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

[David Jordan]

I think it was mentioned... about 100years ago ;-)

73,
dave
wa3gin

----- Original Message ----- 
From: <n8de at thepoint.net>
To: <towertalk at contesting.com>
Sent: Monday, December 16, 2013 12:54 PM
Subject: Re: [TowerTalk] Fwd: Fwd: RG-149: 50 ohm/70 ohm - does it matter?


>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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