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

[David C. Hallam]

I guess my typing got ahead of my brain.

-----Original Message-----
From: Tomm Aldridge [mailto:KD7QAE at ARRL.NET]
Sent: Sunday, February 27, 2005 2:23 PM
To: dhallam at rapidsys.com
Cc: David Kirkby; amps at contesting.com
Subject: Re: [Amps] close to off topic - how to read antenna R + jX ??

Of course you mean inductive reactance at 220MHz.  There will be very
little change in inductance with frequency unless there is a permeable
material in or very near whose mu changes with frequency (ferrite,
etc)or the geometry of the conductor is such that the mean area enclosed
by the "loop" changes as a fuction of frequency (skin and proximity
effects) enough to be seen.

David C. Hallam wrote:
> Admittedly you need a little more math than 1st year high school algebra,
> but it's not all that daunting. You also need to be able to navigate
through
> the Smith chart.
>
> On top of that, it's fun just to connect components to the terminals and
see
> just how much inductance the leads of a resistor have at 220 MHz.
>
> David KC2JD
>
>
> -----Original Message-----
> From: David Kirkby [mailto:david.kirkby at onetel.net]
> Sent: Sunday, February 27, 2005 1:02 PM
> To: dhallam at rapidsys.com
> Cc: amps at contesting.com
> Subject: Re: [Amps] close to off topic - how to read antenna R + jX ??
>
> David C. Hallam wrote:
>
>
>>You can also use a Boonton Model 250A RX meter or one of the later models
>
> of
>
>>the 250A made by HP after they took over Boonton.  They appear on eBay
>>fairly regularly and seem to sell for prices in the $50 to $100 range.
>
> They
>
>>are not exactly a portable piece of equipment though as they definitely
>
> fall
>
>>in the boat anchor class of test equipment.
>>
>>David C. Hallam
>>KC2JD
>>
>
> I've not used the Boonton 250A, and would suggest anyone looking at
> buying any of these professional instruments did a bit of research
> first. Perhaps buy a manual before the kit - you can always sell the
> manual if you find the kit it not suitable.
>
> You really need to be *serious* about wanting the decent impedance
> (R+jX) measurements, and able to do the maths, but if you are, then I
> personally think some money spent on old used professional test
> equipment might be better value than spending the same money on brand
> new glossy kit from the likes of MJF. I've not used any of their antenna
> analsers, but having seen some of their kit, I'm not too impressed.
>
> Others might disagree of course.  Is there anyone using such
> professional kit for ham radio use on here, who might like to comment?
>
> My BSc project report would be useful here, but it was written using the
> old word processor 'runoff' on a VAX running VMS. I don't have the
> runoff file, which is a shame, as I'm sure converting a runoff file to
> pdf would not be too hard. I can't be bothered to scan it.
>
> Dr. David Kirkby, G8WRB.
>
>
>
>>-----Original Message-----
>>From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]On
>>Behalf Of David Kirkby
>>Sent: Sunday, February 27, 2005 10:48 AM
>>Cc: amps at contesting.com
>>Subject: Re: [Amps] close to off topic - how to read antenna R + jX ??
>>
>>If anyone is serious about making antenna impedance measurements you
>>might want to consider getting a used HP vector voltmeter (make sure it
>>has the probes). With a coupler, and some attention to calibration
>>(short and open circuit loads), you should be able to make reasonably
>>accurate measurments, even without the expense of getting it
>>professionally calibrated. I used one on the final year project of my
>>first degree, looking at a vertical antenna with stacked quarter and
>>half wave sections. I measured R+jZ, then used a section of transmission
>>line of the right impedance to convert this to 50 + j0.
>>
>>Looking on eBay, an HP vector voltmeter can be bought for the same sort
>>of prices of these amateurish instruments.
>>
>>Read up about them before shelling out $$'s, but I think for real
>>measurments, you would hard to beat for the money. They are not battery
>>powered, so not much use up an antenna, but there is no reason you can
>>not make the reference plane the end of a bit of coax.
>>
>>A vector network analyser would be better, but then, even used, these
>>are serious money.
>>
>>--
>>Dr. David Kirkby,
>>G8WRB
>>
>>Please check out http://www.g8wrb.org/
>>of if you live in Essex http://www.southminster-branch-line.org.uk/
>>
>>
>>
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>>
>>
>>
>
>
>
> --
> Dr. David Kirkby,
> G8WRB
>
> Please check out http://www.g8wrb.org/
> of if you live in Essex http://www.southminster-branch-line.org.uk/
>
>
>
>
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