One side of the dipole and the feed makes 76 feet, not far from 66 feet
which would be a half wave on 40. Even with the slightly short radiator
on 40, it makes a low impedance that transforms through just over a
quarter wave of 600 ohm feed line to something high so the tuner arcs.
If the dipole was 60 ohms, it transforms to 6000 ohms through a quarter
wave of 600 ohm feed. 100 watts at 6000 ohms is 775 volts. If you cut
those feed line spacers in half to get a 300 ohm line or doubled the
conductor size to get a 300 ohm line, and were those spacers for #14?
Wire size is a part of the impedance formulas. With a 300 ohm feeder
that low antenna impedance would transform to only(?) 1500 ohms so the
voltage would be 1/4 what your getting. I see www.73CNC.com claims 14
THHN makes 600 ohm line. 600 ohms takes 4.48" spacing, 300 ohms takes
0.406" spacing.
73, Jerry, K0CQ
On 2/13/2011 6:17 PM, tom stewart wrote:
OK,
Many came to my aid when I asked for suggestions. I made some choices and
built an antenna. I tried hard to maintain spacing and avoid metal by at
least a foot. It did not come out well so I need help. Here are the details
if anyone has some suggestions I would be grateful-again.
1. Purchase "Ladder Snap" spreaders to build 600 ohm feeder.
2. Purchased plenty of 14 gauge THHN wire.
3. Because of room constraints, the legs of my inverted Vee are 31 feet.
Center is 35 feet high and the legs end at about twenty feet.
4. Legs to feeder is continuous- feeder is 45 feet from center and
terminated by grounded shield coax through window pane(vinyl).
5. Tuner is for now an MFJ balanced- model 974-I think.
6. reactance is high on all bands. It will only tune up on 14MHZ. tuner
arcs if I persist at 7mhz
7. readings with antenna analyzer are as follows:
Readings are at the bottom of the bands beacause I operate CW only.
7.020mhz r57 x158 swr 10.0
10.110mhz r168 x 135 swr 6.0
14.020 r21 x58 swr 7.5
18.075 r20 x70 swr9.1
Thanks in advance- I know it is difficult to say with this limited
information but I am at my wits end- about 10 hours into this
Tom N7ESE
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