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Author Topic: Ladder line  (Read 3344 times)
WB6BYU
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Posts: 13029




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« Reply #15 on: February 22, 2012, 08:56:13 PM »

Here is VK1OD's analysis of the original "Cobra Jr." antenna:

http://www.vk1od.net/antenna/cobra/index.htm

Notice there are some frequencies where the losses in the antenna itself are very high.
That's because the currents in the folded wires mostly cancel each other, requiring a
large current to flow to get much power radiated.

This analysis is for a 72' antenna, so for a 41' version with the same construction you
would scale the frequencies by 72/41 = 1.756.  So, for example, the performance that
VK1OD shows at 10 MHz would apply instead at 17.56 MHz.  (In both cases it means
that efficiency would degrade significantly just below one of the WARC bands.)  It
also means that frequency below which the efficiency is generally poor, shown as about
6 MHz, would translate to 10.5 MHz on your antenna.

So basically 20m would fall on about 8 MHz on the chart - just below a major problem
range, but perhaps not too bad as long as environmental factors don't shift the
resonance downwards.  The spike shown between 8 and 9 MHz would fall between 20m
and 17m instead, and 10m comes in just short of the bad area shown starting at 17 MHz.

So, as long as the antenna is far enough from other objects (such as roofs, buildings, etc.)
that might shift the resonant frequency in an unfortunate manner, the antenna may work
reasonably well on the ham bands from 20m through 10m.

On those frequencies where efficiency is poor, the impedance presented to the tuner at the
end of the ladder line can be pretty extreme, and it wouldn't surprise me if you find it
difficult to match for some line lengths.
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W8JI
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Posts: 9304


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« Reply #16 on: February 24, 2012, 03:50:33 AM »

Well I finally got it to tune in, All the advise paid off!!!

The manual would have solved it, also.   :-)

Read steps 1 and 3 in particular, which tell you to tune your radio at very minimum power. This avoids the "constant power effect", and is much easier on the radio than tuning while watching current.  The manual for the tuner says:

Quote
Adjustment Procedure

To use the MFJ-949E for transmitting, follow the steps below:

1. Select the 30W (out) METER switch scale. Place the PEAK AVG button in the AVG
(out) position. Turn the transmitter's power control fully down.

2. Position the TRANSMITTER and ANTENNA MATCHING controls and the
INDUCTOR SELECTOR switch in the bottom Tuning Chart position for the operating
frequency.

Tuning Chart
Band Transmitter Antenna Inductor
160 1.5 1.5 F
80 1.5 1.5 J
40 1.5 1.5 K
30 4 4 Halfway between K and L
20 5 5 L
17 5-1/2 5-1/2 L
15 6-1/2 6-1/2 L
12 8-1/2 8-1/2 L
10 9 9 L

3. Apply just enough power on CW (or AM / FM / RTTY) to obtain noticeable deflection
on the reflected power meter.

4. Carefully adjust the TRANSMITTER and ANTENNA MATCHING controls for the
lowest reflected power.

NOTE: These controls interact. Adjust the TRANSMITTER control for minimum, then
adjust the ANTENNA control for minimum SWR. Go back and forth between
these adjustments as many
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KJ6TSX
Member

Posts: 116




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« Reply #17 on: February 24, 2012, 06:39:37 PM »

Great Advise
I actually read the manual, but figured something was wrong when it things didn't go as planned  Huh So just like driving a car, there's a learning curve and Ill get it figured out Soon!!!
Thanks For the Help!!! the curve would be longer with out it
George
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