Question regarding antenna bandwidth for coax vs: open wire line

[K9MB]

Hi Folks,

My comments are based upon experience (some of it bad) and a bit of theory.

I believe what we are discussing is minimizing feedline radiation due to unbalanced feed from coax to a balanced dipole.  If you run the feedline away from the dipole perpendicularly for a good 1/2 wave or more to get it out of the inductive field, then you may not even need the "balun". Equal but opposite currents would be generated on the feedline on the outside of the shield but cancel out. I have done this in the past and minimal (milliamperes) outside feedline current was measured taken at several points along the feedline (important!) at 1 KW.  

Years ago I had stacked KLM yagis up for the high bands which used ferrite based baluns.  After a lightning strike VERY close, I noticed the F/B ratio on two of the antennas had dropped from the usual 25 dB or so to around 10 dB the day after the storm.  No visible damage anywhere but the SWR had also gone up a bit so I decided to replace the baluns.  That fixed the F/B problem.  I sent the baluns back to KLM and their analysis was that they had become very lossy.  I mentioned the lightning and they said they had seen this before.  My guess was that they had ceased being baluns and the feedlines now became part of the antenna destroying the pattern.  These were 5 KW baluns with extra ferrite too!   I never put more than about 2500 watts into them. This happened once again and then I went to air wound solenoid choke baluns on all the antennas, never had the problem again.  Anecdotal for sure but.....

On solenoid choke "balun" design, I use an HP 4815A vector impedance meter to optimize the # of turns on 6" PVC.  I have 2 of these on my 2 stacked 40M beams.  To optimize this design, I connected the meter to the shield on either side of the coaxial choke and adjusted the # of turns to get the parallel resonant frequency to 7200 KHz.  It took 12 turns of RG-213 to accomplish this.  At resonance the impedance was 30K ohms and purely resistive.  At 7100 it dropped to about 16K and was slightly inductive, perhaps 10 to 20 degrees if I remember correctly.  At 7300 it was slightly capacitive and about 15K ohms.  Not bad.  Clean text book pattern with no measurable feedline currents.

Having not completely adhered to my no outside ferrites rule yet, in 2010 when I put up the latest 4 stack of 10E log periodic antennas, I used two ferrite based power combiners outside mounted up the tower to reduce the feedline count  coming into the shack.  I used this combiner set up for several years with no issues until another nearby lightning strike.  The next day when I used the logs, I blew up one of the combiners after a short transmission which started out fine.  I remember watching the SWR creeping up thinking maybe some water got into something until it went to infinity.   The combiner had split into a number of pieces and looked like it was overheated since plastic enclosure it was in had started to melt.  I wound up running all four transmission lines into the shack where the ferrite combiners are.  It all gets disconnected when storms come through.  

This is why I will NOT use ferrites outside.  

Back when I designed high power SS amps professionally,  the higher permeability ferrites would sometimes get permanently magnetized if an output device failed and sent a high peak current through an output transformer.  This would skew the hysteresis curve of the ferrite and render it lossy.  The ferrites would still work but run very hot.  Avoid ferrites if possible.  We eventually changed over to powdered iron which is much less susceptible to this failure but it isn't available in a higher permeability unfortunately.

-Chuck

That is very interesting, Chuck.
I assume that your combiners were transmission
Line devices and I can see how they might be destroyed if the reactances dove to nothing. Those are inherently reactive applications where low loss ferrites are essential.
What kind of ferrite did you use?
Have you ever used #2 powdered iron for that kind of combiner?
You certainly have a great deal of experience and I would love to know
How you solved the combiner problem?
I used custom coaxial designs for my moon bounce arrays in the
Late 70s, but that was 2meters where a wavelength is 82-84 inches snd my application was dividers, not combiners, also, do far different from your 40 combiners. 73, MB

[W1ITT]

Mike and all...
In the HF broadcast world, balanced line is still used. 
(although I'm unaware of any new SWBC stations being built and some of the ones that I built have since been taken down.)In some installations the "open wire" is actually copper tubing an inch or so in diameter, hung from insulators.  Some is multistrand alumoweld cable of the sort that would make darn good guy wires on most of our towers, also hung from insulators. The American transmitters seem to gravitate toward 50 ohm coaxial outfput, fed through switch matrices depending on what antenna was wanted, then often to  a broadband transmission line balun outdoors, then balanced to the curtain.  Other folks use balanced tanks at the outfput of the transmitter and go from there, into balanced switch houses and then out to the field  Most of the world operates at 300 ohms but, very Britishly, the Brits liked 330 ohms.  One used to see big oil filled baluns with presumably ferrite cores, but now the transmission lines rule the roost.
I was always bad at photographs, sometimes because I was never  a camera nut and sometimes because the end users were not always happy about picture of their installations, even though all the hardware appeared in manufacturers' brochures.
Back here in the Lower 48, I content myself with the almost 500 feet of open wire  (540 ohms) with number 12 Flexweave and feed it with either link coupled   tuners or balanced L nets with ferrite at the 50 ohm input.I solve the bandwidth problem on my 80m phased array by having a tuner down on CW and the other in the SSB DX window and big honking knife switches.
By the way, I appreciate Tom's habit of bringing his projects into the room.  Plenty of food for thought and education, and even some of the folks who aren't into this level of construction can learn along with the rest of us.
73 de Norm W1ITT

[Chuck...K1KW]

Mike & company,

The "combiner" is a 1:2 impedance ratio ferrite transformer to transform 25 ohms to 50.  I haven't read Sevick's book in years but I believe he covered this design.The top two 10E log antennas are fed in phase and in parallel with equal length lines to the input of the 1:2 transformer.  Same with the bottom two log periodics.  To run all 4 antennas I use a third 1:2 transformer where the output of the two combiners are combined.  This way I can drive either the top two antennas or the bottom two antennas separately or all four together all with 50 ohms.

Previously the combiners were on the tower.  Now I run all four equal length feedlines into the radio room where the combiners are now.  I disconnect the feedlines from bulkhead connectors on my house in case a storm is a possibility.  I do this with all outside connections (rotor controls and other feedlines) for total isolation.  Problem solved.

-Chuck

[K9MB]

Mike & company,

The "combiner" is a 1:2 impedance ratio ferrite transformer to transform 25 ohms to 50.  I haven't read Sevick's book in years but I believe he covered this design.The top two 10E log antennas are fed in phase and in parallel with equal length lines to the input of the 1:2 transformer.  Same with the bottom two log periodics.  To run all 4 antennas I use a third 1:2 transformerl where the output of the two combiners are combined.  This way I can drive either the top two antennas or the bottom two antennas separately or all four together all with 50 ohms.

Previously the combiners were on the tower.  Now I run all four equal length feedlines into the radio room where the combiners are now.  I disconnect the feedlines from bulkhead connectors on my house in case a storm is a possibility.  I do this with all outside connections (rotor controls and other feedlines) for total isolation.  Problem solved.
Thanks Chuck,
Good practical solution.

72, Mike

-Chuck

Chuck, I was just thinking- have you considered a system of linear transformers for feeding your logs?

What I mean is that you build up a system of 75 ohm 1/4 wave Q sections for each of the 50 ohm feeders and switch them in for different bands.
If it were just-say 20 meters, it would be simple. Two 1/4 wave stubs feeding a Tee connector. You would have two 100ohm loads in parallel and in phase.
If you wanted other bands, you just use high current vacuum relays to switch in stubs that would give you 15meters, 10 meters, etc.
I know this smacks of Rube Goldberg (I am an original cult member😉😂) but it would have the virtue of being relatively immune to EMP attack.
It would work. I am sure. Used something like it in my 80 element Moonbounce array of 16-5 element beams…. Talk about a tribute to old Rube?!!😎😁
Comments? MB

[K9MB]

Mike and all...
In the HF broadcast world, balanced line is still used.  
(although I'm unaware of any new SWBC stations being built and some of the ones that I built have since been taken down.)In some installations the "open wire" is actually copper tubing an inch or so in diameter, hung from insulators.  Some is multistrand alumoweld cable of the sort that would make darn good guy wires on most of our towers, also hung from insulators. The American transmitters seem to gravitate toward 50 ohm coaxial outfput, fed through switch matrices depending on what antenna was wanted, then often to  a broadband transmission line balun outdoors, then balanced to the curtain.  Other folks use balanced tanks at the outfput of the transmitter and go from there, into balanced switch houses and then out to the field  Most of the world operates at 300 ohms but, very Britishly, the Brits liked 330 ohms.  One used to see big oil filled baluns with presumably ferrite cores, but now the transmission lines rule the roost.
I was always bad at photographs, sometimes because I was never  a camera nut and sometimes because the end users were not always happy about picture of their installations, even though all the hardware appeared in manufacturers' brochures.
Back here in the Lower 48, I content myself with the almost 500 feet of open wire  (540 ohms) with number 12 Flexweave and feed it with either link coupled   tuners or balanced L nets with ferrite at the 50 ohm input.I solve the bandwidth problem on my 80m phased array by having a tuner down on CW and the other in the SSB DX window and big honking knife switches.
By the way, I appreciate Tom's habit of bringing his projects into the room.  Plenty of food for thought and education, and even some of the folks who aren't into this level of construction can learn along with the rest of us.
73 de Norm W1ITT

Thanks Norm, very interesting rundown. No doubt, that 1 inch copper tube open lime is very low loss but the  rub being at the balun.
I really like the synchronized twin L network tuner that someone published and I have accumulated most of the parts for it have a couple of nice RCA rotary inductors for the inductors.
I would like to switch a vacuum variable across either end to allow the impedance match to extend above and below 50 ohms. Any suggestions appreciated on that.

Tom is a great contributor- not only for his large history as a builder snd experimenter, but his knack of stimulating the discussion to improve the art by allowing the vast experience of members and the creativity that goes with it, to make this forum a great place.
Design and experimentation and creative solutions have always been my meat and drink, so this is great fun. I feel like I learn a lot on here and on Class E Forum all the time.

[K1JJ]

I appreciate the answers, Mike, Norm and Chuck!

Yes, these threads will outlive all of us on the web.  I could be wrong, but I think 97% of the hits on this forum are from Google and web based search engines. That's how some of these threads get 100K + hits.  We learn and the world learns too.   Goto:  Home, Users Online,  Click on: 67 Guests, 3 Users (1 Hidden) to see the hits over the last 20 minutes or so... There are sometimes hits every 10 seconds  - mostly to old topics...absolutely amazing.

I have a few more questions that are still fuzzy...

1) Will a balun do the same job if mounted 1/2 wave away (or any 1/2 wave multiple if the coax is long enough) from the antenna feedpoint?  Or why is the antenna feedpoint the best spot, if any?  It would be so much easier for servicing if 1/2 wave away worked as well.

2) Back to Chucks balun idea...can the MFJ-259B ant analyzer work in place of an HP network analyzer to find the resonant point of a coaxial solenoid wound balun?  It appears the distributed capacitance from the turns is creating the capacitance, not an external capacitor.  Maybe if this balun is operated strictly within its 7.0 - 7.3 resonant design with 15K-30K impedance, it can work well. In my case for the delta loops I'm looking at 3.8 MHz so it would be a big coil. I'd like to try one at the 1/2 wave away point as discussed.  

My openwire experiment feeding both loops, one as a driven and one as a tuned reflector stub did not work as well as it did with coax in the past.  It had forward gain into EU, (+10 DB over the fan dipoles) but the f-b was lacking.  I brought the open wire closer together, 6" spacing but still poor f-b on the air. The antenna beacon off the back nulled sharply, however.  It's like the band has been broken.  The locals are down -20 to -30 DB as desired, but the far away W4s should also be way down but are not. Strange.  I'm hoping a forced driven feed coaxial system will create a tighter pattern.


# BTW, I ran a 1500 watt carrier thru the new broadband fan dipole for a few minutes. The swr stayed at 1:1 on 3785.   So it looks like the RG-11 and nine stacked ferrite cores are working FB. I will use this as a baseline test for lightning or other damage.


T

[K9MB]

I appreciate the answers, Mike, Norm and Chuck!

Yes, these threads will outlive all of us on the web.  I could be wrong, but I think 97% of the hits on this forum are from Google and web based search engines. That's how some of them get 100K + hits.  We learn and the world learns too.   Goto:  Home, Users Online,  Click on: 67 Guests, 3 Users (1 Hidden) to see the hits over the last 20 minutes or so... There are hits every few seconds to old topics...absolutely amazing.

I have a few more questions that are still fuzzy...

1) Will a balun do the same job if mounted 1/2 wave away (or any 1/2 wave multiple if the coax is long enough) from the antenna feedpoint?  Or why is the antenna feedpoint the best spot, if any?  It would be so much easier for servicing if 1/2 wave away worked as well.

2) Can the MFJ-259B ant analyzer work in place of an HP network analyzer to find the resonant point of a coaxial solenoid wound balun?  It appears the distributed capacitance from the turns is creating the capacitance, not an external capacitor.  Maybe if this balun is operated strictly within its 7.0 - 7.3 resonant design with 15K-30K impedance, it can work well. In my case for the delta loops I'm looking at 3.8 MHz so it would be a big coil. I'd like to try one at the 1/2 wave away point as discussed.  

My openwire experiment feeding both loops, one as a driven and one as a tuned reflector stub did not work as well as it did with coax in the past.  It had forward gain into EU, (+10 DB over the fan dipoles) but the f-b was lacking.  I brought the open wire closer together, 6" spacing but still poor f-b on the air. The antenna beacon off the back nulled sharply, however.  It's like the band has been broken.  The locals are down -20 to -30 DB as desired, but the far away W4s should also be way down but are not. Strange.  I'm hoping a forced driven feed coaxial system will create a tighter pattern.


# BTW, I ran a 1500 watt carrier thru the new broadband fan dipole for a few minutes. The swr stayed at 1:1 on 3785.   So it looks like the RG-11 and nine stacked ferrite cores are working FB. I will use this as a baseline test for lightning or other damage.


T

Tom,
We should use the term common mode choke to describe what I have advocated and the purpose is to prevent any common mode current flow on the feedline.
Putting the first one 1/2 Wave from the feedpoint is like leaving th barn door open and them relying on the stock femce gate to keep the horse “in”. He is,obviously already “out” 😉.
The second one I advocated was there as a secondary backup, not a promary solution, to scrub any remaining current at a place where the standing wave would present the lowest voltage, rather than at 1/4,wave where it is max voltage and impedance.

Good news about the tests on the new wideband fan. Run a contest through it with hours of operation and I will be convinced, however…😉.  Still- of there was a big problem,it would have been apparent already…
You have achieved a lot already on that nice 80 meter fan…😎 Mike

[KD6VXI]

If you want to go air core coaxial balun, the work has already been done.

http://www.k1ttt.net/technote/airbalun.html

I will add, you want NO metal near the coax.  It will increase C and cause the balun to be less baluny....  Balunistic?  Whatever.

If you are still considering the ferrite, the 4 inch cores work quite well with LMR600.

Just don't exceed the bend radius.  They look goofy but they work well.  And LMR600 will take anything you've posted pics of....

I've not lived where lightning was a big threat....  So I defer to Chuck and his experiences.  I guess I will be dealing withightning now, though.

--Shane
WP2ASS / ex KD6VXI

[Chuck...K1KW]

Mike,

No on your idea.  As you said, Rube Goldberg!  The solution I did was very broadband requiring a 2:1 frequency range for the log periodics and that could only be accomplished with a broadband approach hence the use of ferrite transformers.  Now safely within in my house, no worries!  Totally different from the transmission line based approach used on VHF where it makes sense.

Tom,

The MFJ will not do the measurement you desire.  It's a 50 ohm based device.  A really great way to do this is with the Nano VNA.  I have one and it pretty much agrees with my HP stuff although really high impedance values make it go nuts.  If you get to that point, your coaxial "balun" is good.

As far as a 1/2 wave away, you would need to make sure everything is very symmetrical to the 1/2 wave point to avoid picking up common mode currents.  If you are that far away, with symmetry, then the balun may not needed anyway.  

Comment:  Given these are dipoles, if 80 to 90% of the power gets to the dipole, why worry about a small fractional dB that might slightly distort the pattern?  It's not a high gain yagi that needs pattern preservation.  (like the VHF guys that need a clean pattern to reduce terrestrial noise)

[K1JJ]

Mike,

Yep,  balun, choke and unun  ....  I often use them interchangeably out of laziness.   I am dealing with chokes here mostly.

Shane, I do intend to use a combination of air core chokes and ferrite beads on both the fan dipoles and the delta loops. Whatever I come up with in the end it will talk, caw mawn...

Chuck,  OK on the MFJ-259 being 50 ohms and not a good tool for the high impedance job.

The fan dipoles are turning into an experiment to see how clean and effective I can make an antenna for 75M.  It covers the whole band reasonably flat and I figured it could also be made as quiet as possible. During the day, the S meter sits at zero... so it's a quiet neighborhood to start with.  It is now my main antenna for local 75M work and will get used a lot, so WTF, might as well. The loops are going to be the real challenge, pattern-wise.  There are a lot of possibilities now for the dipoles and the loops since I've learned a lot of things from this thread.


It's been one of those days...  I took Yaz for a long walk and a thunderstorm rolled in.  Got drenched beyond recognition.

I get home and figure I'd do some coax feedline work by putting some connectors on the RG-11.  It is a slow, difficult job.  After doing a great job two hours later I realized that I forgot to put the ferrite beads on first, before the connectors.  They will not fit over the connectors and I have no clamp-ons, just a box of donuts. Took out the dikes and madly snipped off the connectors to start over.  Am I the only one who does stupid crap like this?  And I reminded myself this morning to put the beads on first.

My Dodge truck blew a brake line.  Gawd darn salt on the roads. That will cost a cool $500.  To add to it, it's been near 100 F.  The summer heatwave.

No worries.

T

[K9MB]

Mike,

No on your idea.  As you said, Rube Goldberg!  The solution I did was very broadband requiring a 2:1 frequency range for the log periodics and that could only be accomplished with a broadband approach hence the use of ferrite transformers.  Now safely within in my house, no worries!  Totally different from the transmission line based approach used on VHF where it makes sense.

Tom,

The MFJ will not do the measurement you desire.  It's a 50 ohm based device.  A really great way to do this is with the Nano VNA.  I have one and it pretty much agrees with my HP stuff although really high impedance values make it go nuts.  If you get to that point, your coaxial "balun" is good.

As far as a 1/2 wave away, you would need to make sure everything is very symmetrical to the 1/2 wave point to avoid picking up common mode currents.  If you are that far away, with symmetry, then the balun may not needed anyway.  

Comment:  Given these are dipoles, if 80 to 90% of the power gets to the dipole, why worry about a small fractional dB that might slightly distort the pattern?  It's not a high gain yagi that needs pattern preservation.  (like the VHF guys that need a clean pattern to reduce terrestrial noise)

Hi Chuck,
😂😂😂 Ok, that was a test balloon to see how much you loathed Ferrite…😉
I get it, I would walk away from Rube on this deal too, but hey- you seemed unhappy with that great Sevick designed Ferrite transmission line transformer, but it was only the lightning- in the end and we all fear and loathe that destroyer…
I am sure that you will never see another fsilure with your protocol, by the way.

I told Tom the same thing about a radiating feedline on a no dorectional antenna that aims high and short. Of course, there is local rfi noise to consider and Tom did say he wanted it quiet and that feedline is a good way to pickup any stray noise maker…😉. MB

[K1JJ]

A friend sent me his interesting explanation of currents on the shield:


"I see guys talking about currents on the shield. A balun will not eliminate the current because they are in the near field. Picture the coax as 3 conductors. The center conductor, the inner shield surface, and the outer shield surface.  The rig drives the center conductor, and inner shield surface. The common mode choke tries to force a balanced out of phase current coming out of the coax so the antenna gets a balanced drive. In doing this drive current is isolated from the outer shield surface. You have unbalanced drive if the current goes down the outer shield and screws up the radiation pattern. Picture the common mode choke in this case is an RF choke in series with the outer shield because flux is balanced between the center and inner shield as long as the antenna presents a balanced load.  So if you have cable with a crappy shield life gets unruly because it will leak.
It was hard to wrap my head around this learning about transmission line transformers."

[K9MB]

A friend sent me his interesting explanation of currents on the shield:


"I see guys talking about currents on the shield. A balun will not eliminate the current because they are in the near field. Picture the coax as 3 conductors. The center conductor, the inner shield surface, and the outer shield surface.  The rig drives the center conductor, and inner shield surface. The common mode choke tries to force a balanced out of phase current coming out of the coax so the antenna gets a balanced drive. In doing this drive current is isolated from the outer shield surface. You have unbalanced drive if the current goes down the outer shield and screws up the radiation pattern. Picture the common mode choke in this case is an RF choke in series with the outer shield because flux is balanced between the center and inner shield as long as the antenna presents a balanced load.  So if you have cable with a crappy shield life gets unruly because it will leak.
It was hard to wrap my head around this learning about transmission line transformers."

Tom,
Your friend has a lot of it right, IMO, but a good choke greatly diminishes common mode current- or the potential of it,  and the near field is intense because there is the antenna currents as well, but the thing that helped me understand it was to stop seeing the choke as merely a “lossless” inductor, but as a very lossy device that has inductance and a great deal of loss reactance u” as well, and like a large dropping resistor for s voltmeter, very little current flows through it, and the current that does gets turned into heat, not transmitted or radiated.
I think that you understand the basic choke balun or 1:1 guanella balun function.
The idea of a common mode choke balun is to make any common mode voltages see a very high complex impedance which reduces common mode current to very small levels.
The differential mode currents are indeed in the center conductor and the inside of the shield.
I do not think that the shield is like the perfect waterhose that is a perfect barrier at all frequencies, though.
Think about the screens used in rf shielding an HF amplifier. As long as the wavelength is large compared to any holes in the shield, it can be effective.
As you go up in frequency, the grid has to be smaller and smaller to be effective.
That is why coax used for Hf can have less dense shielding and still be effective, while coax used for satellite (3.5GHz or higher) uses solid shield or double shield plus foil. Common mode chokes stop currents at the end of feedlines where the inside and outside are connected together. The reason certain Ferrites are more effective is that they have high permeability and high loss resistance and the vector sum will have a very high resistive component that is most effective in killing common mode currents before they can start flow9mg on the outside around the open end of the line.
If you look carefully at the K1TTT data, you will see that the impedance is positive in some ranges for certain coils and negative above a certain frequency and that frequency is the self resonant frequency of the choke. It therefore gradually stops being an inductive reactance and becomes more of a coupling capacitor that actually can effectively couple common mode current to the outer shield of the feedline.
This is why you cannot just make a huge coil to make it better, because above self resonance, it will become worse and even harmful.
Inductive coils actually work as single band chokes at or above 7mHz. You need to custom wrap them for a single band for best performance well below self resonance and where inductive reactance is maximum.
The trouble is that they are useless for wide frequency range and the impedances are not very high compared to FT240-31  RG-400 choke baluns.
If you want to make a single band 40 meter coax choke balun, or single band higher frequency bands with proper design parameters.  They can give 500-2000 ohms for these applications. This seems evident based on the K1TTT data.
I do agree that a coil of coax will be cooler than an inadequate choking impedance ferrite common mode choke because the currents that result from the low choking impedance can over heat the bead(s).
Jim Brown and G3TXQ explain all this in their articles.
73, Mike

[Opcom]

I really envy you guys with tall tower antennas and space away from city power lines. It's very educational to read and understand these experiments and gratifying when they work so well.

[K9MB]

I really envy you guys with tall tower antennas and space away from city power lines. It's very educational to read and understand these experiments and gratifying when they work so well.

Yeah Patrick, A lot of us have unfulfilled fantasies combining these two photos in one location. Trouble is, if you get the first one- property laws prevent the second. We all make compromises , but in my dreams!… 😉😂😂

[W1ITT]

In Mikes second picture, the mountaintop shot, that six-bay sidemount FM array in the back is one that I built years ago when I was doing FM antennas by the truckload.  I'm fortunate that I've been able to make my living playing with transmitters and antennas and getting paid for it, and flown all over the world.  Just the same, I'm happy with my 70 foot tower and 100+ foot pine trees in the back yard.  This fall's project is a 9-circle receive array for 160, 80 and 40 across the trout brook in my swamp.  Common mode is the potential problem on that as unwanted pickup can mess up the directivity quickly.
Again, I'm enjoying the continuing education facet of this thread.
73 de Norm W1ITT

[K9MB]

In Mikes second picture, the mountaintop shot, that six-bay sidemount FM array in the back is one that I built years ago when I was doing FM antennas by the truckload.  I'm fortunate that I've been able to make my living playing with transmitters and antennas and getting paid for it, and flown all over the world.  Just the same, I'm happy with my 70 foot tower and 100+ foot pine trees in the back yard.  This fall's project is a 9-circle receive array for 160, 80 and 40 across the trout brook in my swamp.  Common mode is the potential problem on that as unwanted pickup can mess up the directivity quickly.
Again, I'm enjoying the continuing education facet of this thread.
73 de Norm W1ITT

Norm, that is fantastic! I screenshot that pic from Google. The caption said it was from New Mexico, but I posted it because it embodies all a radio guy dreams of having in his backyard on that mountain.
The Martha’s Vineyard estate is more to my XYLs delight and as out if my reach as the amazing
Mountaintop antenna farm.
I wondered if one of the guys on here would recognize it when I posted.
It constantly astonishes me that there are centuries of radio engineering expertise among the members of this forum. AM Heaven!😎
Thanks for letting
Me know that you were one if those guys who did that amazing array close to the heavens . 73, MB

[KD6VXI]

Chatsworth Peak, California is awesome as well.

Even private homes got in on the antenna farms for commercial profit.

There is a TCI Log pointing at the Central America / Caribbean area there.  It was a single xmitter site using the last Amplifuzz on shortwave.  Now has multiple xmitters connected and running simultaneously so the poor engineer doesn't have to climb the mountain twice a day.  Jim appreciates that!

--Shane
WP2ASS / ex KD6VXI

[K1JJ]

New question concerning tuning two identical antenna elements:

Let's say I have two identical 75M delta loops  spaced 40' apart on a boom.  I want to resonate/tune them in place for 3.790 MHz -  to make each close to 75 ohms J0 at each  feedpoint bottom.   I can climb the tower and access each one (no feedlines attached ) to add or subtract wire.  After tuned, when I connect the coaxes, I would like both coaxes on the ground to read 75 ohms J0 so that I can feed them directly with the proper phasing lengths of coax.

Should I tune each loop alone without the other in the field?  -  Or should I try to jockey back and forth and tune them both to give 75 ohms (both in place) at the same time?  I wonder about mutual coupling of the two loops.   I know if I tune them each alone and then put them together they will pull off freq.  Or is this OK?

I could lower one and get it out of the field or even clip lead lengthen it so that it is invisible while I tune the other.  Or tune each one and get them BOTH to read 75 ohms at the same time.

Which is the proper way to do this?  I won't contaminate the question by saying how I did it in the past... I just want to know the proper technique..  I've  always used parasitic elements, thus all is tuned in place at the same time.  But I don't know about driven elements.

Is this a clear enough description?

T

[KD6VXI]

You're running 75 ohm coass from each antenna to ground level, correct?

--Shane
WP2ASS / ex KD6VXI

[K1JJ]

You're running 75 ohm coass from each antenna to ground level, correct?

--Shane
WP2ASS / ex KD6VXI

Shane,

Yes, after they are each resonated to 3790 in the air, I will attach a 150' long feedline from each feedpoint to the ground level for driving with RF..    I will use my MFJ-259 antenna analyzer at the center bottom feedpoint of each loop (at 120' high) to initially tune them.  

T

[K9MB]

New question concerning tuning two identical antenna elements:

Let's say I have two identical 75M delta loops  spaced 40' apart on a boom.  I want to resonate/tune them in place for 3.790 MHz -  to make each close to 75 ohms J0 at each  feedpoint bottom.   I can climb the tower and access each one (no feedlines attached ) to add or subtract wire.  After tuned, when I connect the coaxes, I would like both coaxes on the ground to read 75 ohms J0 so that I can feed them directly with the proper phasing lengths of coax.

Should I tune each loop alone without the other in the field?  -  Or should I try to jockey back and forth and tune them both to give 75 ohms each at the same time?  I wonder about mutual coupling of the two loops.   I know if I tune them each alone and then put them together they will pull off freq.  Or is this OK?

I could lower one and get it out of the field or even clip lead lengthen it so that it is invisible while I tune the other.  Or tune each one and get them BOTH to read 75 ohms at the same time.

Which is the proper way to do this?  I won't contaminate the question by saying how I did it in the past... I just want to know the proper technique..  I've  always used parasitic elements, thus all is tuned at the same time.  But I don't know about driven elements.

Is this a clear enough description?

T

What are you trying to achieve with this array? You are are about 3/10 wave apart - nearly long enough for a 3 element quad, but you are talking two

You must have something special in
Mind to go to the trouble.
I guess you have shorted stubs that are adjustable for tuning?

[K1JJ]

Hi Mike,

This spacing worked well in the past when I fed the loop driven element with RG-213 and then tuned the second loop as a reflector using the other RG-213 coax as a stub.  I am trying to achieve?  a bodacious low angle signal at about 25 degrees take off. 

In this case I wish to make identical loops that are each forced driven with equal length 150' RG-11, hopefully to a 75 ohm 1:1 match for each loop..   One of these  equal length coaxes  will get a 90 degree (or whatever is needed) phasing coax to make a unidirectional beam pattern into Eu.  Switch the phasing coax to switch directions. (USA)

So, I want to initially tune the loops each to 3790 by just trimming the loop's wire length. Once I get 3790 for each, the coaxes get attached.

I need to know when I do the initial tuning do I tune each loop alone or do I keep both loops in position (spaced 40' apart) and tune them that way, to take in account the mutual coupling detuning effects?

T

[W1ITT]

Tom.. Your best bet is to tune one element, then lower it out of the field of fire and tune the other to make it look like a twin.  Then put it all together and combine things with a phase angle of your choice.  You'll probably have to use an L network at the common point to get things to come into match.
The mutual impedances are not the same for driven elements as they are for elements that are parasitically coupled.  In broadcast work I have measured FM panel arrays in 3- around and 4-around configurations, as well as arrays stacked.  And to make it worse we had to maintain circular  polarity in cross dipole panels.  It involves a fairly complex  drive everything network analyzer setup, matrix math and just plain screwing around until it works.
Check out ON4UN's book, Ch 11 on "arrays" and Ch 12 on "other arrays'.  The first antenna in Ch 12 is a bit like what you are doing except that it involves a 180 degree phase shift to make a Figure-8 pattern.  I know your elements are loops but it's all the same.  I use something similar on my open-wire fed array and I like the bidirectional pattern because it keeps those inland boys reminded that I'm there.  High angles are suppressed and the main beam is shoved down below 30 degrees.  Take notice of one of the first charts in Ch12 that shows the effect of element spacing on feed R.  I space at 1/4 wave.  I use a separate receive  antenna so I don't worry about front-to-back in the transmit array.  There are a few good ways to skin this kitty...  ON4UN's book did a lot to raise the bar for lowband aerials.
73 de Norm W1ITT

[K1JJ]

Norm,

Tnx for the info! OK on tuning the loops separately with the other one out of the field.  That's not what I was hoping to hear...

Once both are tuned, how much do you think the impedance will change when the second loop is hoisted back into position?  Starting at  75 ohms J0,  if it changes like down to 50 ohms, that is a built-in swr of 1.5 :1 with the RG-11.   Whereas, maybe they can be tweaked for length and pulled back to 75 ohms or 1:1 each when in position.  

The last time I did this I tuned each loop with the other in the field. I eventually got close to the 50 ohm match for each that I was looking for at the time. I'm trying to be cautious cuz it seems like something that can be controlled to the benefit of a better match.

On the other hand I have seen guys stack Yagis without changing anything to do with tuning.   BUT... I remember  an antenna manufacturer, I think it was M2 but not sure, that offered to optimize my two store bought Yagis for stacking. He insisted that the mutual was strong enuff to require this.   Maybe Chuck will chime in with his experience and practice on stacking.  

I plan to make the climb as soon as the choke materials arrive.

I did expect to have to match the coax junction of the two loops to the run to the shack, however. An L network or 1:2 step down transformer would do.  (75/2 = 35 ohms  X2 = 75 ohms to the shack )

T