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.
Friday, January 30, 2009
Tech Net Notes January 30, 2009
The topic for January 30th's Tech Net was Echolink. Questions regarding Echolink were invited.
ke5okt - Jim - has had an echolink node set up for about one year. He offered his advise to anyone who might have a question.
wa5psa - Brad- checked in and mentioned his interest in QRP contacts.
ke5ysh - Justin - regarding LK500 amplifiers, asked if anyone had any experience and advise on new tube burn-ins.
ke5wga - Ray - Remarked that he found Echolink nodes very useful while driving a truck in communicating between the Preston and Durant nodes.
kd5njr - Scott - had two questions
1. Does ke5okt, Jim, know of any good conference servers? ke5okt, Jim's answer was that he did not have an occasion to use conference servers.
2. Addressed to wa5psa, Brad, regarding QRP contacts, is that with code and what kind of antenna do you use. wa5psa, Brad answered that he uses an 80 meter dipole running through his attic, with each end running through a peak at the side of the house and then down to fences on each side of the house. He added that he usually runs 5 watts using CW.
ke5ehm - Paul - checked in
kd5cny - Tom - checked in and said he may have more later.
ke5lnn - Dave - checked in
k16und - James - checked in, visiting Tahlequah from San Diego.
w5ggw - Greg - checked in and advised that he was going to try to link up the 147.045 machine, and advised that it had been successfully linked.
ke5iig - Mike - checked in
kd5cny - Tom - asked "Can a switch box be built for two different microphones to be connected on the same radio? Has anyone done that?"
Another ham - said he didn't see why not, that it would be the reverse of one mike for two radios, but that worked with only compatible radios.
ae5ft - Gene - mentioned that he believed he had seen a QST article regarding the building of such a box.
ke5ncp - Tom - checked in.
ke5urg - Steve - checked in.
k5bbm - Brian - checked in and said he had tuned into 147.045 and heard the net. (confirming the linking of the VHF repeater to the UHF system.)
kf5rd - Pete - referenced a September 1988 QST article, page 18 that addresses building a switch-box for two microphones going to the same radio.
ki6und - James - said that one can be found at "tinyurl.com/coxde5", made from an old style computer switch.
wa5psa - mentioned that all QST articles are now available on-lin. He also said he found one made by MFJ model# 1262 at Ne Communications Solutions for $300, nscradio.com.
w5ggw - Greg - added that you should be able to us a 25 pin parallel switch, and that it's just a matter of figuring out the wiring. One PTT wire, two audio wires, and whatever else there is.
ki6und - James - said that what Greg mentioned is like the one he found at "tinyurl.com". He added that the cost was $90, but that you could buy the parallel switch by itself for $7, and then it's just a matter of matching what goes to what.
No other traffic or check-ins past this point.
If you notice corrections or additions that need to be made to this, feel free to leave that info as a comment below.
Don
ke5omv
Oklahoma Spotter Training
Click on this link for Spotter Training in your Oklahoma County
wild weather watchers
wild weather watchers
Thursday, January 22, 2009
Tech Net Notes January 21, 2009
THE BELOW IS A SUMMARY OF WHAT WE TALKED ABOUT ON THE TECH NET. IT IS NOT COMPLETE, BUT IT COVERS MOST OF THE POINTS DISCUSSED.
1. ke5omv, Don asks, " When can you use ladder line as a feed line for your HF antenna? In other words, how careful should you be on where you run your ladder line? or, what are the parameters that exists in the running of ladder line?
kc5uoq, Larry, asks a trivia question, "How many counties in Oklahoma voted for McCain?" The answer is...all of them; Oklahoma was the only state that carried MCCain in all of the state's counties.
kc5uoq, Larry added, "My scanning antenna works good with ladder line. No experience with transmitting through ladder feed line."
ke5wga, Ray, had no experience with ladder line or HF gear at this point.
ke5otq, Dennis, also uses ladder line for listening only.
n5fem, Steve says that in his experience, ladder line is too difficult to use. If you use it, it is best to keep the ladder line in the open. Rather than using it he would rather use a balun with coax back to the radio.
kd5cny, Tom, "No experience with ladder line."
kd5jkh, Martin, "The advantage of ladder line is low loss."
wa5psa, Brad says about ladder line, " I run 80-6 meters on one dipole. It is low loss. The SWR on ladder line on the antenna side of the antenna tuner does not create the loss as it would on coax. I use it for that reason. Cross metal objects at a 90 degree angle. Mine runs loosely behind my desk into a closet, up to the attic, runs along some coax and rotor wire, and on to the attic antenna. Each end of the dipole goes through an eve of the house and then angles down to a fence. It works good.
2. ke5uiu, Chris questions - Why can you not use larger than 12 gauge wire on a dipole antenna?
ke5yoo, Mike, "I believe it doesn't really matter what size wire you use, most programs don't address larger than 12 gauge. The size may change the capacitive characteristics of the antenna.
n5fem, Steve, "Over 12 gauge you may be getting into solid wire. I normally use 14 gauge wire.
kd5cny, Tom, " My opinion is that stranded vs. solid signal travels on the outside of the wire, on a solid core the signal travels smoothly...if there is a kink or a bend it can affect the signal. Solid core should (normally) be better. The bigger diameter of your driven element, the broader the bandwidth. 8 gauge wire weighs more and will be harder to support.
wa5psa, Brad, says, "My gues is that the calculations shut off past 12 gauge due to structural considerations.
Note: If you have an opinion, feel free to leave it as a comment. And of course, if you have a question for a future net topic, leave it in the comment area as well.
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