Fw: Re: [DX-CHAT] Shortened Coiled Ants - Caution
Larry, Your comment that an impedance of 50 ohms at 84.3 degrees results in an SWR of 1:1 is NOT correct and may lead to confusion about SWR and complex impedances. To obtain an SWR of 1:1, the Load Impedance must be REAL (NO reactance) and match the Characteristic Impedance of the Line. For 50 ohm coax, the load impedance must be 50 + j ZERO to produce a perfect match with SWR = 1:1 Plotting Z = 5 + j 49.7 on a Smith Chart shows an SWR of approximately 20:1. The formula for SWR as a function of Reflection Coefficients is given in the ARRL Antenna Book and other texts. Regarding dipoles, the feedpoint impedance varies with height above ground in a diminishing oscillation centered around 72 ohms as shown in the ARRL Antenna Books and Handbooks. For heights above 1/4 WL, Dipole Radiation Resistance reaches it's highest value of ~95 ohms around 0.3 WL high and it's lowest value of ~58 ohms around 0.6 WL high. Note that antenna current will be a Maximum when the Radiation Resistance is at a Minimum which maximizes the antenna gain. Tom N4KG On Tue, 28 Jan 2003 K4WLS [EMAIL PROTECTED] writes: Shortened coiled dipoles and verticals exhibit a large amount of inductive reactance. Even if the overall impedance is 50 Ohms, the large voltages on the feed line cause by the large inductive reactive component can result in ATT or final transistor failure. Never use the ATT in your rig or directly connect such antennas to your rig. Always use an external tuner with such antennas. Suppose the impedance of your shortened coiled antenna has a Resistive component of 5 Ohms and an Inductive Reactive component of 49.7 Ohms: Z = R + j XL = 5 + j 49.7 = 50 Ohms at 84.3 Deg Oh yes, that will give you an SWR of 1:1, but Ouch !!! on the inductive kick. The perfect antenna has a resistive component of 50 Ohms and no Capacitive or Inductive Reactive component. Unless a simple dipole, then 72 Ohms of course !! Larry (K4WLS) Sign Up for Juno Platinum Internet Access Today Only $9.95 per month! Visit www.juno.com Subscribe/unsubscribe, feedback, FAQ, problems http://njdxa.org/dx-chat To post a message, DX related items only, [EMAIL PROTECTED] This is the DX-CHAT reflector sponsored by the NJDXA http://njdxa.org
Re: [DX-CHAT] [DX-NEWS] KH3 - Good news bad info
[Reply moved to DX Chat for comment] Mome, The future DXpedition status of Johnston will depend on how they end up classifying the atoll and what limits are placed on future non-scientific human visits. Desecheo and Navassa are both under US FWS control, and no legal access has been permitted to either in at least 10 years. This is not to say that Johnston will end up being as restricted... we just don't know yet. I also wonder what the future holds for Palmyra and Kingman Reef. I'm hoping that the new owners don't slap similar restrictions on one or both, and I'm also hoping that the K5K group made such a favorable impression that they won't do so. Bottom line is this: AH3D is the best chance in years to work Johnston, and it may be your last chance to do so for a long, long time -- if ever. 73, ron wn3vaw 'Never attribute to malice that which is adequately explained by stupidity.' --Hanlon's Razor - Original Message - From: Mome Z32ZM [EMAIL PROTECTED] To: BARF-80 BBS [EMAIL PROTECTED] Cc: K1B DXR [EMAIL PROTECTED]; NJDXA DXR [EMAIL PROTECTED] Sent: Wednesday, January 29, 2003 3:13 PM Subject: [DX-NEWS] KH3 - Good news bad info KH3, JOHNSTON ISLAND. GOOD NEWS IS : ''' and a near-future administration change will put the U.S. Fish and Wildlife Service (FWS) in charge of the overall operations of this U.S. territory BAD INFO IS : .' Future DXpeditions to Johnston Island, KH3, may be much more difficult.''...I don't think so ! PEOPLE from US FWS are verry nice , DX-peditioners just need to follow the instructions from the US FWS - Officers ! 73,GLDX!!! de: MOME - Z32ZM www.qsl.net/z32zm --- Subscribe/unsubscribe, feedback, FAQ, problems, etc DX-NEWS http://njdxa.org/dx-news DX-CHAT: http://njdxa.org/dx-chat To post a message, DX NEWS items only, [EMAIL PROTECTED] Archives http://www.mail-archive.com/dx-news%40pro-usa.net --- Subscribe/unsubscribe, feedback, FAQ, problems http://njdxa.org/dx-chat To post a message, DX related items only, [EMAIL PROTECTED] This is the DX-CHAT reflector sponsored by the NJDXA http://njdxa.org
[DX-CHAT] Complex Ant Z - SWR
SWR meter compares the relative magnitude of the forward and reverse voltages at a point on a transmission line. The forward and reverse voltages are complex numbers, and add to form Vmax and Vmin along the line. VSWR = Vmax/Vmin The magnitude of the reflected voltage is a function of the complex antenna impedance, the impedance of the transmission line, and the distance in wavelengths that the reflected voltage is measured distant from the antenna. This is an all inclusive stand-alone statement. An SWR meter has no way of knowing whether this reflected voltage is the result of the resistive or reactive component of the antenna impedance. For a transmission line not terminated it it's Characteristic Impedance, the complex impedance of the antenna is also reflected back from the antenna toward the transmitter. At multiples of 1/2 wavelength, the reflected impedance will be the same as the antenna impedance. At the other points along the line, the reflected impedance will possess inductive or capacitive reactive components. Such a line is called a tuned line, since along it's length it possesses the characteristics of a tuned curcuit. So, for a tuned line, the SWR will actually be different at different points along the line. Such tuned lines are often used to for antenna matching. For a complex load impedance, the Smith Chart is used to determine what the complex impedance is at various points along the line back from the load (antenna) toward the source (transmitter). So for an Antenna Impedance of Z = 4 + j 49.7 Ohms (or whatever it was), I wonder what distance in wavelengths you used for your Smith Chart to get the Impedance you came up with Saying that only the resistive component of a complex antenna impedance determines the SWR along a transmission line is like saying .well try this: Wind a 1 diam coil of 30 turns of # 22 nichrome wire - closely spaced. Cut a piece of RG-8 to an electrical wavelength of 1/2 at 10M. Measure the resistance of your coil. Connect one end of RG-8 to SWR meter using very short length from SWR meter to transmitter. Connect other end of 1/2 wavelength of RG-8 across your coil. See if your SWR at 30 Mhz is strictly the function of the resistance of your coil Use very low power, as the nichrome wire will change resistance a bit as it gets warm I rest my case !!! Larry Subscribe/unsubscribe, feedback, FAQ, problems http://njdxa.org/dx-chat To post a message, DX related items only, [EMAIL PROTECTED] This is the DX-CHAT reflector sponsored by the NJDXA http://njdxa.org
