T-lines for dummies ...
A 1/4-wave of anything* will match a 50-ohm antenna back to a 50-ohm radio....
73
Scott
KD5NJR
" Myth #4 - A quarter wave transformer can always be used to match the antenna to the transmission line.
Basic Truth #4 - A quarter wave transformer has no magical properties. It can be used between two impedances for which the characteristic impedance of the transformer is the geometric mean. For example, a quarter wave, 75 ohm line will match a 112 ohm antenna to a 50 ohm line. Great! However, if you are familiar with the Smith Chart or other techniques for designing matching sections, you can use lengths of transmission line, both open and shorted, to obtain transformations or to make impedance corrections. (See "Quarter Wave Transformers" below)"
* It will help, I think, if the velocity factor of the material is close to 1.
Friday, February 27, 2009
Thursday, February 26, 2009
Tech Net Notes February 11, 2009
We had a question from a local area amateur radio operator who wanted an explanation of what forward and reflected power means on an SWR meter. He was very aware of the use of the meter as it applies to antennas, but he was unsure about it's use in tuning an amplifier.
The topic of the net was SWR meters...forward...reflected...what's the deal?
To start off, the net moderator read from an excerpt found at:
http://searchcio-midmarket.techtarget.com/sDefinition/0,,sid183_gci852555,00.html
(Note: you may have to copy and paste this link to get it to work)
"Under ideal conditions, the RF voltage on a signal transmission line is the same at all points on the line, neglecting power losses caused by electrical resistance int he line wires and imperfections in the dielectric material separating the conductors. The ideal VSWR is therefore 1:1.
When the line and load impedances are identical and the SWR is 1, all of the RF power that reaches a load from a transmission line is utilized by that load. When the load is an antenna, the utilization takes the form of EM-field radiation. If the load is a communications receiver or terminal, the power is converted into some other form, such as an audio-visual display. If the impedance of the load is not identical to the impedence of the transmission line, the load does not absorb all of the RF power (called forward power) that reaches it. Instead, some of the RF power is sent back toward the signal source when the signal reaches the point where the line is connected to the load. This is known as reflected power or reverse power."
Among the check-ins there were two who gave an opinion:
kd5cny, Tom said, "You have it right. Reflected comes back through the coax to the radio. When tuning an amplifier you want to keep the reflected power as low as possible.
w2csl, Bob added, "In my 70 years of being in the hobby, I've never worried about SWR. I just built the antenna according to the figures presented and it either worked or it didn't. It usually worked and it wasn't a problem.
In additon, on the blog ae5ft, Gene, had to say, "...sorry I missed the Tech Net, and you may have your answers already. Just in case you didn't get what you were after, here's my take on your SWR meter:
1. The SWR reading is where the needles cross.
2. The forward power is the amount of RF measured going to the antenna.
3. The reflected power is the amount of RF being sent back toward the meter - reflected.
4. Set the forward power as high as you can get it and the reflected power as low as you can get it. Normally, what increases forward power reduces reflected power, so the needles should work together to achieve this. When reflected power is at "zero" the forward power should be your xmtr power.
5. I don't have an amp, but I never tune my xmtr at full power (100 watts). My suggestion would be to tune at low power (25 watts maybe) then retest at 100 watts. After that you can safely bring your amp into the mix.
6. Make some notes of the settings needed on your favorite frequencies and you can dial right in. With some experience, you will be able to tune up without a big hassle.
73, Gene (AE5FT)"
Note from moderator: There were other opinions and comments but the notes were lost. My deepest apologies.
Sunday, February 1, 2009
Hams and Eggs Breakfast
THE NEXT BREAKFAST:
Saturday, December 5th
Broken Arrow Amateur Radio Breakfast
"Hams and Eggs"
Golden Corral
9711 East 71st Street
71st & Mingo, Northeast corner
8:00 a.m.
"Hams and Eggs"
Golden Corral
9711 East 71st Street
71st & Mingo, Northeast corner
8:00 a.m.
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