SE

CHANGES TO CLEAN SCOOP CERTIFICATE REQUIREMENTS

We have had horrible cooperation from the weather for the first 3 events and our on the air time has been limited so the club decided to change the qualification for the certificate. Now you only need to QSL 5 of 7 of the Special Events to get the clean scoop certificate !!

Good Luck and 73 from SEKARC

SE link

Thursday, October 31, 2013

David's Narrow Copper J-Pole Antenna

This is another post from one of our members!  David sent me a bit of this in an email and I thought it would be great to share with the group!  If you would like to post something here about your projects, experience or thoughts, let me know and we will see if we can post it up!

Narrow Copper J-Pole Antenna
Written by David Keath – N0WKZ

The goals I had in mind for this antenna were relatively simple.  Specifically the low takeoff angle of the Slim-Jim mixed with some of the durability characteristics of the larger cactus copper J-Pole.  Another desirable feature I was seeking that the Slim-Jim does not tend to have due to it's small conductor size is a wider usable bandwidth.  The final result is not a magic bullet, nor does it exhibit miracle performance.  Within reason, it does seem to have the qualities I was trying to achieve.

The dimensions and materials I am listing here come directly from the antenna, which is currently in use at the time of this writing.  I tuned and tested this version using nothing more than the built in VSWR meter on my radio.  Like any other antenna device, this antenna was tuned to my own equipment in my operating environment.  Please be mindful that the J-Pole design is generally somewhat sensitive to environment. Cutting and tuning it on different equipment in a given location may alter the final tuned dimensions slightly. Please also remember that this project was and is an experiment.

Materials Used
¼ Inch Diameter Soft Copper Tubing – at least 6.7 Feet
Common Black Electric Tape
Small Diameter Nut, Bolt, Washer – 2 Sets
1 Card Board Toilet Paper Tube
Fishing Line – Enough to make a loop for hanging the antenna

Tools Used
Ordinary Pliers
Screwdrivers – Phillips and Flathead
Utility Knife
Scissors
Cordless Drill and 7/64 or 1/8 inch Bit

Measurements Approximate
Long section of the J – 60 inches.
Short section of the J – 21 inches.
Bottom section of the J – 0.5 inches.
Feed-point location from the bottom of the J – 2 inches.

The total length here is long for construction purposes.  It should initially be approximately 81.5 inches/6.7 feet.

NOTE: If you feel confident in working with the copper tubing, you can use lengths closer to the final dimensions which are listed below for reference.  I would however recommend leaving at least an extra 2 inches on the long element and perhaps an extra inch on the shorter one.

Final Tuned Dimensions – Approximate
Long section of the J – 54.5 inches.
Short section of the J – 18.75 inches.
Bottom section of the J – 0.5 inches.
Feed-point location from the bottom of the J – 2 inches.

Construction
Measure the tubing  and mark off the points where the two bends making the bottom of the J should be located.  Be careful to leave the extra length of tubing on each element before making the bend.

Pliers may be used to flatten the  bottom section of the J.  Being careful to work slowly and carefully, bend the tubing into the J shape.  Used gently, the Pliers can aid in this process.  The average gap or space between the J elements should be approximately 0.5 inches.  This gap is between the tube surfaces, not center to center.

Cut small rectangular strips from the toilet paper tube.  Use a number of them together to create a simple spacer between the elements.  I used electric tape to hold them together and in place.

The area of the feed-point should be flattened and drilled.  Use the feed-point location listed here.  There is no need for the extra holes.  Feed-point holes should be located 2 inches from the bottom of the J.

Choke Balun
5 Turns at about 5 Inches Diameter of RG-58

Handling
While this device is certainly more durable than a Slim-Jim, soft copper tubing can be warped or bent.  Too much rough handling or a good hard drop could damage it.  Some care should be exercised in handling the device.

Mounting
I intended this device for indoor use, either ceiling hung, or installed in an attic.  The main idea is to keep the antenna as far from nearby dense objects or metals as possible, choke the feed-line, and isolate it from direct contact with conductive surfaces within reason.

Feed-Point Polarity
Depending on what publication you read, there seems to be a difference of opinion about which element on the antenna the coax center conductor should be attached too.  Based on most articles I have read, I chose to attach the coax center conductor to the long element on the antenna.  Supposedly this is the difference between having a lower verses a higher takeoff angle.

Tuning
Coax with the choke formed at the antenna end should be attached to the antenna feed-point allowing most of the choke to hang off below and too the side of the antenna within reason.

NOTE:  Before going any further, please remember that trimming off too much Copper cannot be undone.  Certainly an extension can be added but that defeats the whole simplicity idea.  Take your time trimming and tuning the antenna.

The short element of the J should initially be just slightly longer than my final tuned length.

The long element of the J should initially be longer by an inch or more.

The antenna should be initially placed or hung in it's desired location or someplace similar and then check the VSWR at the top, middle, and bottom of the 2 meter band.

Most of the tuning should achievable by trimming the long element until VSWR is low across the entire 2 meter band.  I would be careful trimming  too much off the short element.

Trimming is done by grasping the Copper near the top of the long element and using pliers to flatten 1/8 to ¼ inch of copper.  Carefully work the flat copper back and forth with the pliers until it breaks off.

Check the VSWR between each trim.

The antenna should initially have a lower VSWR value near the bottom of the band.  Shortening the long element by small amounts and re-checking the VSWR should move the low VSWR region up until eventually the VSWR is acceptable across the band.  That is where I stopped trimming.

When tuning is complete, a small section of copper at the top of the long element can be flattened and formed into a small hook or drilled for a fishing line loop to hang it.  Be certain to hang it securely.

NOTE:  Because I do not own a device to analyze the antenna, it may have positive or negative characteristics I am unaware of.  I would be grateful to anyone who builds the device and tests it with an analyzer to send or share the results.  pu239u235@gmail.com

References:
NOTE: There are more web sites about J-Pole antenna devices and designs than I can list here.  These sites I have listed lead to many others and provide useful information.  Simply searching the word jpole on the Internet will also yield many other sources of information.

Jpoles - Mythbusting and the perils of rogue RF currents. By John Huggins, KX4O
https://docs.google.com/presentation/d/1uJ_INHlJEXUFeDrEiBgRcpQRKZ3TNSx6ahN_lWdX4ms/edit?pli=1#slide=id.p14

JPOLES Handbook, 4th Edition Buck Rogers, K4ABT
http://www.buxcomm.com/jpoles4ever.htm

Web Station, N9TAX
http://www.n9tax.com/Slim%20Jim%20Info.html

I wish to thank the SEKARC Club, Pittsburg KS, USA  for allowing me to submit this project so that others may perhaps use and even improve it.  If errors, comments, improvements, or anything else regarding this antenna comes to light, please do submit that information in the comments below so that others might benefit.

The J antenna design has been around since the time of the rigid airships prior to WWII.  The Slim Jim has also likely been around for some time.  Neither design is my own.  As well, it is unlikely that I am the only operator to attempt to make a rigid narrow J pole.  In essence, I can only state that I constructed this project as an experiment based on existing sound designs.  This is what Amateur Radio is all about.  Experimentation, improvement, and a sharing of those experiences with others.  Of course, there is always the inevitable failures on the way to successes but that is the way of it...

73's
Sincerely, D Keath – N0WKZ – 2013

Element Closeup

Balun Coil

Feed point Closeup

Complete

Hanging Method


3 comments:

  1. Wow! Thank you for allowing me to share Jeff..

    ReplyDelete
  2. David, I have heard you on this antenna and it is an improvement over your old one. Good job, and your explanation is clear and complete -bill

    ReplyDelete

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