[RVRC] 40 Meter Dipole antenna

[John Manna]

Ham radio 40-meter dipole: your first antenna
Posted on February 17, 2011 by Dave
Okay, as I noted before, my first antenna wasn’t a dipole. But I’ve worked with lots of new hams, and a dipole is cheap, quick, and easy to construct. But, you say, dipoles are only single-band. Yep, and I recommend the 40m band.
Why 40? For several reasons:
§ The antenna isn’t really all that long, just 66 feet
§ The band is often open in the evening
§ Unlike 80m, which is full of nets, 40m is more open and you can call CQ and hope to get an answer
§ If you want to play with digital modes (such as radio-teletype or PSK-31), there’s a fair amount of activity
§ If you want to try out your Morse code, there’s a spot on the band where people use slow code.
So, what does a 40m dipole consist of? Well, 66 feet of wire, split in the middle. You can use pretty much anything for wire as long as it’s a good conductor. I don’t recommend house-wiring type wire because that’s soft-drawn copper and will stretch, but if that’s what you’ve got, by all means use it. Radio Shack sells bare stranded copper wire made specially for antennas, but it’s a bit pricey. You can use either insulated or not—it doesn’t matter.
An insulator goes at each end and in the middle—electric fence insulators work fine and are dirt cheap. To each insulator you’ll tie rope to hold up your antenna.
The RF feed point of a dipole is in the middle, so there’s 33 feet of wire on each side. Yes, this is a balanced antenna, and coax is unbalanced, but I still recommend you solder the coax directly to the antenna. The shield goes to one wire and the center conductor to the other. This is a tried and true method and it works. You’ll want to cover this well with electrical tape and also make sure that the coax doesn’t hang by the soldered connections, but rather by something physically tied to the center insulator.
There are two ways to mount the antenna. The first is to get each end up as high in the air as you can so that the antenna is a straight line (it’ll droop a bit in the middle). This is the standard configuration. The second is to get the center of the antenna up as high as you can and pull the ends out as far as you can get them. This creates an inverted V configuration. Either works.
Safety comes first in either case. Make sure the antenna is high enough in the air no one will run into it. Further, if you use bare wire (which is fine, by the way), the ends of the antenna is where the high voltage is, so keep these up and out of reach! And do not go even close to electric utility lines—getting to close creates noise anyway. Sadly, hams have been killed ignoring this guidance.
How long should the coax be? The standard answer is “as long as it needs to be to reach the transceiver!” And you can take that literally. There are no restrictions. And on 40 meters (7.1 to 7.3 MHz), coax line losses are so low that they aren’t worth bothering with. What kind of coax? I happen to be particular to RG-8X, but RG-58 works too. The stuff you get at Radio Shack kind of skimps on the braid coverage, but will work.
Put a drip loop where the antenna enters your house. Slamming a window on the coax is not a good idea as coax really doesn’t like to be squeezed. I bring my coax through a vent into the crawl space, then I pull back the carpet and drill a hole near my desk. I push the coax up through this. Pushing the carpet back keeps cold air from coming up through the hole. Make sure you have enough slack that you’ll be able to move your transceiver around easily.
Now is the time to solder the connector, usually a PL-259, to the end of the coax so you can connect this to your transceiver. Don’t scrimp on the connector—get the silver ones like these at Ham Radio Outlet—order several as you’ll end up using them! And make sure to order the reducer—RG-8X and RG-58 take different reducers. You’ll want to make that connection where you can reach it easily, as you’ll want to disconnect the coax when thunderstorms are near. In fact, during the summer, I leave my antennas disconnected unless I’m actually operating, and I don’t operate during thunderstorms! Yes, I’ve had lightning hit an antenna and yes I had to repair some damage.
And that’s all there is to it! This is a great first antenna! Your next step, after getting some air time, will be to insert some traps in that dipole so you can use it on 20-meters. Oh…you can use the 40-meter dipole on 15-meters without modification.
Do you need an antenna tuner? Nope. The antenna is close enough. If you happen to have a tuner, then by all means use it. In another post I’ll talk about trimming up that antenna so it has a low SWR, but with a simple 66-foot antenna, you’ll be close.