In a message dated 9/19/01 4:30:17 AM Pacific Daylight Time,
alsopb@gloryroad.net writes: <<
What's wrong with your bridge? Show me the math that says this is so.
de Brian/K3KO
> I used only 1/4 wave of 90 ohm coax as that reached
> right to the rig. So that actually was the only coax used and a 50 ohm
> bridge read 1:1 SWR. K7GCO
> >>
Brian: I thought I'd have to explain this even more and didn't do it
enough. The 1/4 wave 90 ohm stub matches 50 ohms to 160 ohms which I
suggested adjusting the L antenna Rr to making it longer than a 1/4 wave and
using a series Xc. 50 ohms resistive is seen at the end of the 90 ohm 1/4
wave stub by the 50 ohm bridge regardless of the coax being 90 ohms or
actually because of it. The formula is Zstub= Sq Root of Z1Z2 and I'm sure
you can do this. You show me your math now. Now it's been many years since I
did this and at 3 AM in the morning these are the numbers that came to mind
typing this while listening to Art Bell and watching the History Channel out
of the corner of my eye. I've been on the computer for 12 hours today also.
I did a quick calculation and a 90 ohm 1/4 wave actually will match a 162
ohms to 50 ohms and that's close enough for RF TT work. Some say the Zo of
this coax is 92 ohms. In that case that wants to see a slightly higher Z at
the antenna and I'll let you calculate it. What do you get?
In any case to take care of all the coax variables the rule of thumb is to
vary the length of the L wire length longer than 1/4 wave and the series Xc
value so that 50 ohms is measured by the SWR or MFJ analyzer using this 90
ohms coax or what the hell ever it is. This concept is so simple that some
have a problem with it. It's a very effective way to reduce the losses at
the feedpoint. If you keep lengthening the antenna you will approach and
arrive at a 1/2 wave and a couple thousand ohms which matches easily with a
very low loss L network. With say 3000 ohms at the tip of the 1/2 wave
antenna running 1 KW there is about .57 Amps and about 4.5A at the other 50
ohm end with fairly low circulating currents. Would you believe it is so
efficient and cool, a "RF Frost" forms on L network coil. That's an
absolutely true story--I just made it up. A ground rod is all you need to
cool the coax shield as the antenna is self resonant. I will be running side
by side tests of a 1/2 wave verticle over a Brown, Lewis & Epstien IRE buried
120 radial field, over a buried 16 radial field, over 16 elevated radials and
then just a ground rod. And then having the base .18 WL high and 16 45
degree sloping 1/4 wave radials and other variations such as extended radials
on the ground like WWVH does out in Hawaii as Ted brought to light. I've
already done some of this but am doing it all over. Be prepared for some
surprises. Are there any more questions of math? I only describe what works.
Brown, Lewis and Epstien actually ended up with 100 radials in their tests as
optimum and had copper wire left over just enough for 120 radials so they
just added it. That's where the 120 came from they claimed. I did some
research and would you believe I found out that all 3 also had stock in
Anaconda Copper. Do you realize how much more copper sold with just 20 more
radials all these years?
It's after 6 AM--I think I'll read the paper and go to bed. I had a very
good day. I had what was potentially a $900 clutch job (145,000 miles) a
crook mechanic tried to tell me. After a quick phone call I fixed it in 4
minutes and 40 seconds. A dentist tried to tell me I had abcesses at the
tips of 2 root canals (which are potentially very bad) that needed pulling
and Big Bucks Bridges. I know how to read negatives very well myself doing
it since 1936 and got a dentist friend who didn't need work to do it over and
he couldn't find any evidence of abcesses. I offered him my Front Row World
Series Tickets at Face Value as my guest. The Mariners won again and have the
best all time won lost record. I'm going to the game tonight. I'd like to
take a portable HF rig, a low 1% loss L network and load up the retractable
roof over Safeco Field on 160 or 80M. What a Field Day antenna! I hope it
doesn't trigger the circuitry that makes it move. If you hear of a strange
"RF Halo" over Safeco well-----it may not be a terrorist. Someone said "when
they heard of the air planes crashing into the twin towers they thought it
was terrorists or a really bad control tower operator."
Since I saved so much money this week also with some other things I looked
at some new pickups. The Dodge Ram is Awesome. Does anyone have any
experience with it? K7GCO
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