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Author Topic: Looking for Sugestions on Heavy Duty DE Zepp Design  (Read 2909 times)
W2WDX
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« on: December 06, 2012, 01:33:16 AM »

Hi all,

I recently got approval to install a large dipole on the top of my 21 story Co-Op in the Bronx. The building is situated on top of one the largest hills in the Bronx making this location one of the highest points in NYC. From this location on the roof I can see the Atlantic Ocean to my south (and the rest of NYC) Long Island and the LI Sound to my East. I can see into New Jersey to my South-West and West. I can even see the Armstrong Tower in Alpine NJ to my North. Lucky me!

Here's the plan. Double-extended Zepp for 80m fed with ladder-line and tuned with a Johnson Kilowatt Matchbox. The antenna should end up being between 309'-311' depending on trimming and the ladderline works out to be 95' total length.

The roof of the building is poured concrete and brick. Looking top down it is about ~320'x100' rectangular and in the center is a 30'x30' brick structure which rises 45' above the roof. The center of the Zepp will be mounted at the top-center of this structure and the ends going out in an inverted V configuration to the ends of the roof. The only thing above the antenna is a samll tower where there is attached a aircraft avoidance light. The largest buildings are miles away and most nearby are either two or three-story houses or eight-story buildings. Here's a link to satellite image of the roof:


Incidentally, it was the promise that I would remove the three large unused satellite dishes off the roof is how I got the Board of Directors to approve my proposal. And I got it in writing!

The antenna will be mounted on the west side of the structure in the center at its top. The feed-line will be coming of it perpendicular and straight down the center of the west-side of the building to my apartment, using feed-thru terminals right through the brick to the outputs of the tuner directly on the other (inside) side of the wall. The superintendent of the building is helping me with all this and suggested the feed-thru, which I never thought he would ever say yes to. He prefers it actually over going through a window!

Now here's the question. I know many large dipole arrays have been built over the years for commercial and military use. These things were massive, like the old three wire cap-hat verticals systems and such. Or the big VLF arrays. I am interested if anyone knows the mechanical design of the hang points and insulators used. If anyone knows of any images online maybe of such things? I have studied and actually been to (and climbed up) the old Russian Duga "woodpecker" array and seen how some of the wire attachments were done there. (Damn ... that thing is massive and you can't tell, unless you're standing below it, just how monstrous it really is).

Now the copper-weld steel aerial wire I have chosen is a cable that has three groups of seven strands woven together then twisted into one cable .162" in diameter. This weave makes the cable much more flexible and easy to handle than a 6 AWG solid copper-weld would be. Extremely heavy copper plating should allow for wear and tear as an antenna. Also it should exhibit lower losses on the low bands over 12awg copper-weld. This wire will actually extend unbroken through the feedpoint and actually become part of the feedline down to the point where it begins to go down the side of the building. I will do the calculations for this section to make sure the impedance is the same as the ladder line. The transition point is where horn gaps will be (see below) bonded to the steel structure of the building. I will be tensioning these wire to reduce sag and movement, so mounting is crucial. Probably going to use big Birnbach end-insulators.

I also plan on using the steel structure of the building to bond grounding at roof level. This way I can mount some big horn gap devices and some DC shunt inductors to help with some lightning protection and static dissipation. I will be able to open the window and by using some connectors (working that choice out too), disconnect the ladder-line from the feedthru's when a real likely chance of lightning is around.

I will be running QRO and QRO AM at times.

I am going to this extreme because of the location. I want too make the installation as permanent and safe as a wire construction can be. Even if that compromises some RF performance parameters slightly. I don't want a break and have 150' section of wire hanging over the side of a high-rise in the middle of NYC.

The words here are tough, long-lasting, durable, safe and the like. I want to see if anyone out there has some first hand knowledge of commercial or military installations that I can apply to this. I am a PE and can stamp and sign off on any engineering drawing here in New York, but I was hoping to hear some first hand information and advice.

Thanks in advance...

John, W2WDX

 
« Last Edit: December 06, 2012, 01:57:17 AM by W2WDX » Logged

K3VAT
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« Reply #1 on: December 06, 2012, 04:03:25 AM »

Hi all,

...  Here's the plan. Double-extended Zepp for 80m fed with ladder-line and tuned with a Johnson Kilowatt Matchbox. The antenna should end up being between 309'-311' depending on trimming and the ladderline works out to be 95' total length.
...
I will be running QRO and QRO AM at times.  ...

John, W2WDX  
 

Congrats on getting approval!  So John, what are your operating goals with this antenna (it is actually called Extended Double Zepp in most of the literature like the ARRL Antenna Handbook [p 8-40])?  For DX'ing this is a killer antenna but has rather narrow lobes in the horizontal axis and 4 additional, lesser lobes in a sort of cloverleaf pattern with some pretty deep nulls in between.  So gain will happen only at quite specific azimuths.  And how far will the antenna be from your operating position?

I ask these Qs so as to understand why you choose the EDZ.  I've successfully used the EDZ on 40M for a specifically targeted area (SE Asia) but my antenna was out in the open with very little nearby metal, not in a very urban setting like Bronx.

73, Rich, K3VAT
« Last Edit: December 06, 2012, 04:05:30 AM by K3VAT » Logged
W5DXP
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« Reply #2 on: December 06, 2012, 05:10:27 AM »

Here's the plan. Double-extended Zepp for 80m fed with ladder-line and tuned with a Johnson Kilowatt Matchbox. The antenna should end up being between 309'-311' depending on trimming and the ladderline works out to be 95' total length.

EZNEC sez the Johnson Matchbox will see an impedance around 376+j1760 on 80m. That may be outside of the Matchbox's tuning range.
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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.
N4CR
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« Reply #3 on: December 06, 2012, 08:48:03 AM »

I think I'd be trying to center feed an inverted V with ladder line.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
WB6BYU
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« Reply #4 on: December 06, 2012, 09:02:11 AM »

Quote from: W5DXP
Quote from: W2WDX
Here's the plan. Double-extended Zepp for 80m fed with ladder-line and tuned with a Johnson Kilowatt Matchbox. The antenna should end up being between 309'-311' depending on trimming and the ladderline works out to be 95' total length.

EZNEC sez the Johnson Matchbox will see an impedance around 376+j1760 on 80m. That may be outside of the Matchbox's tuning range.


Putting 47 pf to 100pf or so in series with each leg would help to handle the reactance.

Once it is up and we can measure the actual impedance at the tuner, I'm sure we can
invent some sort of added matching to shift it into the range of the tuner.


Somewhere on the internet is a copy of Laporte's classic Radio Antenna Engineering
book, which has a lot of photos of actual wire antenna installations.  That would be a
good place to start.
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WB2WIK
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« Reply #5 on: December 06, 2012, 09:18:27 AM »

Congrats on the approval, I'm sure that wasn't easy.

Is the raised brick structure you describe (that rises above the roof) for an elevator by any chance?

If so, have you been on the roof with a portable AM radio or similar when the elevator is operating to see what kind of noise level it generates?

I've (unfortunately) heard many cases where hams couldn't operate on HF at all in the proximity of such stuff -- elevator electrical system generated so much noise it would wipe out all the bands with S9++++ noise every time someone used it.  I hope that's not the case in your situation, but you might want to check it out just in case, as if it's a noisemaker you may want to keep the antenna as far away from it as possible.
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AJ4WC
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« Reply #6 on: December 06, 2012, 10:04:06 AM »

Congrats, but don't blow it.   Grin

It's already a great location, so you don't need massive gain or power.  If you go too big on the antenna or cause RFI, your time on the roof might be short lived.  If it were me, I would go with a multiband antenna that's not overly large or prone to RFI, and keep the power down during peak TV viewing hours.  Also, make sure everything is grounded per NEC.  Otherwise, first time they get a permit for some unrelated work on the roof, your antenna will become an issue. 
« Last Edit: December 06, 2012, 10:06:25 AM by AJ4WC » Logged
N4JTE
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« Reply #7 on: December 06, 2012, 02:31:41 PM »

A couple of electrical points, I will leave the durability issues to others more informed.

1; The edz will be highly directional at that height assuming little interaction with roofing materials, I mean that sucker is a bidirectional lazer so pick your aiming points accordingly. I had to build other 40 antennas to fill in the missing lobes, hi.

2; Cecil can give you an exact length of 450 ohm feedline that will give you 50 ohms at the rig, thinking 3/4 wl but I forget.

3; As an engineer you already know the trade off of weight vs duribility, in this case the sag, windload etc, I am concerned that the wire you plan on is overkill but alas I don't live below you, hi.

Good luck.
Bob
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W5DXP
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« Reply #8 on: December 06, 2012, 04:42:12 PM »

2; Cecil can give you an exact length of 450 ohm feedline that will give you 50 ohms at the rig, thinking 3/4 wl but I forget.

A 450 ohm series section transformer of 0.18WL is about right for an EDZ.
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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.
WX7G
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« Reply #9 on: December 06, 2012, 05:41:06 PM »

Why a double extended Zepp? You gain 3 dB broadside and give up gain elsewhere.
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W2WDX
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« Reply #10 on: December 07, 2012, 01:03:26 AM »

Extended Double Zepp ... DOH! I knew that ...  Roll Eyes

Well ... Let's see. As far as the tuning range of the Matchbox I have had this on a similar antenna albeit a bit shorter. I didn't see any problems in tuning with it. My understanding is this should be well within the range of this tuner. This tuner does much better with high impedance reactive loads, and is poor on low impedance. Am I wrong? (It's a Johnson Killowatt Matchbox). However I don't have a problem putting a few puffs on the output of the tuner to get the reactance down a bit; makes sense to me. I have a bunch of doorknobs that would work well.

I do plan on using this antenna not just on 80m, but also multiband. Of course that depends on the tuning.

The antenna will be fed with 12awg 450ohm ladderline. (DavisRF copperweld)

The operating position is about 90' below. Like I said a preliminary calculation says about 95' of feedline physical length. It may fall somewhere between 85' to 120'. There may be other variables involved here though, like capacitances at the feedpoint (depending on the mechanical requirements) and the shunt inductors and horn gaps (read below). These may end up modifying the electrical lengths, so keep that in mind.

I was wondering about elevator noise myself and was up on the roof today with an HT on a variety of bands (including BC and HF, VHF) I didn't notice anything when the elevators were running. They just recently replaced the motors, so maybe that has had a positive affect. I do have more problems, particularly on 80m with Plasma TV's in the building when using a receiving loopy thingy on the terrace. Hopefully being that high I will be less prone to that problem. What I am concerned about (refer to the image of the Da Roof) is the structure contains a water tower. Now I wonder how much detuning will occur as the level in the tank changes!?  Huh

As far as RFI issues, I usually do not have any. I am quite diligent about reducing causes for RFI (i.e. imbalance of feed-lines, improper tuning, mismatches, etc). I also monitor with both an HP scope and an HP Spectrum Analyser when transmitting. I bought and recalibrated an old HP 141T just for that purpose. I use QRO power level gear for headroom, not output. I don't push things, I try to use big antennas so I don't have to. Fortunately, in the big cities most people are using cable or sat. So RFI issues are usually only a problem on peoples computer speakers. Right now I have been operating 6m AM on a horizontal loop on the terrace and haven't run into any problems, and I have asked neighbours about it directly. So I will keep my fingers crossed and watch my transmission times.

Well as far as use, I want the West. I like to talk to the AM guys in CA. So this would be a good antenna for this. The roof image I posted earlier is oriented as North is top. I think by putting this in an inverted V configuration it may broaden the patterns a bit as well. I should also do fairly well into Europe. I know as I go up in frequency the number of lobes begins to grow into alot of different vectors, so that's good too. I hope.

Adherence to NEC is a must, both from a practicable point of view as well as a smart way to go. I do plan on using some pretty big horn gap suppressors I picked up a number of years ago, waiting for an application. The are rated for about 50kV. Here's a Pic:

Also, I am using DC shunt inductors to ground for static bleed off. They look something like these, but are homebrew with higher reactance. I measured about ~+j 2,400 at 3.8MHz. Here's an image of similar ones for reference:

Both the inductors and the horn gaps will be housed in very large plastic NEMA box, to keep them clean. Horn gaps need to be kept clean to work well, especially with near hit discharge. There is a large I-beam exposed right at the edge of the roof right where the feedline will make its way over the edge and downward. So that part of the I-beam will be ground down to bare metal and everything will be electro-bonded and mounted to that part of the structure. This should be an effective ground point. I have been on my CAD software all day today doodling out the mechanical design for this.

I found a proposal online for a major overhaul at the VLF site the Navy has up in Maine. Some of the hollow core cables where going south on it, so this was a proposal to repair these VLF wire based arrays. In it there are detail drawings for wire and cable attachment methods for the miles of wire they use on those two arrays. I'm also pouring through the literature on steel cable specifications for structural design. And I will check out that link. However if anyone knows of more information I would be thrilled if you could pass it along here.

I forgot about Laporte's classic Radio Antenna Engineering, of which I had copy many years but lost it somewhere over antiquity. I am delighted it now in the public domain and available online for download. Thank you for mentioning it, I forgot all about that one.

The project is slated for construction in the Spring, so I have a few months to work out the mechanical design and get it on paper and run some mechanical stress modelling. However please keep the electrical modelling, opinions, and experience coming. If you need any additional information, or for me to make more detailed measurement I of course will give you any data you could use to help me with this one. It's just one wire, but it's a big one. And the biggest and tallest I have ever made. Help is most appreciated!!!


« Last Edit: December 07, 2012, 01:24:24 AM by W2WDX » Logged

W5DXP
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« Reply #11 on: December 07, 2012, 05:52:37 AM »

My understanding is this should be well within the range of this tuner.

What I am remembering probably applies to the 300 watt Matchbox, i.e. a matching range of 25-1200 ohms.
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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.
WB2WIK
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« Reply #12 on: December 07, 2012, 10:09:39 AM »



I do plan on using this antenna not just on 80m, but also multiband. Of course that depends on the tuning.

As you know, such a long doublet will likely work best on 80m and on the higher bands have lots of long, skinny lobes with gain; and between them will be deep nulls with a lot of negative gain.  But it's a whole lot better than no antenna, or any sort of indoor antenna.


Quote
Well as far as use, I want the West. I like to talk to the AM guys in CA.

Good goal.  3000 mile 75m AM contacts aren't that easy, but if you're a nightowl and your noise level isn't bad, should be doable -- especially with some of the high-powered AM Superstations we have here in California.  A lot of these guys are running converted AM BC transmitters at -- ahem -- "1500W PEP output."  I suspect in many cases it may be a tad over that. Wink  But many of those guys do have big, high and clear antennas on large properties in the country, so no matter what they're running, they hear very well also.  I checked into the "AMI" pre-net Wednesday early evening (gathering of folks before the net actually starts) with 25W output on AM and everyone heard me fine.  Testimony to their good antennas and quiet locations.

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K3SF
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« Reply #13 on: December 07, 2012, 10:53:05 AM »

i only have one comment....

safety for all should be your prime concern

take extreme care for lightening protection.....
and
mechanical part failures ..should fail safe


I had a similar opportunity as part of PECO Radio Club in Philadelphia PA ( K3LDD)
We installed antennaes on top of 23 story office building..
This was the highest tower monkey experience i ever had ;-)

Installed Molsey TA33 on a 15ft of rohn45 and fan dipole in inverted-vee of corporate communications tower (75 ft)
 
Lightening protection and mechanical design was done by guys in club who did that as their day job...

Paul K3SF


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W2WDX
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« Reply #14 on: December 08, 2012, 12:32:14 AM »

Safety ... hell yes!!!

That's why I am giving myself so much time in the design phase and looking for methodology for mechanicals that already exist and have been tested and proven. "Fail safe" is important. I have to look at how things like mechanical load factors apply to insulators and mounts, and then try to balance that against corona and capacitive loading issues. However I will lean towards safety over performance, within reason.

I already have a weather monitor on the roof, which is hooked up to a dedicated PC, so I will be able to see average and min/max wind speeds over the next five months. I already have the data from Sandy, so most likely that will be my max speed baseline (not including factoring additional working load limits).

In regards to building the center insulator and making sure it has the mechanical strength required, I have some questions I would like to throw out there. It's my understanding that the gap is more a function of the feedline spacing and not the length of the antenna. To quote Cebik, "...the gap is a function of the feedline, which supplies energy to the antenna. Any gap in the element itself has some effect on the feedpoint impedance, but with normal materials at HF, these effect is negligible for practical installations. ... The actual gap is the distance between the conductors of a feedline, viewed as the energy source for the antenna element."

So now the idea comes to me to use larger polymer insulators used in the utility power field. Some of these can handle 12000 lbs of working load and have capacitance values that are very low. (5pf-20pf) I can get them in lengths as small as 5" giving a center insulator gap size of about 11". By mounting the wire in such a way I can now bring the feed wire into the the center, resembling a old style "delta match" to the width of the feedline spacing. The "delta match" will not function as a true impedance matcher, since its physical size would be to small for that effect. Besides even if it did, it would lower the feedpoint impedance anyway, which is probably a minor step in the right direction.

So my question is what is the experience with using things like clevises, thimbles, deadends, Crosby clips, vise type connectors and such on element wires? Pro and cons?

John
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