[Amps] close to off topic - how to read antenna R + jX ??

[David Kirkby]

Dan Sawyer wrote:

> Well,
>
> Based on the replys I purchased an HP vector volt meter to measure 
> antenna impedance. 

I don't think that is a bad choice (I think it was me who oringallly 
suggested it, so I've got to say that).

> 1. How do I use the vector volt meter in place of the above to read 
> phase? The hookup is an RF generator, a coax feed line, and an 
> antenna.  I would like to measure the antenna and ground charactoristics. 

I would start by getting the manual - there must be dealers with them. 
That will give you the accessories too, which you might want to purchase 
- or make yourself and describe how to use it.  It will also describe 
exactly how to use the instrument to do this. You might not be able to 
download the manual on an old one, but a more modern one might well have 
a downloadable manual.

I did it many years ago, but can't recall how, although my undergrad 
degree project must have some details on it. I'll dig that out for you 
and see what I can recall. I think I used a dual directional coupler to 
sample the forward and reverse voltage. I suspect I used an open (180 
degree reflection) and short (0 degree reflection) loads to calibrate 
it. What you measure will not be 0 or 180 degrees. So you just add an 
offset, to correct the measured values, from what you know to be true. 
This would work for perfect couplers. As a starting point that will get 
you going. Then look at correcting for inadequacies in the coupler later.


Also, take a look at some Agilent/HP application notes on vector network 
analysers.

> 2. Is there a way to determine the antenna and ground charactoristics 
> ffrom the shack with the vector volt meter? 

All phase measurements *must* be made at a reference plane. So you need 
to make the reference plane the end of the bit of coax you attach to. 
Hence use a coupler in the shack, put the coax on the coupler, and 
calibrate your  setup at the end of a bit of cable. Then attach that to 
your anteanna.


> 3. I have not used Smith Charts. I assume they are part of the answer 
> to 2. 

On a Smith chart you can place a point at any impedance you want. The 
centre is at 1+j0, but you can just multiply all values by any constant 
(say 50), so the centre is a pure 50 Ohms. But if your antenna is 100 + 
j25 (for example), you would plot that at point 2+j0.5. Then with a 
compass at the centre, you can draw a circle showing all the impedances 
that the will be seen for any length of coax. A half wave is all the way 
around the Smith Chart, since any impedance (say 100+j25) will look 
100+j25 if you have an integer multiples of a half wave.

A Smith Chart is useful for understanding things, but you are probably 
better getting a bit of software to make life easier and more accurate.

I would be tempted to do some experiments at 70MHz or so, where you can 
make up cables of various managable lengths using a cable of known 
velocity factor, so you can compute (from the Smith chart or a bit of 
software) how the impedance should change as you put longer or shorter 
bits of coax in.  You can also assemble youself some loads where you can 
calculate the impedance, so you can build confidence that you can make 
measurements proplerly.



-- 
Dr. David Kirkby, 
G8WRB

Please check out http://www.g8wrb.org/ 
of if you live in Essex http://www.southminster-branch-line.org.uk/