Tulsa Tech Net for 10/8/10

The subject is antennas

I'm sure to those that have noticed that I'm having a time keeping up on the blog !!!
The tech net started with me having some mic troubles and i got them resolved. Then Clarence, AE5UB, was wanting some help on choosing a directional array for 6 and 10 meters. I suggested a rotatable dipole for 6 meters. One issue that was brought up is antenna regulations for being in the vicinity of an airport. If anybody has a link to this info please send me a line so I can post it. Multi-band quad was also brought up as an option and there are many plans for multi-band quads on the internet. There was also metion for the log periodic antenna for multi-band use. The Log Periodic is an array that consist of multiple dipoles to cover a wide range of frequency's. The look similar in construction the the old vhf television arrays that everyone had on their house until the mid 90's. As you change frequency the active region shifts along the array.

The windom is a dipole that is commonly used for multi band use. It is an off-center fed dipole. At certain bands and a certain distance that the dipole is fed off center there comes a point that the feed point impedance will be the same . Then the dipole is fed with an impedance transforming balun to bring that high impedance down to 50 ohm.

Tim, KF5FSJ, had an oddity about pouring water at the base of the push-up pole of his 2 meter beam to cure some performance issues he was having.

Dave, NB5B, Is putting up a digipeater in Manford to bring APRS coverage to that area.

Thanks to all that participated and it was a good turnout being that the football game was on.

here is a link to the MP3 of the net to save it right click on the link and click Save Link
http://www.w5bbs.us/?file_id=11

The Grid Dip Meter

The Grid Dip Meter

The grid dip meter also known as a grid dip oscillator is variable oscillator circuit that uses a variable capacitor for tuning and external coil for the L part of the circuit. Most also have a potentiometer to control the level of oscillation of the circuit. The value of the coil determines the frequency range of the meter and most meters come with a set of coils that are calibrated to the frequency dial on the meter. The actual values of the coils is not critical if you have a receiver with a calibrated readout on it to compare to. The indicator meter on the unit measures the grid current in the circuit and is what is used to make measurements. You can find used meters at hamfest and swap meets in the price ranging from $5 to $50 depending if they have the coils and the quality and condition of the meter

Well the grid dip meter is used to check the resonant frequency of a tuned circuit whether that circuit be a pi-network in an amplifier or a filter or the most common an antenna. How this is accomplished is that rf energy is coupled into the circuit under test and you vary the frequency of the oscillator in the meter until resonance is found. At the point of resonance the circuit will absorb the most rf from the meter much like an antenna will take rf from a transmitter that is matched. Once this happens the grid current of the tube in the oscillator will drop thus the "dip".

If you want to know the value of that inductor or coil you just wound then you can use the meter and a little math to do that. What you would do is take a high quality capacitor of known value and put it in parallel with the unknown inductor and now you have a tuned circuit. Then you just take your meter and find the resonate frequency of that circuit. Now you have a lot of info now what? Well here's the formula to find the inductance of that coil in uH.
.............1
L=--------------------
..........4π2 f2 C

π=3.1416
f is in MHz
C is in uF
L= unknown in uH

The same thing can be done with a known inductor for capacitance and the formula is the same but reverse value C and L in the formula.

You can find the Q of an inductor and here's how to do it:
Find resonance of the industor then tune lower and then higher until the dip is reduced by 30%. When you go higher this is F1 and then lower is F2 and the formula is as follows....

.........F
Q=------------
......F1 - F2

and that Q!!!!!!

Here's a link to follow along with on tonites net

http://g4rvh.files.wordpress.com/2008/08/waht-you-can-do-with-a-dip-meter.pdf

The Tech Net Is Back!!!!!

Well its been a while but the Tech Net is back!!


Last week 9/24/10 was the first net in a while and we had just a handful of check-ins but overall I think that it went well. We had Tim KF5FSJ ask about how to tune in a psk-31 signal and Dan KF5HYD talked antennas and I recomended that he put his dipole for 10m in a vertical configuration to help him work local stations. Also I am in the process of getting some specs on a roll-up J-pole made from 450ohm ladder line that would be great for a go box.



Curtis KC0FHF was looking for a good base antenna ideas for 2m local repeater work and he mentioned an antenna that he used to run that he enjoyed. It was described simular to a discone but with solid cones for the groundplanes. I said that I would try to look up the manufacturer and model number for him and i ran across it in an ad in the Sept. 1995 issue of QST when i was looking for plans for a 2m quad that i was refered to. Still havent found the plans but the Ant was an IsoPole144 made by Spectral Antennas. The wbesite is http://www.isopole.com/



I realy enjoyed running the net and i will try my best make sure its held on a weekly basis. I send many thanks to Don KE5OMV for starting this net that our community has needed for a long time and his support in helping me continue it.

The Topic for the next net is GRID DIP METERS! I look forward to the next net and until then keep the solder flowing and the RF going.

Travis
KD5UND

Rain Gutter Antennas

http://www.sgcworld.com/raingutterinstall.html

Green Country Hamfest Prizes

March 12th and 13th - Claremore, OK
http://www.greencountryhamfest.org

GRAND PRIZE Elecraft K3 160M - 6M Tranceiver
2nd Prize - Yaesu FT 897D
3rd Prize - Yaesu FT 8800R
Hourly prizes - Yaesu VX-3R (Must be present to win)

Tech Net Notes January 29, 2010

Loop Antennas

nb5b, Dave, (I believe it was) mentioned that he was thinking about putting up a loop antenna and suggested Loop Antennas as a topic several weeks ago. Having been intrigued with the loop antenna concept for some time, I was very interested in pursuing that topic when I had a chance. That chance came with the January 29th, 2010 Tech Net.

I received the following information from: http://www.bloomington.in.us/~wh2t/loop.html

From WH2T - Dr. Ace's Full Wave Loop Antenna Info

"I have personally used a 160 meter band Horizontal Full Wave Loop antenna with very good success. I worked all 50 states and several countries with 100 Watts using the antenna on the 160 meter band. With a good antenna tuner, the antenna will work 6 thru 160 meters. I currently use a 75 meter Full Wave Delta Loop on the 6 thru 75 meter bands with a tuner. I am very pleased with it."

He goes on to mention that on horizontal polarization he gets about 2.1 dB gain.
He claims the loop is:
Much quieter than a dipole or vertical
Has a broader bandwidth
Usually outperforms a dipole antenna

He feeds his with 75 ohm coax but says 50 ohm can be used.

Dimensions: The formula is 1005/Freq in Mhz = Length in feet
160 Meters at 1.9 Mhz = 529 ft.
75 Meters at 3.85 = 261 ft.

Trimming may be necessary to obtain a low VSWR.

The impedance of a Full Wave Loop is theoretically in the vicinity of 100 ohms.

Connect the center conductor of the coax to one end of the wire and the shield to the other end. Be certain to seal the shield end to protect against water. \

Any shape will work - square, octagon, pentagon, triangle (delta loop), or circle. "The larger the area or aperture inside the loop, the better. A circle has the largest area but is impractical. If you use a triangle shape try to make each leg an equal length as this gives the largest inside aperture or area."

"Note 1 - Don't use a Balun on this Antenna! On a horizontally oriented loop you can feed a corner, center of a side or anywhere. It is unimportant."

"Note 2 - If you know you will be using a Loop, Dipole, Zepp, etc. on Multiple Bands and you want the most efficient performance of the antenna system you will always get less feedline loss if you use Open wire 450-600 Ohm window/ladder line."

"If you feed the Full wave loop antenna direct with a single piece of coax you can only adjust add/prune the antenna til the VSWR gets down to about 1.7:1 at resonance, so you will probably want to use a tuner if you want to cover an entire HF band.. And yes the tuner will work fine as long as you are not using a linear amplifier."

In the article he also includes information on how to determine the length of the feedline using a combination of 75 ohm and 50 ohm coax.

With that we began the net.

kd5avk, Harold, on his mobile from Greenwood, Arkansas, asked if a turner was needed with a loop. After looking back at the information from WH2T it was determined that yes, a loop is needed if using the antenna multiband. Otherwise the antenna will work best only for the frequency for which it is cut.

ae5mn, Hank, asked if the loop antenna can be strung aroung gutters, downspouts and etc. Having installed a myster antenna around the eves of my house with good results, I relayed that I had kept the elements of the dipole in the middle of the eve which was at a minimum the recommended 5 1/2 inches away from most metal objects (including gutters, downspouts, cable TV wiring, and AC service wiring.

kc5zqm, Doug, mentioned that he had used 2 Meter loops for fox hunts and tracking balloon beacons with kc5trb, Harry. In those instances he's used them mostly in the vertical orientation. He mentioned that in SSb mode the horizontal orientation would more likely be used.

k5bbm, Brian mentioned that the loop antenna that kc5zqm referred to may be able to be found at arrowantenna.com.

kf5if, Wade, mentioned a 40 meter Delta Loop that he and the late ae5ft, Gene built at a field day that had a 40 ft peak. The broadsides were to the North and South and that is where they made most of their contacts. It was bottom fed, near the center. No turner was used and SWR was below 2:1 across the band. He mentioned that he was a little disappointed in the amount of noise he had on it and that the high noise may have been due to the vertical orientation and it being in the heat of summer. He did not use an analyzer during the construction, but did use an analyzer afterward to confirm it's characteristics.

End of Net