Welcome To the Tulsa Tech Net Blog

NOTE: THANKS TO AD5PE, JAY, FOR SENDING THE PIC OF THIS ANTENNA. TOWER HEIGHT IS 330 FT. THE 80M BEAMS ARE AT 300 FT. AND THE 160M AT 270 FT. TOTAL WEIGHT IS 80,000 LBS. THE 160M IS 3 ELEMENTS, 12.9 dBi GAIN, 26 DEGREE TAKE OFF ANGLE. THE 80M IS 5 ELEMENTS, 15.7 dBi GAIN, 12 DEGREE TAKE OFF ANGLE. FRONT TO BACK RATIO FOR THE 160M IS 20-30 dB AND 20 dB FOR THE 80M.



The purpose of this blog is to support and promote the 2 meter Tech Net hosted by Broken Arrow Amateur Radio Club's w5bbs repeater on 146.910 on the second Wednesday of the month and Tulsa Amateur Radio Club's w5ias repeater on 145.110, and 443.850 and the linked system, on the last Friday of the month.

The purpose of the Tech Net is to focus discussion of technical issues related to Amateur Radio. A big part of that is to provide a forum in which inexperienced (or less experienced) Amateur Radio operators may ask questions and thereby receive advice.

So if you have a question that you’ve been dying to get an answer to, you can do a “shout out” on Tech Net Night. Or feel free to leave us a question/comment on this blog in any one of the posts.

Tech Net Notes March 11, 2009

The net started with a query that wb5vst, Ben, relayed to me on the March 10th RACES Net. He was interested in knowing how to go about hooking up several radios to the same speaker.

With four or so radios in his car and speaker space sparse, having the radios share a speaker would help him manage the available space. What he wants is the ability for one speaker to carry the signals from four radios all at the same time (as if you have four radios on). So a switched speaker system would not necessarily work.

On the RACES Net, kd5und, Travis, suggested a mini-mixing board. A mixing board would tend to isolate the signals from each other and send them to one speaker output.

To start things off on the Tech Net, ke5yoo, Mike, relayed some information he discovered that related to the last Tech Net on February 27th, regarding coax. There was an issue of how to use various types of coax. Mike mentioned that RG58 is not a very good coax to use. It's only good for about 150 watts of power on VHF. In addition, Mike told us that as the frequency goes down, all coax types tend to work better.

ke5zol, Casey, checked in and had a question about the performance of his station. He said that yesterday evening, he copied the RCWA Net quite well, but could not make it in from about 31st and Harvard. He is talking on 10 watts with a magnet mount antenna on a piece of sheet metal in his garage.

kc5wsi, Ray, replied that Casey's equipment performance was understandable and that it is really performing well. Getting into the RCWA repeater with that set up, that far away, is not likely. In fact, Casey is doing well to get into the BA repeater as well as he is. ke5omv, Don, agreed.

ae5ft, Gene, related to Casey that he had a similar set-up when he placed an ultra-light MFJ magnet-mount antenna upside-down on his ceiling vent. Gene went on to mention that with that set-up, at his location, he was able to get into all of the Tulsa area repeaters.

ae5fa, Rob, (regarding 4 radios/1 speaker) suggested that installing a 4 to 8 ohm resistor on the signal side of the audio line coming out of each radio and hooking into the speaker with the return lines (or shield) wired in common and to the shield side of the speaker may work to keep signals from back-feeding into the other radios.

ke5wsu, James, suggested using diodes instead of resitors.

ae4fa, Rob, mentioned that diodes would just "clip" the audio and it would not work well.

kb5vdb, Ron, said that he found a device that you can use to hook up 6 radios to one speaker in the AES catalog, but that it may not allow you to listen to all of the radios at once. The part number is NCS 3220.

wx5lib, Matt, said that he has used a commercially available product to hook up four radios to one speaker in emergency vehicles. It is called a 4-channel combiner amplifier. The website for that is: http://www.fleetradioproducts.com/htm/model301b.htm

kb5vdb, Ron mentioned that you could re-wire a quadrophonic headphone to allow for a 4-input headset.

kc5wsi, Ray, said he found the four into one device from AES and it is called an interface kit with catalog numbers 3240 and 3230.

kc5uoq, Larry, announced the Storm-spotter training in Claremore for March 17th.

Tech Net Notes February 27, 2009

The theme for the February 27, 2009 edition of the Tulsa Tech Net was coax. As most of us have found out by now, there are a lot of varieties of coax out there. Big coax, little coax, medium coax, stiff coax, flexible coax, and stranded vs. solid center conductor. (not to mention the electrical characteristics variations) Some coax jackets are more durable than others. What we would like to know is, "What kind of coax do you use?" What problems have you encountered? What works best? What is your coaxial philosophy?

aa5ss, Glenn, said that he didn't have very good luck on one of his two meter antenna installations using coax. He said it was too lossy. He said that he was able to make some twin-lead work better for the installation of the antenna in his tree.

ke5lep, Stan, had a question, "What works well for jumpers? Big coax is cumbersome; small coax is lossy. Plus, it's understood that the more connections, the more the loss. What is the recommended size and type of coax between switches, meters, equipment, and antennas?"

ki6und, James, mentioned that using equivalent products would be best, in that the entire transmission line will take on the characteristics of the least common denominator, or "weakest link".

kd5njr, Scott, said that a good rule of thumb is that each connector will cost about .2db a piece. He said he agreed with the "weakest link" comment.

kd5vsf, Terry added, "Ground your system with heavy wire to a good ground. Check your specs for the frequency you'll be using most. One of the best instruments for checking things is the MFJ antenna analyzer. Also get good reference material, such as those publications from ARRL."

kd5njr, Scott, came back in and said that the Belden 9913 (or compatible) cable is a good type for jumpers.

ki6und, James, followed with another question, "Is there a way to negate the loss from a connector?"

kb5vdb, Ron, had to say, "Get the best connector you can get. A lot of the loss is relative to the frequency you are using. In some instances you can reduce loss by matching the connector with the frequency you'll be on."

kd5njr, Scott said "...just keep an eye on your connectors. Keep them clean. Use a light electronics cleaner. Keep them clean and free of debris. The worst thing you can do is splice your coax. LM400 is tough to beat."

ke5y00, Mike, says, "As far as coax goes, I just use what I have. As long as you keep it short, you can get away with a lot. I try to keep my length down."

kb5vdb, Ron, added, "RG8 is a military designation. There are several companies that manufacture it...just because it says "RG8" does not necessarily mean that it meets stringent operational specifications. There are a few quality companies out there that manufacture good products, such as Belden and LMR."

kd5vsf, Terry, said, "Keep an eye on that RG59, that can be a bad mismatch on an HT."

kd5njr, Scott said that compared to RG8, RG6 is better for antennas, loss-wise, even though it is 75-ohm and a mismatch. If you have a lot of RG59, look up "1/4 wave transformer" on the internet, for ways to use it.

w5taz, Dave said, "When they talk about balums, there is a big impedence, but when you're talking about the difference between 50 and 75 ohms, there's not that much difference."

Transmission Lines for Dummies

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.

Ham Pests

Here is a story about interference...
http://c2.com/cgi/wiki?HamRadioPests

73
Scott
KD5NJR

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.

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.