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1-9 of 9 messages
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fencetop antenna
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by ALCO141 on October 16, 2009
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how many have made a fence top antenna, how did you make it, did you buy something how did you handle the feed line?
i need something to fit a fence 72 ft then turns 90 degrees and runs 80 ft the fence is six feet off the ground.
any and all help is welcome.
alex cocco
kc9pwt
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RE: fencetop antenna
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by WB6BYU on October 16, 2009
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What bands do you want to operate?
What is the fence made of?
What other constraints do you have?
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RE: fencetop antenna
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by ALCO141 on October 16, 2009
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would like to work HF 80 m and down, the fence is wood,would need to bury some coax to feed the antenna, could attach the antenna to the top of the fence.
alex cocco
kc9pwt
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RE: fencetop antenna
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by N5LRZ on October 17, 2009
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Well you have several problems...
1) Height above ground...Loop antennas radiate perpendicular to the plane of the antenna and as such need to be as high as possible above ground. 6 feet above ground is not even high enough for 10 meters to escape the earth directing your signal strate up rather than in a horozontal direction. You need your loop to be at the very least over a full quarter wave length high for the operating frequency in use to escape the effect of the earth.
2) That low to the ground, unless you have great dirt, earth losses will be severe. Close proximity to the ground, even for 10 meters, will cause severe signal loss due to proximity ground losss.
3) The length of wire is no where near long enough for an 80 meter loop. The length of a skyloop antenna, which is what you are attempting, per the ARRL Antenna Book is 1005 divided by the frequency in Mhz. If you do the math you will see that you need a far bigger loop for 80 meters. What you are making is a random length loop rather than a calculated length proper length loop.
4)Visibility: even IF you were to put the loop on top of the fence, the fence will have to have gaps to allow wind flow thru the fence (to keep the fence from blowing down in a strong wind). The wire could then still be seen by the HOA Inspection Team thru these breaks in the fence. Believe it or not HOA Antenna Inspection teams are not stupid. They know where to look.
5) You did not mention it but no matter what kind of feedline you use, Ladder or Coax, you will need at the very least a half way decent tuner. Your random loop antenna will definately require a tuner to provide a 50 ohm imp point to keep your radio happy.
6)IF you live in an HOA that has in its contract either directly or indirectly any kind of clause that can be used to prohibit RFI to TVs and such at 6 feet your antenna will be right at that level where RFI will be a real problem--about 4 or 5 feet off the ground.
7) Being a loop in close proximity to your house, you might/perhaps also get RFI in your own home. Home electronic devices are notorious for NOT having any filtering these days.
BUT THERE is some good news....
IF IF IF you restrict your activities entirely/100 % to the digital modes such as RTTY, PSK and such you can operate using a dipole rather than a loop on the fence and get away with open operations. You see, people KNOW voice as mumble mumbles. People know how the code/CW sounds believe it or not. BUT PEOPLE do not in general know what PSK, RTTY and other computer digital modes sound like. To them it sounds like Internet Dial-up. AND TOO, you can legitimately state in all honesty that you were NOT talking if the neighbors bitch--because you were NOT talking/you were sending computer sounds which is NOT the same thing as talking.
You can even send SSTV and Digital SSTV mode transmissions if you increase your power output a bit. You do not have to ID via voice or CW as long as you put your call on the picture you are sending. Just remember to turn off auto cw id at the end of transmission. SSTV sounds like birds chirping so the neighbors will be confused. AND Digital SSTV sounds like a wall of static.
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RE: fencetop antenna
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by WB6BYU on October 17, 2009
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I don't think you'll find a single antenna that will work
well over multiple bands in that location, especially a
commercial one. (You can certainly get something that
will give you a low SWR, however, if that is more important
than a good signal.)
For 80m, a dipole along the top of the fence isn't a bad
start. I'd either feed it in the middle (regardless of
where that is with respect to the bend) or at one end,
depending on which is more convenient in your situation.
This is because 80m often supports high angle (NVIS)
propagation, so even though most of your signals is going
straight up (which will be the case for a horizontal antenna
less than 1/4 wavelength off the ground) it will still
get reflected back down to Earth out to a few hundred
miles.
40m may also give reasonable results with a low horizontal
antenna, especially as the sunspots pick up (hopefully!)
in a couple of years. If you are using an end-fed wire
on 80m, the same antenna will work on 40. If you are
using a center-fed dipole, just tack on a second set
of wires resonant on 40m.
Can you run a wire back from the far end of the fence
to the beginning (across the diagonal, or around the
other sides) to make a loop? That could be used on
multiple bands.
While you'll hear some signals on the higher bands
with a low horizontal antenna, and that might be a good
place to start, the high angle radiation from a low
horizontal antenna isn't reflected back from the
ionosphere on the higher frequencies. So if you are
limited to a maximum height of 6', you probably will be
better off with something vertically polarized. A
common solution would be to use some sort of loaded
vertical whip placed on the ground with radials running
though the yard. Another option would be a vertical
loop of wire attached to the fence, so about 5' tall
(so the bottom wire is 1' off the ground) and as long
as necessary to reach resonance on the desired frequency.
(About 30' for 20m, 22' for 17m, or 19' for 15m.) This
will give maximum radiation broadside to the fence.
Plan on carefully tuning whatever antenna you use.
Performance and tuning will depend on your local soil
type, how the antenna is mounted to the fence, etc.
Ground losses will be rather high with any sort of
antenna at such a low height, but that doesn't mean
that you can't make contacts.
On the practical side, installation depends to a large
part whether you have kids, or are otherwise concerned
with people coming in contact with the antenna on your
side of the fence. While I have been known to simply
drape an insulated wire over the fence posts, a better
installation would be to get some electric fence
insulators at the local farm supply store. There are
many varieties, including several that are designed
to hold the wire spaced from various sorts of posts.
You may have to improvise a bit, depending what you
can find. The old screw eye insulators for 300 ohm
twinlead have also been used for this purpose, but they
are getting harder to find.
If there is a danger of someone reaching over the fence,
or touching it from your side, you'll want better insulation
on the wire. Running it through plastic tubing of some
sort (at least in the critical areas) would be a good
idea. The actual installation will depend on the fence
construction and the allowable visibility. (In an
extreme case you could staple the wire to the fence and
cover it with a coat of paint to hide it.)
It is unlikely that you'll find any commercial antenna
that is directly applicable and will provide good
performance. That doesn't mean it can't be done, just
that it will require you to do some planning and
experimenting to find what works best for you. You
may need several antennas to cover all the bands of
interest: I'd suggest starting with a wire down the
top of the fence for the lower frequency bands to see
how it works, then looking at other options for the
bands where that doesn't work well enough.
Good luck!
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RE: fencetop antenna
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by N3LCW on October 18, 2009
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No antenna = 0 power output means you are off the air and not enjoying amateur radio.
Regardless of the low height of the fence my advice is go for it. Use the entire length of the fence and then run a counterpoise along the ground away from the fence (behind bushes, along the house foundation, etc) and use a remotely mounted auto-tuner at the fence with a bias-T to run power to the tuner through the coax. I did this for a few years using my 4ft wood fence before my Leyland Cyprus trees grew to a useable antenna-hiding height. The tuner loaded the antenna 10-80M and I had a great time, most being spent below 20M. I had some DX contacts and excellent state side SSB, CW, and digital contacts running 100w. No neighbors were the wiser, no interference problems, and it worked better than my attic wire which was prone to noise and lots of interference/TVI.
You can also mount the tuner where the corners meet and load the wires as an off-center fed doublet. Try both to see which works better for you.
The bottom line is you will get better than expected results by making use of your available fence.
If you can plant fast growing trees along the fence they will eventually make great camo for hidden antennas. My Leyland Cyprus grew from 3ft planted to 20ft in 6 years. There are other variations of Cyprus that are sturdier and still fast growing.
Andrew N3LCW
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RE: fencetop antenna
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by K5END on October 20, 2009
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I made a 40 meter horizontal loop on a 6' wooden fence top and worked daytime stations around as far as 1,000 miles away.
But hurricane IKE blew the fence down and I have not had the time to replace it. :(
For close in work or up to several hundred miles, have you considered a terminated folded dipole? I haven't used one, but have read about it and plan to install it on my fence I build in November or December.
It's not the most efficient antenna, and you probably won't work DX with it, but at fence height you should be able to use a single antenna for 160 and 80 meters, plus 40 meters in the daytime at least.
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RE: fencetop antenna
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by KH6TDM on October 23, 2009
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Don't listen to the nay-sayers. Nothing tried, nothing gained. Right after I got my current license (1983?) I strung a piece of #26 magnet wire about 40 feet along the top of a wooden fence, bent it around a tree about 60 degrees, and brought it into the house via a bolt drilled through a wooden plank. The insulators were rubber bands thumb tacked to the fence, and the plank. The antenna was no higher that six feet above the ground, and over concrete most of the way. It was tuned with a itty bitty MFJ tuner into my TS-520S. I made contacts from just south of San Jose California to all over the Pacific basin, to include Australia, Alaska, Hawaii and Asiatic Russia via CW. String your antenna and give it a try. If it doesn't work, you've at least learned something. Good Luck,
73, Terry
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RE: fencetop antenna
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by WB1FPA on October 31, 2009
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Alex,
Build your loop and give a try. I'd tune it right at the loop and bring coax into the station. I use LDG RT-11 remote tuners for my wire antennas. They are placed at an inverted-L feedpoint, and at the end of a G5RV in some trees (at the end of the twinlead). The remote tuner should allow you to avoid the additional loss that hi SWR may result in inside the coaxial feed. The RT-11 is far from the only remote tuner available in 2009. MFJ , SGC, Icom (AH4), and Yaesu (FC-800) and a Chinese company producing the CG-3000 and CG-5000 are all available. The RT-11 is out of production, but you can find them used for under $200. When building your loop simply make the wire as long as possible (pretty much dictated by the shape of the fence). Use insulated wire. Mount the wire for minimum visibility. The HOA people may not know this, but many loops like this are placed just under the ground for 'The Invisible Fence' company's dog collar system. If you've got a dog (are they allowed?) then that could be a 'cover' .
73, Tom Howey WB1FPA (NH)
"Live Free or Die , there are tyrannies worse than death" - John Stark
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