[TowerTalk] Amplifier Impedance Matching

[XV4Y (Yan)]

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 at contesting.com a écrit :

> Message: 1
> Date: Mon, 28 Oct 2013 09:43:52 -0700
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: towertalk at contesting.com
> Subject: [TowerTalk] Amplifier Impedance Matching
> Message-ID: <526E9448.5080300 at 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