Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!ginosko!usc!apple!agate!ucbvax!hplabs!hp-ses!hpdml93!tom
From: tom@hpdml93.HP.COM (Thomas Wheless)
Newsgroups: sci.electronics
Subject: Re: Antenna Publication
Message-ID: <970006@hpdml93.HP.COM>
Date: 5 Oct 89 17:27:39 GMT
References: <4918@cbnewsm.ATT.COM>
Organization: Hewlett Packard - Boise, ID
Lines: 77

> 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