I am picking up a DC voltage on my receive antenna.

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prc117f

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Strange, I hook up two alligator clips to my coax connector to an antenna not plugged into anything and I am getting 156 millivolts, it is decaying down slowly and its now at 143.5 millivolts.

I have nothing transmitting near the antenna. Where is this voltage coming from, is this normal?

I am using a Fluke 289 multimeter. I am recording the trend to see what happens in the next few hours.
 

WyoDuner

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I would short center conductor and shield, confirm zero volts and see if it climbs up from there or what. I'm guessing it's just the capacitance of the coax and am again guessing if you shunt center and shield it will go away. I would also put the meter on AC and see if you see any AC voltage. DC can't be induced onto your coax but AC sure can.
 

krokus

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Either induced AC, as mentioned, or DC from wind. Many aircraft have a discharge to the ground or ship, when they are landing.

Sent via Tapatalk
 

majoco

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VOM's don't necessarily measure DC alone when switched to DC! They may measure AC in isolation by the addition of a capacitor in the way, but not the other way round. Older multimeters (Simpson 260 etc) had no differentiation at all between AC and DC, only a rectifier and a scale change - of course the DC goes right through a rectifier!
You could be reading the AC from a nearby broadcast transmitter, regardless of the frequency response of your Fluke.
I feel an experiment coming on........ :roll:
 

prc117f

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I get the DC between the coax center connector and the outside.

I shorted it first to see if it was acting like a capacitor but the voltage came back.

Now I am not getting any voltage this morning. it did rain a little.

The reason I decided to measure for voltage is I noticed the noise levels increased quite a bit on my SDR and I got curious to see what was going on, ie maybe some kind of static buildup. I also noticed weird noise bursts that were broad band.

It looks like that was the case. or something generating very strong RF.

I reconnected the SDR this morning and the noise floor is back to normal.
 

n0nhp

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One thing you can do to help bleed the static off of a long piece of wire is to place a 100K to 10M ohm resistor across between the center and ground.

The noise you were experiencing was probably due to wind or thunderstorm activity in the area. It may not have built enough potential to actually cause lightning but it can still raise your noise floor substantially.

Usually when I start hearing that noise, I will unplug the antenna lead at my grounded entry panel and I will plug in a coax connector that I have soldered a neon lamp into. The when the potential reaches 90 or so volts the neon will light. You can watch the polarity change when a cloud or ground stroke occurs. Lots of fun, if you don't mind sitting within a few feet of a very long lightning rod!

Another way to protect the front end of your SDR would be to place a pair of general purpose diodes back to back between the center and shield of the coax.

Be aware that none of these techniques will protect the radio if lightning decides to use your antenna as a path. Unplugging the coax and providing a substantial air gap is usually enough to protect things.

Bruce
 

prc117f

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One thing you can do to help bleed the static off of a long piece of wire is to place a 100K to 10M ohm resistor across between the center and ground.

The noise you were experiencing was probably due to wind or thunderstorm activity in the area. It may not have built enough potential to actually cause lightning but it can still raise your noise floor substantially.

Usually when I start hearing that noise, I will unplug the antenna lead at my grounded entry panel and I will plug in a coax connector that I have soldered a neon lamp into. The when the potential reaches 90 or so volts the neon will light. You can watch the polarity change when a cloud or ground stroke occurs. Lots of fun, if you don't mind sitting within a few feet of a very long lightning rod!

Another way to protect the front end of your SDR would be to place a pair of general purpose diodes back to back between the center and shield of the coax.

Be aware that none of these techniques will protect the radio if lightning decides to use your antenna as a path. Unplugging the coax and providing a substantial air gap is usually enough to protect things.

Bruce


Looks like that is what is happening. it is windy again and looks like a storm in the distance, now I measure 395 millivolts.

Thats messed up. I left the cable unplugged, and going to look at setting up a bleed resistor. Interesting experiment, I will need to look for a neon bulb. :)
 

SCPD

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I would just buy a Polyphaser and be done with it. Just make sure it's properly grounded for it to work correctly.

PolyPhaser Coaxial Lightning Protectors IS-50UX-C0 - Free Shipping on Orders Over $99 at DX Engineering

PolyPhaser protectors are suitable for all types of antennas, even "floating" antennas that are not DC grounded. With PolyPhaser DC Blocked protectors, any slow voltage build up from wind driven rain, snow or dust, will not get to the equipment. As the protector reaches threshold for turn-on in a dc blocked circuit, it will go into a momentary soft turn-on as the gas barely ionizes and bleeds the static charge to ground. This does not create noise since it will not get to the arc mode and lasts only a short time.
 
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