resitance vs impedence
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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 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Ken,
I would rather deal with the antenna having the higher 'R'. The reactance can be easily tuned out with a capacitor or an inductor, but the return path losses (usually ground loss) on the antenna with the low resistance become a higher percentage of the total, and the antenna having the higher R will be more efficient - the SWR alone does not tell the whole story. To illustrate - assume a 12 ohm return path loss resistance and the reactance component properly compensated - with the 6 ohm antenna, the total resistance is 18 ohms and 1/3 of the signal is radiated, but with the 48 ohm antenna and the same 12 ohm return loss, the total resistance is 60 ohms and 4/5 of the total signal is radiated. The actual return resistance may vary, I just used 12 ohms because it made the resulting numbers come out easily - the principle is still the same with any other value. 73, Don W3FPR ----- Original Message ----- 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. _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Ken H
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 > >_______________________________________________ >Elecraft mailing list >Post to: [hidden email] >You must be a subscriber to post to the list. >Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > >Help: http://mailman.qth.net/subscribers.htm >Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Ken H
Ken H 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. The efficiency of the antenna is defined as 100 * Rr / Rl where Rr is the radiation resistance and Rl is the resistance (at RF) of the conductors making up the antenna. Assuming the antennas you describe have the same or similar conductor resistance, then, the one with the higher radiation resistance will be more efficient. Of course, tuner losses also come into the picture. In your example the reactance is inductive, and that is easy to cancel out with a capacitor. If there was a lot of capacitive reactance that had to be canceled by an inductance, you might have more tuner losses. In the case of a non-ground-independent vertical, the equivalent resistance of the ground path is also in the circuit, and this can make a big difference in efficiency. -- 73, Vic, K2VCO Fresno CA http://www.qsl.net/k2vco _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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