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[TowerTalk] ferrites & fires

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Subject: [TowerTalk] ferrites & fires
From: (Chuck Counselman)
Date: Sat Jun 28 12:09:16 2003
At 10:57 AM -0400 6/28/03, Martin Ewing wrote:
>I appreciated W1HIS's tale about his line isolator burn-up.  I was 
>thinking about getting some of these products, so the timing was 
>A few thoughts in response:
>-Life is simpler with QRP. :-)

No question.  Most of the time I transmit between 10 and 15 watts. 
Only when conditions require it (just twice in the last month -- I 
just reviewed my log.) do I QRO to 100 or 200 W.  Only when 
conditions really, _really_ require it (the last time was on May 1st, 
for a 20-m SSB QSO) do I switch on the power amp, for 1500 W.

>-Unless you have high confidence in the product you are buying _and_ 
>you understand the your operating conditions, you need to over-rate 
>your components....

That is _extra_ true for commercial products in the ham-radio market. 
Fortunately, I enjoy rolling my own stuff.  Email me for a photo of 
my 160 - 10 m, 4:1 balun transformer, which I believe based on a 
combination of theoretical arguments [using published data on the 
insulation (Teflon, 0.072" thick), the ferrite (six FT-240-61 
toroids), and the wire (10-gauge, silver-plated) that I used to build 
it] and low-power measurement data could handle 29 kW at SWR = 1, and 
a photo of my 45-uH common-mode choke, wound with Heliax LDF4-50. 
After my experience with Radio Works' Line Isolators, I made sure 
that every transmission-line component (a low-pass filter, 
directional coupler, c-m chokes, balun, and ATU) as well as the line 
itself within my house was not only over-sized, but also completely 
enclosed in metal.  My coax is all Heliax (LFD5-50 except for a few 
short jumpers of LDF4-50), with a solid metal outer conductor.

>-Everybody uses PVC, but PVC is not known for its ability to 
>dissipate heat, and it makes pretty nasty smoke.

Thermally, the worst aspect of Radio Works' Line Isolators is not the 
PVC outer shell but the plastic _foam_ that fills this shell and 
drastically reduces heat transfer from the small-diameter, tightly 
wound, 23-turn coil of small-diameter coax.  An academically 
interesting modification would be to replace the foam with a 
non-flammable, thermally-conducting, dielectric liquid, perhaps a 
Freon, or uranium hexafluoride (which is used in some 
extremely-high-voltage electric power cables).  Chemical 
compatibility with the coax-jacket material and the PVC shell would 
be an issue.

73 -Chuck, W1HIS
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