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

[n8de at thepoint.net]

I wonder why no one has mentioned the fact that the "50-ohm antenna"  
is NOT 'etched in stone'.

ANY antenna can be made to have a 75-ohm feedpoint, thus allowing the  
use of 75-ohm feedine.

When the transmitter/amp/tuner looks at the feedline, it will be  
75-ohms, which MOST GOOD rigs/amps/tuners can easily match.

73
Don
N8DE


Quoting Hans Hammarquist <hanslg at aol.com>:

> Hi Jim,
>
>
> -----Original Message-----
> From: Jim Lux <jimlux at earthlink.net>
> To: towertalk <towertalk at contesting.com>
> Sent: Mon, Dec 16, 2013 12:07 am
> Subject: Re: [TowerTalk] Fwd:  RG-149: 50 ohm/70 ohm - does it matter?
>
>
>
>> On 12/15/13 5:40 PM, Hans Hammarquist wrote:
>>
>>> 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.
>>
>
>> I'm not so sure.  If I make a short length of 75 ohm transmission line
>> (or, for that matter, use a PL-259/SO-239 barrel, which is NOT 50 ohms)
>> in a typical 50 ohm system.  So it's, say, 10 feet of 50 ohm line, 2" of
>> 75 ohms, and 20 feet of 50 ohms, terminated in a perfect 50 ohm load.
>
>> I doubt that if you hooked up an SWR meter it's going to show anything
>> remotely like 1.5:1.
>
> Correct, an SWR meter (made for 50 ohm) would show 1:1 but your 75   
> ohms line is now not matched as it sees a 50 ohms load. The short 75  
>  ohms line does not ?upset? your set-up. That's why it is called   
> ?short?.
>
>
>> Sure, if I hook up 10 feet of 50 ohm line, then 20 feet of 75 ohm line,
>> terminated in a 75 ohm resistor, THEN the SWR meter is going to read 1.5:1.
>> (neglecting the loss in the line in all cases)
>
> In the above set-up it doesn't matter the length of the lines (if we  
>  can neglect the losses)
>
>> I contend that it is EXACTLY the same if the BIRD or whatever has a 50
>> ohm line inside, and you're operating it in a 75 ohm or 92 ohm or
>> whatever system.  The meter will show forward and reflected readings,
>> but they won't be numerically correct (because the meter is calibrated
>> for a 50 ohm system), but the ratio is probably right..
>
> I suggest you try it. Forward and reflected power is just one way of  
>  dealing with SWR. The ?true way? is actually to measure the voltage  
>  along the line, take note of the highest and the lowest reading.  
> The  SWR is the highest divided by the lowest reading. This is   
> transmission-line-independent way of measure SWR. (It is easy to   
> understand why we don't do that though.) The distance between the   
> high and the low reading is, by-the-way, 1/4 wave-length.
>
>>> 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.
>
>> Yes, but the "bridge" or "line" doesn't have to be exactly the same as
>> the system impedance.  It can be close, and even if not close, the
>> numbers can probably be calculated for a new system.
>
> No, I didn't say that. E.g. you can measure a 15 kOhm resistor in a   
> bridge without using 15kOhm anywhere else in a Wheatstone bridge. If  
>  you care, you can build a transmission line SWR meter using any  
> type  of transmission line as long as you balance it for the desired  
>  impedance.
>
>> This is really nothing different from taking a VNA and using a set of 75
>> ohm standards to calibrate it instead of 50 ohm standards.  The couplers
>> inside the VNA don't change, just the relation between "voltage on the
>> detector" and "power" changes.
>
> When you ?calibrate? you do the balancing. If you calibrate for 75   
> ohms you set up the system for 75 ohms. If you now try to use it   
> (without recalibration) in a 50 ohms system you will get the wrong   
> readings.
> Hans - N2JFS
>
>
>
>
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