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Dipole with 102s

T

The Jerk

Active Member
May 6, 2008
647
66
38
Reading, PA
Well, after going through the thread about building a dipole out of different materials, I broke down and bought two Radio Shack 102s to build one. I was actually surprised Radio Shack had not one, but four 102s IN STOCK! $20/each.

I have the materials to make up the female 3/8x24 connectors with bolts (will allow me to use ring terminals for both center and shield) in place of the SO. I intend to use two mirror mounts bolted back to back and flipped (sort of like a "Z"). This will place the threaded ends about three inches apart.

This will allow me to "clamp" a piece of 1" pipe (yet be electrically isolated from it), and erect it probably 10 or so feet in the air (whatever a standard length of black steel pipe is). If I can get ten feet above the peak of my shed, that would put the tip around 25 to 30 feet above the ground.

I guess this is all experimenting, but since the 102 is "short", will the antennas need the added 6 inches to make it a 108?

Now I need to buy around 50 feet of coax...nothing big, just running a barefoot Grant XL. I may actually set it up in the shed temporarily just to see if it works!

I will take pictures next week when I get it together and possibly put in the air.
 

The Jerk said:
Well, after going through the thread about building a dipole out of different materials, I broke down and bought two Radio Shack 102s to build one. I was actually surprised Radio Shack had not one, but four 102s IN STOCK! $20/each.

I have the materials to make up the female 3/8x24 connectors with bolts (will allow me to use ring terminals for both center and shield) in place of the SO. I intend to use two mirror mounts bolted back to back and flipped (sort of like a "Z"). This will place the threaded ends about three inches apart.

This will allow me to "clamp" a piece of 1" pipe (yet be electrically isolated from it), and erect it probably 10 or so feet in the air (whatever a standard length of black steel pipe is). If I can get ten feet above the peak of my shed, that would put the tip around 25 to 30 feet above the ground.

I guess this is all experimenting, but since the 102 is "short", will the antennas need the added 6 inches to make it a 108?

Now I need to buy around 50 feet of coax...nothing big, just running a barefoot Grant XL. I may actually set it up in the shed temporarily just to see if it works!

I will take pictures next week when I get it together and possibly put in the air.
Click to expand...
my homemade wire dipole came out to about 15 feet,i think that was channel 20.however if you need to add length try this cut the ball off at the top and crimp on 3/8s copper brake lines!i did this to my wilson 1000 to lengthen it for other frequencys.
 
well hook up an swr meter and cut or lengthen it for whatever frequency you want it in.
 
Will it work? Absolutely! Basically just using different conductors for the antenna elements.
Here are the 'catches'. A 'flat' dipole never has an input impedance of 50 ohms, more like 75 ohms (not exactly, but close). 'Drooping' those two dipole 'legs' tends to lower the input impedance. Depending on everything around the thing, the amount of 'droop' varies, so experiment till you find the 'sweet spot'.
An SWR meter can tell you nothing about resonance. It can only make comparisons of impedances (and that set of impedances has to have 50 ohms on the input side of that SWR meter, otherwise, all bets are off). The easiest/simplest/cheapest way of determining resonant lengths is by formula. Set the length by the usual formula that takes things into consideration (468 instead of 492) and you're close enough. Then work on matching impedances, which is a totally separate thing. When you get both of those characteristics right, then you've got a 'tuned' antenna.
Did things just get harder? No, but understanding what you did might have. It's still the same as it ever was.
- 'Doc
 
I assume by "droop" you meant the antenna is horizontal...I was intending to mount it vertically, will that make a difference?

I can mount it either way just as easy, but figured to talk locally, vertical would be better? If I mount it horizontally, it will only be around 18 feet high to the top.
 
I know the calculator is to get you close...but the 102 with the short mount will be nearly 103. Through the calculators, I should be around 103.25. However, my mount will have around 2 inches of space between the two antennas...essentially making the overall length grow to 208.

So I guess that will make it a tad long, but probably still useable.
 
Horizontal or vertical, will it make a difference? Sure. That 50 ohm match will be just a bit harder to get close to, can't play with the droop to get it right. Actually, you can, but it isn't going to 'look' very good so I figure most people won't do it. That's basically what you are doing when you play with the 'angle of the dangle' with radials on a groundplane.

Length. That can be done several way. You've got some 'play in the amount of wire that makes connections to those two 102" whips. You can use that to make up for any shortness, just make them a little longer/shorter/whatever.

There's lots of 'wiggle room' with a dipole, they aren't all that 'exact' or critical in shape, sort of. They tend to work no matter how you shape them or position them. That doesn't always mean they will work well, but nothing does all the time anyway.
- 'Doc
 
So the feed point of any dipole is 70ohm, and you adjust that by the angle of the legs?

I could also turn this into an inverted V also...I guess I am trying to avoid being directional since I won't be able to turn this easily.
 
What's the angle between the two legs of an inverted V supposed to be? I am guess it typically around 90 degrees, or each leg roughly 45 degrees from horizontal?? With varying degrees in between (whatever works)??

I was reading and I assume the "angle" discussed is the one from vertical, and not the angle between the legs?

I'm probably better off with an inverted V configuration.
 
Don't get too worried about a dipole being directional. That depends a lot on how high the thing is. Until it get's above about 1/2 wave length above ground, it isn't all that directional at all. Even if it's very high, that directional thingy is never a 'spot light', more like a 'flood light', pointed in two directions.
Inverted 'Vee's.
There are a lot of antennas called inverted 'V's, but are really 'droopy' dipoles instead. An inverted 'V' has an angle between it's legs of 45 degrees, and a feed point impedance other than 50 ohms. They are typically fed with ladder line and a tuner, not coax. They cetainly do work, but there's really not all that much going for them.
The 'typical' inverted 'V', or that 'droopy' thingy, is actually what you see most of the time. That angle between it's legs is whatever it takes to get the input impedance close to 50 ohms. It isn't difficult to do at all, just takes a little time to find the right 'angle of the dangle'. They are also typically horizontally polarized so might not be your best bet for what you want to do. Most CB antennas are vertically polarized, that's because of there being a lot of mobiles which are also vertically polarized. Want both polarizations? Easy! Just make two of the things. (I said make them, not buy them. If you can't find enough wire for a bunch of dipoles for less than you'd give for one bought one, you just ain't trying.)
The hardest thing about using dipoles is finding a place to hang them. Got trees??
- 'Doc
 
dipole

102 in. each side works perfect for me as an inverted v on 11 meter. Use a 1:1 balun as the feedpoint and you will be good. This comes from experience AND the 1999 ARRL handbook.
 
I don't know why I'm worrying about polarization...its easy enough to change this setup, so I can take a previous suggestion, and experiment. I figure I'll try vertical, then switch things up once I get talking with the locals.

I had some questions related to this install.

The first is the coax...I'll be buying new. This setup will be used with a barefoot (tuned/aligned) Grant XL. So power handling isn't really an issue, but I'm not sure attenuation will be with the lower power? I have two placement options, one (the preferred) would be to the adjacent shed and the other would be the house eave. Obviously the house would have the length around 25 feet, the shed will be between 75 and 100 feet (will be cut to length). While LMR-400 Ultra seems to have the best number, it just looks BIG at nearly 1/2 inch...I'm thinking LMR-240 Ultra at 1/4 inch?

The second is lightning/surge protection...is it worth looking into, besides grounding the mast and radio? I was looking at a cheap air core suppression deal that went in the feed line. I know nothing (cheap) will stop a direct lightning strike.

Lastly, relating to the first question, if I mount the antenna to the shed all of the antenna will be above any close obstructions. If I mount the antenna to the house, only the top section of the dipole will extend past the side of the house. Both setups will net about the same overall height above ground surface (around 18 feet). How would that affect the performance, having the lower section below the roof line?
 
What coax to use? The 'best' that you can afford within rea$on. Nothing wrong with using LMR-400, or the '240, but there are others to pick from. There is going to be some losses no matter what you select. The 'trick' is to minimize those losses as much as -practical-. Larger sized coax tends to lessen the resistive losses, doesn't necessarily mean a lessening of other kinds of losses. Take a look at the loss per 100 feet of coax, use the one with the least amount of loss. Pay attention to your wallet, it'll make about as much difference as anything will, and you'll feel it a lot more than some undetermined amount of loss that you don't even know was there.

Safety, lightning etc. Nothing wrong with minimizing that as much as possible. A dipole antenna requires no grounding for radio purposes. For safety purposes, the simplest solution is to disconnect the antenna and ground it, or just throw the end of the feed line outside. Considering the height you will be putting it, does your house, trees, etc, ever get hit by lightning? Maybe something 'close' at one time or another? If your answer to either of those is 'no', then I wouldn't waste a lot of good worry on it. I'd still disconnect when applicable though.

How close is too close? Uh... good question. Wish there was a good answer. Best is when an antenna is not close to anything (good luck!), close defined as in miles? From there, it's a compromise to some degree. 'Further' away is almost always better. Do what you can, then live with it. 'Above' stuff is better than 'beside' stuff. It's a good idea to think of an antenna as one lump, not in pieces. Getting the whole thing away from stuff is better than just part of it away from stuff. But, sometimes getting just a part of it away from stuff is more practical. Which of the two locations you gave would be 'best'? Beats me! Try them both, see for your self.

Nothing very specific in all that, huh? Sorry 'bout that, any more specific and it turns into an opinion, which isn't all that good, sort of. Good luck.
- 'Doc


Oh, one 'opinion' that I will 'share' is that 'spark-gap' type lightning arrestors are totally worthless. Don't waste your money. Why? Cuz by the time the voltage is strong enough to jump that gap, it's already inside the equipment you wanted to protect.
 
Using cheap coax is false economy (IMHO). Get the best and you can use it forever (almost) on all your future antennas. Get 100' and you can use it at any height (up to 100'). Coil the unused part on the ground (etc)

"The chain is only as strong as it's weakest link."
 

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