Wednesday, October 19, 2011

How to build a mono band 80 meter 1/2 wave wire antenna

How to build a mono band 80 meter 1/2 wave wire antenna

I decided to build a 80 meter, half wave wire antenna that takes full advantage of the phone portion of the band known as 75 meters, also this antenna will not need a tuner.  I looked at some of the various web sites and still did not get a single idea as most of the designs were attached to trees, houses, or towers.  There are no big trees in my back property and the ground is very hard and rocky, so any design that uses trees or buried ground radials as part of the antenna design was out of the question.... besides I hate to dig.

I did not want to be one of those hams that has wires all over the place.  I also did not want to have to hand out special, designed maps to family and friends so they could navigate safely through my back yard without the possibility of being in-tangled in wires.  I wanted something less obtrusive, something seamless, something that looked neat and most importantly something that worked well.

Be sure you pick a place for your New 80 Meter Wire Antenna that is not near Power Lines, Metal Roofs, Buildings, Trees, other Antennas etc.. I have seen times when I have made a complete resonate wire antenna and then gave it to a friend only to find out it did not work for him with out adding additional length or cutting length because there were way more obstacles on his property than I have on mine.  There are many factors that will affect your wire antenna from functioning optimally..... your best bet if you can, get it up as high as possible, keep your antenna away from everything, like in the middle of a large dirt field.  Have Fun & Good Luck with your New 80 Meter Wire Antenna!

My final 80 meter wire antenna design is simple.... and it works great!

1 - 20 ft Flag Pole Kit ( Lowes $150.00 )
200 - ft LMR 400 Flex ( Ham Radio Outlet $258.00 )
2 - PL-259 Amphenol Connectors ( Ham Radio Outlet $4.98 )
1 - Alpha Delta Center Connection Package with the 2 tie off insulators ( Ham Radio Outlet $24.95 )
500 - ft Green 14 Gauge Solid Core Plastic Shielded Wire ( Lowes 500 ft Spool $80.00 )
2 - 50 lb Bags of Quick Crete Cement ( Lowes $10.00 )

1st  I cut my wire at 2 lengths of 70 ft each.  This gives me an Idea ( when stretched out ) on where exactly my wires will be tied off.  This also enables me to hook the wire ends to a fence or what ever I know exactly where the completed antenna tie off rope will start.  We know that the formula for a 1/2 wave antenna is 468 divided by frequency.  I am a General Class Ham and I want to use phone ( or voice ) on the phone portion of the band also known as 75 meters which is 3.800 MHz: to 4.000 MHz: I will make initial measurements for the middle of the band which is 3.900 Mhz.
So, 468 divided 3.9 = 120 ft ( that's 120 ft of total length...don't cut anything yet!! )
Now!!! this it not my first wire antenna rodeo, and I know that if the formula says 120 ft you better cut about 140 ft ( 2 - 70 ft same length pieces ).  This way you will have enough wire to tie the 2 insulators and the center feed point ....also you will have enough wire left to trim, so you can get to your desired frequency range of 3.900 MHz: without any need for an antenna tuner.

I like to mock up everything before any final assembly.  That way I do not make too many mistakes when it comes to the final assembly process. For mock up I was able to use an old patio furniture cement umbrella base to build the the flag pole on worked great and the base of the flag pole fit perfect over the cement umbrella base stub.  When I use coax from my shack to feed my antennas I always wind a RF Choke 6 inches wide about 8 turns. I use 2 ball bungees to hold the winded coax coil and try to get it near the antenna, in this case it will be on the flag pole itself.

I also like to goop up everything with 20 year clear silicone sealer ...I never have those problems that others do because of rain getting into their coax, antenna or corroding connections.  This silicone sealer dries clear so it will look better in a few hours.  I also slipped on 2 small sections of automotive vacuum line and used the silicon sealer as a glue, just incase the wire was to move around and rub against the flag pole causing a possible short by wearing into the wire coating.
        I try to think years down the road, in design and construction of any antenna.

2nd Dig the hole, put the Flag Pole into the hole, level and add cement. Be sure to fully assemble the flag pole kit and level prior to adding any cement.

IMPORTANT: I know this sounds crazy but do not, just rely on your level, as you will see with your eyes, if you stand back you will notice the pole will lean one way or the other.  I used the level then looked at it from each direction. I still had to move it back a bit because it was noticeably leaning.
Remember it does not matter if your level shows perfect...stand back and look at the Flag Pole fully assembled to be sure its straight.  This has to do with, if your bubble is less than 100 percent perfect in the viewer of your level on the first 2 pole sections as the additional pole sections are added it will magnify the smallest amount off, causing a noticeable lean.     the worst thing is to have your friends say it's crooked, it's leaning

3rd  Wait a day or two before trimming your wire.  That way you do not disturb the fresh cement you just pored.

 4th  Raise the antenna to its final installed height on the flag pole and start to trim both ends of the wire evenly, constantly checking your SWR Meter or Antenna Analyzer.
IMPORTANT:  either use a External SWR Meter or a Antenna Analyzer for checking your match during this trimming stage.

Do not rely on your Ham Rig's Automatic Antenna Tuner.  The new ham radios with built in antenna tuner's will tell you your receiver "has a good match" but in the real world, we first want to have a resonate antenna and there is simply no substitute for an external SWR Meter or Antenna Analyzer at this trimming stage.  I trim each end, come into the shack, push my automatic antenna tuner on my rig, then calibrate and check my external SWR Meter while finding my flat match spot.  I do this several times till I finally get real close to that 3.900 MHz: spot.  Now!!!, like I have said "this it not my first wire antenna rodeo" I have actually cut my wire antennas to short in the past in an attempt to be more than be careful. If you cut it too short you will have to solder another length to each end or replace the whole wire and start the trimming process all over..which sucks!.
After I completed this antenna my 75 meter SWR readings are:
3.800 MHz = 2.0
3.900 MHz = Flat match
4.000 MHz = 2.1
The signal reports and receive are wonderful !!

The best part about this 80 meter antenna is not that it works great, looks cool, needs no tuner and will last for many years...... is it also comes with an American Flag.

We fly the American Flag in front of our property 24 hours a day 7 days a week and wind up replacing them about every 4 months. So it's extra nice to have another back up.

Life is about learning, and helping other as we go.

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