Amps
[Top] [All Lists]

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

To: Dan Sawyer <dansawyer@earthlink.net>
Subject: Re: [Amps] close to off topic - how to read antenna R + jX ??
From: David Kirkby <david.kirkby@onetel.net>
Date: Sat, 05 Mar 2005 13:15:42 +0000
List-post: <mailto:amps@contesting.com>
Dan Sawyer wrote:

>All,
>
>I have become interested in leaning more about what is actually going on 
>at the antenna. I have a vertical and would like to measure it.
>
>First I have an Autek RF-1. It measures swr, Z, L, and C at a preset f. 
>I am not certain what the L and C are reading. When I measure a 50 Ohm 
>terminator at 7.2 MHz it actually reads a Z of 50 and an SWR of 1.0. 
>However the L and C readings are of 2 uH and 2500 pf.  What are these 
>actually measuring? The values are very close to those from the antenna. 
>
>How can I actually measure the L and or C component of the antenna? If I 
>use an impedance bridge at the frequency will it measure L (or C)?
>
>I hope this is close enough to topic.
>
>Dan
>

I mentioned some time back in a sentence or two of another way of 
measuring R+jX, so I thought I'd elaborate a bit.

Here is a commercial lock-in amplifier

http://www.thinksrs.com/products/SR844.htm

which can measure the amplitude and phase of signals up to 200MHz. I 
hope to get one on loan soon, with a view to buying one.

You *might* get some useful info by downloading the manual on that, 
although the main aim of lock-in amplifier is to measure small signals 
buried in **lots** of noise - in this case, 80 dB below the noise. I 
best most hams would like to be able to hear other hams 80 dB below the 
noise.

I bought this model

http://www.thinksrs.com/products/SR810830.htm

which only goes to 102 kHz when I started my PhD. It can measure phase 
with a resolution of 0.01 degrees, but I can't recall what the accuracy 
is. I've used that, along with a couple of sig gens and mixers to make 
what is effectively a vector network analyser that worked at 500MHz, 
although in principle there is no limit to what you can make it work to. 
Here's a diagram of such an instrument.

http://www.medphys.ucl.ac.uk/research/borg/homepages/davek/phd/chapter1.pdf

see page. 64 of the thesis (which is page 36 in that chapter).

Although not the subject of my PhD, details were included as it was a 
competing technology to what my thesis is about. Here's a picture of me 
standing by the "DIY vector network analyser".

http://www.medphys.ucl.ac.uk/research/borg/homepages/davek/phd/phd.html

but it would not need to be that big. That unit 'Bambi' as it was known, 
RF modulated a number of lasers and detected the phase of the light 
collected on a very sensitive detector (as per the diagram in my thesis).

If you did think of using a lock-in to do this (and I don't think it is 
the best way to go, but mention it since it is definitely one way to do 
it), make sure the lock-in you get is a dual channel unit, as single 
channel units can only measure amplitude, not phase.

PS, if I do get one of the 200MHz units on loan (which Standford 
Research Systems will need to do if they want us to buy one), I'll do a 
quick few checks to see how it works as a VNA and report them back here.

PS, there a few Vector Network Analysers on eBay at the minute. This one 
is sitting at just over $500 with 15 hours to go.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=97190&item=3878457428&rd=1&ssPageName=WDVW

and this one will set you back a bit more, as the starting price is $57,000

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=97191&item=3879305896&rd=1&ssPageName=WDVW


-- 
Dr. David Kirkby, 
G8WRB

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



_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

<Prev in Thread] Current Thread [Next in Thread>