LF: Re: Was Fire in the wire

["Markus Vester" <markusvester@aol.com>]

Warren, Laurence,

 

the earth losses beneath the loop could either be inductive or capacitive. In reality probably both, but which effect dominates depends on voltages, surroundings (branches etc), and also ground conductivity (higher conductivity increases eddy currents but lowers capacitive losses). Ideally you would have maximum voltage at the capacitor terminals and zero in the middle of the loop. One way to generally reduce voltages for a given inductance would be to split the resonance C into several larger pF capacitors in series, distributed around the circumference. This is often done with MRI loop antennas ("surface coils") to minimize resistive coupling to the patient's skin.

 

Best 73,

Markus

 

----- Original Message -----

From: Laurence KL1 X

To: rsgb_lf_group@blacksheep.org

Sent: Saturday, April 27, 2013 1:09 AM

Subject: LF: Was Fire in the wire

Thanks Warren - interesting stuff !
 
Certainly the ground beneath the loop is probably a couple of Kv pd above the conductor at my Kw level - I can create colorful violet coronas :-) - and the capacitive affects of raising the or lowering the horizontal section of the loop just a few feet make a lot of change to the tuning -
 
Ive followed a suggestion to "relieve" earth losses/add a little more tuning stability by shoving a Earth mat under the loop here - at the moment it is untuned but installing it did cause an increase in loop current by a couple of percent - I saw a paper tuning this earth and it is was said to improve the stability and increase the effective power in this NVIS set up at 137.
 
Im using the underslung pulley system on mine at the moment which has survived one Alaskan winter :-)  Rough diagram attached - there are two parallel conductors in this loop but a single black line is shown for clarity - adding a second conductor didnt change the efficiency much on 505kHz where its nearly resonant, but helped  on 137
 
http://kl7uk.com/loopy.jpg
 
Cheers
Laurence KL 1X  WE2 XPQ
Alaska

 

---

Date: Fri, 26 Apr 2013 17:47:51 -0400
From: wd2xgj@gmail.com
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Fire in the wire

Hi Stefan,

       There are thousands or perhaps tens of thousands of volts from the loop to ground.

If I grounded the loop with and there was a large potential difference, wouldn't that cause a large current to flow to ground and effectively shunt my signal to ground?

73 Warren

On Fri, Apr 26, 2013 at 1:44 PM, Stefan Schäfer <Stefan.Schaefer@iup.uni-heidelberg.de> wrote:

Warren,

What is the voltage between the tree (GND) and loop? I thought that the voltages are rather small, i.e. below 100V, especially when a large loop is used. Isn't it possible to ground the loop?

73, Stefan

Am 26.04.2013 19:21, schrieb Warren Ziegler:

Just a quick report on a failure mode for LF TX antennas.
Last Fall my original TX loop made up of approximately 500 feet of RG-11 coax suspended from trees went open circuit (both center conductor and shield), some time later part of the wire actually came down. I had thought that abrasion from contact with the trees had eventually worn through it.

What I found was a little more interesting. Looking at the downed end, it appeared that the insulation had melted for a considerable distance and there were burn marks further up the cable. My hypothesis is that the outer insulation was abraded away and the coax shield came into contact with the tree limb causing the burning/melting.
Will try to get some pictures this weekend and put them up on my website.

Long term if one wants to operate at the kW level either better insulation or a better way of supporting the antenna than trees would be required!

--
73 Warren K2ORS
                WD2XGJ
                WD2XSH/23
                WE2XEB/2
                WE2XGR/1

--
73 Warren K2ORS
                WD2XGJ
                WD2XSH/23
                WE2XEB/2
                WE2XGR/1