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KB9YZL
Member

Posts: 16




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« on: December 17, 2003, 10:24:38 AM »

I was recently reading an article on the fabrication of a loop antenna using the ever-present copper plumbing fittings.  In this article, the author stated that he avoided the use of aluminum mounting hardware in contact with the copper antenna element, because he felt that the electrical action between the two dis-similar metals contributed “noise”.

In real life, how serious is this concern?  Has anyone had noise issues that were traced to this cause?

Kent Carroll
KB9YZL


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WB2WIK
Member

Posts: 20666




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« Reply #1 on: December 17, 2003, 01:46:45 PM »

Sure it can cause noise.

Whether that makes any difference or not depends on what you're using the antenna for, and whether the noise could possibly bother you.  It also depends a bit on your location, since dissimilar metals forming a semiconductive joint can generate signals that could bother your receiver if there are strong signals generated nearby, such as those from broadcast stations, powerful pagers, etc.  If you're in the middle of nowhere with no neighbors and no transmitters around, this probably won't occur.

Hardware-generated noise is most problematic in duplex (repeater) systems which use the same antenna for transmitting and receiving simultaneously.

WB2WIK/6



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WB6BYU
Member

Posts: 13571




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« Reply #2 on: December 17, 2003, 04:54:14 PM »

I've seen quite a bit of dissimilar metal corrosion
on antennas, though can't necessarily say I've heard
specific noise from that source.  However, it does make
a bad joint due to the corrosion, and any RF voltage
applied across that may be rectified to create
harmonics or other distortion products.

Certainly a number of cases of RFI have been traced
to the harmonic generation in rusty metal contacts or
similar semi-conductors.  And any motion across such
a joint could generate noise (just like any other
intermittent connetion in an antenna.)  Remember the
loop is a very high Q antenna, and an aluminum bracket
may have enough capacitance to change the loop tuning.
An intermittent connection there could your loop tuning
to change as the antenna moved in the wind.

So there are a number of possible conditions where it
could cause "noise", plus a few others why it wouldn't
be a good idea.
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FRANKM12
Member

Posts: 46




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« Reply #3 on: December 17, 2003, 11:23:10 PM »

KB9YZL,

The Peltier Effect does produce a constant voltage.  Even independently of temperature.  However, I think Thermocouples operate via this effect, so the voltage may be more as the temperature increases.  But you will get a small voltage at any rate.  As WB6BYU said, the corrosion factor is probably the worst aspect of it.  However, this kind of voltage might just cause problems with noise as well, especially if the joint became loose and it started to move around and cause an oscillation.   But the voltage is small in milliamps at best.  But this might mess up an antenna mounted pre-amp or something like that.  Copper and Aluminum are both good conductors, so the problem might be noticeable.

73
frank
KG4VLQ
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