Hi Jim,
I remember from my old Line Theory lessons than the choice of 50 ohms for
coaxial lines is not accidental.
I cannot remember the exact background but in TEM mode the lower losses (and
then better power handling because of less heat generated) is at 51,2 ohms.
50 ohms was then a good choice.
For receiving however, 75ohms was still good regarding losses but offers a
better match to a dipole. The core wire is also smaller allowing some savings
on copper...
I also think I remember than for high-power application when we use "rigid" air
dielectric coaxial lines the best impedance is different. However I don't
remember if it lower or higher... I have been working more on waveguides than
on coaxial lines... still we used "off the shelf" devices so I was not spending
much time thinking about impedance choice.
Unfortunately I have no time to do extensive researches but I found this by
ON9CVD:
http://sharon.esrac.ele.tue.nl/~on9cvd/E-Waarom%2050%20Ohm.htm
By the way, for small powers (100W), I often use cheap and widely available
CaTV 75 ohms cable. It works very well.
On a fixed frequency a 1/2 wave line offers no mismatch and mismatch losses.
For a multiband antenna, the internal ATU of my TS-590s handles the mismatch
and using only short runs the mismatch losses are acceptable.
50ohms cable is quite expensive here and I have to go to Saigon (7 hours trip
by car) to buy by 100m cuts.
73,
Yan.
---
Yannick DEVOS - XV4Y
http://www.qscope.org
http://xv4y.radioclub.asia/
Le 29 oct. 2013 à 02:16, towertalk-request@contesting.com a écrit :
> Message: 1
> Date: Mon, 28 Oct 2013 09:43:52 -0700
> From: Jim Brown <jim@audiosystemsgroup.com>
> To: towertalk@contesting.com
> Subject: [TowerTalk] Amplifier Impedance Matching
> Message-ID: <526E9448.5080300@audiosystemsgroup.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> On 10/27/2013 7:14 AM, Jim Lux wrote:
>
> The fact is simply that because 50 years ago when the first solid state
> ham rigs were designed they were designed to be happy with a 50 ohm load
> was nothing more than a good design choice given available devices and
> commonly used transmission lines. And I suspect that 50 ohms might have
> been chosen over 75 ohms because the first solid state amps (and their
> output devices) were likely made for use in 2-way radio, which was
> almost exclusively a vertical ground plane that matched 50 ohm coax.
> When I got started in ham radio in the mid '50s, we mostly used dipoles,
> which we learned were a closer match to 75 ohm coax than to 50 ohm coax.
> What we missed in those days was that height affects feedpoint Z, and
> that low antennas tend to be a closer match to 50 ohms. So 50 ohms
> wasn't as dumb a choice as it first looked. :)
>
> 73, Jim K9YC
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