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
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