[Lowfer] Matching transmitters to transmission lines

[Bill Ashlock]

Johan,

Thanks for your return on my post. I think I am going to have to 
respectfully disagree with your statement that there is no problem in 
driving a coax with a 0 ohm square wave. I have tried this numerous times 
and large ringing occurs at the transmitting end even though the signal at 
the receiving end looks fairly close to a square wave. In the lowfer 
application the driving transistors run very hot and the efficiency is well 
below 100%. In the case of my exciter that drives the final out in the woods 
(terminated in line Z), adding a series 47 ohm resistor makes the square 
wave into the coax look nearly perfect and the transistors become cold to 
the touch! I could use an LC impedance matching network in place of the 
resisor but the loss in the resistor is of no consequence.

I just did a search in the ARRL handbook and I can find no example of a 
class D PA driving a coax unless there is a impedance matching network 
between. In fact in every example I can find relating to transmission line 
theory the source impedance is always matched to the transmission line with 
the same impedance.

Comments please.

Bill A


>From: "Johan Bodin" <sm6lkm.jbeab@swipnet.se>
>Reply-To: lowfer@mailman.qth.net
>To: <lowfer@mailman.qth.net>
>Subject: Re: [Lowfer] Re: Amplifiers for 136k
>Date: Sun, 25 Aug 2002 11:28:01 +0200
>
>Bill,
>
> > 3. The output impedance of the stage, in spite of the filter, is close 
>to 0
> > ohms, and hence, makes a poor match for a coax line (take notice Mike).
>
>That is no problem. In fact, the TX output impedance (resistance) should
>be as close to zero as possible. Anything else means loss. A TX output
>impedance of 50 ohms would result in 6 dB loss!
>
>If the far end of the coax is terminated in 50 ohms (the antenna), the TX 
>will
>see exactly 50 ohms when "looking into" the cable (assuming 50 ohm coax).
>
>However, "back terminated" cables are sometimes used in small signal
>systems (video distribution for example) to avoid re-reflections if the 
>load
>end of the cable happens to be slightly mismatched. In the video case, this
>would lead to blurred edges or "ghost pictures" depending on cable length.
>
>73
>Johan SM6LKM
>
>
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