[RVRC] Fwd: 40 Meter Dipole antenna

Thu Nov 2 11:56:46 EDT 2017

Robert,

Thank you for sharing with the club. 

73, Drew

*sent from my ,iPhone 7+-forgive typos*

Begin forwarded message:

From: Robert Oppen <oppen.robert at yahoo.com>
Date: November 2, 2017 at 9:27:06 AM EDT
To: E Drew Moore <w2oudrew at gmail.com>
Subject: Re: [RVRC] 40 Meter Dipole antenna
Reply-To: Robert Oppen <oppen.robert at yahoo.com>

Hi Drew,
The antenna discription is confusing... My phone number is 973 699 7355
Take 1/2 wave of wire (468 divided by frequency in Mhz )  This is the radiating element and is hung vertical from a tree,  I use string trimmer line as it acts as both an insulator and is very abrasion resistant. String trimmer line is impossible to tie in knots.  I use swage crimps available in small quanities at Tractor Supply in electric fence section.  It is not perfect but is a simple solution and has the added benefit of being streamlined to be able to pull through the tree branches ..These swages are aluminium and easy to crimp using a hammer against a sledge hammer.  No need to buy a $60 crimper which I have and you can borrow if you are a purist.
The antenna is end fed via  a quarter wave section of 450 ohm ladder line...  again 234/frequency in MHz ..An end fed antenna presents an fed point impedance of like 3000 ohms  The quarter wave matching section will transform this high impedance to something more manageable.   Connect one leg of ladder line to antenna   let the other leg float free...  This quarter wave part is suspended horizontal on fiberglass electric fence posts... Again at Tractor Supply... If you have a 110 foot tree then by all means hang it all vertical.  The attachment point of the coax is at the other end of the quarter wave.  This you can determine with an antenna anylizer   Beyond the antenna connection point you will need about another foot and half of 450 ohm line short circuited at the very end.  I made my last piece out of bare household wire as it allowed me to fine tune the connection point of the coax with alligator clips.  This stub is more like an inductive shunt to tune out the capacitive reactance of the antenna.
I probably have you completely confused by now, so Iet me say this is not an original design. Visualize it is just a scaled up version of a two meter J pole.  Most of the radiation is at a very low angle, 9 degrees   I believe the matching section radiates but the received local signals are way down like S3...  Finally the big challenge will to remove the RF from the shield of the coax . this I accomplished by 3 turns of coax rg58 through 3 clip on ferrites ..  RG8 coax will require physically larger ferrites...
When the antenna was hit with lightning the coax was disconnected from the flex radio. The entire antenna was vaporized only small plastic squares of ladder line remained...Under the vertical part was a hole blown in the dirt about the diameter of my arm and maybe 3 feet deep. Induced voltages caused a lot of damage.  Flex radio was nailed because it was still  connected to  ground and also the modem destroyed via computer destroyed via fire wire to flex radio  destroyed  Also destroyed was a DVD player A TV antenna amplifier and our dish washer.  It started a fire which smoldered and went out that burned up a topo map and also blew up some wire nuts in junction boxes and blew the plastic cover off a wall switch.  What was odd is my tower 70 feet high and heavily grounded and 30 feet away wasn't touched Go figure.
Bob Oppen  NE2C

On Tuesday, October 31, 2017 10:50 PM, E Drew Moore <w2oudrew at gmail.com> wrote:

Hi Bob, I would love to see the specs. 

73, Drew

*sent from my ,iPhone 7+-forgive typos*

On Oct 31, 2017, at 8:33 PM, Robert Oppen <oppen.robert at yahoo.com> wrote:

I agree,  an excellent article and I think you should submit it to QST.  I myself built a vertical dipole end fed with a quarter wave of 450 ohm ladder line.  Pro, an outstanding dx antenna, I would start working Europe four hours before sunset..  Con,  requires a tree at 75 feet plus to hang it from.  With this antenna I confirmed 140 countries on 40 meters before it was vaporized by a lightning hit.  I could give you the specs if you like to make one.
Bob NE2C

On Tuesday, October 31, 2017 2:35 PM, E Drew Moore <w2oudrew at gmail.com> wrote:

Good write up John. 

73, Drew

*sent from my ,iPhone 7+-forgive typos*

On Oct 31, 2017, at 10:32 AM, John Manna <asajlm at verizon.net> wrote:

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.
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