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Author Topic: OCF wire antenna in deed restricted area  (Read 7012 times)
KD0EEZ
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Posts: 4




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« on: June 26, 2012, 09:51:52 PM »

I looked around on the web for a off center fed dipole that I could transmit on 40, 20,10 and 6m and found one that said it should be 68 feet long with a 4:1 balun 23 feet in from one end so it would be 45/23 feet.  I live in a deed restricted area and put it up at the top of a 6 foot tall block wall along the back of my yard.  Any antenna I put up can not be visible from the front of the house.  I had a friend come over with an antenna analyzer and it wasn't even close to 2/1 swr an any frequency.  What happened and what can I do?

Thanks for any help.
Bob
W1RDS
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VK2WF
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« Reply #1 on: June 27, 2012, 12:31:12 AM »

Some of the the range of options that I know of are-

- a just the length longer or shorter to adjust the SWR.
-put a tuner on it
-change the position of the feed point along the wire.
-you could try spacing it up a bit higher from the wall.
- you could try a mix of the above.

I would put a tuner on it as this is also likely to give you access to more bands.

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W1JKA
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« Reply #2 on: June 27, 2012, 03:47:06 AM »

    Along with the other suggestions you might try putting a wire that is 5% longer than your total antenna length on the ground directly underneath your antenna wire.I did this with a 40m dipole atop a 6 ft. high stockade fence and my signals improved considerably.This is basically an NVIS set up with the ground wire acting in a ground/reflector capacity and I have no problem making contacts in N.Eastern states and E.Canada with QRP power.Won't hurt to try,good luck.  Jim
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K5LXP
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« Reply #3 on: June 27, 2012, 06:10:02 AM »

I looked around on the web for a off center fed dipole...
put it up at the top of a 6 foot tall block wall along the back of my yard...
it wasn't even close to 2/1 swr an any frequency. 
What happened and what can I do?

I doubt the installation instructions suggested putting it on top of a block wall.  Between the close proximity to ground and whatever rebar or whatever is inside the wall, you've detuned the antenna.  What can you do?  Suspend it in the clear at least 30 feet above ground.  If you can't do that, don't expect it to work as advertised.  If your only antenna option is along the top of the block wall, it won't matter what you put on it, it's not going to work well.  In my experience there are always other options, so consider where/how else you can put an antenna in a different place, OCF or otherwise.


Mark K5LXP
Albuquerque, NM
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KG6YV
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« Reply #4 on: June 27, 2012, 07:40:18 AM »

Here, read this.  This guy is one of the acknowledged experts in OCF antennas.  Also it might help you to join the Yahoo
 "OCF antenna group"...

Gud luck,

Greg

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=15686

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K4SAV
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« Reply #5 on: June 27, 2012, 09:05:15 AM »

There are two problems, it's too low, and it is on the top of a concrete wall.  Most OCF antennas don't have a very low SWR even when they are placed at high heights.  For an antenna of the dimensions you gave and with the antenna at 30 ft, a simulation gave SWR = 2.5 at 7.2 MHz, 1.8 at 14.3 MHz, 1.8 at 28.8 MHz, and 1.7 at 50.5 MHz.  At 6 ft it's worse.  I don't have the conductivity and dielectric properties of concrete (they vary widely anyway) but I would suspect that would detune the antenna so that the low SWR points are no longer hit in the bands.

You could try using it with high SWR but the low height will make the gain very low.  However, no doubt you will be able to make a few contacts with it.  

Consider disguising a vertical, either trapped or matched at the base with band switching.  Can you put up a bird house or a flag pole?  If you have any trees, hiding an antenna in a tree should be easy.
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K0ZN
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« Reply #6 on: June 27, 2012, 05:19:45 PM »


There are several things going on here. 

1.)
Unfortunately the hard reality is that an OCF is NEVER optimum on ANY frequency or band. It is an electrical compromise in ALL conditions. It is not a symetrical, balanced antenna and it is not a properly fed UNbalanced system. A mechanical analogy is that an OCF is kind of like trying to force a round peg into a square hole.
This fact, "amazingly" never shows up in any of the commercial antenna ads/propaganda. You simply cannot take a simple wire antenna and feed it with coax or via a balun  and expect it to show low SWR on several bands. That is electrically impossible. I think people get caught up in the coax feed thinking it is "plug and play" because it is coax; that is not the case. They try to make those antennas look like they will do everything and that simply defies the laws of electrical physics. This is not to say they don't work, because they do radiate, and can work pretty well, but because of the compromises, there is always "something", be it feedline radiation, SWR issues, etc. involved
on some bands. Fed with coax, an OCF simply will not offer a good match on all bands....again... it is just electrical physics. Usually, a matching network (antenna tuner) is needed with an OCF.

2.) 
That antenna is very close to the ground and the proximity to ground will definitely impact the antenna feedpoint impedance and point of resonance.
Although, there is some question about how much impact actual resonance may have on an OCF on some bands. If the wall is pure concrete block, it may....
hard to know for sure..... have some interaction or effect of some kind too. Are there any steel fences or utility lines, etc., near it?

Your best bet, it seems to me is some form of stealth (very small gauge wire) as a sloping center fed system fed possibly from the peak or highest point on your
house, or along the roof, etc., with balanced line (e.g. ladderline) to a good balanced antenna tuner. Or a very small gauge wire up under or attached to the eves of your house, etc. A very small gauge wire as an inverted L against a similar wire as a counterpoise can work surprisingly well too.....and here again, the trick is how do
feed it effectively. The beauty of a tuner fed system is that you don't care what is going on SWR wise out on the line and antenna because the tuner is a
happiness box" that lets the radio work into 50 ohm coax with low SWR....and that is all the tranceiver cares about, so to speak.

Most respectfully:  At this point, the best investment you could make in your station is to pick up a copy of the ARRL Antenna Book (an older copy, which is cheap, off of Ebay, etc. would be as good as a new one in this case) and put in some study time on antenna basics and how to feed them. The main challenge with stealth or
HOA restricted antennas is how to feed them. A little good knowledge of antennas is a major asset in this situation. You CAN make a decent antenna almost
anywhere....the catch is how to match it and feed power to it.....which is where the Antenna Book knowledge comes in so handy.

Bottomline: stealth situations very often require a homebrew antenna system because this is necessary to get the greatest height or best positioning because of the physical/esthetic limitations and that situation is not agreeable to most commercial products.

Unless your neighbors are out to get you, etc., it should be easy to come up with something "decent" that is not VISIBLE from the front of the house. Keep in mind
the key word here is VISIBLE from the street !!  Maybe you just installed a "wire" to hang a bird feeder on !...and then HANG the bird feeder from it !! (with a small insulator between it an the antenna wire.)  Ya gotta be creative !   The antenna is NOT a problem until somebody complains about it !! .....and if it is not VISIBLE, it can't be
a "problem".

 Good Luck,   73,  K0ZN

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N4RSS
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Posts: 263




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« Reply #7 on: June 27, 2012, 07:20:08 PM »


There are several things going on here.  

1.)
Unfortunately the hard reality is that an OCF is NEVER optimum on ANY frequency or band. It is an electrical compromise in ALL conditions. It is not a symetrical, balanced antenna and it is not a properly fed UNbalanced system. A mechanical analogy is that an OCF is kind of like trying to force a round peg into a square hole.
This fact, "amazingly" never shows up in any of the commercial antenna ads/propaganda. You simply cannot take a simple wire antenna and feed it with coax or via a balun  and expect it to show low SWR on several bands. That is electrically impossible. I think people get caught up in the coax feed thinking it is "plug and play" because it is coax; that is not the case. They try to make those antennas look like they will do everything and that simply defies the laws of electrical physics. This is not to say they don't work, because they do radiate, and can work pretty well, but because of the compromises, there is always "something", be it feedline radiation, SWR issues, etc. involved
on some bands. Fed with coax, an OCF simply will not offer a good match on all bands....again... it is just electrical physics. Usually, a matching network (antenna tuner) is needed with an OCF.



The other excellent comments notwithstanding, best to avoid generalizing.  My OCF measured at 50' below feedpoint:

3.8mhz   1.9

7.180      1.2

14.200     1.6

18.120      1.5

21.300      2.5

24.960      1.3

28.480       1.8

...and in general any antenna can only be made optimum at  one frequency.

Cebik and many others also demonstrate the theory behind the OCF if one cares to look
« Last Edit: June 27, 2012, 07:22:09 PM by N4RSS » Logged
KA1RUW
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« Reply #8 on: June 30, 2012, 12:19:52 PM »

Hi Bob

First of all, you’re being tossed too much expert opinion and not enough practical seat of the pants real world advice.

I suggest you go to http://hamwaves.com/cl-ocfd/index.html and read all the pages associated with Serge Stroobandt’s (ON4AA) site as it relates to CL-OCFDs. That's Center Loaded-Off Center Fed Dipole. Its well worth the time and you will understand a lot about OCF doublets when you’re done. You do not need Center Loading to have a functional OCF Dipole.

As previously stated, height above ground greatly effects the aerial’s input impedance.

An OCF doublet’s standard design goal is to offset the input feed point so that a 200 ohm input is realized with the areal raised to ¼ wavelength above nominal ground. This allows use of a readily available/fabricated 4:1 current balun.

In practice, the height above ground and the type of ground (salt marsh, pig sty (my preference, hi hi), grassy yard, veggie garden, paved lot, etc makes more difference to input impedance than height above ground when you’re much less than ¼ wave length above ground.

Also, heavy rains over a grassy back yard will change your input impedance quite a bit so no matter what your height or ground conditions are, you might as well plan on an antenna system tuner in the shack from the get-go.

Positioning any horizontal radiating wire at the top of a mason wall is less than ideal. As suggested, move it away from the very top of the wall either up or to the side. My suggestion is to move it to the side by a few feet and up to a height that satisfies the condo Nazis so they can’t see it from the street. Also as suggested, add a ground reflecting wire under the aerial wire. One directly below it is reported to help lots with NVIS propagation and some suggest one each set 3/4/6 feet to either side of the aerial wire. YMMV!

Then, if needed, adjust the feed point position so you have the best SWR you can get at the lowest design frequency. Since your feed point is at arm’s length, borrow an antenna analyzer and adjust feed point to 200 ohms at the feed point without the balun or as close to 200 ohms as possible.

I used to have 1:1 @ 3.8Mhz with 140 foot of wire @ 30 ft using a 4:1 current balun and 100 ft of RG213. My 2:1 SWR without a tuner was +/- 50 KHz. Yes, low Q but it work great! My feed point is at 30%, from the short end of course! Hi hi.

At the new QTH, I haven’t bothered to check it because my Kenwood almost instantly auto tunes to all but 30 Mtrs.

After I find my ‘Round To-it’, I’ll add a 1:1 current choke to the balun’s input. At my new QTH, I’m resetting my USB’s port when I transmit. It’s a new problem but no other signs of RF in the shack.

And by the way, there is absolutely nothing wrong with the radiation pattern(s) of an OCF doublet. The current distribution is identical to a half have dipole. IE: Max current is in the physical center of wire where we want it when we operate at the fundamental frequency. And, regardless of the off-band frequency we operate at, there is always a current node in the center if you follow Serge’s center loading and cutting/tuning recommendations. Just like any doublet, when we operate off-frequency, we have multiple radiation lobes into space and multiple current nodes on the wire.

On 10Mtrs, my 80 Mtr doublet is essentially omni-directional because of all the weird and wonderful nodes generated.

In summation: Don’t give up. Stay with a 4:1 current balun and adjust the input feed point for best SWR at the lowest operating frequency. Move wire to the side and if possible, up from top of wall. Use an antenna tuner.

Also, add a 1:1 current choke to the input of the balun.

You will have a great NVIS antenna on 80/40 and one which will also get you into the east coast and pacific on 100 watts.

Happy HAMing and good DX

73
Russ, KA1RUW
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KD0EEZ
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« Reply #9 on: July 01, 2012, 11:46:22 AM »

Thanks for all your input.  One of the problems I didn't mention is that the back wall is against the main street that goes through the subdivision so I can't go any higher than the back wall with out at least the supporting posts on each end being seen.  Eves are out because it is spanish tile and has metal trim the whole length of the roof on all sides to form to the bottom of the tile.  We don't have any trees that are useable as they are young and small.  There is two Palo Verde trees about 10' outside the side wall but don't know how I could get the feed coax to it with out being seen at least by the land scape company.  They would probably pull it down.  My next attempt might be using a couple of ham stick dipoles mounted next to the little trees.  At least the tree would hide the pole.  My back yard is only 68' long by 20' deep and the side is only about 6'X80' long with the block wall running the full length.  The other side area is open with only a 3' high steel dog fence but open to the neighbors view.  I have looked into the MFJ 1788 magnetic loop antenna but it is a little pricy for me right now.  I'll look into the ideas you all have presented.

73
Bob
W1RDS
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KC9TNH
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Posts: 304




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« Reply #10 on: July 06, 2012, 01:22:41 PM »

Bob, don't be afraid to re-read what Russ posted above. Lots of good practical things to consider in the approach. And I've got an OCFD not because it's a fad but because my narrow residential lot pretty much dictated where the feedpoint was going to be. No issues. 75 is a local/regional ragchew, 40 basically day NVIS and a bit further sometimes, anything else competes because the higher bands don't need something real high to be productive (in real, restriction-laden suburban hell environments). And yes, I've worked plenty of farrrrr DX with it too.

Get a GOOD 4:1 current balun, get an even BETTER tuner (than some of the budget auto tuners) with alot of inductance & capacitance to it, Palstar & some other good manual ones come to mind). Don't be afraid to adjust your feed point even to an 80/20 offset, it will take some playing if you don't have an analyzer like Russ said but, when you come out the other end, you'll know your antenna & it will be there for you.  W8JI makes a good case for the 80/20 offset although he uses an 80m example. Do the math & scale it back as best you can for your physical constraints.

Good luck & Never give up!   Grin

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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
K5KNE
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Posts: 65




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« Reply #11 on: July 06, 2012, 07:52:55 PM »

Trying to make an antenna work very close to the ground and other objects is a tough situation. You might think about trying a black vertical (with ground radials) that you only put up after dark and take down before dawn. There are some fiberglass rods that lend themselves to make a long whip by putting a wire around the rod.  You can put an antenna tuner, preferably at the base of the antenna in a waterproof box, to make it work best on the band you want to use.  Use an antenna analyzer like the MFJ 259B or similar to adjust the antenna tuner then hook it up to the transceiver. I don't know where I got my 32' black telescoping fiberglass whips that look a lot like long fishing rods.  MFJ makes a little simple tuner that is about $100 - no meter and it would be fine for even putting outside in a box.
I have thought about putting them on the eave of the house like long rabbit ears and using the little tuner to set it to a band I want to try.

I don't know what time of day you do your hamming, but I hope that it is dark ouside.

 I don't like verticals, but if it is the only thing you can put up - O.K.  I recently talked to a guy clear across the country using two back to back ham sticks on 20 meters - like you were thinking about doing. You again need the analyzer to set the ham sticks and poke them up as high as you can get away with - maybe just at night. You can rotate the pole to get some directional effect.

There are a lot of stealth antenna articles on internet. However, all of them put the secret wires up in the air.

If you have a metal patio cover or table in the back - it is possible to make some contacts on a mag-mount with a whip. I play with antennas and I have taken a CB mag-mount whip and made the coil do 10, 15, and 20 meters - and made contacts.  40 would be a stretch for it.

Good Luck,   Walter  K5KNE
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KI5SO
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« Reply #12 on: September 27, 2012, 02:51:34 PM »

End fed tuner like the Icom AH-4 is a magic box.  Use it for an configuration.  A thin magnet wire thrown up in a tree, ho about a loop around the house?  If there's a will there's a way.  Here's one:  connect a wire to your raingutters and one to ground with the AH-4.  Lots of success stories on the internet.
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N4UM
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« Reply #13 on: September 27, 2012, 03:48:06 PM »

Sounds like you have room for a modest radial field in your back yard.  Why not consider a "flagpole" with an autotuner at the base.  My 23 footer works reasonably well on 40 thru 10. I'm ven able to use it on 80 and 160 under cover of darkenss by using top loading.  You can check it out on my QRZ.com page
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N4JTE
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« Reply #14 on: September 27, 2012, 04:42:43 PM »

All I know is that I have a buckmaster OCF at 65 ft. flattopped and the only band below 2 to 1 SWR is 20 meters, so don't expect much of anything with your layout.
Bob
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