[Antennas] Help with feedpoint impedance measurement

[Robert Lay (W9DMK)]

I would recommend that you make measurements of the impedance at the 
frequencies of interest. Then, I would consider using some matching sections 
that can be switched or plugged into the line when operating on bands that 
have awkward impedances.

I would also recommend the tuner designed by Richard Measures. I don't have 
a reference at hand for the QST article on his design, but it is a good one.

73 de W9DMK, Bob in Dahlgren, VA
http://www.zaffora.com/W9DMK/W9dmk.html
----- Original Message ----- 
From: "John Lawson" <jpl15 at panix.com>
To: <antennas at mailman.qth.net>
Sent: Sunday, February 18, 2007 7:12 PM
Subject: [Antennas] Help with feedpoint impedance measurement


>
>
>   I have a loop around my back fence (and rear eaves of the QTH) of about 
> 480' total length, rectangular in about a 4:5 ratio.  It is an average of 
> 5'5" from the ground, save for the house which is nearly 7'. Topography is 
> high desert, ground is alluvial clay/sand - and quite dry most of the 
> time. Dunno what the actual ground resistance is - but salt-marsh it 
> ain't. (North-western Nevada, Carson City / Virginia City area)
>
>   Currently I am running 450 ohm ladder-line from the feedpoint of the 
> loop (the interface is a large bakelite barrier strip with approximately 
> the correct 'spread') back about 30 feet to an Ameritron ATR-15 tuner - 
> which my Johnson Valiant feeds - 160 is a bit touchy to tune, 75M can be 
> gotten down to 1.1:1 across the fone segments, 40 tunes 1.3:1, 20 gives 
> the same, it doesn't like 15, and 10 is tunable but also quite tricky.  I 
> am primarily interested in 80, 40, and 20 - perhaps 160 too.
>
>   I have a rescued broadcast transmitter that I am planning to use - it 
> will put out full legal limit when locked into 'low power' mode... so my 
> Plan is to use a home-brew automatic (or semi-automatic) tuner at the 
> balanced loop feedpoint, and employ a coax balun there, and run 50-ohm 
> line back to the transmitters: they will always see 50 ohms regardless of 
> what the feedpoint impedance happens to be at that moment.
>
>   I have the parts and overall design data for the tuner - but I am 
> wondering what the advice would be to actually measure the impedance of 
> the feedpoint over the range of my frequencies of interest - as opposed to 
> using a modelling program (like EZNEC) to derive this information.  I have 
> a good selection of 'high-end' test gear, and I can think of a couple of 
> ways to do this - but I'm by no means an antenna expert - so I thought to 
> ask before I leap...  ;} Once I know what the range of feedpoint 
> impedances is, then the design of the tuner can proceed from there.  I 
> will primarily be using a pair of large roller-inductors, with fixed 
> capacitances.
>
>
>   Thanks!!
>
>
> John
> KB6SCO
> DM09fg
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