where to search information
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sometimes it is hard to find specific information, I am trying to understand the relationship between power out put and antenna length and its line losses .
example: suppose I have a random length of wire hook-up of 20 meters (ok, would that be 60 feet) and I have two watts output. if I shorten the wire to 10 meters what will my watts output be, frequency would stay the same for this example. _______________________________________________ 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 |
RE: where to search information
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mc,
There is no relationship between power output (measured at the transmitter end) and the antenna length. Any antenna will radiate, and all the power produced by the transmitter is radiated - losses may occur most anywhere (and everywhere) in the system, in the feedline, in the ohmic characteristics of the antenna, in losses due to absorbsion by objects near the antenna. We normally don't consider the losses by absorbtion until the frequency is well above the HF bands because it gets worse as the frequency increases (feedline loss does too). Part of the answer depends on how every part is matched to the other parts. The RF power radiated by the antenna will be the power produced by the transmitter minus the power absorbed by all the loss contributors - so the real answer to your question is that it all depends on your specific antenna system. The antenna, its positioning, and the feedline are all a part of your antenna system, and must be considered together. 73, Don W3FPR > -----Original Message----- > > sometimes it is hard to find specific information, I am trying to > understand the relationship between power out put and antenna > length and its line losses . > > example: suppose I have a random length of wire hook-up of 20 > meters (ok, would that be 60 feet) and I have two watts output. > if I shorten the wire to 10 meters what will my watts output be, > frequency would stay the same for this example. > _______________________________________________ > No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.323 / Virus Database: 267.6.9 - Release Date: 6/11/2005 _______________________________________________ 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 |
RE: where to search information
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In reply to this post by mc-20
"mc" asked:
sometimes it is hard to find specific information, I am trying to understand the relationship between power out put and antenna length and its line losses . example: suppose I have a random length of wire hook-up of 20 meters (ok, would that be 60 feet) and I have two watts output. if I shorten the wire to 10 meters what will my watts output be, frequency would stay the same for this example. ---------------------------- The power delivered to the antenna would still be 10 watts, mc. Now, what would the strength of the electromatic field produced by the antenna be? That's a tougher question that requires some exotic measurements usually well beyond the capabilities of Radio Amateurs. In the first place, such measurements require a stable area over which to make them that extends for at least several wavelengths in all directions! Even commercial companies where I've worked, such as Sylvania Electronic Systems, tested antennas on a range a few hundred yards across by scaling down antenna designs to VHF size. That's why Hams are allowed to estimate the "Effective Radiated Power" (ERP) in the one instance where we are required to do so: operation the shared 60 meter frequencies. The ERP is a function of the Antenna and the environment in which it is used. For evaluation, many antennas are presumed to be in "free space". That makes calculations easier but it's very hard to put an antenna in free space on a city lot. Besides the earth can actually help in certain circumstances. It can double the ERP of a horizontal dipole for either long-distance skip or short skip (near vertical incidence) communications, for example, depending upon the height of the antenna. Antennas smaller than a 1/2 wavelength - the smallest "self resonant" antenna - suffer from ohmic losses mostly. As the antenna is made smaller, the circulating currents in the conductors increases very rapidly. RF travels only in the very surface of a conductor, so the resistance of a conductor is usually much, much greater at RF than it would be for direct current. That's why very thin tubing is as good a solid copper for RF, and why a very thin layer of gold or silver plating greatly helps conductivity at RF. The day we can buy cheap, room-temperature semiconductors we'll have really efficient small antennas! Another area of losses is the earth itself. If the earth is near an antenna, the antenna will induce RF currents in the earth. The earth is a relatively lousy conductor, as poking your ohmmeter probes into even a damp garden soil will disclose. The RF currents in the earth are almost all used for warming the worms. This is a particular problem when using a "Marconi" antenna - a 1/4 wave antenna working against "ground". Heavy RF current will try to flow from the ground connection. Without a lot of help, almost all of this power will be lost in heat. The losses skyrocket at the antenna is made shorter than 1/4 wavelength and the currents get stronger. A short whip such as you see on a car or a pedestrian mobile uses may lose 95%, or more, of the RF in the earth with only 5% being radiated. Historically broadcast stations, which use "Marconi" antennas that are short electrically in spite of their physical size, use very extensive ground radial systems. The old "standard" ground was 120 0.2 wavelength long radials extending from the base of the antenna. Such radials intercept the RF wave and cause current to flow in the wire instead of through the lossy earth. Conversely, if you end fed a 1/2 wavelength wire, it will show a voltage loop at the end so almost no RF current will flow there. It will show a very high RF voltage but almost no current. So a ground becomes almost unimportant in such a setup, although some grounding is usually needed to keep the whole rig from being 'hot' with RF voltage. Antenna and antenna efficiency is probably one of the least-understood, hardest to measure and, for many of us, one of the most interesting areas of Amateur Radio. There are a number of excellent books on antenna theory and practice out there. Two of my personal favorites are as Heye's (G3BDQ) little volume "Practical Wire Antennas" and Moxon's detailed manual "HF Antennas for All Locations". Both of these are published by the RSGB and available through the ARRL. Heye's book is more "nuts and bolts" ideas and descriptions of popular antennas with little math, where Moxon takes a much more rigorous engineering look at subject. In addition there's the ARRL Antenna Handbook available through the ARRL and through Elecraft (www.elecraft.com). One tool that's been invaluable to hams doing "thought experiments" about antennas are the antenna simulation programs that run on a PC such as EZNEC by Roy Lewellan (W7EL): see: www.eznec.com. One thing to keep in mind about these simulators: they are based on what we know about antenna behavior and theory. Any errors in the theory, assumptions or conditions will be an error in the predicted performance. Still, they appear to be very good. At least, few, if any, Hams have the resources to do actual measurements and dispute them <G>. An excellent on-line source for Antenna information and theory are extensive the web pages created by L.B. Cebik, W4RNL: http://www.cebik.com/ Cebik has a wealth of information about antennas, tuners, simulators and other stuff written for the interested Ham. Ron AC7AC _______________________________________________ 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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