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Author Topic: How about a fence dipole - will it work?  (Read 2859 times)
AB3OL
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Posts: 18




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« on: May 18, 2012, 10:38:29 AM »

Thinking about constructing an HF dipole at the top of a 6 foot high wooden fence.  Have 80 feet in the back which connects at right angles to the two sides which are 72 feet and 80 feet long.  Could I run an 80 thru 10 meter horizontal dipole fed at approximately the center of the 80 foot section in the back? How long should the elements be?  Would I have to run separate wires for each band? 
Have a MFJ949E tuner so I should be able to reduce SWR to acceptable levels.  If fed with Coax should I use a 4:1 BALUN at the antenna feed point?
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KG4RUL
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« Reply #1 on: May 18, 2012, 10:56:35 AM »

Start by going to this page http://transition.fcc.gov/oet/rfsafety/#sec5 and reading the section entitled "
OET RF Safety Bulletins, Fact Sheets, Guides and Reports" near the bottom of the page.  Use the OET Bulletin No. 65 (August 1997) to evaluate your potential installation.  IF it meets the criteria then you can go ahead, otherwise it is a non-starter.
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WB6BYU
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« Reply #2 on: May 18, 2012, 11:35:32 AM »

Will it work?  It might, depending on your definition of "work".

At such a low height (less than 1/4 wavelength across the HF range) most of your
radiation will be upwards, and relatively little will be at angles less than 45 degrees
above the horizon.  This should give usable local propagation on 40m and 80m out
to several hundred miles as long as the ionosphere is cooperative.

The big problem is that the ionosphere rarely reflects RF at such high angles on the
higher bands, so most of your tadiated power wanders off into space rather than
coming back down to Earth where there are other hams.  Not that you won't make
some contacts, but you signal will be much weaker than if your antenna were mounted
higher in the air.  If that is the only antenna you can manage, it is better than nothing.
But an antenna up 10' will be noticeably better, and performance will continue to
increase with height.


You really do NOT want a 4 : 1 balun at the feedpoint.  If you want to use coax feed,
put up a separate wire for each band and trim each one for best SWR.  (Starting with
the longest wire first reduces the interactions among them.)  If you want to use a
single wire on all bands, feed it with ladder line or twinlead to a tuner in the shack.
Try to avoid running the feedline along the fence parallel to one of the elements - that
induces a lot of RF on the outside of the coax and generally makes a mess.
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AA5WG
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Posts: 494




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« Reply #3 on: May 18, 2012, 12:01:33 PM »

WB6BYU gave some excellent advice.

This morning I was testing my 75 meter Center Fed Zepp at a height of 3 - 4 feet off the ground.
I was working stations within several hundred miles on 40 meters CW. 

Chuck
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AD4U
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« Reply #4 on: May 18, 2012, 12:14:17 PM »

WB6BYU is right on! 

"Will it work"? 

It all depends on how you define "work".  With open wire feeders and a good "tuner" you should be able to get the SWR down on most bands to something your transmitter will be happy to operate into. 

Even at such a low heighth you will make some contacts.  By far most of them will be close by.  You will "hear" a lot more stations than you can talk to.

If this is the only option you have for an antenna, then go for it.  However I feel that within a very short time you will get frustrated with HAM radio under these conditions.  Try to put your dipole up higher if at all possible.

Dick  AD4U
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W4VR
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« Reply #5 on: May 18, 2012, 03:01:21 PM »

I did just that several years ago but I used it only for receiving.  It worked well on 160 as a short beverage antenna.  If you transmit into it you end up with an NVIS.
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KJ6TSX
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Posts: 116




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« Reply #6 on: May 18, 2012, 08:03:16 PM »

I currently have a center-feed dipole on my fence it creates a u shape. It works but not very well... I can hear pretty good but cannot get out. If thats all you can do, I guess it would have to do. is there any place to install a mast on the fence to be used in a inverted v antenna? or how about a mast on top of you roof with the wire coming down to your fence with maybe masts Huh
Good Luck
George
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K0ZN
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Posts: 1542




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« Reply #7 on: May 18, 2012, 08:20:09 PM »


Short version:  That antenna is so low that a very high percentage of the transmitter energy will get absorbed in the ground.

You "can" make some contacts, but to give you a very candid response: that would be a VERY poor antenna.

If you have severe antenna limitations, try some form of a vertical or inverted L. Either one will put a LOT more energy into the air than
a horizontal antenna that low.

Respectfully:  pick up a copy of the ARRL Antenna Book and put in some study time on antennas. You will learn that there are probably several
good options available that are going to be much better than that antenna. "Knowledge is power."

73,  K0ZN
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VE3FMC
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« Reply #8 on: May 19, 2012, 04:47:09 AM »

A friend of mine has no trees, no tower. His G5RV is attached to the wooden fence surrounding his property.

He works DX almost daily with CW and digital modes.

So it works, but there will be no comparison to an antenna in the air.

Like I always say "A low antenna is better than no antenna"  Grin
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N3WAK
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Posts: 274




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« Reply #9 on: May 19, 2012, 05:48:36 AM »

AB3OL:  Respectfully, my advice to you is to buy (or obtain from the library) an antenna book.  You ask very good, but basic questions.  There are a lot of great, easy to read antenna books available.  "Simple and Fun Antennas" from the ARRL is one, and is a good book for beginners.  You will end up with a better antenna, and a lot of satisfying, practical knowledge, if you take some time to learn about antennas first. 

In answer to your question, you might try a coax-fed dipole with a 1:1 balun at the feedpoint, or no balun at all there, with legs about 60' long for 75 meters or (my choice) legs about 33' for 40 meters, which would be useable to an extent on 15 meters.  However, far better would be put up even a 20' PVC mast and set your antenna up as an inverted vee.  The 40/15 combination would probably work very well in an inverted vee configuration.  Don't use your 4:1 balun at all for a 1/2 wave inverted vee. 

I wish you good luck and 73, Tony
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W1JKA
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Posts: 1619




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« Reply #10 on: May 19, 2012, 06:31:54 AM »

  WB6BYU and others are right,basically a NVIS antenna.My 40M dipole sits atop a 8 ft.stockade wood fence with a 5% longer reflector on ground beneath it.I use it for close in and in state (0-200 mi.) qrp contacts that my higher wire antennas skip over.No balun,coax fed and works well on 40/30 mtrs.      
« Last Edit: May 19, 2012, 06:33:33 AM by W1JKA » Logged
K7KBN
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Posts: 2782




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« Reply #11 on: May 19, 2012, 06:18:36 PM »

I agree with N3WAK.  You have your license.  Among other things you can do with it is experiment and find out in person what will "work" and how well, and what happens if you change something.

The desire to know something, whether about antennas or automobiles, is great.  It gives you something to direct your actions and studies.  But what if Marconi hadn't TRIED his theories about what would "work" for his first tentative transmissions?  Get a good basic antenna book and study it.  Build whatever antennas you think might perform and try them out on the air.

Do it safely and with due regard for RFI.  And enjoy the hobby.
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73
Pat K7KBN
CWO4 USNR Ret.
W9KDX
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Posts: 770




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« Reply #12 on: May 20, 2012, 06:29:04 AM »

I had a similar problem where I had to mount my dipole between two trees that were just not high enough.  One night I realized I could increase the height of the trees by just adding a ten foot piece of wood to each tree.  That got me 8 more feet.  You could consider adding posts to the ends of your fence and could increase the height with very little change to the existing structure.
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Sam
W9KDX
K3AN
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Posts: 787




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« Reply #13 on: May 21, 2012, 01:05:22 PM »

Height is everything- if your antenna didn't come down in the last storm it wasn't high enough. That said, you CAN make short range contacts with a low wire. Years ago I finally moved to a QTH that was suitable for hamming. I was so intent on getting back on the air right away that I stapled a wire to the ceiling joists of the basement (where the shack was being assembled). The wire was about four feet above average ground, and inside the house! On 40 CW I worked out to about 400 miles, but all the signal reports I got were pretty lousy. SSB contacts? Forget about it.

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