[TowerTalk] Stupd question about measuring Z and SWR

[Roger (K8RI) on TT]

An AIM in the shack can do all that for you without ever leaving the rig 
and it will do it to two or three decimal places. An accuracy that is 
way beyond being useful. .
The difference between 50 ohms and real life coax is close enough that 
it really doesn't need to be taken into consideration.
The AIM can show you impedance bumps, SWR, loss, return loss, Z, R, 
etc.  Even with a dipole the AIM will display changes in parameters 
caused by the wind moving the antenna, trees, and any nearby objects.

Save the initial traces, add an antenna and overlay the traces to see 
changes.
The AV640 vertical is a great working antenna, but is very sensitive to 
nearby antennas.  I found a 40 meter dipole over 50 feet distant made 
substantial changes as shown by the AIM.  It gives far more information 
than SWR, but for most of us, the SWR as seen in the shack is good 
enough for who it's for.

I knew the real SWR was at the antenna, but for me and my rig the SWR as 
seen in the shack was what was important.
A tuner in the shack only changes what the rig sees, which is usually 
good enough.

True, if you want every last watt out of the antenna  that the coax can 
deliver then it needs to be a perfect match at the feed point. 
Unfortunately a simple antenna like a dipole is unlikely going to be 50 
ohms,
However, there is likely more loss in a tuner at the feed point between 
the 2:1, or possibly 3:1  SWR, points than in the mismatch.

In my case, I do use good, low loss coax, with exceptions. Most runs 
from the rigs to the common point ground (CPG), the Polyphasers, the 
6-pack, the grounding at the bottom and top of the tower, the rotator 
loop and the antenna may result in as many as 10 connectors in over 200 
feet.  The loss in all those connectors does add up, but on 160 through 
10  the total is still minuscule (even with UHF connectors).

OTOH I even use RG-8X (on 40)  to the sloping dipoles.
If I ran QRP, I might be more concerned, but then, I'd not be running 
SO2R with all the switching and connectors.

73

Roger (K8RI)


On 6/1/2016 Wednesday 7:29 PM, Roger D Johnson wrote:
> Real coax is seldom exactly 50 Ohms
>
> Real coax has a small amount of reactance
>
> Real coax has loss
>
> If you don't do a SOL calibration at the antenna end you're just 
> guessing!
>
> 73, Roger
>
>
>
>> On 6/1/16 1:20 PM, Hans Hammarquist via TowerTalk wrote:
>>> When you want to measure the "real impedance" at the antenna, isn't 
>>> it important that you have a feeder (coax) that has the correct 
>>> Zo=50 ohms all the way?
>>>
>>>
>>> I guess that the bridge will not read a correct antenna Z if the 
>>> coax is not perfect.
>>>
>>>
>>
>>
>>
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-- 

73

Roger (K8RI)


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