cab@cbnewsm.ATT.COM (carl.a.benevit) (10/02/89)
Posting for a friend with no net access... Is there a recommendation for a recent publication on antenna theory and use. My friend had built a transmitter for use in model rockets that he uses. It works fine but he would like to make sure he is getting the best possible range per ounce of transmitter payload. Please email any responses. Thanks! ______________________________________________________________________________ Opinions and commentary are personal and not those of AT&T and subsidiaries. ______________________________________________________________________________ Carl Benevit - att!cbnewsm!cab - 215-770-3564 AT&T Bell Laboratories - 1247 South Cedar Crest Boulevard, Allentown, PA 18103 | Think about it... ---|--- JOHN | 3:16 | ______________________________________________________________________________
myers@hpfcdj.HP.COM (Bob Myers) (10/04/89)
>Is there a recommendation for a recent publication on antenna >theory and use. My friend had built a transmitter for use in >model rockets that he uses. It works fine but he would like to >make sure he is getting the best possible range per ounce of >transmitter payload. Please email any responses. Thanks! Well, the good ol' ARRL Antenna Handbook is still probably the best introduction to the subject for a beginner, without getting into the fairly heavy-duty math of serious antenna theory/design. However, I think that I can provide some additional help in this particular case. The problem your friend's facing is one of providing the strongest signal AND an omnidirectional pattern, with an antenna which I assume must be kept as small and as light-weight as possible. (By the way, knowing the intended frequency of operation would help a LOT. For this application, you really want to be in the VHF or UHF range.) If an antenna is properly "matched" to the transmitter, such that we can assume that all of the power makes it into the antenna, then the only way to provide actual "gain" is to make the antenna directional; that is, to squirt more of the signal in one direction, at the expense of having less power radiated in others. This is the principle behind "gain" antennas such as the Yagi beam or the log-periodic (which actually is a compromise between gain and a wide bandwidth). Omnidirectional antennas MUST (to qualify for the name!) radiate equally well in all directions, and there's only so much signal available to begin with. This sets a limit on the signal power which can be radiated in a given direction. BUT, you say, what about these "omnidirectional" antennas which claim "gain of Xdb over a quarter-wave whip" (which is itself an omnidirectional antenna)? Well, consider that the radiation pattern acutally extends in three dimensions, not two. If a perfect "isotropic", or "point" source were possible, it would radiated equally well in ANY direction - in other words, a plot of equal-signal-strength points would be a sphere. Real antennas - such as vertical whips - do not do this, but instead have a radiation pattern which is roughly toroidal. The "flatter" the toroid, the greater the signal strength in that *plane*, at the expense of signal strength in other directions. The usual means of getting the pattern "flatter" (besides moving the antenna up away from the ground) is to make it *longer*, and thus a 5/8 wave vertical antenna can be said to have "gain" (toward the horizon!) over the shorter quarter-wave whip. For a model-rocket based transmitter, though, we have the restriction of having to keep the antenna short and light, plus we cannot count on any particular orientation with respect to the ground-based receiver. A small whip antenna is probably the best that can be done here, and likely this will take the form of a helically-wound whip (or "rubber duckie") for size and ruggedness reasons. The key, then, is to provide the best possible *receiving* antenna; if you have someone to track to rocket, a dish antenna provides tons of gain, but a narrow "beamwidth" - you've got to aim it pretty well. A good choice might be a small Yagi on a hand-held mount; check the amateur radio handbooks for suitable designs for the VHF frequencies. (And again, exactly WHAT frequency did you have in mind? Gentle reminder: are you *licensed* to use the frequency in question?) Good luck! Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers%hpfcla@hplabs.hp.com | sentient life-form on this planet.
barry@hprmokg.HP.COM (Barry Fowler) (10/05/89)
cab@cbnewsm.ATT.COM (carl.a.benevit) writes: >Posting for a friend with no net access... >Is there a recommendation for a recent publication on antenna >theory and use. My friend had built a transmitter for use in >model rockets that he uses. It works fine but he would like to >make sure he is getting the best possible range per ounce of >transmitter payload. Please email any responses. Thanks! >______________________________________________________________________________ >Opinions and commentary are personal and not those of AT&T and subsidiaries. ______________________________________________________________________________ >Carl Benevit - att!cbnewsm!cab - 215-770-3564 >AT&T Bell Laboratories - 1247 South Cedar Crest Boulevard, Allentown, PA 18103 Try the ARRL (American Radio Relay League) Antenna Book. You can get it at most ham radio or electronic stores for $18.00. ISBN 0-87259-206-5 for 1988 edition. It's geared to amateur radio but the formulas don't change. It has lots of stuff for verticals, phased arrays, Yagis, etc. Barry
tom@hpdml93.HP.COM (Thomas Wheless) (10/06/89)
> Is there a recommendation for a recent publication on antenna > theory and use. My friend had built a transmitter for use in > model rockets that he uses. It works fine but he would like to > make sure he is getting the best possible range per ounce of > transmitter payload. Please email any responses. Thanks! To get the best possible range you first want to orient your antenna correctly. For example, I assume your transmitter uses a whip antenna or a dipole because they could be glued or taped along the side of the rocket body. Both whips and dipoles radiate most strongly in a direction perpendicular to the axis of the antenna and so you would want to orient the antenna vertically to get the most range horizontally. Secondly, you want to match the output impedance of your transmitter to the input impedance of your antenna using a matching network (typically an L-network or Pi-network made of inductors and capacitors). Both impedances are typically complex (have both a real and an imaginary part). The output admittance of the transistor itself can usually be found in the transistor data sheet in the form of y-parameters for the lower frequencies or s-parameters for the higher frequencies. This output admittance is in parallel with your collector load (assuming you're operating common-emitter). So now you've calculated the output impedance of your transistor and you now have the more difficult task of finding the impedance of your antenna. Assuming you don't have an expensive vector voltmeter which will measure the impedance directly, you will have to use an equation (which can be found in the ARRL handbooks) which will give you an approximate impedance based on your operating parameters (frequency, length of antenna, etc). This approximate impedance can be used to design your matching network. The inductors and capacitors in the matching will probably be adjustable so that you will be able to tweak in the network in the final step of the process. Once the matching network is designed, you can maximize the output power by measuring the field strength of your signal and adjusting your matching network until your field strength peaks. The ARRL handbook describes several field strength meters you can build. You could also use your receiver to to get an idea of the field strength. The voltage at the output of your IF amplifier will be proportional to the input RF voltage (assuming your input signal is not so strong that your IF amplifiers are limiting (clipping)). If they are clipping, you could either move the receiver further away from the transmitter or put an attenuator between your receiver antenna and the receiver itself. Once you can measure the field strength, adjust your matching network until the field strength peaks. By the way, if your pulsing your transmitter to save power, you will want to modify it temporarily for this step to make it operate continuously. When you make an adjustment to the transmitter, step a little away from the transmitter before taking a reading to make sure the capacitance of your body doesn't affect the transmitter. References: "High-Frequency Circuit Design" by James Hardy, Reston Publishing, 1979, about $45.00 (A good introduction) "Solid-State Radio Engineering" by Krauss, Bostian, and Raab, Wiley, 1980 (More theory than the first book) Priced at about $35.00. "Antenna Handbook" Henry Jasik, Editor, McGraw-Hill, my version is dated 1961 but I believe there is a newer version available. Somewhat pricey at around $60 - $70 but contains a lot of good information. The only reference I've seen that addresses dipole antennas of less than one-half wavelength. Hope the above will be of some help to you. By the way, would you let me know if you find some good circuits for model rocket transmitters and receivers. In another five or six years, I'm sure my son will become interested in model rockets and I'll be building the same type of things for him. Tom Wheless Hewlett-Packard Boise, Idaho