[TowerTalk] Polarity question..on a dipole
patrick_g at windstream.net
Sat Apr 1 09:25:53 EDT 2017
All the fence chargers I have seen have a pulsed output. I have not
examined the waveform of their pulses with an o'scope but suspect the
pulses contain significant RF since I can clearly hear them on my Flex
5000A on multiple bands. A minor annoyance listening to an approximate
1/sec noise burst but not enough to motivate me to walk out to the barn
to shut it down then back again to turn it back on, especially if it is
Luckily my antennas are not close to any of the electric fence spans.
Just guessing but if the RF component of the pulses were to be
penetrating the plastic insulating tubing, i.e. shorting out to grounded
metal in contact with the tubes OD or if the RF were getting through the
tubing to the damp earth where it is buried I would have thought that
would have selectively attenuated the RF before it was propagated down
the fence wire and radiated where my antennas could pick it up.
The fence charger itself is inside a well grounded all metal building.
If the charger is running but not connected to the output wire the radio
does not detect pulses so any noise emitted by the charger itself is
contained within the building and additionally does not get to the shack
via the AC wiring as no pulses are heard if the output is not connected
to the distribution system/fence wires.
The tubing is cheap so I would give it a try before writing it off if I
needed to insulate a wire antenna from something.
On 3/31/2017 10:57 PM, Grant Saviers wrote:
> I put up some wire electric fence screw in insulators into beach side
> coconut trees for a 160m one 1/4 wl radial on a DXpedition. CW, 1kw.
> They promptly melted and arced over to the screw. So what works for
> insulating animal fences or DC may not work for RF. It looked like a
> good insulator but couldn't handle those RF voltages. And I thought
> proven, since after 4 years the same insulators on my 160m T 120' long
> elevated 10' aluminum fence wire radials in the trees look ok (SSB
> QRO), but there are 8 radials so voltage stress is much reduced.
> Grant KZ1W
> On 3/31/2017 19:25 PM, Patrick Greenlee wrote:
>> Farm and Ranch stores like Orchelns, Atwoods, Tractor Supply, etc
>> sell insulating tubing (plastic, don't know composition) for use with
>> electric fencing. It is 1/4 inch or maybe a tad less. My fence
>> charger is rated for over 200 miles of fence with a fairly high joule
>> rating and puts my digital meter (good for up to 10,000 Volts) into
>> over scale , i.e. has more output than 10KV. I can have a wire
>> insulated with this tubing against grounded pipes or buried in
>> conductive soil and not get arcing to the pipe or the damp conductive
>> This stuff might just help stop an antenna from arcing to any part of
>> a tree. I haven't tried it with antennas as I have no need for that
>> but have used a lot of lot for underground distribution of the hot wire.
>> Patrick NJ5G
>> On 3/31/2017 3:39 PM, jimlux wrote:
>>> On 3/31/17 12:25 PM, Jim Thomson wrote:
>>>> Ok, the peak V on the tips of a dipole is sky high. But what
>>>> about the polarity between
>>>> the ends ? If one end is + 10 kv, is the other end at – 10 kv
>>>> ?? Is there a 20 kv
>>>> potential difference between them at all times ?
>>> It's very hard to calculate (or measure) the actual voltage at the
>>> ends - best you can say is "it's high". You can infer a voltage by
>>> calculating the feedpoint impedance of a 1 wavelength long doublet,
>>> then putting in 2x power (each half radiates full power), and using
>>> Ohms law.
>>> Yes, it would be opposite polarity. but not "at all times" because
>>> it goes through zero every half cycle.
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