[TMC] 50 ohms vs. 70 ohms

[John Vendely]

TMC's power amplifiers, such as their 350, 500, and 1000 watt models, will 
match to a wide range of load impedances, from around 20 ohms up to 600 ohms 
(unbalanced, of course) with phase angle +/-45 deg.  They  are intended for 
use in either 50 or 70 ohm systems without modification.  The wattmeter's 
directional coupler can be set up for either 50 or 70 ohms by simply 
performing the alignment procedure with the appropriate termination 
impedance.

Cable characteristic impedance is not critical when the cable length is very 
short in terms of a wavelength--1/10 wavelength max at the operating 
frequency would be a good rule of thumb.  Short coax jumpers of the type 
used to interconnect amplifier, T/R relay, and coupler are typically 
sufficiently short in the HF range, except perhaps near 30 Mc.

As a matter of principle, I wouldn't mix lines of differing impedance unless 
you have to.  But sometimes it's justified.  For example, I came across a 
3000' spool of 93 ohm double shielded teflon dielectric coax virtually for 
free, and use it to distribute the 1 Mc house reference frequency around the 
shack.  Good shielding was the primary concern there, and 30 or 40 feet of 
93 ohm line in a nominally 50 ohm system was of no consequence at a 
wavelength of 300 meters!

73,

John K9WT




----- Original Message ----- 
From: "H.L." <hl at sfsu.edu>
To: <TMC at mailman.qth.net>
Sent: Tuesday, September 08, 2009 3:21 PM
Subject: [TMC] 50 ohms vs. 70 ohms


>
> Hello again:
>
> I would like to raise another more general issue that I have not
> found in ER or QST.  How serious is the mismatch between 50-Ohm and
> 70-Ohm systems and cables? Many TMC transmitters and linear
> amplifiers are 70 ohms, but most ham radio antenna tuners,
> wattmeters, baluns, and antennas are specified as 50 ohms.  Also,
> useful one-foot up to ten-foot coax cables found at ham flea markets
> are usually 50 ohms. Can we ignore the differences? Is the power loss
> trivial--especially for short length cables?
>
> My apologies if this issue has already been covered--please let me
> know where to find the discussion or article.
>
> 73 Hal KK6HY
> email: kk6hy at arrl.net
> ______________________________________________________________
> TMC mailing list
> Home: http://mailman.qth.net/mailman/listinfo/tmc
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:TMC at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>