Fw: Topband: Noise and reception

[Donald Chester]

>From: k3ky@erols.com

...I will add those comments to my
>webpage writeup of the K6STI loop.
>Those who read the articles will end up building an actual open wire
>450 ohm line. Here, I used some of the old TV ladder line with the
>plastic rod spacers about every foot. Yes, you can use the so-called
>ladder line with the brown plastic that looks like oversized 300 ohm
>ribbon with a lot of rectangular holes punched out-  but *only* if you
>can confirm that the characteristic impedance is actually 450 ohms
>or higher. Apparently a lot of the stuff runs more like 370 ohms or
>so. K6STI said that stuff will not work. Further, you will get pretty
>bad detuning of the antenna when it gets rain, snow, or ice on it,
>and you will hear a drop in output signal level. Using real open wire
>construction minimizes that.

I read the articles in Sept 95 QST,  but I don't recall anything magic about 
450 ohm line impedance.  They did say that 300 ohm line would work only with 
small loops.  I suspect the 450 ohm figure comes from the standard impedance 
of commercially made line, and that you could give or take 10% or so with no 
noticeable effect.

I use #12 wire spaced 1 1/2 inches with standard EF Johnson spreaders every 
couple of feet, which calculates to something around 420 ohms.  I can't see 
how this value could be that critical.  I find very little shift in resonant 
frequency between wet and dry weather.  The winter this year so far has been 
so mild I haven't been able to observe the effects of ice.

Has anyone experimented with any effects of varying the imedance of the 
line?  I recall from the article the main concern was maintaining perfect 
balance and resonance of the tuned circuit with the antenna.

My antenna performs noisewise about the same as the beverage, but the signal 
output level is so low that I can't hear anything less than moderately 
strong signals.  Weaker ones are buried in the background hiss.

Don K4KYV

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