[TowerTalk] Fwd: RG-149: 50 ohm/70 ohm - does it matter?

[Roger (K8RI) on TT]

On 12/15/2013 8:40 PM, Hans Hammarquist wrote:
> -----Original Message-----
> From: Jim Lux <jimlux at earthlink.net>
> To: towertalk <towertalk at contesting.com>
> Sent: Sun, Dec 15, 2013 7:41 pm
> Subject: Re: [TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?
>
>
> I think the real question is whether a  such a hybrid/coupler/line
> sampler would have a characteristic impedance? That is, if I have a 75
> ohm source and a 75 ohm load and I put a line sampler (designed for 50
> ohm systems) in between, will the 50 ohm source see a mismatch.  I think
> not.
> Wrong! If you put a "line sample, made for 50 ohm, in a matched 75 ohms line, the sampler will show a 1.5;1 SWR. It doesn't care what source you have, only what impedance the load have.

As I under stand, auto tuners work on the magnitude of the SWR where
SWR = 1 + (sqrt)(Pr/pf)/1-(sqrt)(Pr/Pf) Note this is strictly a power 
forward and power reflected equation or ratio and can not be less than 
1:1. An auto tuner works to "minimize this figure" regardless of the 
sample line standard. So, I would think that unless the manufacturer 
went to unnecessary lengths the tuner will go for minimum.
It appears to me that the trigger point, or maximum allowed before the 
tuner would "kick in" would be the only thing referenced against the 
relative standard.  Even though the meter says 1.5:1, the amp would see 
1:1 at minimum ahead of the tuner.

Regardless of what's at the antenna, if the tuner (internal, or 
external) can match the impedance then the amp is seeing the proper load.

73

Roger (K8RI)


> You will find some type of referrence inside every SWR meter, may it be a transmission line with a characteristic impedance or a resistor in a bridge. As you might remember the "old" phone patches used a combination of resistors and capacitors for the referrence in order to achieve a good isolation between the incoming and the outgoing audio.
> I know you can buy SWR meters made for 50 or 75 ohms but I haven't seen anything for ladder lines (300 - 600 ohms). Maybe because nobody is looking it. Who cares what SWR you have on a ladder line, you always connect that to a tuner anyhow.
> Hans - N2JFS
>
>
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