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Re: hardline



This (50 ohm to 75 ohm matching and loss)  is an often misunderstood
transmission line problem.  Additional loss due to mismatch between a
transmission line's characteristic impedence versus it's terminiated load
impedence will be very small if the transmission line's matched line loss is
low at the frequency of use.

For example,  per the program TL (Transmission Line) by N6BV (published by
the ARRL),  100' of 3/4" 75 ohm hardline has 1.62 dB of matched line loss at
440 Mhz.  If that line is terminated into a 50+j0 ohm load impedence, the
SWR at the load is 1.5:1 and is 1.32:1 at the line input.  The additional
loss due to SWR on the line is only 0.09 dB for this transmission line type
at 440 Mhz, for a total loss of 1.71 dB.  Assuming an efficient impedence
match is made at the line input from the tx/rx, this additional loss may be
considered negligable, depending upon the application.  Some ARRL antenna
books contain rough charts that demonstrate this same effect.

For the same parameters of 100' length,  440 Mhz  and 50+j0 load impedence,
1/2"  75 Ohm hardline is:

Matched Line Loss = 1.94 dB      Add'l loss due to mismatch = 0.11 dB.  SWR
at line input 1.29:1

I have used 1/4 wave impedence transformers (copper lines, air dielectric)
to match 50 to 75 ohm transitions on hardline.  The additional 4 connector
losses plus the loss of the 1/4 wave transformer itself added up to more
additional loss (approx 1.2 dB at 432 MHz as I recall, as measured on a
laboratory test instrument) than if I had not matched the impedance at all
(for my application of line length, line type and frequency).

73,

Dennis - N4NR
n4nr@amsat.org

>From: Mark West <MarkW@instantdocuments.com>

>
> Not! This is nonsense, do not believe this. Impedance
> matching is very important, especially in transmission.
> Mismatched impedances cause reflections that set up
> standing waves in the wire, and just like sound waves,
> these waves create interferance patterns, which consume
> power and degrade the signal.
>
> - Mark West

>>>From: K3ROJAL@aol.com <K3ROJAL@aol.com>

>>>ohm cable used for CATV drops is also very good. Don't worry about any
>>>matching for 75  to 50 ohm .......


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