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Author Topic: Rigid Dipole using Hustler Mobile HF Antennas for Apartment in the Bronx  (Read 21747 times)
W2WDX
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Posts: 188




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« on: November 11, 2012, 05:25:51 PM »

Hi all,

I am currently located in a co-op apartment in the Bronx in NYC. (Sandy drove me here.)

Here's the plan. I cannot put up an antenna of any size. Nineteen feet width is the limit (the width of the balcony). I cannot string up wires. I do not want to use any of these little window verticals since I know the counterpoise is crucial and most likely I'm going end up inducing currents all over the place (ie balcony railings, windows, AC lines, etc.).

So I figure a dipole is the way to go. Obviously, it's balanced so the currents will have something to "push" against in the antenna itself, instead of anything metallic on the building as would be the case with anything end-fed.

So I happen to have two Hustler 54" MO-3 masts and a bunch of resonators for different bands. I have enough resonators to make an 75m dipole. (2 masts, 2 75m resonators, & the dipole mount). I can use a piece of masting on the balcony which would extend the dipole out about 17' out from the balcony and away from the building. Also we are on the 9th floor, so we are about 95' above ground.

Now I know I could probably just feed this with some good coax and tune it well. But I was also thinking of feeding it with some ladder-line (450ohm) and use a Johnson Kilowatt Matchbox and try to use the dipole as a multiband. 75m is of the most concern for me but multiband would be nice.

The resonators are the big ones so I could probably do a kilowatt, barring no dangerous RFI on the surroundings. The other thing it would be easy to rotate the antenna from horizontal to vertical.

So what is everyone's opinions about this? I have no other alternatives. Everything must be portable, no permanent installations. No wires on or coming off the building. Building management is here 24/7, so no cheating or stealth. The building is concrete (with re-bar) & steel and concrete internal walls. Inside the building is an RF dead zone as a result. However I can push out a mast from the balcony, as long as it is not permanent and removed after I am finished using it. So if your going to post "just put up a dipole" or "hang a wire out the window", you're wasting your time as well as mine. :-) I just want to know what the experience may be on this subject. I know the efficiency will be low, I know the pattern will be unpredictable, etc. I just want to know about any possible ways to make sure this works as well as it can and whether I can tune this type of antenna with a balanced feedline and tuner.

I know anything will be a compromise, but that's OK. I think this idea is the best option. What are the thoughts on this?

John, W2WDX
« Last Edit: November 11, 2012, 06:13:21 PM by W2WDX » Logged

NA4IT
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« Reply #1 on: November 11, 2012, 06:27:01 PM »

Not sure how well it will work as a multi band antenna, but as a dipole, it should work. Might think about more resonators.
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K0OD
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« Reply #2 on: November 11, 2012, 10:40:21 PM »

For several years I used such a dipole from the 18th floor of a 23 story apartment building. Also tried individual Hustler whips mounted perpendicular to the aluminum balcony railing.   The later was the best approach. Easy to change bands. Used a simple direct coax feed. Spent a lot of money on Hustler stuff while at the QTH. Nothing worked great despite being quite high.

Antennas seemed to do best on 20 meters. They were too short for the lower bands and 180' was too high for 10 meters.  

You are correct the zero RF makes it thru such a building.  
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N4UM
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« Reply #3 on: November 12, 2012, 05:43:38 AM »

You asked for thoughts and opinions on your idea. 

Here's a thought... You have to be crazy to extend anything out from the building on the ninth floor where it could fall and kill someone.  The support mast and those Hustler masts and resonators could become deadly weapons.  If you insist on trying this crazy scheme I suggest that you greatly increase the coverage on your personal liability insurance and prepare to spend big bucks on a really good defense lawyer.  All this to get on the 75 meter cesspool?
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AD5X
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« Reply #4 on: November 12, 2012, 06:27:04 AM »

You might take a look at the portable dipole I use from the balcony of a condo when we go on vacation (in the "Articles" section of my website at www.ad5x.com).  The feed is at the center of the balcony railing, and I extend the dipole wires up to the balcony corners and then let them drop down (sort of a "M" configuration).  Works great on 40-10 meters.

Phil - AD5X
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WX7G
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« Reply #5 on: November 12, 2012, 07:17:49 AM »

It will work well enough with the correct resonators for each band. Fed off band it will be a short, unloaded dipole and the Matchbox may exhibit high loss. But, it does no harm to give it a try though.

The Buddipole antenna is the same but covers all bands with one set of loading coils. They have accessories to allow rotation of the antenna, matching, and masts.

http://www.buddipole.com/

And the MFJ HF Sticks work as well as the Hustlers and cost $15 each ($20 for 75 meters). MFJ has a center mount to make an HF Stick dipole.

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WB6BYU
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« Reply #6 on: November 12, 2012, 09:10:15 AM »

The matchbox is unlikely to work well on other bands - basically you'll be loading up the
54" masts on each side, which is rather short even for 10m.  I'd recommend using it as
a dipole with the appropriate resonators for each band instead.

The larger kW resonators are actually more lossy than the low power ones due to the
metal end caps.

Such a system can work reasonably well on the higher bands, but may be very tricky
on 80m.  The bandwidth will be narrow, and you can have significant common mode
current that will shift the resonant frequency as you rearrange the coax.

But, if that is what you have available, try it and see how it works.
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K0OD
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« Reply #7 on: November 12, 2012, 01:25:03 PM »

Quote
I can use a piece of masting on the balcony which would extend the dipole out about 17' out from the balcony and away from the building.

17 feet out?  I tried some wild stuff when I lived in a high rise but nothing extended more than about 8' out. And then it was pretty scary. In my case my antenna was over a rarely used patio. Probably lost a bolt or two over the side but nothing more.
 
You don't want to snag some window washer.  A lightweight whip pointing straight out or at 45 degrees is the way to go,
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W2WDX
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« Reply #8 on: November 12, 2012, 01:38:52 PM »

Hi again,

Well the idea was to not spend any money. :-) Except for rigging, so buying something right now is not an option. Trying to buy stuff like couches, chairs, clothes, refrigerators after Sandy. Lost everything but my electronics which was in my shop at the time. I was one week from moving when the storm came. Moved as much as I could beforehand. Oh well ...

So I plan on using what I have. I am sure the big resonators are more lossy than the smaller ones, but I can overcome those losses with power, which I have in abundance. Poor radiation efficiency notwithstanding.

Phil - I did see your article before I posted. I think I checked out everything out there before deciding on trying this method. The problem is the balcony is laden with steel everywhere. The railing is solid steel mesh and support which is at chest height. Overhead there are steel I-beams and aluminium flashing all around the top. This is the reason I have extend a boom out to try and make some kind of distance (still not enough tho) from the antenna.

Anyway, would there be a problem with tuning (via balanced line and balanced tuning) tuning an 75m antenna with coils? I don't see why, but maybe somebody could explain better. I usually use doublets (Double Zepps w/ balanced feedline and balanced tuner). My last antenna was a 160' doublet flat top at 90' above ground, which obviously worked great on most bands. Right now I'm just toying around to get my HF fix and I fully realize the performance is going to be really bad by comparison. But of course I want to maximize whatever compromise I end up with.

And of course, on eHam I figured I would get one troll to say something ridiculous.  Roll Eyes If one does not know how to secure ANY antenna properly you should not attempt to put anything overhead, ANYWHERE. Obviously, the mounting while temporary would be substantial and would involve safety straps (nylon) on every component. Applying my 25 years of experience in engineering overhead rigging for concert touring, gives me some intuitive understanding of how to do this safely. Any antenna put up has to be rigged properly. Even a Balun hanging at 30' can kill someone if it falls on their head. Of course this will be rigged for safety, even though the area it would fall down into is the side of a ridge where no pedestrian traffic can go. If it falls down there, there is no way to retrieve it without jumping over fences and climbing up rock faces. The building is built into the side of a cliff.

Bandwidth is not an issue, I don't plan on doing DX work (unless I hit on someone who can hear me) and I do not engage in CW. I'm just looking for ragchew, and do not need to be frequency agile. And like I said, I do not plan on using coax, I generally do not use coax on HF anyway. Too lossy as SWR rises. So I guess my main question is the balanced tuner and ladderline and whether tuning into coils actually works. I ask because I've never done this before since I always use much more simpler (and IMO more effective) antennas like true doublets.

Thanks again ...

John
« Last Edit: November 12, 2012, 02:34:32 PM by W2WDX » Logged

W2WDX
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« Reply #9 on: November 12, 2012, 02:05:55 PM »

After thinking about it, maybe I should explain the rigging.

Overhead on the balcony are I-beams that extend out from the structure of the building supporting the balcony above. I plan on using a swingarm system we used to use in concert touring that clamps onto these I-beams. We used to use these to hang loudspeaker rigs from structures. Each I-beam clamp is rated to carry a working load limit of about 2 tons. The rig uses four of these The swing arm which is an L-shaped contraption that can accommodate a arm length of twenty feet supporting 1 ton (WLL) at the furthest point. It's designed to be a temporary rig, easily removed. The swing uses thrust bearings.

I plan on not using the steel extension arms but instead will be using 2" aluminium tubing (.120" wall) with 1.75" inch inside. Double tubing in effect. I have calculated the span working load limit to be about 570lbs, which will be well beyond the weight of this antenna by a factor of about 7.

With this I can put the mount on one end of the balcony and the end of the arm will be at the opposite end. This way I can mount, adjust and fiddle with the antenna (if I leave the window open Smiley ), and then swing it out 90 degrees into the working position. The arm locks into various position with steel locking pins.

It's a perfect rig for this application. It's a little heavy, but it takes no time to rig up. And I can totally remove the rig in about fifteen minutes if the building management has a pissy fit. As far as its structural worthiness I would have no problem slinging on a harness and sliding out to the end of the boom, if I was that crazy. (I wouldn't due to my aversion to heights, but I have in the past at shows.) Granted it's way overkill for the loads involved, but I don't have liability insurance Roll Eyes . I'm just lucky I have used it before and happen to have a couple of them left over from my former profession.

Now its just a matter of getting this thing to radiate to some degree so I can say hi to some friends!

John
« Last Edit: November 12, 2012, 02:23:25 PM by W2WDX » Logged

WB2WIK
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« Reply #10 on: November 12, 2012, 02:19:32 PM »

Balanced line and balanced tuner should work fine.

The issue with the loading coils on the Hustlers (the RM-75S or RM-80S or whatever you have) is they have so much inductance they'll act like traps on 40m and below (although I'm sure they go through self-resonance in the HF spectrum somewhere, and above that they'll act like capacitors), so the whips beyond the coils are likely to be choked off from the rest of the antenna, leaving only the two 54" masts as the actual radiator on the higher bands.

80: 54" masts+extension whips beyond the coils, all radiating.

40m and higher frequencies: Likely only the 54" masts, the RF current is unlikely to reach the whip extensions.

This makes for poorer efficiency. 

One thing I'd try is using a pair of clip leads to short out the 80m loading coils (both of them) for operation on 40m or higher bands, which gives you more active radiator.  In fact, if you have space for longer extension whips than the ones the Hustler resonators come with, using those to extend the antenna to the maximum available space, and then reducing the number of turns on the RM coils is always a great idea, and hams have been doing that for decades.  You can change the RM coils fairly easily if you remove the white insulator sleeving over them.

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W2WDX
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« Reply #11 on: November 12, 2012, 02:51:10 PM »

Ah ... yes ... of course. That makes perfect sense.

OK ... so if I attempt multiband these act like chokes and actually make the antenna shorter at higher frequencies. That was what I intuitively suspected and now that you explained it better ...

It might get a bit squirly to extend those masts, being they are normally designed to take a load perpendicular to gravity and not horizontally.

Well I guess I'll have to change the resonators for the respective bands. I have two each for 75, 40, & 20 and I have only one for 10. Of course I could get that silly adapter that goes on the top of the mast that allows you to screw on three resonators at once. Ha ... that would be pretty! It probably would work but the stress load on the mast would be insane if made it horizontal. Probably would be ok vertical. But I think I'll just try it doing the swap out the coils thing.

BTW, the antenna shows very little sag when horizontal, even with the bigassed 75m coils.

I was just hoping I could go multiband without any fuss, but of course there is never an easy way out with antennas. Hey but that's always the case isn't it with antennas. If it wasn't I'd just strap on a bunch of Isotrons. HiHi

John
« Last Edit: November 12, 2012, 03:04:11 PM by W2WDX » Logged

WB6BYU
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« Reply #12 on: November 12, 2012, 03:55:08 PM »

One problem with the Johnson MatchBox is that it is designed for a relatively high
impedance.  The dipole will be perhaps 20 to 30 ohms or so, depending on losses
due to the building nearby.  The twinlead will increase the resistance a bit, but
also add an inductive component.

As an example, using VK1OD's transmission line loss calculator
a 20 ohm impedance through 18 feet of Wireman 552 twinlead looks like about
30 + j200 ohms on 75m.  It seems to me that the Matchbox had a range of something
like 200 - 800 ohms or so (though I can't find my notes to confirm that.)  There are
various ways you could step up the impedance to the range of the tuner, including
possibly an appropriate shunt coil across the feedline at some point, or some sort
of transformer, or possibly using some sort of alternative feed arrangements (such
as shorting the bases of the two antenna together and adding wires to make some
sort of delta match.)  But, at first glance, I think the matching range of the tuner
will be the limiting factor to this approach.



I had pretty good results in one apartment using a mobile whip stuck horizontally out
the window and fed against the aluminum window frames.  That would be similar to
using the steel railings of your balcony as a ground plane.  With multiple conductors
bonded together, the current in any one portion shouldn't be high enough to cause
problems.  (The more likely problem will be interference due to radiation from the
antenna itself.)

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WX7G
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« Reply #13 on: November 13, 2012, 06:41:27 AM »

I had a Johnson Matchbox and it was almost useless due to the narrow impedance range it could tune.
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W2WDX
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« Reply #14 on: November 14, 2012, 12:21:44 PM »

I had forgotten about the low impedance range on the Johnson.

And as far as that is concerned, most of the Johnsons out there were modified in such a way as to accommodate SS transceivers, which decreases the low end range even further. So tuning coax cable, which is a pointless endeavour anyway, isn't what that tuner was designed to do. It was designed to match higher impedance loads, like on doublets or random length dipoles. So if you are trying to tune coax near its own impedance, it's down in region the Johnson is not best suited for. And if you are tuning for a high impedance and using coax, your losses are so high from the coax anyway that tuning is a moot point. Except in terms of saving the output devices.

Ha ... another rant. I was always frustrated with great reduction in size approach the radio manufacturers used. They said, "Well ... we gotta make the radios smaller. Ok so let's pull out out the Pi-network and limit the tuning range of the radio. After all, the Hams all have tuners anyway." So now you have tiny radios AND a tuner is required in most cases so what space savings is there really? And most tuners made these days are terrible lossy things. And it's a more complex set-up. My Hallicrafters HT-37 doesn't have all the cool bells and whistles, but it sounds far better than any modern radio (both AM & SSB), and I will probably be able to attach this antenna directly to the radio without a tuner.

The Johnson tuner when used for its intended purpose is the best tuner I have ever used. It can tune twice the legal limit easily. The "kilowatt" rating is not PEP and in actuality it can easily handle about 3KW PEP, probably more. It can tune very high impedance on balanced systems. You can even internally adjust the caps as to achieve perfect balance in terms of current, voltage and phase on the feedline. That's a key issue when using ladder-line. You cannot do that with any auto-tuner or even the so-called balanced tuners available today. And I really like the link coupled input and no toroidal balun approach Johnson used. Very low loss design.

I say matching coax is pointless because all you are doing with a tuner and coax is making the transceiver see a 50ohm load on its end, so with most modern rigs you get full output. But the losses get so high on the coax you lose most, if not all, of the power you are gaining anyway. The better approach is tune the antenna to the feedline impedance, and use a current balun if you have to transform the balanced to unbalanced. If you run a multi-band dipole, use a balanced tuner or link-coupled tuner and balanced feedline. Otherwise, you are just wasting ALOT of power.

Thanks again for all your input. I may have to rethink the approach a little based on what everyone is saying. If my impedance is that low (20-50) it may be better to run coax in my case. I guess I will have to do some measurements en situ and see how it goes. I hate compromise situations, but so be it.

John
« Last Edit: November 14, 2012, 12:47:28 PM by W2WDX » Logged

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