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Re: [TowerTalk] Polarity question..on a dipole

To: Grant Saviers <grants2@pacbell.net>, towertalk@contesting.com
Subject: Re: [TowerTalk] Polarity question..on a dipole
From: Patrick Greenlee <patrick_g@windstream.net>
Date: Sat, 1 Apr 2017 08:25:53 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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 cold out.

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.

Patrick        NJ5G


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 dirt.

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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