Re: resitance vs impedence

Posted by Stephen W. Kercel on Jan 17, 2005; 2:44am
URL: http://elecraft.85.s1.nabble.com/resitance-vs-impedence-tp373670p373672.html

Ken:

Couple of points to consider:

1) Is the complex impedance measured at the antenna feed point, or at the
transmitter end of the transmission line? As you are no doubt aware, as you
move along the transmission line from the load to the source the impedance
of the transmission will roll along a circle of constant SWR (assuming the
line losses are negligible). Thus, if you're measuring complex impedance at
the transmitter end, you need to do a Smith Chart computation to back out
the impedance at the antenna end.

2) Be very wary of the R+jX meters being sold to hams. The Autek and MFJ
are junk. The CIA-HF is pretty good, but even it shows considerable error
for impedances far from 50 ohms.

3) The quantity that really matters is the R in the R+jX measured at the
antenna feed point. Essentially, that value is the sum of the radiation
resistance (the part of the energy coupled to the antenna that turns into
radiated RF) and the losses (the part of the energy coupled to the antenna
that turns into heat). With reasonably good materials and construction in
your antenna, the losses should be well less than one ohm. Since the
radiation and losses are essentially forming a resistive voltage divider,
if you have choice of radiation resistances, always go with the high one.
That will put the greater fraction of your signal into radiated RF. For
example, suppose the losses are 0.6 ohm, and your radiation resistance
is  6 ohm. Suppose your power is 100 watts. For the resistive part of the
circuit, you end up with 25.69 volts across the load, of which 2.33 drops
across the loss, and 23.36 drops across the radiation resistance; that
works out to 0.82 dB of loss (if I did the math right). On the other hand,
suppose your radiation resistance is 48 ohms. Now if you crank through the
same math, your loss works out to be about 0.1 dB. A difference of 3/4 dB
is close to being audible.

4) Much more significant is the fact that you can and probably should try
to knock down the reactance at or near the feed point. That 6.5:1 SWR is
causing added line losses, and for any practical length of affordable
feedline, those will be well in excess of a dB, and possibly many dB.

5) Effectiveness has to do with how much of your signal actually gets
radiated. From the perspective of the transmitter, if your tuner gives you
a 1:1 SWR, all the energy (except for the 0.1 dB or so being turned into
heat in the tuner) is being coupled from your transmitter to the
transmission line. Effectiveness then turns on what fraction of that energy
becomes radiated RF, and what fraction contributes to the heat death of the
universe. There, the two rules of thumb are very simple and consistent. The
lower the actual SWR on the line (as opposed to what the transmitter thinks
it is seeing), and the higher the radiation resistance, the better you get out.

73,

Steve
AA4AK

At 07:39 PM 1/16/2005 -0600, you wrote:

>While setting up a dipole antenna tonight a question come up while
>discussing the tuning with myself:  Which is the "most effective" antenna
>- understanding it will be matched with a antenna tuner, either the
>internal autotuner or an external manual tuner - a dipole type antenna
>with a resistance of R=6 ohms and impedance of X=16, or the same dipole
>type antenna with R=48 and X=146?  Both of these settings have the same SWR=6.5
>
>I guess that same question would apply to a short vertical, or end fed
>zepp type antenna.
>
>Anyone care to comment on this?
>
>Thanks,
>
>73 de Ken
>K9FV
>K1 #1951
>
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