Home |
Search |
Today's Posts |
#4
![]() |
|||
|
|||
![]()
Gary
I have very little confidance in any calculations I do without outside help. Maybe you can help me. I understand the 100 foot long center fed dipole antenna is horizontal to the earth and is 35 feet up above it. At the operating frequency of 3.8 MHz, its input impedance is 17 -j343. Although I didnt realize the real part of the dipole fell that rapidly with frequency and nearness to effective ground, I assume these numbers are accurate. I'll consider the 39 foot long shorted stub made from 300 ohm twin lead to be about 0.166 wavelength long (if the velocity of propagation for that line is 0.9). That makes the length of 300 ohm TV antenna twin lead from the antenna to the place where the 50 ohm line gets connected, to be about 0.153 lambda. And the remaining 3 feet of 300 ohm line to be 0.012 lambda long. Would the antenna's impedance plot at 0.057 -j1.14 on a 300 ohm Smith Chart? Thats what I calculated. If I go 0.153 lambda from the antenna along that 300 ohm line toward its shorted end, the impedance looks like 0.02+j0.18, which might be something like 6 ohms in series with 54 ohms of inductive reactance. The three foot length of shorted twin lead seems to look like 21 ohms of inductive reactance. I figured the impedance at the location where the 50 ohm line connects to the 300 ohm twin lead is something like 6 ohms resistive in series with 54 ohms of inductive reactance all in shunt with 21 ohms of inductive reactance. What am I doing wrong?? The way I'm looking at the problem, there will be a serious mismatch to 50 ohms. at 3 feet from the shorted end of the "matching stub". If I had any confidance in my thinking ability, I'd suggest using a shorter stub, so the impedance at the location where the 50 ohm line conects is still R-jX. Then the shunt L will have a chance of providing a more resistive match. Jerry "JGBOYLES" wrote in message ... Just for practice, I'd like to know your numbers. that three foot stub of 300 ohm line seems like it would impose alot of low impedance inductance in shunt with the feed Sure Jerry, I was hoping someone would check it. Eznec says at 3.8 MHZ a 100' dipole about 35' up over average ground is 17-j343. .126 lambda or 32.7' of 300 ohm line transforms to 7.5-j17.6. ..011 lambda shorted stub or 2.9' connected at the 7.5-j17.6 point gives 50-j0. The .011 lambda stub is .874 uh at 3.8 MHZ . This is according to Mr. Smith and his chart. 73 Gary N4AST |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
Inverted ground plane antenna: compared with normal GP and low dipole. | Antenna | |||
40 meter dipole or 88 feet doublet | Antenna | |||
Dipole Next To Home-Is That A Problem?? | Antenna | |||
Pls comment on this dipole | Antenna | |||
shortened dipole loaded | Antenna |