Hi, gang,
I finally got my 102-foot doublet put up. It's only up 35 feet or so, and it's
held about 4 feet away from the Rohn 25 tower by a yard arm. It's fed with 45
feet of homebrew open wire line (2.75 inches separation, number 14 THHN
insulated house wire, same as antenna). I think it's 300-400 ohms or so.
I plan to feed it with a ground-mounted SGC 231 autocoupler, which I haven't
tested just yet. If the coupler is bad I can always extend the open-wire line
into my basement shack. I replaced a small basement window with lexan. There
are some cable pass-through ports and a pair of brass feed-through bolts in
case I need to bring the open-wire line to the shack.
I haven't connected my MiniVNA or my spectrum analyzer to the antenna yet (cold
and dark when initially tested) because I have a 10k/1k AM broadcast station
3.5 miles north of this QTH, and I don't know how much RF might be present on
that big wire (TV stations, too, 3-4 miles away). I can't yet find my inline
attenuator, so I don't want to risk blowing the analyzer front end just yet!
Let's just say that I'm not yet very skilled with those tools...
What I did do was hook up an MFJ antenna analyzer right to the feed line wires
(no balun or impedance matching goodies...just the wires). The following table
lists the readings at several useful "dips" on the SWR meter:
2.82 MHz R=16 X=16 5.2:1 SWR6.43 MHz R=85 X=0 1.5:1 SWR12.39 MHz
R=85 X=0 1.2:1 SWR16.1 MHz R=108 X=0 2:1 SWR21.96 MHz R=85 X=0
1.4:1 SWR25.68 MHz R=108 X=0 1.9:1 SWR31.35 MHz R=79 X=0 1.5:1
SWR50.38 MHz R=85 X=29 1.9:1 SWR
Does this show anything meaningful or unusual? Or are these readings too goofy
because they're normalized to 50 ohms or only really exist if I keep the feed
line at its present length? (I will need to shorten it a bit). I can't really
test at the antenna feed point...
I'd think that the 6.43 MHz "dip" is closest to the antenna's fundamental
resonance?
What's that hiccup at 2.82 MHz? The nearest AM station (3.5 miles or so) is
KOLM on 1520 (3.04 MHz second harmonic).
Does the tower noticeably affect the readings being only 4 feet away?
Anything look to bizarre?
Thanks,
--Kirk, NT0Z
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