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Quarter wave stub to prevent coax radiation - anyone here ever done it?




 
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Author Topic: Quarter wave stub to prevent coax radiation - anyone here ever done it?  (Read 913 times)
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steve_qix
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« on: November 10, 2019, 09:42:07 PM »

I've got a situation - not at my hose - where I have 2 very tall trees where I can place a 75 meter dipole between them.  There is NO center support available.

So, first test, I put up the antenna, coax fed, and it works.  No decoupling yet for the feedline.  It's up about 80 feet.

Now I want to remove the obvious radiation from the coax.  If I had a center support, it would be done - use some cores, make a balun.  Done.  Same thing I do in Townsend.

But, with no center support, the cores are just too heavy and the center would droop down way to much...

So, how about a 1/4 wave stub (aka, shorted 1/4 wave section) ?  This is essentially another wire run right next to the coax, 1/4 wavelength long, connected to the center conductor of the coax/antenna, shorted at the other end to the shield of the coax.

I have never used anything like this, but, in theory it looks as if it should work.

Any experience out there with stubs (shorted 1/4 wave sections)?  I'm trying to stay away from open wire line - a tuner would add another piece of equipment - yada, yada..

Let me know - thanks !

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« Reply #1 on: November 10, 2019, 11:24:02 PM »

Steve, that sounds like an ideal opportunity to change to a delta loop- no sag caused by feedline. If you can get the circumference greater than 180 ft, it will work well on 80/75. Do you do antenna modeling? 4nec2 maybe? 73, and thanks for all your contributions to class e.
 Rod KM6SN
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« Reply #2 on: November 10, 2019, 11:30:45 PM »

You don't need a piece of coass to make the stub with, a piece of 16g wire will work as well.

I believe the vf is about .98.

This being said, I believe the 'choking impedance' isn't near what a single toroid with a few turns would be.

How about a rope strung between both trees to hold up the torroid(s) and keep the weight off the antenna wires?

You can make the rope pretty right w a 5 gallon bucket o water weighing one end and a pulley.

--Shane
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« Reply #3 on: November 11, 2019, 01:49:44 AM »

Steve,

If feeding a dipole with coax and ferrite cores/balun works for you, then just run OWL between the antenna and the balun. If the antenna is resonant the OWL will run flat and can be of any length and not require a tuner.


Don



Edit:

Looking into this further, it seems that 70 ohm balanced transmission line is not easily found these days. Unless you have some old 70 ohm balanced line in the junkbox, I can only think of a few alternatives:

-Any impedance balanced line of a halfwave length.

-Some sort of twisted magnet wire line (might be a fun experiment).

-Dual coax lines.

Why is nothing ever easy???



Edit again:

A resonant folded dipole can easily be fed with balanced line and thence to the balun and coax. Make the whole thing from 300 ohm twinlead, ladder line, or homebrew.
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« Reply #4 on: November 11, 2019, 05:58:36 AM »

...So, how about a 1/4 wave stub (aka, shorted 1/4 wave section) ?  This is essentially another wire run right next to the coax, 1/4 wavelength long, connected to the center conductor of the coax/antenna, shorted at the other end to the shield of the coax. ...

Wouldn't the stub connect to a location where it would produce a high impedance at around 3.9 MHz for currents flowing along the outer diameter of the outer conductor of the coax running from the dipole back to the transmitter, rather than across the feedpoint terminals of the 3.9 MHz antenna?

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« Reply #5 on: November 11, 2019, 09:19:59 AM »


So, how about a 1/4 wave stub (aka, shorted 1/4 wave section) ?  This is essentially another wire run right next to the coax, 1/4 wavelength long, connected to the center conductor of the coax/antenna, shorted at the other end to the shield of the coax.

[/quote

When I had my antennas on the roof of my barracks in the USAF circa 1967 I used a couple inverted vees as guys to support the mast for my 15 meter yagi.  I had RF
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« Reply #6 on: November 11, 2019, 09:28:27 AM »

Thanks for the replies!  As much as I do not want to build another piece of equipment, I may have to go with open wire line for this particular installation.  Still thinking about it...  I like coax because, if decoupled from the antenna (such as my installation in Townsend), the line does not radiate at all, nor does the line allow outside signal sources (such as from house wiring, computers, etc.) to penetrate.  

I have found that it is extremely difficult to prevent open wire line from radiating at least a little because it is, as a practical matter, very challenging to ensure absolute balance in the physical antenna itself.

If using open wire line, will probably have to go with a remote tuner...we'll see.


Delta loops:  That is/was a good suggestion, and I did try it last week.  I cannot configure the loop in the horizontal plane due to the location and nature of available high supports, so I did put up a delta loop in the vertical plane, with the horizontal wire at 80 feet.  It sort of worked, but the local signal was very lackluster, probably due to the large amount of low angle radiation due to the vertical orientation.

The dipole is quite a bit better than was the vertical loop, even with the radiating coax.

Putting a balun at the center:  This just not possible unless I can accept the center of the antenna being lower than optimum  Undecided

There is a bit (maybe more than a bit) of antenna perfectionism (obsession?) here - and I admit it.  There are probably support groups out there somewhere that can help !

Continuing ideas and thoughts are truly welcome.  Thanks !
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« Reply #7 on: November 11, 2019, 09:31:42 AM »

My previous post failed, I edited and corrected it here:


So, how about a 1/4 wave stub (aka, shorted 1/4 wave section) ?  This is essentially another wire run right next to the coax, 1/4 wavelength long, connected to the center conductor of the coax/antenna, shorted at the other end to the shield of the coax.


When I had my antennas on the roof of my barracks in the USAF circa 1967 I used a couple inverted vees as guys to support the mast for my 15 meter yagi.  I had RF in the shack due to feedline radiation.  I was feeding the vees with RG-59,  no balun.  Dad, W2DU, suggested what you are considering, a wire attached to the center conductor at the feed point of the vee, run along the outside of the coax, and attached to the shield (using a connector) 1/4 wavelength down the feedline.  These vees were for 20 and 40, with the 40 also usable in 15.  This trick not only cured the RF in the shack, but it also brought the noise level down two s-units.  If it worked for 20 and 40, it should also be effective on 75.  

A bead balun that is good for QRO would likely be too heavy at the feed point.  Using the original W2DU bead balun, with the thin coax and smaller beads, would probably not hold up to QRO AM use, so the shunt stub would probably be the most practical approach since you cannot support the center of the dipole.  I am envious that you are able to get it up into the trees eighty feet or so.  My tree situation gives me forty feet elevation at best, not so great for 75 meters, but you do the best you can....

73, Rick
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« Reply #8 on: November 11, 2019, 10:22:33 AM »

How much do a pair of stacked cores (I think you're only running 400 w at Mirror Lake) with 10 turns of RG-303 weigh as compared to at least 80 feet of coax? I would think the majority of the weight, thus sag, would be due to the coax. How about slipping a few dozen high permeability ferrite beads over the coax at the feed point?

Rich
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« Reply #9 on: November 11, 2019, 11:39:19 AM »

If you decide to use a stub, the easy way would be to get some coax with an integral messenger ground wire. Such as:
https://www.amazon.com/Messenger-ANTI-CORROSION-Compression-SATELLITE-PHAT/dp/B01NBGYZK1?th=1
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« Reply #10 on: November 11, 2019, 12:55:43 PM »

Hi Steve,

The main problem seems to be the lack of a center support. Any way you look at it, a balun, coax run, open wire, etc., will mean a few pounds at the center of the antenna and during NE ice storms, wind storms and the like, the antenna will come down.

Instead, I would focus on the center support and then put up anything you want.

A suggestion is to run a strong non-metallic cable between the two trees like Phillystran  using steel cable to go thru the tree end supports. Smooth, standard 3/16" EHS guy cable will not saw into the tree limbs. This will handle everything the trees will.  Then attach three rope/pulley setups to the Phillystran - run to the ground - one supported at the center and one to each end of the Phillystran.  You now have a universal breadboard to hoist and hold anything you want into the future with excellent center and end support.  Use a bow and arrow to get the steel cables through strong tree limbs and you will never have to climb again… do everything from the ground.

If you decide to go with a lightweight 1:1 center un-bal  balun, a standard 2" donut ferrite core using Teflon wire will handle many KW and is very light if hung without an enclosure.


T
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« Reply #11 on: November 11, 2019, 02:22:57 PM »

Hi Tom,

What you suggested is the so-called "final solution" - have a non-conductive messenger between the trees.  That will take the force off the antenna itself.  Once the testing is complete, I'm going to pull the messenger up there, which will actually support the antenna itself and handle all of the forces of wind, etc.

I just have to be careful not to put so much force on the tree that it breaks the top off, or breaks all of the high branches.  A tree of that height takes a LONG time to grow  Smiley
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« Reply #12 on: November 11, 2019, 02:24:24 PM »

My previous post failed, I edited and corrected it here:


So, how about a 1/4 wave stub (aka, shorted 1/4 wave section) ?  This is essentially another wire run right next to the coax, 1/4 wavelength long, connected to the center conductor of the coax/antenna, shorted at the other end to the shield of the coax.


When I had my antennas on the roof of my barracks in the USAF circa 1967 I used a couple inverted vees as guys to support the mast for my 15 meter yagi.  I had RF in the shack due to feedline radiation.  I was feeding the vees with RG-59,  no balun.  Dad, W2DU, suggested what you are considering, a wire attached to the center conductor at the feed point of the vee, run along the outside of the coax, and attached to the shield (using a connector) 1/4 wavelength down the feedline.  These vees were for 20 and 40, with the 40 also usable in 15.  This trick not only cured the RF in the shack, but it also brought the noise level down two s-units.  If it worked for 20 and 40, it should also be effective on 75.  

A bead balun that is good for QRO would likely be too heavy at the feed point.  Using the original W2DU bead balun, with the thin coax and smaller beads, would probably not hold up to QRO AM use, so the shunt stub would probably be the most practical approach since you cannot support the center of the dipole.  I am envious that you are able to get it up into the trees eighty feet or so.  My tree situation gives me forty feet elevation at best, not so great for 75 meters, but you do the best you can....

73, Rick


Hi Rick - Yeah, that's exactly what I'm thinking about doing.  And it *should* work.  Going to give it a try next weekend, assuming the WX cooperates.

Thanks!
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« Reply #13 on: November 11, 2019, 05:04:58 PM »

Steve,

Yes, a messenger is the key.

What is cool about using Phillystran as a messenger is that it's ultra light... maybe a few ounces for a 150' run. Much lighter than rope and much stronger. So the load on the tree supports is greatly reduced, like it is barely there until the antenna load gets put on it..

Another idea is to make the WHOLE dipole completely out of 3/16" EHS guy cable. Use strong insulators and EHS end supports thru the tree limbs. No messenger needed. It will never break. Loss-wise: Larger diameter 3/16" steel cable at 75M / 50 ohms feed will not be much lossier than thin #12 copper wire. In this case, no need for a messenger but somewhat heavier than a Phillystran and copper dipole.

I once used  1/4" EHS guy cable as a 160M dipole and it doubled as a boom for a 75M wire Yagi.   Worked FB.

There are lots of ways to solve the tree/mechanical support problems.     No center support = fail...   and a saggy antenna while it lasts.


T
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« Reply #14 on: November 11, 2019, 07:11:23 PM »

ANother solution, although it may be too much weight, is a loop balun.  6-8 turns of coass, a couple feet in diameter.  You could use RG-8X or similar even if you're running high power, so long as you stay mindful of the voltage limit.
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« Reply #15 on: November 11, 2019, 10:00:48 PM »

Put up a folded dipole. They are self-balancing.
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« Reply #16 on: November 11, 2019, 10:57:07 PM »

Steve,

Thinking more about it... wanna do an overall first class job?

To better protect the trees, I would add a pair of long garage door springs to the ends of a dipole made from EHS 3/16" cable. Paint them well with black spray paint. Use guy wire "Johnny Balls" as the center and end insulators. Use two insulators in series at each end for the HV.   Then add a #12 copper wire "ghost" flattop in parallel with the EHS to reduce the steel resistance.  Use small wire clamps to keep the copper wire held to the EHS cable throughout the flattop run.

Silver plated lugs can be used to bolt the copper feedline and #12 copper wire to the steel EHS using wire c-clamps..

So essentially we have a 3/16" EHS dipole (full support structure) with a #12 copper wire dipole in parallel.  The antenna will stay tight as the springs adjust for the wind.  The EHS support cables go thru the tree branches to ground and will double as an effective lightning path.  Electrically we have a #12 copper dipole fed with any type of feedline we desire.

Position the springs a few feet from the tree crotches to minimize their effective loads on the system.  IE, it's easier to support them at the ends rather than farther out towards the middle.

As a balun using coax feed, I would wind 15-20 turns of the coax feedline onto a 4" PVC plastic form and hang it below the center insulator using rope.  Let the balun hang at right angles (not parallel) away from the dipole to reduce coupling... IE, the open end of the PVC points towards the ground.

Mechanically we have an ass kicking 3/16" steel cable support structure with wind compensation throughout -  with low resistance copper wire for RF.   Cost under $200. What's not to like?

The tension pull of the springs can be tailored by the diameter of the coil stock used..

Springs sample:
https://www.365garagedoorparts.net/springs/garage-door-torsion-springs/garage-door-torsion-springs-for-any-wire-size-or-length-up-to-39.html?gclid=EAIaIQobChMIk9KMvOTj5QIVi5-zCh0BbQA8EAQYASABEgKFA_D_BwE


T


* Torsion_spring_.png (1053.06 KB, 1920x4658 - viewed 48 times.)
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #17 on: November 12, 2019, 07:20:51 AM »

Hi Steve,

Going back to your original post...

If it were me, I see two options I'd try:

1.) Feed flattop from one end with vertical wire making a Inverted L antenna that is a little longer than 3/4 wave long. A large series tuning capacitor might be needed, as well as some ground radials. The Inverted L antenna on 160m is a proven performer. Why not on 75m?

2.) Use your coax dipole as you have it now, but put a coax balun coil at ground level, and earth the shield of the coax side going to the shack (gnd rod). This will embrace coax radiation instead of minimize it. Then tune antenna length along with adding something at the bottom such as an inductor or capacitance to ground across the coax common mode coil to ground. I do this technique here, but I have an inverted V with peak at 38'. In my case I use a coil  (abt 5 uh) to ground across the common mode choke. For a longer feedline (> 1/4 wave), a variable Cap to ground might be better.

I am no antenna expert, more of a Hack actually, but my use of option 2 gives me nice SWR where I want it, and my signal reports are very gratifying. I think of this as an inverted vertical ground plane antenna where the dipole wires are the radials, and the top of the vertical coax is at a current maxima. The bottom of the vertical coax above the common mode coax coil is RF HOT.  Not good for a male dog looking for a spot to spray upon.

Jim
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« Reply #18 on: November 12, 2019, 09:18:12 AM »

... 2.) Use your coax dipole as you have it now, but put a coax balun coil at ground level, and earth the shield of the coax side going to the shack (gnd rod). This will embrace coax radiation instead of minimize it. ...

Below is a NEC4.2 study showing how that configuration might perform:

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« Reply #19 on: November 12, 2019, 11:17:33 AM »

Why avoid a vertical component?

I'd put the chokes where the coax turns horizontal and can be easily supported from the ground.
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« Reply #20 on: November 14, 2019, 09:41:42 AM »

Hi Clark, it's not so much that it's vertical per-se, but that there are increased ground losses (due to imperfect ground), and also the significant low angle component does not help the signal locally (within 500 miles), which is where most of the action is most of the time.

There is already some vertical component because the antenna is not purely horizontal.  This is also the situation in Townsend with the antenna being somewhat inverted V shaped, however there I have a real radial ground system with 100+ 1/2 wave radials (1/2 wave on 75 meters that is).
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