Question regarding antenna bandwidth for coax vs: open wire line

[K1JJ]

Thanks for the additional info, Mike -

It's certainly getting heavier with the load of insulators in the center.  

I'm looking at your other approach of using "straight thru" double ropes.   How about these changes?....   The results would be no need for spacers, only one insulator in the middle (and one on each end leg) as well as a stronger structure by using  two rope supports on each side. Use a "bow tie" fan dipole config with 10' spacing on the support ends that would give an average of 5' spacing (or more) thruout the two dipoles.

It would mean another two tower climbs and two more ropes and pulleys.  But I think it would be a more manageable thang. No wind spinning either. Less ice loading.  I wonder if increasing the support rope  spacing at the ends to 20' would increase the bandwidth more?    The lower dipole could be near horizontal -   and for mechanical advantage the upper dipole could be sloped upwards by 20' higher at each end.  (like a truss that would actually help support the center - feedline, insulators, ferrite core weight and lower dipole)

Also, what if the two dipole legs were cut for 3790 and 3870?  This could produce a broader 1:1 "double dip" in swr between the desired  3700 - 3885 MHz band that is desired. (Assuming I can match the 75 ohm feedline to the dipoles well.)


Just thinking....

T

[K9MB]

Thanks for the additional info, Mike -

It's certainly getting heavier with the load of insulators in the center.  

I'm looking at your other approach of using "straight thru" ropes.   How about these changes?....   There would be no need for spacers, fewer insulators as well as a stronger structure by using  two rope supports on each side. A "bow tie" dipole config with 10' spacing on the support ends would give an average of 5' spacing thruout the two dipoles.

It would mean another two tower climbs and two more ropes and pulleys.  But I think it would be a more manageable thang. No wind spinning either. Less ice loading.  I wonder if increasing the support rope  spacing at the ends to 20' would increase the bandwidth more?   For mechanical advantage, the lower dipole could be near horizontal -   and the upper dipole could be sloped upwards by 20' higher at each end.  (like a truss that would actually help support the center - feedline, insulators, ferrite core weight and lower dipole)

Just thinking....

T

Tom,
If you stay with the wires being parallel, then you may want to have at least little insulators in the mid spans for stability, though, if the wire is tight, it is unlikely to twist together.

Your thought about separating the ends brings up another type of multi-wire antenna.
Guys that have run fan dipoles have discovered that adding a second-in band wire and spreading the ends gives a nice broadbanding effect. Twenty feet might be necessary to limit interactions and detuning the most effective versions have the second dipole going off to the side by 20 or more feet.
Usually, the first dipole is set at the low end of the desired range- maybe 3600khz and the second at 3900khz. The low one is put up and length worked out for that range and then the second is added and trimmed for best overall coverage.
Seems like the swr curves are similar to a critically coupled double tuned circuit with two small  dips at the end and a flat dome in the middle less than about 1.6 swr.
This is not fro, experience though, so in my case I thought to make it a couple of inverted vees, so the ends would be there to trim.
At 100+ feet and a nearly flat top, I think that you might just go with calculating the lengths closer together - like 3700 and 3900 and put it up and scan the antenna with a impedance meter and trim the bottom to pull it up.
For these things, there does not seem to be any reason to join the ends, by the way and they will act like a skeleton bow tie antenna.
Here is the article on one version. They successfully added a 40 meter dipole in the middle at no extra charge either…😉

https://wireless-girl.com/Projects/Antennas/Broadband80mDipole.html

If you were going to use the cage dipole idea, the dimensions are apparently somewhat critical in terms of impedance and also, the height.
If you spread it st the ends, it might look more like the fan version, even if you kept tue connection together. In my opinion, adding a dacron rope between the center insulator to each side 10 feet above the cage straight through terminations would add a great deal of strength and allow your copper wire to work well.


Even better idea, IMO:
Of course, a 3/8 dacron rope could be run between the cage elements, too and keep it more compact. That stuff can hold a ton easily, I think.

By keeping the ends compact, the ends could terminate in an aluminum tube or large fiberglass rod maybe 6 ft long so it is below the cage, with a single rope passing through a big pulley and a second check rope at the bottom of the tube or rod dropping straight down to prevent the end from twisting.
With this setup, you could lower and raise the entire array from the ground, if you are clear of guy wires, etc to the tower bottom at that angle.
You get a cage that can be raised and lowered and optimized from the ground.
Best solution, IMO and what I had decided to do on my shorter towers.
73, Mike

[KD6VXI]

A string of beads isn't the best option.

I purchased 4 inch cores and wrapped coax through them


A lot lighter and a LOT more choking impedance.

--Shane
WP2ASS / ex KD6VXI

[K9MB]

Agreed:

http://www.karinya.net/g3txq/chokes/

http://www.audiosystemsgroup.com/K9YC/K9YC-old.htm


The bead string will help a little but insufficient to be effective according to these researchers.
It is why I recommended 16 turns of rg400 through two 240-31FT cores. Will give about 12K choking at 80 meters…
Two single 12 turn windings in separate cores even more effective and easier to wrap

[K1JJ]

Ok on all, tnx!

Some more ideas...

Since the RG-11 is so light and I have plenty of it, I wonder how small  can coils be wound with it?   I have three large ferrite cores, about 3" in diameter. I could wind three large coils, two turns thru each big core, with the three placed in series hanging under the dipole center.  I was thinking that 14" coils might be big enuff to stop any coax migration. What you think?

I would have to mount the coils at right angles to each other to minimize coupling I think... That could be a challenge to mount.

Today I put up a 30' high mast under the dipoles to support the RG-11.  This will leave about 65' hanging down from the dipole and reduce the strain on the coax and the dipoles.

Also, I will be replacing the end ropes with 3/16" aircraft cable.  I think using a pulley with a heavy load just asks for the rope to fail.

I want to go with the fan dipole idea... with two or maybe even three dipoles that are staggered between 3.7 to 3.9 MHz frequencies.  It will be an experimental thing to find the best swr curve. Drop, adjust, raise, test over and over.

 I still like the overhead truss dipole idea.  I have some old Phillystran fiber guy rope that can take a few thousands pounds. Lighter than rope. I was thinking of using that it an overhead trussing arrangement, but not sure how to do it yet.

T

[K9MB]

Ok on all, tnx!

Some more ideas...

Since the RG-11 is so light and I have plenty of it, I wonder how small  can coils be wound with it?   I have three large ferrite cores, about 3" in diameter. I could wind three large coils, two turns thru each big core, with the three placed in series hanging under the dipole center.  I was thinking that 14" coils might be big enuff to stop any coax migration. What you think?

I would have to mount the coils at right angles to each other to minimize coupling I think... That could be a challenge to mount.

Today I put up a 30' high mast under the dipoles to support the RG-11.  This will leave about 65' hanging down from the dipole and reduce the strain on the coax and the dipoles.

Also, I will be replacing the end ropes with 3/6" aircraft cable.  I think using a pulley with a heavy load just asks for the rope to fail.

I want to go with the fan dipole idea... with two or maybe even three dipoles that are staggered between 3.7 to 3.9 MHz frequencies.  It will be an experimental think to find the best swr curve. Drop, adjust, raise, test over and over.

 I still like the overhead truss dipole idea.  I have some old Phillystran fiber guy rope that can take a few thousands pounds. Lighter than rope. I was thinking of using that it an overhead trussing arrangement, but not sure how to do it yet.

T

Tom,
Jim Brown K9YC published this in about 2008. He gives information for using big large coax in big loops like you describe.
He specifies 5-FT240-31 stacked for 80 meters. See article here:
They seem bulky and heavy to me…
http://www.w4ava.org/articles/k9yc-rfi.pdf

In his later work, he went to rg-400 teflon coax  or twisted insulated wire like teflon awg 10 aircraft wire and it made the chokes much lighter and more compact.
This was published in 2018.

http://k9yc.com/2018Cookbook.pdf

The smaller chokes can be enclosed in plastic Electrical boxes at the antenna center of the antenna at the feedpoint and the rg-11 feedline.
I had initially recommended rg400, but you may be better off using the paired teflon coated wire he discusses in the 2018 cookbook.
The impedance of the twisted wire is 80-90 ohms and he says it will be fine for the match of a high dipole. Check it out…
So many choices…😉

Edit: Just found some more data on rg11 catv cable. Loss at 1.pmhz is 0.22db/100ft.
Should be fine at HF low end.
Also, min bend radius loaded is 8 inches or 16 inch circles.
Unloaded is about 4.5 inches, but wrapping into a toroid is a loaded application I think, so you need to use big loops. Maybe two stacks and wind them like a binocular core?
This is going to be bigger and bulkier than paired Teflon awg10  or teflon coax in one core like k9yc used in 2018 cookbook…

[K1JJ]

FB on the RG-11 specs, Mike.   Good info that I printed out.  It appears to be acceptable for this use.


So it looks like it will take 260 pounds of pull. Is this what they mean by "loaded" at 8" bend radius?  (16" diameter at 260 pounds)   So if it is well supported, maybe the proposed vertical drop of 65' will not hurt it.    I always use a 15-20 turn "pipe hitch" rope knot, like a hangman's noose - like a Chinese finger grip, to distribute the coax weight pull more evenly.

The 300V rating at 75 ohms seems low to me considering RG-213 can take a few thousand volts.

Think I will try a hank of 16" diameter RG-11 turns thru my big ferrite cores at the dipoles' feedpoint and be done with it for now. That's less than one pound of extra coax weight. The ferrite is a couple of pounds, so not bad. Continuous run with just one connection at the dipole input. I can always improve on it later.

I wonder if we supported the RG-11 coil hank by itself with no pull on it at all, would a 4" bend radius be permitted under the "unloaded" category?  (8" coil diameter)


Tnx again for the great info!

[K8DI]

If you’re worried about the loaded radius of the coax, you could use something like this to have a coil/choke with no load on the coil itself..

Ed

[K9MB]

FB on the RG-11 specs, Mike.   Good info that I printed out.  It appears to be acceptable for this use.


So it looks like it will take 260 pounds of pull. Is this what they mean by "loaded" at 8" bend radius?  (16" diameter at 260 pounds)   So if it is well supported, maybe the proposed vertical drop of 65' will not hurt it.    I always use a 15-20 turn "pipe hitch" rope knot, like a hangman's noose - like a Chinese finger grip, to distribute the coax weight pull more evenly.

The 300V rating at 75 ohms seems low to me considering RG-213 can take a few thousand volts.

Think I will try a hank of 16" diameter RG-11 turns thru my big ferrite cores at the dipoles' feedpoint and be done with it for now. That's less than one pound of extra coax weight. The ferrite is a couple of pounds, so not bad. Continuous run with just one connection at the dipole input. I can always improve on it later.

I wonder if we supported the RG-11 coil hank by itself with no pull on it at all, would a 4" bend radius be permitted under the "unloaded" category?  (8" coil diameter)


Tnx again for the great info!

Tom,
The 16 inch loops sound fine to me.
You might tighten the loops to 12 diameter if you relieves stress completely so no pull was present in the turns except the pressure of the turn itself.

The 300volt spec is rms so you are talking 420-430 peak.
This stuff is low density foam so it cannot be compared to solid PE for dialectic strength.

425volts peak is equivalent to over 2kw at 75 ohms on a flat line.
I suspect that the voltage rating is conservative, Also and it will stand over 1000volts on peaks .
You could take a few feet - remove shield from each end for a few inches and run a hipot test on it to see how much it will stand.
By the way, foam dialectic rg8 is rated at 600volts rms.
Foams are for low lids, not high voltage. MB

[K1JJ]

FB Mike -


For end supports, I decided to go with 3/8" polyester rope and pulleys instead of the wire aircraft cable.  Same price.  Two ropes per side oughta make a solid structure and with 20' end spacing at 115' / 95'  it will allow a decent droop to lighten the load.  The bottom dipole will be flat-ish and the upper one will have an upward truss slope.

Still not sure about the balun fine details, but close..  I may mount some 10" diameter RG-11 turns loosely sitting around  a 4" diameter ~8 inch long ABS pipe and also use the pipe as the center insulator for the legs. I can also mount a few beads there too. Low weight is important to me.   It's a big project to get this up and working.

If I have, say, 10 turns of RG-11, where is the best spot to add the three large ferrite cores?  Should I put one on the input of the coil, thru the coil and the third one after the coil at the dipole input?  Or put all three on the coil - or all three at the dipole input? Any feel for this?

Yes, the RG-11 should take the power.  I ran it on 40M at full DXing power for years with no problems.  The thang that killed it was the water contamination I told you about. And even then, I didn't even see the signs of contamination until I replaced it and looked at the coax -  dark brown for its full length...  

Ed:  I like your idea of attaching the coil with heavy duty heatshrink/glue.  That will keep it floating with the stress off it. I'm looking into that. Also looking into tying it on with rope lashing. I'm good at that from the Boy Scout days... :-)   I will keep the coil ends facing the dipole legs to minimize coupling.    The main feedline below will be supported by a rope pipe hitch as I described before.   Sounds like a plan!


T

[K1JJ]

The sunlight and shadows near sunset caused me to get out the camera.  

T


Pic #2 is a rare view of the delta loops wire. They are usually washed out by the sunlight. The sun is JUST at the boom - before disappearing.  BTW, that boom hangs on a pin and is guyed by the loops. No torque stress on the tower. It sways slightly in the wind. Just below the loops is a 40M reference dipole at 100'.

Pic#3: Just mowed the lawn yesterday.  

[K1JJ]

More:

Pic #1:  Yaz puts up with a lot around here, but gets plenty of exercise following me around.

Pic #2 shows the 40M stack. Top of the hill.  Gotta climb up there to finish the job soon.

Pic#3 is a rare telephoto shot of what it looks like at the top of the 190' tower. That is the 40',  4.5" boom holding up the two 75M delta loops.  I put that up in 2013 and still works after recent repairs. Worked a lot of Russians and Euros on 75M with that array.  I still remember leaning out there to attach the boom to the sidearm.  Scares me just looking at it.   

T

[K1JJ]

For the balun, I broke down and ordered (20) type 43 ferrite cores, just like the ones used on the class E rigs.  FB-43-1020.   No coils of RG-11.   Now I can keep it light with just one insulator in the middle and six ferrite cores slipped over the dipoles' feedpoint.  It will solve a lot of problems and worth a try.  

This is the classic W2DU choke balun version.   A friend suggested it and I read a bunch of old threads written by Walt W2DU and decided it was the best choice at this point.

Hopefully the simple 20' leg spacing and staggered frequencies of the two dipoles will do the same broadband job as the heavy, complicated cage dipole. The cage would never stay up in an ice storm here....   Now the center weight will be one ceramic insulator, about 65' of light RG-11 and six ferrite cores. Maybe a few pounds at most... and held up by two ropes per side with an overhead trussing effect.   I like it!  

T  

Pic#1: The garden is growing like crazy. We usually end up giving tons of tomatoes away.  

Pic #2:  My first tower, Rohn 45, 150' put up in 1986.  After repainting with zinc paste, it's still solid and could last another 40 years.  Star-guyed and uses three sets of anchors going out 30', 90' and 140'.   That tower has seen a lot of big antennas on it over the years. It's the only tower not holding up an antenna right now. Been thinking hard for something new...

[K9MB]

FB Mike -


For end supports, I decided to go with 3/8" polyester rope and pulleys instead of the wire aircraft cable.  Same price.  Two ropes per side oughta make a solid structure and with 20' end spacing at 115' / 95'  it will allow a decent droop to lighten the load.  The bottom dipole will be flat-ish and the upper one will have an upward truss slope.

Still not sure about the balun fine details, but close..  I may mount some 10" diameter RG-11 turns loosely sitting around  a 4" diameter ~8 inch long ABS pipe and also use the pipe as the center insulator for the legs. I can also mount a few beads there too. Low weight is important to me.   It's a big project to get this up and working.

If I have, say, 10 turns of RG-11, where is the best spot to add the three large ferrite cores?  Should I put one on the input of the coil, thru the coil and the third one after the coil at the dipole input?  Or put all three on the coil - or all three at the dipole input? Any feel for this?

Yes, the RG-11 should take the power.  I ran it on 40M at full DXing power for years with no problems.  The thang that killed it was the water contamination I told you about. And even then, I didn't even see the signs of contamination until I replaced it and looked at the coax -  dark brown for its full length...  

Ed:  I like your idea of attaching the coil with heavy duty heatshrink/glue.  That will keep it floating with the stress off it. I'm looking into that. Also looking into tying it on with rope lashing. I'm good at that from the Boy Scout days... :-)   I will keep the coil ends facing the dipole legs to minimize coupling.    The main feedline below will be supported by a rope pipe hitch as I described before.   Sounds like a plan!


T

Tom,
A 4.5 inch diameter 8 inch long coaxial coil will have about 18 turns and 15uH which has about 500 ohms of reactance at 80 meters.

I have included some charts showing chokes made with rg400 on FT-243-31 and FT400-31 toroids that have 8-12k ohms of choking impedance.
This method allows layouts that can be weatherized inside 4 inch sewer pipe caps.
Much better choking and light and compact.
See charts here and link to the cookbook below.

See page 14 for layout designs
http://k9yc.com/2018Cookbook.pdf

Your physical support system sounds fine from your description.
73, Mike

[K9MB]

Tom,
I know you are leaning toward a choke balun that uses the rg11 straight through approach, so here is a lot of stuff that can help with that. IMO, Jim Brown has done more research than anyone and his presentation at tje bottom- and old one can be of great assistance to understand balun chokes.
73, Mike


https://k6jca.blogspot.com/2018/06/transmit-common-mode-chokes-11-current.html

https://www.google.com/search?q=ferrite+balun+choke&tbm=isch&ved=2ahUKEwif4bjJs_H4AhWYBc0KHVX1DvYQ2-cCegQIABAC&oq=ferrote+balun+choke&gs_lcp=ChJtb2JpbGUtZ3dzLXdpei1pbWcQARgAMgQIHhAKOgQIIxAnOgYIABAeEAg6BwgjELACECc6CAgAEB4QCBANULQYWK4tYPFjaABwAHgAgAHWAYgBlwmSAQUxLjcuMZgBAKABAcABAQ&sclient=mobile-gws-wiz-img&ei=fmPMYp-qM5iLtAbV6ruwDw&bih=897&biw=1366&prmd=svin&rlz=1C9BKJA_enUS931US931&hl=en-US#imgrc=N16WCM8NxUCqGM

https://www.nonstopsystems.com/radio/frank_radio_baluns.htm

https://www.nonstopsystems.com/radio/frank_radio_baluns.htm#chokes

http://www.audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf

http://audiosystemsgroup.com/CoaxChokesPPT.pdf

[K9MB]

Image of a big coax wound choke using a stack of toroids

[KD6VXI]

Since the choking impedance goes up with the square of the turns, a string of beads today is foolish.

I have seen well over 8kohms of choke on an optimized choke balun made of LMR240.  Typically you can get over 5k on a hf choke.

How many beads is it going to take to get 5k?  A LOT!  Like over 50, IIRC

--Shane
WP2ASS / ex KD6VXI

[K1JJ]

Lots of info, thanks, Mike!   I read thru most of it.  There certainly are a lot of different opinions and approaches.

I think the main thing in this application is that I need something lightweight due to no support at  the center - and a minimum impedance that will still work.

I understand that  a minimum X5 of the input impedance will  still do the job for the majority of unwanted current. Maybe -20 dB isolation?  So at 75 ohms X  5 = 375 ohms.  I can eek out this impedance with a string of beads based on info I've read.  If I start to see  it get squirrelly, I can always add some coil turns or more beads.  But for now I want to keep the package small to see how it flies.  

Each FB-43-1020 bead = 1.75 uH.     9 beads slipped over RG-11 =  15.75 uH.      At 3.800 Mhz =  376 ohms inductive reactance.   75 ohms X 5 = 375 ohms.  Right at the hairy edge....  assuming X5 is an acceptable margin.

I should have it working within a week when parts arrive and will report back.

Thanks again and I appreciate all the extra effort, OM!!

T

[K9MB]

Lots of info, thanks, Mike!   I read thru all of it.  There certainly are a lot of different opinions and approaches.

I think the main thing in this application is that I need something lightweight due to no support at  the center - and a minimum impedance that will still work.

I understand that  a minimum X5 the input impedance will  still do the job for the majority of unwanted current. So at 75 ohms X  5 = 375 ohms.  I can eek out this impedance with a string of beads based on info I've read.  If I start to see  it get squirrelly, I can always add some coil turns or more beads.  But for now I want to keep the package small to see how it flies.  I should have it working within a week when parts arrive and will report back.

Thanks again!

T

Ok, Tom. Good luck.
One thing you should note from Jim Brown’s and others’ presentations is that choke baluns work best when they use materials that have as high loss resistance as possible. I believe that W2DU used type 73 material that had very high mu and high R value and later work with toroids utilized the high R value of Type 31 material.
The secondary consequence of harnessing loss impedance is that any residual current will result in heating of the bead or toroid.
We all use very high values of resistance as voltage dropping resistors because it also limits current, say- for driving the screen of a tetrode. In tjose cases, we want residual voltage, so calculate how big the resistor needs to be to handle the power  and still provide the voltage needed.
In this case, we want zero volts-ideally below our “dropping resistor” (choke), so the higher the total impedance the better and the higher the R part of the impedance, the better it covers a wider range of frequencies  without becoming capacitive.(above self resonance).
That is why Jim Brown and others recommend as high an impedance as possible because the current is body slammed to nothing, even when high power is involved.
If the impedance and R value are lower, then any significant current can cause the beads or toroids to get very hot and self destruct.
The 500 ohm rule makes sense when talking about lossless inductive reactance, but if you are talking about a lossy balun, which these things are in the common mode, then any current will inevitably cause it to heat up.
The thing that mitigates this for bead strings is that they get a lot of surface area, so they can dissipate heat easily, compared to a toroid in an enclosure.
My preference is the enclosed toroid, but making the toroid have 16-18 tirns and having 12-16k of choking impedance, so currents are effectively reduced to nearly zero. They are also pretty compact and light when made with rg400 or paired awg10 teflon wire.

Still- the experiment may work for you if you have the #73 beads that W2DU recommends.
73, Mike


https://owenduffy.net/balun/W2DU/index.htm


Edit: just checked and W2DU used #73 beads that can accomodate 0.195 diameter coax, but they do not make the big beads in 73 material.
If you use #31 beads, the mu is 1500 vs 5000 for 73 ferrite, so the choking impedance is lower. Tough choices here…MB

[K1JJ]

Mike,

After doing some thinking, another thing that comes to mind is the real world applications...  

I think the common mode isolation requirements of a simple dipole with the feedline at perfect right angles is less stringent than say, a stacked, phased driven Yagi array.   The difference in a 375 ohm balun performance could be -20 dB  vs: -25 to -30 dB for a 1000 ohm - but still perform acceptably.    The radiation patterns may be the difference between lazy, broad lobes to very sharp, -40dB nulls.

For a "local" higher angle  dipole application, I think the broader lobes (if present) might even reduce communication fading by filling in the sharp nulls. Especially the moderately sharper pattern of a 75M dipole at 100'.  Whereas for a phased, driven array, we like to get all the sharp nulls and performance we can, thus baluns that are 1500 ohms or even more.  On my 40M triple Yagi stack, I used 14 turns of RG-213 on a 4" ABS form and 8 ferrite beads at each Yagi feedpoint.  The f-b and high angle suppression can approach -30dB or higher at times. I know it works well.  They are supported at the feedpoint by the boom, so no structural issues there.

But I think the broadband dipoles planned will do fine as a local and refence dipole with -20 dB isolation AND as a benefit being physically light at the feedpoint to survive the storms..

As for local noise suppression, I pay close attention to this. The noise level on 75M during the day is below S1... really nothing there to suppress. Neighbors far away -  antennas and power lines far away.  At night noise will propagate in based on storms across the country.   So this is my rationalization to getting away with a more marginal beads-only balun for the broadband dipole.  

BTW, the 75M loops use an openwire antenna matcher -  and uses ferrite beads  into the  50 ohm link, so is well covered there for common mode problems. That is a performing antenna system that needs good isolation for pattern sharpness and DX hearing..

The thing that does concern me is about the possibility of wasted or damaging heat generated by the beads.  I will test this with a 1500W  carrier for 5 minutes and then quickly drop the dipoles down to do a touchy-feely...     The beads will be supported below the feedpoint in open air for cooling with their weight transferred to the center insulator..

T

[K9MB]

Mike,

After doing some thinking, another thing that comes to mind is the real world applications...  

I think the common mode isolation requirements of a simple dipole with the feedline at perfect right angles is less stringent than say, a stacked, phased driven Yagi array.   The difference in balun performance could be -20 dB  vs: -25 to -30 dB but still perform acceptably.    The radiation patterns may be the difference between lazy, broad lobes to very sharp, -40dB nulls.

For a "local" higher angle  dipole application, I think the broader lobes (if present) might even help communication fading by filling in the sharp nulls. Especially the moderately sharper pattern of a 75M dipole at 100'.  Whereas for a phased, driven array, we like to get all the sharp nulls and performance we can, thus baluns that are 1500 ohms or even more.  On my 40M triple Yagi stack, I used 14 turns of RG-213 on a 4" ABS form and 8 ferrite beads at each Yagi feedpoint.  The f-b and high angle suppression can approach -30dB or higher at times. I know it works well.  They are supported at the feedpoint by the boom, so no structural issues there.

But I think the broadband dipoles planned will do fine as a local and refence dipole with -20 dB isolation AND as a benefit being physically light at the feedpoint to survive the storms..

As for local noise suppression, I pay close attention to this. The noise level on 75M during the day is below S1... really nothing there to suppress. Neighbors far away -  antennas and power lines far away.  At night noise will propagate in based on storms across the country.   So this is my rationalization to getting away with a more marginal beads-only balun for the broadband dipole.  

BTW, the 75M loops use an openwire antenna matcher -  and uses ferrite beads  into the  50 ohm link, so is well covered there for common mode problems. That is a performing antenna system that needs good isolation for pattern sharpness and DX hearing..

The thing that does concern me is about the possibility of wasted or damaging heat generated by the beads.  I will test this with a 1500W  carrier for 5 minutes and then quickly drop the dipoles down to do a touchy-feely...     They will be suspended below the feedpoint in open air for cooling with their weight transferred to the center insulator..

T

Tom,
I had not considered specific criteria for this individual application.
If local (less than 500 miles is going to be most of your working area, there is good reason to think that a radiating feedline could be a good thing, given the fact that you are up 3/8 wavelength with a flat top dipole.
If local rfi is not a comcern, one might forego the balun at the feed point completely
and insert a very good choke at 1/2 wavelength from the feed point where it is a voltage minimum on any common mode currents.

I need to confess that I am a fruitcake for over designing everything… Old habits!😬🤪😁

I,doubt that the beads will get very hot, because it will be only a few watts and it is open air.
Hey test it at night and if it looks like a sparkler- try something else…😉😂😂
Let me know your results. 73, MB

[K1JJ]

Mike,

You said:

"If local rfi is not a concern, one might forego the balun at the feed point completely
and insert a very good choke at 1/2 wavelength from the feed point where it is a voltage minimum on any common mode currents."


Could you elaborate on this technique, reasoning and benefits of doing this?  Why not do it all the time instead of at the feedpoint to reduce weight and easy accessibility?  I read about it but not sure I understand what is going on there...

T

[K9MB]

Mike,

You said:

"If local rfi is not a concern, one might forego the balun at the feed point completely
and insert a very good choke at 1/2 wavelength from the feed point where it is a voltage minimum on any common mode currents."


Could you elaborate on this technique, reasoning and benefits of doing this?  Why not do it all the time instead of at the feedpoint to reduce weight and easy accessibility?  I read about it but not sure I understand what is going on there...

T

Full disclosure- I have not personally tried it.
The theory is that if you do not put a choke at the feedpoint or a balun, there will be common mode current flowing on the shield of the coax.
Normally, we do not want common mode current because it can result in RF getting into the shack, and also causing RFI on the feedline pathway.
Also, the shield will change the impedance of the antenna because it will effectivel look like a finite parallel load.
It will also make the directional antennas likely to have extra spurious lobes and interact with the beam pattern.

In this specific case, a secondary vertically polarized radiator can actually benefit local communications for short hops and ground wave, if the current is significant.

Still- the presence of rf in the shack and burned lips from touching a hot mic is not attractive, so we still need to eliminate rf currents at or near the tower bottom.
Since the common mode current will- like the antenna have current loops and nodes and alternate voltage variations, the placement of the choke on the ground can be more effective if it occurs 1/2 wavelength or a multiple of that from the feedline and we are talking about the shield, so it will not involve velocity factor in locating that point. For 80 meters, it will be 130 feet from the feedpoint therefore, I believe.
Maybe someone has actually implemented this and can correct or confirm these ideas?
It might be worth trying…
I put a good choke at the feedpoint of my Tennadyne T8 and another 66 feet below it- likely overkill…😉 MB

Edit:I have toyed with doing this very thing on a flatop for 80 meters, but my height is only 65 feet, so I would need to extend the feedline another 1/4 wavelength, so it would not be as good as your 95ft height where you just need 35 feet more before the good choke.
Besides-mat 65 feet, my antenna is already high angle radiator..

[K1JJ]

UPDATE  7-16-2022:

Mike and those interested in some results for the fan broadband dipoles project:

I'm very pleased with the initial results! Over the last week all the parts came in... 500' of RG-11 75 ohm coax, weatherproofing supplies, rope, etc.   I got some nice Polyester 3/8" braided rope. Beautiful stuff for the pulleys. Now that everything is up on the tower, I can work from the ground.

As planned, I made up some homebrew RG-11 connectors as well as weatherproofed everything. I made the center very light with nine ferrite cores and just two insulators. I supported the RG-11 at 30' high off the ground. The dipole centers are at about 85' because they are loose and sag by design.  The double ends are at about  105' and 85'.  I think this is the optimum height for local/moderate distance on 75M.

Today I raised it all up and finished the connectors and ran the feedline.  I plugged the MFJ-259B in and nervously anticipated the results.   The 75 ohms J0 point was about 5% lower. (as predicted by the articles)  The system resonated at about 3650.  I want it at about 3825.  This is about 6 feet too long.  I can fix that tomorrow and trim it to bring it up to 3825.  The great part is that the swr stayed below 2:1 from about 3.4 Mhz all the way up to 3.9 MHz!  So once I get it trimmed up to 3825, it should do 3700 to 3900 easily with 1.3:1 or less based on my brief tests today.

Bottom line is the 1:1 swr dip is very smooth and broad. It's like a dummyload within the first +- 100 KHz.    I really should have made an antenna like this long ago.  Every ham needs one accurate antenna...   I see no signs of antenna interactions / or feedline radiation.  I'll have some accurate measured curves soon.

So I will be keeping the legs both the same length. The swr curve is so clean and flat that I don't want to mess it up with staggered "W" double dips and that stuff. The 20' leg spacing with equal legs is very FB as-is. My other single wire dipole at 90' had a very poor, sharp swr curve in comparison.  I think this may be better than a heavy cage dipole certainly for  weight - and performance.

It's hard to see from the pics, but I put a lot of time and effort into the center insulator area.  The RG-11 is supported by a long rope pipe hitch, like a hangman's noose, a Chinese finger grip knot.  The weight is well distributed.  Also the 9 ferrite beads are supported by the center insulator to keep the weight off the RG-11.  I have the RG-11 supported on a mast so that only 65' is pulling down.  The RG-11U is happy and really VERY lightweight.  Small wind profile overall.

I made up copper sleeves as the RG-11 shield connector.  I then used three different waterproofing techniques using 3M tape, rubber tape and liquid tape.  Best made wire antenna I have.  The stress on the ropes and pulleys is surprisingly light considering no center support.. I expected a very hard pull, but as you can see from the pictures, the dipoles are just loafing along. I think it will do fine in wind and ice storms.  There are no trees swaying since all is anchored to towers.

Listening on the air with A/B tests, there is a tremendous difference between the high DX delta loops and fan dipoles. The locals are weak on the delta loops and strong on the broadbanded fan dipoles, sometimes by as much as 20 dB difference. This tells me I have the heights dialed in pretty well.

There will be more testing once I bring the system up to the 3825 area.   The swr should be reasonably flat around 1.2 : 1 on both 3885 and 3775 exceeding my two goals.

Thanks again for the suggestions on this project. It appears to be the best of a few worlds;  light weight, broadbanded, durability, efficiency...the design that is.

T
 
Pic #3 shows the 30' high "relay" support of the RG-11

[K1JJ]

Pic #1:  Notice the lazy droop on the dipole legs.  Very little stress. It could be pulled much higher, but it will last longer loose. If I wanted it higher, I'd climb higher...  

Pic#2:  Ferrite support, RG-11 support, #10 antenna wire, rope pipe hitches and weatherproofing all in one lightweight, low wind profile package

Pic #3:  The 75 ohm aluminum hardline uses my standard homemade connectors. Uses 3/4" to 1/2" copper plumbing adapter, SO-239 and an inner pin from Andrew Co.  The RG-11 uses a copper sleeve and SS hose clamps to connect to its aluminum braid and foil.  It should handle a KW no problem.