What can I learn?
|Apr 26th 2013, 21:26|
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I am about to set up my antenna farm. For sake of this discussion we will limit it to dipoles, inverted Vs, and loop antennas.
I have a relatively basic understanding of the Smith Chart. I'm not a pro, but I have a fair amount of industrial experience. I have the following capabilities in my shack: SWR (simple: forward and reflected power), SWR built into my rig, Power Meter (old military job), McMillan GDO, and home made standard terminations.
Question 1) If I were to make standards of common impedances using non inductive resistors, and terminate a feed line with a known physical length and a known velocity factor, I should be able to determine by the SWR which impedance matches the impedance of the cable. True or false? I would first dip out the terminations and make sure I knew exactly what the impedance is of the termination.
Question 1A: I should then be able to determine the nominal impedance and velocity factor of an unknown cable of known physical length, True or False?
Question 2: I am planning on making my dipole with a type N connector at the top so I can disconnect it from the dipole, and also to prevent moisture contamination of the coax. If I substitute various known standard impendances, and plot them on the Smith Chart, hung in place 30 feet above the ground, then I compare that data with the data from the dipole, will I be expected to be able to determine the impedance of the feed point of the antenna from the SWR measurement at the rig?
If I am able to measure this, I am much more interested in winding baluns. Yes, my goal is to continue my study of the Smith Chart, and yes, I am studying on the Extra Class material. I can do all but the math problems, and those just take a bit more time than I have applied. Please give me your comments.
My question is: What can I learn with my current equipment that will be insightful rather than just measuring the SWR to determine if it will burn out the finals or not? Oh, and yes, I do have my own copy of Terman.
|Apr 27th 2013, 15:49|
Joined: Apr 4th 1998, 00:00
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(It's customary to give your callsign and first name. Most of us use our callsign as the ARRL username. You can change your username if you want.)
I think the answers to your questions are mostly yes, except that with just an SWR measurement, you won't be able to calculate the actual complex impedance at your feed point - or anywhere else. SWR does not tell you about phase, so you can't distinguish inductive from capacitive from resistive terminations. It will tell you the frequency where your antenna is resonating, so it will tell you whether to lengthen or shorten it for a desired center frequency.
If you want to do a complete measurement, you'll need a vector antenna analyzer. They're not cheap, and most hams get by without one, I think.
Without an analyzer, you can still do interesting experiments besides finding resonance. You can terminate a line with a short circuit, an open circuit, or any known impedance and get an idea of line loss and nominal impedance.
Many people obsess about getting 1:1 SWR. It's really not necessary, especially for HF frequencies. As long as your SWR is less than 2:1, most transmitters will work fine. If you have an antenna tuner, you can use 3:1 or even more, accepting some extra attenuation.
It sounds like you'd enjoy reading the ARRL Antenna Book! (It's a bit more practical than Terman.)
73 Martin AA6E