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Re: [TowerTalk] KF4BWG

To: towertalk@contesting.com
Subject: Re: [TowerTalk] KF4BWG
From: David Gilbert <xdavid@cis-broadband.com>
Date: Thu, 18 Apr 2019 16:30:36 -0700
List-post: <mailto:towertalk@contesting.com>

I do have a question for the experts about what I posted below. What effect, if any, is there on skin effect for individual wires when you have several in close proximity?  Since skin effect is a field-related phenomenon, does having multiple fields in phase mitigate the effect on the individual wires?

Just double checking ...

73,
Dave   AB7E



On 4/18/2019 2:06 PM, David Gilbert wrote:

I had no idea the wires could be that thin, but I checked around online and saw horse fence tape with wire thicknesses ranging from 6 mils to 12 mils for 15 strand tape, so 7 mils (0.1778 mm) for 18 wires sounds like a good ballpark figure.

You make a great point about the ferromagnetic effect on skin depth.  I found this calculator for skin depth:

https://chemandy.com/calculators/round-wire-ac-resistance-calculator.htm

The calculator only has a few typical metals in its menu.  The closest I could get to stainless steel was nickel, so I used that to get an AC resistance of 43 ohms per meter per wire at 3.5 MHz. Dividing by 18 for the number of wires gives about 2.4 ohms per meter compared to about 0.1 ohm per meter for a single strand of 14 gauge copper ... using the skin effect calculator in both cases.

Stainless steel would be even worse since it has higher relative permeability than nickel and the formula for skin depth is proportional to the square root of the inverse of permeability. Taking midpoint values for the range of permeabilities shown in various tables the actual number for the horse fence tape with 18 strands of 7 mil stainless steel would probably be closer to 5 ohms per meter.

I then generated a model for a 75m Inverted-V in EZNEC+ with an apex at 50 feet, both with and without a 5 ohm load roughly every meter of length.  Radiated power is a function of current so used a 1 amp forcing current in both cases.  Without the five ohm loads it took 46 watts to get 1 amp at the feedpoint.  With the 5 ohm loads it took 141 watts to get the same 1 amp feedpoint current. That's a power ratio of roughly 3, meaning that the horse fence antenna loses roughly 2 thirds of its power to resistivity loss in the thin stainless steel wires.

By the way, the feedpoint impedance appears to be about 140 ohms at resonance so it's no surprise that KF4BWG used an antenna tuner in his videos.

If anyone spots any errors I've made here please point them out to the list.  The last thing I want to do is spread bad information, but this doesn't look to me like a very efficient antenna.

73,
Dave   AB7E



On 4/18/2019 11:18 AM, Grant Saviers wrote:
The wires can be 0.007" diameter.  Not only is the DC resistance higher for stainless, but the ferromagnetic loss at RF can be significant.

https://www.statelinetack.com/item/field-guardian-2in-polytape-classic-14-wire/BXC33/?srccode=GPSLT&gclid=Cj0KCQjwwODlBRDuARIsAMy_28UA2BShpSxPs83FvSIvuOjQW1dQFVyMhvXhgK7CQK05EtlHaYD0YFYaAgg7EALw_wcB

Grant KZ1W

On 4/17/2019 15:42 PM, David Gilbert wrote:

I'm curious what anyone thinks is unique about this antenna. Yes, it is a bit wider than wire and it appears to roll up easily for changing lengths, but fundamentally it's just an Inverted-V.  I think it's kind of ridiculous that he advertises it as having "more gain".  As far as loss in the stainless steel conductor is concerned, the 18 wires might offset the higher resistance depending upon how thick the wires actually are, but who knows.

If he's still active, his QRZ.com web page says he's on 3976 Khz every morning at 7:45 Tennessee time so WP3C might check there.

73, Dave  AB7E


On 4/17/2019 1:59 PM, john@kk9a.com wrote:
I am curious about this product. It appears that these Horse Fence antennas use a wide stainless steel mesh radiator. Since the conductivity of stainless steel is significantly less than copper which is typically used in dipoles, how does this material effect the efficiency?

John KK9A


wp3c wrote:


HiDid someone knows or known KF4BWG the creator of the Horse Fence antennas?I visited his website and sent some emails but didn’t receive any answer. I’m curious about the broadband of those antennas. If someone knows about that please let me know. ThanksAlfredo WP3C Sent from my T-Mobile 4G LTE Device

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