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Author Topic: Multiple Dipoles Single Ladder Line Feed Questions  (Read 22453 times)
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K3ZS
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« on: June 08, 2008, 03:38:58 PM »

After finding out that my 135 ft doublet fed with ladder line has a crazy pattern on 10M, as pointed out by K1JJ, I was thinking of putting up a separate dipole or extended double zepp (43 ft) for 10M.   Seeing that hams put up multiple dipoles fed with a single feed, I was thinking adding another 10M dipole to the 135 ft antenna and feeding it with the same ladder line.    I was wondering how such an antenna system would be tuned with the balanced tuner.    Would there be two tuning points, one with low SRW that matched the 135 ft doublet as I have now, and another tuning point that would produce a match to the new 10M dipole?    If that is true could one produce an antenna system with multiple patterns and change the antenna pattern by choosing different tuning matches?   Or is this just wishful thinking?

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K1JJ
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« Reply #1 on: June 08, 2008, 04:07:08 PM »

Hi Bob,

For your first question about changing the pattern by the match.... no.  The antenna pattern remains basically the same no matter what the match is.  This is assuming you are using the same feedline without introducing an imbalance, etc.  The only difference match could cause would be more or less power being delieverd to that existing antenna pattern.  ie, Pattern is not affected by a properly performing feedline.

The second question:  Good news. I modeled your 135' dipole on 10M and saw the normal circular eight-legged octapus.  However when the two 8' 10M legs are added to the common feedpoint, the pattern now changes into a reasonable dipole figure- eight on 10m. The pattern does have some minor ripples in it from being affected by the 135' dipole, but they are no more than 3db ripples, not big nulls like with the 135' dipole alone.

Bottom line is, yes, definately add on the 10M common feed dipole to your open wire line to achieve a nice figure-8, predictable 10M pattern, broadside. 

BTW, I don't know of anyone doing this with openwire, probably cuz of not really caring about the pattern. But your reasoning is valid and will give you the performance of close to a single band dipole on 10m.

I would not use a double extended Zepp for 10M, just a 1/2 wave dipole. The 1/2 dipole will hog the power on 10M, which is what you want. A 10M DExt Zepp will not cuz of a higher feedpoint impedance. See my posts below for more details.

Good thinking, OM.

T
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« Reply #2 on: June 08, 2008, 04:49:17 PM »

Tom-

Thanks for your expert advice.    I didn't want to put up another antenna and add switching and all of that.   Adding the 10M half wave to the present antenna looks like the way to go.   I generally operate 80, 40 and 10 and rarely the other bands.   Looks like the cat's meow (wherever that expression came from?)
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« Reply #3 on: June 09, 2008, 08:52:20 AM »

looked it up ...What does the "cat's meow" mean? - Yahoo! Answers.webarchive

something considered as excellent

Coined by American cartoonist Thomas a. Dorgan (1877-1929) whose work appears in many American newspapers. (see also Cat's whiskers)

Tom ...  this is a very practical solution to spotty coverage ... to extend the paradigm ...  I AM interested in upper bands coverage for a 80 mtr loop ... I dont possess antenna modeling software ...do you mind?   tnx   73   John
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« Reply #4 on: June 09, 2008, 09:15:41 AM »

Tom, Any advantage in extending the length of the 10 meter dipole to, maybe a 5/8 wavelength or more? I wonder how that would model?

Another question, the Johnson match box has a shorting strap on the back to short out one side of the doublet for 10 meters? This is puzzling. What is the purpose? Thanks.

Mike
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« Reply #5 on: June 09, 2008, 10:25:32 AM »

Beefus, as I remember now, the Cat's Meow is (or was) a bar on the corner Bourbon St. and Peter St. in New Orleans that had reasonably priced milkshake size beers and no pressure to keep buying.   Sorry not on topic.
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« Reply #6 on: June 09, 2008, 10:43:08 AM »

indeed ...there were several more items showed up in the cat's meow search ..drinkup!
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« Reply #7 on: June 09, 2008, 11:45:44 AM »

I would love to know how this project works out for you. I have been thinking of doing something similar to my 256 foot open wire center-fed antenna. It works great on 160, but seems kind of ho-hum on 75 and it has pretty deep nulls on 40 meters making it difficult to predict performance there. Adding to the complexity of the pattern is the fact that it is configured as an inverted vee, with the apex at 70 feet and the ends at around 40 feet. If I add elements for 75 and 40, my hope would be that it would produce a cleaner pattern on each of those bands.

I guess there's nothing to lose but some wire and a little time. Good luck with your project!

73,

Rob W1AEX


* 160-40 meter open wire feed.jpg (7.26 KB, 653x256 - viewed 1062 times.)
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« Reply #8 on: June 09, 2008, 11:55:15 AM »

I have been doing this for years. works FB.
Current setup is 135' center fed, with an extra pair of loaded wires for 160. I use center loading and made sure the loading coils weren't traps on any hamband ( I may change this). I use separate, feedpoint placed, balanced tuners for every band--all switching is remote. The ladder line feeds a bus that uses large solenoid relay pairs for each tuna.
I am usually heard with this haywire setup so I guess it works.
Skip
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« Reply #9 on: June 09, 2008, 12:00:42 PM »

Tom, Any advantage in extending the length of the 10 meter dipole to, maybe a 5/8 wavelength or more? I wonder how that would model?

Another question, the Johnson match box has a shorting strap on the back to short out one side of the doublet for 10 meters? This is puzzling. What is the purpose? Thanks.

Mike

Hi Mike,

I checked out extending the 1/2 wave 10M dipole to a larger length and the 10M figure-8 pattern starts to fall apart. It seems that we need the lower impedance presented by a resonant 1/2 wave to "hog" the power on 10M from the 135' length. Otherwise the 135' dipole starts to take more power and distort the figure 8.  Normally, on 10M, the 135' dipole is about 500 ohms. The 10M 1/2 wave is about 50 ohms. As you can see, there is a 10:1 power division. (not counting reactance)   This pattern looks like a reasonable figure 8 when 10:1.  When we lengthen the 1/2 wave 10M dipole more, the 1/2 wave goes up in impedance and the pattern starts to get bumpy and nasty again due to less power flowing to the 1/2 wave dipole.   So, make sure you cut the 10M dipole to a  resonant 1/2 wave length for best results.

On the MatchBox... it's been years since I've used one -  maybe someone here knows what the jumper does on the back. However, the shorting of one leg sounds like a gimmick to find a match. You normally do not want to ground one side of openwire since it will case feedline imbalance and possible feedline radiation. Your antenna pattern may not be what you think.



"Tom ...  this is a very practical solution to spotty coverage ... to extend the paradigm ...  I AM interested in upper bands coverage for a 80 mtr loop ... I dont possess antenna modeling software ...do you mind?   tnx   73   John"

John,

I modeled the 80M loop. Again, this is cutting hairs and most people won't care about pattern. But from what I can see, the full wave 80M loop is really a 160 and 80M antenna. It shows a nice figure 8 on 160M at 33 ohms input (not too low) and a sharper figure 8 on 80M with about 1/2 db gain over a 1/2 wave dipole.  However, as you'd expect, on 40M we have a cloverleaf with four lobes and four large nulls. The broadside is down about 15db, which is not good if you have your loop favoring a broadside direction on 160/75.  On 10M the pattern is 16 legs and nulls.... pot luck and most radiation off the ends of the antenna.

There is a solution to this for all bands. (Even 160/75M have nulls off the side like any dipole).  If you can use remote relays to disconnect the feedline from one side and feed it on the ADJACENT side of the full wave loop instead, (90 degrees away) you will then see a complete 90 degree horizontal shift of your patterns on all band. (Disconnect the feedline at the antenna feedpoint) ie, You could actually see full coverage 360 degrees by switching A/B. The mechanical layout would take some planning to insure proper isolation of the control cables and two feedlines, but it would be a nice way to make a multiband loop work in all directions....

Later -

T






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« Reply #10 on: June 09, 2008, 12:14:43 PM »

thanks Tom for taking the time and effort to answer both questions....excellent..73...John
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K1JJ
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« Reply #11 on: June 09, 2008, 12:54:04 PM »

I would love to know how this project works out for you. I have been thinking of doing something similar to my 256 foot open wire center-fed antenna. It works great on 160, but seems kind of ho-hum on 75 and it has pretty deep nulls on 40 meters making it difficult to predict performance there. Adding to the complexity of the pattern is the fact that it is configured as an inverted vee, with the apex at 70 feet and the ends at around 40 feet. If I add elements for 75 and 40, my hope would be that it would produce a cleaner pattern on each of those bands.

I guess there's nothing to lose but some wire and a little time. Good luck with your project!

73,

Rob W1AEX


Hi Rob,

I think that's a great idea. Yes, you can extend this concept to any band using 1/2 wave legs cut to the desired freq to give a reasonable figure 8 pattern.  The only exception to this trend is if the bigger flat top shows a lower impedance at the operating freq, then it will "rob" more power from the 1/2 wave dipole cut for the band. For example, lets say on 15M your 260' flat top is 250 ohms, and your 15M 1/2 wave is 50 ohms. This is now a 5:1 power division. The 15M pattern will still be a quasi-figure 8, but will have deeper bumps/nulls.  A 10:1 division on any band is very good showing nulls no greater than 3db or so.  This is all general talk, but I just wanted to show the point.


There's really much to be said about planning your antenna patterns, if you can.  A power increase of X2 is good for only 3db, transmitting only. But putting in some time to enhance an antenna  pattern (from nulls) can sometimes mean 10-20db and it also works on the receive end too....  Grin


**  I would really like to see someone try that loop idea using relays to switch the feedline feedpoint to the adjacent leg. It would be slick to see a null, then, "click"  - full lobe strength.  Full 360 degree coverage.  It wouldn't be that hard to do, really.

Though, another simpler version would be to put up two multi-leg dipoles fed with openwire (like your design, Rob) at RIGHT angles to one another.  Use two antenna tuners. By selecting the proper tuner/antenna in the shack you have full 360 degree coverage with clean figure 8 patterns.  That's what I'd probably do it I had just one high support and one (actually two in this case) antenna system.


Let me know how your plans work out, OM.

T

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« Reply #12 on: June 09, 2008, 02:04:10 PM »

The shorting strap is when you want to use the Johnson as a long wire or coax feed tuner and is not used for balanced feeds. The Johnson tuna has a limited range in this configuration.

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K1JJ
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« Reply #13 on: June 09, 2008, 02:16:03 PM »

I have been doing this for years. works FB.
Current setup is 135' center fed, with an extra pair of loaded wires for 160. I use center loading and made sure the loading coils weren't traps on any hamband ( I may change this). I use separate, feedpoint placed, balanced tuners for every band--all switching is remote. The ladder line feeds a bus that uses large solenoid relay pairs for each tuna.
I am usually heard with this haywire setup so I guess it works.
Skip

Hi Skip,

Your signal actually stands out from Minnesota whenever I hear you on 75M.  One of the beacons.. :-) 

So let's see... you have one tuner for the common-fed 160M and 75M legs... and how are the upper bands set up?



OK on the matchbox jumper used for unbalanced loads only. That's makes good sense.

Later-

T

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« Reply #14 on: June 09, 2008, 02:41:15 PM »

Your right Skip. I dug out the manual and read it again. It is still a bit questionable as to what the tab dose. For instance "Using a single wire antenna the unused terminal is grounded  on 80/40 and 20 meters and lifted on 15/11 Grin and 10 meters" Using an unbalanced antenna feed line  the same tab lift applies on 15/11 and 10. I don't want to hijack the thread so I'll leave it at that.

I used a multi element (160-20 meter one dipole per band) antenna for years. It used an unbalanced feed line and a balun. Tuning was straight forward and performance was good. Actually I think 20 meters was handled by the 40 meter wires?

Tom. Thanks for answering my questions. While we are on the subject. Please model adding a second dipole (135') at a 90 degrees. No switches. Like the Collins fan dipole. Any advantage at 75 or 40 meters? Thanks in advance.

Mike
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« Reply #15 on: June 09, 2008, 03:10:17 PM »

Tom. Thanks for answering my questions. While we are on the subject. Please model adding a second dipole (135') at a 90 degrees. No switches. Like the Collins fan dipole. Any advantage at 75 or 40 meters? Thanks in advance.

Mike

Mike,

By adding a second 75M dipole at right angles (zero degrees, common feed) it will have little effect on the 75M or 40M patterns.

However, you are really talking about making a turnstile antenna. By feeding the second 75M dipole with a 90 degree phase shift you get a nice omni-directional pattern on 75M. The overall gain is down by -3db because it's omni compared to a single dipole figure 8.
A 42' length of RG-213 (.66 vel factor) or equivalent 62' of openwire  in this case, will produce the required 90 phase shift - by connecting one   end to the common feedpoint and the other end to the second right-angle dipole.


The 40M pattern is unaffected by either of these changes.



Your second question:  For 50 ohm coax, power is matched mostly into a 1/2 wave dipole of the operating freq. (~50 dipole) On 20M, a 40M dipole would be a very high impedance. (mismatch)   So your multiband coax dipole most likely had a 20M 1/2 wave dipole leg to work correctly.  There's other ways/gimmicks to do it like with G5RV's etc, but for simple dipoles, direct fed coax needs resonant 1/2wave legs to transfer power.



T
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« Reply #16 on: June 09, 2008, 06:55:33 PM »

Thanks Tom Vu,

You never get rich giving away your genus. I'll run now and make millions using your secret plans.

Seriously, your time and expert advice is appreciated.

Best Regards,
Mike
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« Reply #17 on: June 19, 2008, 12:53:27 PM »


Hi Rob,

I think that's a great idea. Yes, you can extend this concept to any band using 1/2 wave legs cut to the desired freq to give a reasonable figure 8 pattern...

Let me know how your plans work out, OM.

T


Ah well, braving the impending rain and thunderstorms yesterday I went ahead and added a 130 foot dipole under my existing 256 foot dipole, both sharing the same open wire line and feedpoint.

Initially I just soldered it in at the feedpoint and used one attachment point at the end of each 65 foot leg, using an insulator to attach it to the 160 meter antenna above, just like in the ARRL antenna book. What escaped my eye was that at one end the 75 meter element twisted around the 160 meter element. I use insulated wire, so when I initially tuned up on 75 meters at around 100 watts it worked fine. As soon as I put some fire in the wire though, a spectacular arc melted the insulation for about 6 inches on each wire and tack welded them together. Umm... quite a bit of voltage there at the end of the 75 meter elements!!!

So I hauled it all down and used 6 inch spacers every 4 feet to suspend the 75 meter dipole from the 160 meter dipole. It looks like old fashioned open wire line. This made for a much more twist proof design and also holds the distance between the two wires steady so that in the wind there is no change in the interaction between elements.

The load settings for 75 meters are quite different from when I simply loaded up on the 160 meter antenna. It's too early to tell how different the antenna behaves, but when I worked a couple of friends on the east coast I got very good reports. Loading on 160 meters is slightly different but nothing drastic. I have not tried it on 60 meters or 40 meters yet so I have no clue what changes will be noted there. My guess is that 40 meters will load up on the 160 meter legs as it always did (roughly at the third harmonic).

I'll keep notes on results as I see them.

Rob W1AEX


* 160-75 meter open wire feed.jpg (6.92 KB, 653x256 - viewed 809 times.)
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« Reply #18 on: June 19, 2008, 02:19:18 PM »

Seeing that this topic is active again, I'll report that I just added the 10M dipole to the 135 ft dipole.    Used some CPVC pipe and made some spreaders, the 10M dipole hangs about 6 inches under the other one.   In the process of lowering the antenna, I found that exposed wires of the window line (#14 copper clad) were badly rusted, even though the copper clad antenna wire was fine.   I guess the feedline was made in China with not much copper over it.    I repaired all of that and coated it with autobody undercoating that I had.     The antenna tuned differently from the 20M band on up to 10M.    Bands lower than 20M remained the same.    Since making the change 10M has not had a good opening, although I am hearing a lot of low power beacons at times, so it must be an improvement.   Thanks to Tom Vue for the info on this.

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« Reply #19 on: June 19, 2008, 02:36:51 PM »

indeed ...there were several more items showed up in the cat's meow search ..drinkup!


701 Bourbon St


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« Reply #20 on: June 19, 2008, 03:43:50 PM »

Since making the change 10M has not had a good opening, although I am hearing a lot of low power beacons at times, so it must be an improvement.   Thanks to Tom Vue for the info on this.


Vely, bery good there, Bob!  What you have done is to seize control of your 10m destiny!  You now have a clean, broad, figure eight pattern on 10M. You have booted out the octapus null monster and can hear in directions where once tho was deaf....  Grin 

 
Rob:  Yikes! Yes, as you know, any dipole length has maximum voltage at the ends, so separation is critical.   Another way to do it without spreaders is to simply pull the new leg out lower, like an inverted vee. Nothing says the legs need to be held close together. However, your spreader method is superior cuz it keeps the new legs as high as possible using only one set of end supports.

BTW, the pattern difference you will see is mainly off the 45 degree front of the broadside, in both directions. ie, Your new 75M pattern is broader and you'll see stations come up that were previously down 10-20db becuz of the previous 160M legs' (used on 75M) sharpness.


The bottom line of all this is optimization. Pick your favorite one or two bands and do something special to enhance your pattern - just like you guys did. It will make the band more fun and give you a slight edge over the average installation. You will begin to hear and work stations that others cannot.

If everyone could transmit and receive equally, we would call it the internet or telephone and get bored pretty quickly.  After all, how many times can you tell Pascal in Yugoslavia, "you're 59, how's the family?" On the air you can do it every night. But call him on telephone and tell him he's 59 night after night and after a few days he'll tell you to hit the frickin road.  HA!


It's always nice when guys try out new stuff and report back with results.  Makes it all well worthwhile.

Later -

T




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« Reply #21 on: June 19, 2008, 04:18:16 PM »

I ran a 198 foot dipole in parallel with a 125 foot dipole once that worked well for 80 through 10. I bet it would have worked on 160.
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« Reply #22 on: June 19, 2008, 04:49:44 PM »

Oh, BTW Bob -

I had another thought. For 10M you might like to try hanging a 16' 8"  wire reflector (for 29.1 mhz AM operation) behind the new 10M dipole legs, if you have the tree supports available.

Space it between 6'-10' away to pick up another 4db to the west/SW for USA.  When you want to work Europe NE, just lower the reflector using pulleys at each end. Not really critical, put keep it as level and square to the other dipole as possible.

What will happen is you can drop off 12-15db of noise coming in from the east (better ears) and get some forward gain to the west as a broad pattern unidirectional 2el Yagi.

It's really a worthwhile thing to do when the USA AM activity starts or even now during occasional scatter activity.

That reflector is invisible on the other lower bands, of course.

T
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« Reply #23 on: June 19, 2008, 05:01:35 PM »

Okay, guys... this thread got me thinking about my own situation, end even propelled me to acquire some modelling software and frigure some things out.

What I've got now is a 200' flat-top up about 35' at the center and 40-45' at the ends. It's fed with 420 ohm window-line crap, and broadside almost exactly east-west.

On 160, it's a Martian Yagi. Maxim would be proud. 3.5 dBi or so straight up, drops dead towards the horizon. Similar pattern on 75, but it doesn't drop below the 2.5 dBi mark until about 45 deg. 40 meters is really interesting: in the CW portion, the primary lobe is broadside and narrow from about 45 deg. takeoff angle to straight up, but as it crosses 7150 it becomes more end-fire with a takeoff angle around 30 degrees. On 20 it looks like a cigar with butterfly wings. On 15 it looks like a porcupine on meth with lots of hair gel. On 10 it looks like the debris field from two cannonballs colliding in mid-air.

So I've started playing around with the model, adding an inverted vee to the feedpoint of the 200 footer, parallel with the primary radiator. If I go 45 degrees on both wires almost down to the ground, I wind up with just under 15 meters on each leg. It's very interesting what that does to the 20 meter pattern: it winds up looking like a waffle with a dorsal fin, with a little over 6dBi on the corners.

I'm just goofing around with it at this point, but I'm curious if anyone else out there has combined a flat-top and an inverted vee in the same ant like that.

(While we're on the topic)

--Thom
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« Reply #24 on: June 19, 2008, 07:01:15 PM »

Joe-W2YR and I designed the antenna below. He's been using it with good success on 160-10 for several years. Coax feed.


* w2yrantenna.jpg (15.19 KB, 540x582 - viewed 743 times.)
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