do 17 so I tend to use that band mobile. Yes, the rebar and metal in bridge may indeed be a factor here.
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NO2A
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« Reply #3 on: September 10, 2013, 06:47:05 PM »

I`ve noticed the same thing while driving under bridges listening to am broadcast. Ignition/alternator noise. My car radio may have a loose ground connection as well.
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W9MMS
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Posts: 121




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« Reply #4 on: September 10, 2013, 07:10:21 PM »

Have you check the SWR on 17m lately?
How high of the road surface (terra firma) is your feed point?
Bear in mind, bridges has much more rebar plus Ibeam than the road surface.

((((73)))) Milverton.


[/quot

Match is good. Feed point is about 30 inches above ground. I have not really listened on another band to see if it does it there. Mobile is only place I can do 17 so I tend to use that band mobile. Yes, the rebar and metal in bridge may indeed be a factor here.
Assuming the Vehicle is adequately bonded. Is there any way to further increase the feed point above (Terra Firma) and beyond 30"
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W5WSS
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Posts: 1742




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« Reply #5 on: September 10, 2013, 07:38:23 PM »

Perhaps the antenna is capacitive coupling to the internal grid work of interlaced Iron re bar and works to enhance the signal strength of both transmit and receiving when crossing over it.
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W8JX
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Posts: 6030




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« Reply #6 on: September 11, 2013, 07:48:55 AM »

Perhaps the antenna is capacitive coupling to the internal grid work of interlaced Iron re bar and works to enhance the signal strength of both transmit and receiving when crossing over it.

No difference in signal strength that I can see. I will try it on 20 and 15

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K5LXP
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« Reply #7 on: September 11, 2013, 07:59:02 AM »

What I find interesting is even on the AM broadcast band sometimes you'll hear a surge in signal strength just before you get to the bridge.

Mark K5LXP
Albuquerque, NM
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W5WSS
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Posts: 1742




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« Reply #8 on: September 12, 2013, 01:03:44 PM »

Yeah Mark, I have noticed the signal increase and then when exiting the bridge the signal resumes as was.

Could be several reasons and even a combination of them.

I have noticed this for many years but never really discussed it.
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N6AJR
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Posts: 9910




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« Reply #9 on: October 02, 2013, 10:40:43 AM »

I spent 28 years working on the bridges in northern california.  when crossing a typical bridge  there is a lot of metal and it is reflecting the engine noise back to the radio.
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