On Wed, 08 Jun 2005 12:52:01 -0400, Buck wrote:
Pardon me while I learn from this discussion.
Hold on Buck.
So far there has been very little "learning" to be done from this
discussion.
I dislike getting personal but I strongly suggest that you reject
anything Fred has put forth. Some of what he says is correct, but for
the most part you are being led astray. Since you don't yet have the
skills to separate the good from the BS, the safest thing to do is
ignore it all.
For example he has suggested that an all-band antenna can be nothing
more than a 130' dipole fed through 100' of 450 ohm line to which you
connect a 50 ohm coax and "your rig will be happy."
Let's examine this premise, shall we.
If you don't already have it, download the free version of EZNEC.
www.eznec.com
If you have MS Excel do the following. If you don't skip down below
the dotted line.
Download the program XLZIZL.xls at:
http://www.qsl.net/ac6la/xlzizl.html
Using EZNEC set up a frequency sweep, for example 3.5 to 4.0 MHz in 50
KHz steps and check Microsmith Files as an output and give a file name
("80meter" for example)
"Build" the 130' long antenna in the wires menu, add a source in the
middle and do the frequency sweep. (I used a height of 50' and 12 AWG
wire)
Open xlzizl.xls and select the "ZIZL" worksheet if it isn't already
active. "Press" the "Clear all freq..." button (near cell A25).
Press the "Read file for Freq, R, X" button (near cell D25). Navigate
to the file location where you specified the Microsmith files be
located. The default is in the Smith subdirectory in the EZW
directory. Select the file "80meter.gam" and open it.
The calculated data will populate the Frequency, R at load and X at
load cells.
Press the "Refresh and show Smith" button (near cell I28). The Smith
chart will open and display the calculated impedance data for the 130'
antenna operated from 3.5 to 4.0 MHz.
Now we are going to add a transmission line. Select the "ZIZL"
worksheet again and press the "Set via Dialog" button (near cell A34).
The "Network definition" window will open with "Element position 1"
selected by default. Change it to "5". Under "Element type" select
Transmission line.
Under "Qualifier" scroll the dropdown menu to either "Generic 450 ohm
window" or if you more more realistic numbers you can select one of
the Wireman Ladder line types. (Ignore "wet" ones.)
Mouse down to the "Element Value" area an type in "100" for the length
and press the "Set This Element" button and then close the Network
definition window.
Press the Refresh and show Smith Chart button and you will now see two
traces, the original "load" data and the feedpoint data at the input
to the 100' transmission line. If you know anything about Smith charts
you will immediately notice that the match is worse at the input of
the line than it was at the antenna. So much for the "magic" 100'
length that Fred proposes.
Going back to the ZIZL sheet and looking in the "results" area you can
see that at 3.5 MHz the SWR is 36:1. Without the "magic" 100' of
ladderline, the SWR is 3:1.
You can repeat this exercise at different frequency ranges and see
just how awful this idea is.
Let's continue...
Assume that your 50-ohm coax is 50' long. Go back to the Network "Set
via Dialog" button. Accept the "1" default for Element Position and
again select Transmission Line for Element Type. For Qualifier, let's
use Belden 9258 (RG8X), although you can use what you use.
Set the length to 50' and press "Set this Element." Close the window.
Press the Refresh and show Smith button and you will now see three
traces on the chart: the load, the impedance at the inut of the 450
ohm line and the impedance at the input of the 50 ohm line.
Return to the ZIZL sheet and under results note that the SWR at 3.5
MHz is down to "only" 17:1. This is what your rig is going to see.
Also note that the network loss (the line loss) is over 3 dB.
Now just for giggles, let's take out the 100' of ladderline and make
the whole 150' run out of RG8X. You can just highlight the cells "G34
through G36" and delete them and then select cell C36 and type in 150
and tab out. Press "F9" and see the new results.
The 3.5 MHz SWR is now 2.4:1 and the total network (line) loss is 1.2
dB. So replacing the "low loss" ladderline with "lossy" coax improved
the match and lowered the loss.
I don't know how much of this "magic" I can stand.
************************************************** *****************
If you don't have Excel there is a more labor intensive method that is
just as accurate.
Go to
http://www.qsl.net/ac6la/tldetails.html
and download the program and open it.
Select a transmission line type, "Generic 450 ohm Window" for example.
The line parameters will populate the boxes to the right. Under Set
Frequency, type in 3.5.
Run EZNEC one frequency at a time, beginning with 3.5 MHz. Look at
the source data and note the Impedance R and X values. Pay attention
to the sign of X.
Copy these values to "R" and "X" in the TLdetails program. Remember
the sign of X. For example I used R = 61, X = -64.
In the "results" area under "At Input" you can see the R and X values
at the input and note that the SWR in the 450 ohm line is about 7:1
and in the 50 ohm feeder Fred would have you connecting at this point
the SWR is as before ~36:1.
If you want to "add" the 50 ohm line, copy down the R and X at the
input (132, -470) and enter them in the the R and X boxes above.
Change the line type to Belden 9258 and the length to 50 feet and as
above, the SWR at the input is ~17:1.
To summarize:
There are ample free tools to work these problems out without relying
on bafflegab. You don't have to take my word or anyone else's; work
the problem yourself and learn something while doing it.