20m Vertical Antenna From Scraps--1/2 Inch Copper Pipe or Rebar?
I'm looking to build a 1/4 wave vertical antenna for 20 meters from some scrap metal that's laying around. The antenna is to go on a sloping steel roof that's up good and high, so there's the ground plane. Now for the monopole I have rebar and copper pipe available. The copper pipe is in pieces, so I would need to sweat them together to get to ~16 feet required, but there are several long pieces of steel rebar on hand that would only need cut to length, so my laziness is tempting me to use it.
Is there any reason to not use the rebar?
Really, go all copper with it. You don't want to involve dissimilar metals, that can lead to problems down the line.
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First thing I see about using rebar is the weight. Any wire just about will work as an antenna. You could plant the rebar in the ground with a sledgehammer and use them as a ground stakes. Make sure you drill a sideways hole in the top of the rebar beforehand to attach some radials. You can likely get a decent ground field with radials tuned by frequency.
If you want to go that route I know there are very tough flagpole sockets that would work nicely to hold some pvc pipe. Simply attach about 17 ft. of wire to the PVC pipe from the bottom upward. Attach the wire with cable ties There is your radiator! Radials are the other half of the antenna, basically. My calculator suggests that you somehow attach a few 17 foot radials to the ground side of the antenna. Solder the radials to the outer braid of the cable. Once you get the antenna made up and SWR tested get a nice big piece of teflon bar stock, maybe 3 ft or so from a wall, hopefully. You can use other types of nonmetallic to make the upper anchor. If Teflon isn't available use PVC and configure it as the upper wall support instead.
Now, whatever you do, don't get advice happy. A lot of guys on here see all our posts, and you can bet they are thinking as well.
I know antennas are almost a mystery to new hams. And again, the Bottom line is this...You will be heard on 20 if you put this puppy on the air.
Last edited by N8WWM; 05-29-2012 at 07:45 AM.
It would really help new amateurs who want to build antennas to find an older copy of the ARRL Antenna book. There are too few explanations and too little data in the current editions. By old I mean say pre-1989. Cruise hamfest tables and Ebay, there are plenty available pretty cheap.
The resistivity of rebar is higher than copper and thus more lossy.
Originally Posted by KF5NZF
73, Cecil, www.w5dxp.com
The maximum power transfer theorem works just as well for a non-resonant antenna as it does for a resonant antenna.
I'd say probably quite a lot more lossy, especially since only the outermost skin of the material is used to carry RF current and that will become "rust" pretty quickly with rebar. If I wanted to use rebar for a radiator, I'd weld it and paint it with aluminum paint or something.
Originally Posted by W5DXP
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-- George Bernard Shaw
Get that antenna at least 16' high at the base and it will be an excellent performer. In the old days it was called a ground plane and was the DX king before everyone had a yagi. Use the steel roof as the ground plane but also string out 4 16' copper wire radials to the corners just in case the metal isnt well bonded.
As a kid in HS I built a copper pipe 20M GP with pipe radials using a short horizontal and then a 45 degree sweep down. It was mounted to the rear of the garage on a steel mast just long enough to get it to 16-17' up. Worked absolutely gangbusters for DX and long haul US from NY.
It would be my choice today if necessary.