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Author Topic: 43 foot verticals  (Read 11911 times)
WA8FOZ
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Posts: 192




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« Reply #15 on: November 24, 2012, 08:50:19 PM »

Quote
For operation on the high HF bands I can telescope the vertical down to a lower height to preserve the low takeoff angle.  I'm not too keen on the verticals with multiple elements (ugly) or on traps (lossy).
Sounds cool. A bit shorter will give you a lower angle of radiation and better DX on 17 and above, as you know. And if it is no hassle for you to walk out when you change bands, you can work out some sort of base loading arrangement to supplement your matching device on 80 and 160.

If I were to do this I would have an autotuner at the base, maybe the MFJ 1500 watt model, at the base; and have the option of going out to the antenna or not, depending on weather and my mood.
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STAYVERTICAL
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« Reply #16 on: November 25, 2012, 01:38:03 AM »

With regards to the long radiator problem on the higher HF bands, has anyone considered putting a sliding ferrite suppressor ring on the vertical?

I can't claim this is original since I saw this in some forgotten antenna book, but it may be one way of stopping having to lower the whole tube.
Perhaps some sort of flagpole pulley arrangement on the top and a ferrite ring free to be hoisted up and down.

I am not an engineer, so I am only surmising this could act as a sliding suppressor trap, electrically shortening the antenna.
It may be totally wrong technically, but perhaps some of the genius antenna guru's could comment on this sliding ring's effect.

Another thing I saw was that if you move a capacity hat down near the feedpoint of a vertical, very little radiation will be above the hat.
If a suppressor ring is not practical, could a sliding capacity hat do the same thing?
 
I am pretty busy at the moment with my own antenna experiments/building, but I will try it when I have some free time.

73 - Rob
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W5WSS
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Posts: 1732




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« Reply #17 on: November 25, 2012, 02:55:09 AM »

The three lengths of wire verticals are not all left in the air and up simultaneously. Their use is superior, though when one is chosen the other two are completely down and not connected but rather rolled up and in the garage ready for deployment when needed.

The system consists of a hoist and halyard fabricated of rope to hoist one wire up vertically and hold it up. Total deployment time about 3 minutes including removing one and adding another.

The techniques used provide for a quick change between the three and allow for better pattern development where longer range dx is desired 22 ft 44ft  66ft provides for better 10m to 75m vertical performance than a single 43ft.

The cost of the system I deployed was about $20 for the rope, wire and hardware.

The antenna system utilized the three lengths and a competent network delivery system capable of handling the band excursions and legal limit power application.

The antenna system can be installed in a way where the coaxial cable feedline linking  the delivery network located at the antenna base feedpoint can be eliminated totally.This methodology completely overcomes the coaxial line losses avoiding an unmatched line.

The system counterpoise consists of tuned pairs oriented opposite relative to each other for each band 10m-40m elevated and sloped downwards to Teflon anchors driven into the ground. Note the 66ft length vertical did not have a dedicated set but relied on the shorter 40m pair as space did not allow for longer radials.

The entire elevated system can be designed and considered in 4 parts.

The three individual wire verticals
The hoist
The delivery system
The counterpoise

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K0ZN
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Posts: 1548




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« Reply #18 on: November 25, 2012, 05:49:41 AM »

"Self supporting" is not a guarantee, especially if you live in an area with high winds or subject to thunder storm gusts. Guys are not a guarantee either but they greatly improve high wind survival for a simple structure made of tubing. I am not going to run stress analysis on this, but a set of guys probably reduces torque on the base by 70%+. That is a very significant gain in strength. Your situation may or may not need or justify the additional support.

73,  K0ZN
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W5DXP
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« Reply #19 on: November 25, 2012, 06:40:00 AM »

... has anyone considered putting a sliding ferrite suppressor ring on the vertical?

Although possible in theory, since it would be a one-turn choke, the problem would be getting the choking impedance high enough to compete with conventional traps. The characteristic impedance of a vertical is in the ballpark of a few hundred ohms (variable with height above ground) so the choke would need to exhibit a few thousand ohms of choking impedance which is tough to do with a one-turn choke but it would be easy to simulate using EZNEC.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K4SAV
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Posts: 1842




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« Reply #20 on: November 25, 2012, 10:07:52 AM »

Although possible in theory, since it would be a one-turn choke, the problem would be getting the choking impedance high enough to compete with conventional traps. The characteristic impedance of a vertical is in the ballpark of a few hundred ohms (variable with height above ground) so the choke would need to exhibit a few thousand ohms of choking impedance which is tough to do with a one-turn choke but it would be easy to simulate using EZNEC.

You would need about 5000 ohms to keep the bead loss below 2 dB at 7 MHz.  It's difficult to calculate what size bead would be required since the impedance isn't a linear function of diameter, but I would expect to see a bead (probably multiple beads) weighing between 30 and 100 pounds for a 3 inch diameter mast. 

This becomes a lot easier if the radiator is smaller diameter.  For a half inch diameter radiator, a quick check shows that you could get 5K ohms with about 15 lbs of beads (based on some beads I have in hand).  With a skinny wire as a radiator, you would only need about 1 lb of 1 inch diameter #75 beads.  Anyone for designing a ferrite tuned vertical?

Jerry, K4SAV
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WB6BYU
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Posts: 13281




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« Reply #21 on: November 25, 2012, 02:18:46 PM »

Quote from: WA8FOZ

...A bit shorter will give you a lower angle of radiation and better DX on 17 and above, as you know.



There is a lot of theory supporting this, but it isn't always the case in the real world.  Let's consider
a full wave vertical for a moment:  the current will be about equal and out of phase in the upper and
lower halves of the radiator, so there should be no radiation broadside to the antenna.  Similarly for
a 3/4 wave vertical:  the radiation from the lower 1/4 wave is out of phase with that from the upper
1/2 wave, which tends to cancel radiation at low angles.

But when the antenna is installed in a subdivision or condo with a lot of buildings around it, the
radiation from the bottom section of the antenna may be blocked or reflected by nearby buildings,
leaving mostly that from the upper section (which hopefully is more in the clear) as the strongest
component.  That means you'll have better radiation at low angles than you would have with the
same antenna in an open field (at least in some directions.)

And even without the surrounding buildings, the signal strength from a full wave vertical really
isn't that bad: my model suggests that compared to a 1/4 wave ground mounted vertical, a
3/4 wavelength vertical is about 2dB better and a full wave radiator is less than 2dB worse
at an elevation angle of 10 degrees.  That doesn't mean that the longer vertical is ideal, of course,
but that the differences won't be huge on DX signals:  even at 10m or 6m, EZNEC suggests the
radiation at 10 degrees vertical angle is less than 2dB down from a ground mounted quarter
wave vertical (though such an antenna isn't particularly popular for 6m weak-signal work.)


If you do want to improve performance on the higher bands, a "sleeve vertical" approach may
be better, using perhaps a pair of stub verticals parallel to the main one and spaced a foot or
so away from it.  This makes the bottom section act more as a transmission line so maximum
radiation is from the top section of the antenna rather than the bottom.  For example, consider
a 3/4 wave vertical with a 1/4 wave grounded section parallel to the bottom:  this would
make a J-pole (in ideal conditions) and may be more effective than a shorter vertical, as well
as being simpler mechanically than some scheme to change the element length.

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W5DXP
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« Reply #22 on: November 26, 2012, 08:19:56 AM »

You would need about 5000 ohms to keep the bead loss below 2 dB at 7 MHz.

Here's another idea. Given a 44' vertical, we could turn it into a 2x5/8WL phased collinear on 10m by installing a phase shifting coil at the halfway point. If we could come up with a phase shifting common-mode bead choke, would that work? Some ferrites are mostly inductive at certain frequencies.

For instance, if I take a 44' vertical and install an EZNEC load of 0-j400 ohms at 22', it lowers the take-off-angle from 56 deg to 10 deg while increasing the gain at 10 deg by about 12 dB. Of course, such a choke is impossible in the real world but the idea might have a grain of usefulness.

I wish Owen, VK1OD, was still posting. This would be right up his alley.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K4SAV
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Posts: 1842




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« Reply #23 on: November 27, 2012, 06:34:06 AM »

Here's another idea. Given a 44' vertical, we could turn it into a 2x5/8WL phased collinear on 10m by installing a phase shifting coil at the halfway point. If we could come up with a phase shifting common-mode bead choke, would that work? Some ferrites are mostly inductive at certain frequencies.

For instance, if I take a 44' vertical and install an EZNEC load of 0-j400 ohms at 22', it lowers the take-off-angle from 56 deg to 10 deg while increasing the gain at 10 deg by about 12 dB. Of course, such a choke is impossible in the real world but the idea might have a grain of usefulness.

I'm not sure that I agree with your numbers for the reactance needed.  (The reactance will depend on the details of the antenna construction and I think it will require a positive reactance.)  But that's not the point. 

For a phase shift network made from a core, I doubt that you will ever find a core(s) with low enough loss because the voltage across that core will be very high (maybe 500 volts at 100 watts, and 1500 volts at high power).  You don't get much inductance per turn with powered iron cores (which you will need for high reactance and low resistance) and for this application you are probably thinking a single turn per core.  That would probably require hundreds of cores.  Multiple turns on a core would very quickly exceed the maximum flux for the core.  It would be much easier to just add an air core coil or a phasing line.

I wish Owen, VK1OD, was still posting. This would be right up his alley.

Yes, I miss Owens comments too.  He is a library of information.

Jerry, K4SAV
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N3OX
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« Reply #24 on: November 27, 2012, 07:34:40 AM »

Quote from:  link=topic=86565.msg635755#msg635755 date=1353836283
This becomes a lot easier if the radiator is smaller diameter.  For a half inch diameter radiator, a quick check shows that you could get 5K ohms with about 15 lbs of beads (based on some beads I have in hand).  With a skinny wire as a radiator, you would only need about 1 lb of 1 inch diameter #75 beads.  Anyone for designing a ferrite tuned vertical?

Are you distributing things over the length of the wire that they'd occupy in the model? We might want to be little careful not to completely trust a single EZNEC load for long bead chokes in high impedance areas.  For an extreme example of the weirdness that can happen because of displacement currents, take a look at this.

I was doing some coil Q tests using the method described here:

http://n3ox.net/tech/coilQ/

I became concerned about common mode excitation of the coax to the bottom plate of that apparatus; after all some small "antennas" are built like that Smiley  But every time I added a choke, whether beads or multiple turns through a core, I found a substantial measurable increase in the measured resistance of the LC circuit; equivalently a sharp reduction in the number I was getting for the coil Q.  The maximum attenuation I could achieve was greatly reduced. The absolute weirdest part came when I removed the bead choke from the coax feedline I was shorting to do the measurements; I simply attached it to the plate with the other end dangling in the air and measured a similar increase in the measured resistance of the "coil!!"

So I re-rigged and did some tests.  I removed all metallic connections between the outside world and the plate on one side of the coil/cap LC circuit.  I put the LC circuit in parallel and coupled to it with an inductive loop to my transmitter.  I attached the choke to the plate with the free end dangling.  That setup looked like this:

http://n3ox.net/files/chokeprob/setup_ann.jpg

I adjusted the coupling loop and LC resonant frequency for a good match, and applied 100W.

I have access to a thermal camera and took a picture after a short time:

http://n3ox.net/files/chokeprob/IR_Choke.png

The coil gets warm and the choke gets warmer, but not uniformly (this is probably less power dissipation in the choke than in the coil because of lower heat capacity, mass, and cooling).  The main weirdness here is that the choke is attached to nothing at all at the far end.  So the "circuit" causing current flow through the choke is all displacement current, and it is apparently distributed along the choke... you could kind of draw a circuit like this if you wanted:

http://n3ox.net/files/chokeprob/beadcirc.jpg

I made a lumped model to understand this a little better (though keep in mind that b/c of the reduction in current along the length the choke impedance R+jX in this model doesn't exactly translate to the string of beads).  I assumed a lossy inductive choke coupled across the coil with a small capacitance.

http://n3ox.net/files/chokeprob/circ.jpg

In this particular case, with a super strong electric field due to the high Q tuned circuit, the choke that you'd need to have to avoid perturbing the circuit is really extreme.  Here's the peak parallel resistance of the circuit above as a function of the choke impedance:

http://n3ox.net/files/chokeprob/R_chokes.png

Now, a long wire antenna isn't nearly as harsh of an environment as this: the electric field is not nearly as strong and if you put a 5000+j5000 choke at the "end" of a long wire section it's not going to affect the overall antenna losses as much as it does a very low loss tuned circuit.  Plus the wire on the other side will have some effect.  But the reason why I bring it up in the first place is that I wouldn't be surprised if a long string of beads in a high impedance part of an antenna has a substantial variation in heating along the length and probably ends up with a different effective choking impedance.

It's possible that this simply isn't relevant outside of this extremely high Q tuned circuit environment with hugely concentrated fields, but I wanted to mention it anyway.  I haven't looked much at models, but playing around with ten 500+j500 loads distributed over a couple feet in the middle of a 43 foot wire turns up some situations with very uneven distribution of dissipation in the individual sections.

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K4SAV
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Posts: 1842




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« Reply #25 on: November 27, 2012, 02:27:44 PM »

But the reason why I bring it up in the first place is that I wouldn't be surprised if a long string of beads in a high impedance part of an antenna has a substantial variation in heating along the length and probably ends up with a different effective choking impedance.

I didn't do an actual design Dan, especially since it didn't seem to be a good solution for a multi-band antenna.  I suspected that there might be a problem with a long string of beads, but the solution to this problem was left as a exercise for the student. Cheesy  That's why I asked if anyone was interested in doing a design.  It didn't seem to be worth the effort for me because I didn't think the solution would yield anything anyone could build.  Power limitations will be a problem too.  The voltage across the choke will be very high and that can very easily cause the core flux to exceed the maximum ratings for the cores.

Jerry
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N3OX
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« Reply #26 on: November 27, 2012, 05:35:50 PM »

  It didn't seem to be worth the effort for me because I didn't think the solution would yield anything anyone could build. 

That's true. 
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KE2TR
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Posts: 155




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« Reply #27 on: December 04, 2012, 09:12:52 AM »

I installed a modified S9 43 ft vertical and laid a bunch of radials into the ground, used a 4:1 Unun and the antenna worked real nice on 20mtrs were it was a 5/8ths wave vertical but on the higher bands its take off angle was too high and on 40ty and 80ty there was way too much loss. I will end up tunning it just for 20 and let it be.
Jim
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W5DXP
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« Reply #28 on: December 04, 2012, 11:32:48 AM »

I will end up tunning it just for 20 and let it be.

You can improve the 20m efficiency by installing a matching network at the base - probably just a tapped loading coil.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K0OD
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Posts: 2557




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« Reply #29 on: December 04, 2012, 02:27:01 PM »

Quote
"on 40ty and 80ty there was way too much loss."

Unless you're running like a mile of coax, loss on 40-meters should be microscopic. How are you determining the loss there?

I use my 43 footer all the time on 40 and it works as well as a 1/4 wave vertical or perhaps better because it's taller than obstructions such as our home.
« Last Edit: December 04, 2012, 02:29:25 PM by K0OD » Logged
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