On Sat, 2010-02-20 at 08:46 -0800, Denton wrote:
> Hi all...a little off the Ten Tec groups, but...
> I have an MFJ 259B that I use and am a little confused about the R and X
> readings...
> I am trying to determine the true losses of my 235 ft horizontal loop, fed
> with 600 ohm ladderline, then to a 4 to 1 voltage balun outside the shack.
> SWR seems reasonable..below 3.8 to 1 80 thru 10 meters with the exception of
> 60 meters.
Most of the R of a decent antenna is radiation resistance, not wire loss
resistance. You can't tell about an antenna's efficiency or looses by
looking at the feed impedance. Unless its a very well documented antenna
like a quarter wave vertical that should show about 35 ohms R if the
grounds are very good. That's at the antenna, measuring through a feed
line will confuse you. Many a ham vertical measures more like 60 or 70
ohms feed R at quarter wave resonance and that's an indication that the
ground resistance is significant and decreasing the efficiency or
increasing the energy dissipated in that ground resistance and not
radiated.
> I run about 15 ft of good quality coax to the shack.
> I also run 450 ohm feeders to the shack and a johnson matchbox to compare
> signal levels...and to be honest, I can't tell much difference on transmit
> nor recieve....except the signal to noise is a bit better with the balun vs
> the matchbox.
That better S/N may be a sign that the balun's better isolation of
ground to the balanced side is reducing reception of noise by the
feedline.
> I have tried 4 to 1 and 1 to 1 current baluns, but the voltage balun gives
> me better matching.
As one might expect when the main feed line is 450 ohms and the antenna
tends to be higher impedance than low impedance. Those ratios are
impedance transformation ratios. 4:1 with a 200 ohm load shows 50 ohms
to the transmitter (or antenna bridge) if the 200 ohm load is on the
high impedance side. 1:1 doesn't change the impedance if its perfect,
though it may show a change because of SWR because R + jX changes with
feed line length anytime the SWR is greater than 1:1.
>
73, Jerry, K0CQ
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