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Dipole Coaxial question.

B

brownstone101

Member
Nov 10, 2010
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hello.

ive been reading and watching youtube vids, and reading some more about dipoles. i plan on making one next week for a base station so i can talk to my friend down the road. and or get better healthy setup for a base station.

my question is about grounding.. the past antennas ive used i have had to drive a wire thin rod into the ground with a wire lead to the ground braid of the coaxial just so my SWR wouldn't pin high from no ground plain.

now i read about the dipoles and see you take the ground braid attach it to one "leg" of the antenna. and attach the center (hot) core for the other leg. then cut or fold it on its self till the proper leg length is achieved for that frequency, but where is the ground if the other leg has it? or what am i missing here.

thanks for any help. and i have been doing my own research and, i could probably build 20 different kinds of dipoles now, but i just don't understand were the ground is.. or if the lengths some how act like a ground?
 

Dipoles don't use a ground as you would expect from a vertical antenna. They are two separate and equally measured length wires that are in phase for that frequency that amount to a 1/2 wave; each side is therefore 1/4 wave. But they act together and in harmony.

The trick to make a dipole work is to make sure that each side is the same length - even it you must change it to tune it correctly. Another is to make sure that there are no objects within 10 ft of any part of the wire - for best results. The center mast can be PVC; as this is cheap to do this way. You can run it level to the ground or even as a inverted "V" shape; so long as the angle is not less than 90 degrees. 18 ft up or higher for best results. You can even suspend it vertically - if you can keep the coax feed to the wires at a 90 degree angle to the ground surface/horizontal and keep that length at ~5 feet . .
 
That 'ground' when used in talking about an antenna means the 'other half' of the antenna. That can be a number of different things, just depends on what kind of antenna it is. I honestly don't know of any currently made commercial antennas that really need a 'ground' as in ground wire, other than a 'safety ground' (lightning, etc.). A safety ground isn't the same as an RF ground at all. The metal in a car's body is the 'ground', or 'other half' of a typical mobile antenna. A 'groundplane' antenna's radials are it's 'other half', or 'ground'. One side/leg of a dipole is the other half's 'other half', or 'ground'. The only antenna type I can think of that requires an RF 'ground is a random length, or long-wire antenna.
Another little 'gem' is that a single ground rod is a worthless ground for either RF or safety purposes. Lot's of controversy about that, but it's still a fact.
Another of those 'gems' is that RF is alternating current. That means that every half cycle the two 'halves' of that antenna, the sticking up part and the 'ground', swap jobs. The sticking up part becomes the 'ground' and the 'ground' becomes the 'hot' element.
:)
- 'Doc
 
yeah it wasn't for safety it just helped bring the swr from 3.0 to 2.5 with the little ground rod....also that antenna was all kinda of messed up. even on my car the swr was 3.0 till i put a 4' firestick on. then it was 1.3 swr.

so the earth was acting like the other half of the antenna. but with a dipole the other half is just the other half.
 
Google dipole, thousands of hits.

hamuniverse website in the antenna section has a lot of info and ideas on how to build and install dipole antennas.

google dipole calculator, it will do all the math for you on the frequency you want to build the dipole for.

they work well, not as good as a yagi and better than some commercial verticals sold on the market.

Have fun.
 
yeah ive searched many calculators and sights and ways to build them. i have an idea for the use i plan on using it for. but i was just confused with the why do you ground it to your car or the earth, but a dipole is not grounded. but im understanding it now some. thanks
 
brownstone101 said:
yeah ive searched many calculators and sights and ways to build them. i have an idea for the use i plan on using it for. but i was just confused with the why do you ground it to your car or the earth, but a dipole is not grounded. but im understanding it now some. thanks
Click to expand...

Glad you are starting to understand, the principals of different type of antennas.

Lots of great people here who have lots of helpful advice.

If any more questions or something you just do not get as some reference material may get scientific in describing the principles of how an antenna works, just come on back and ask away.

But most of all have fun building and trying out your antennas. Once you get bitten by the antenna building bug it will stay with you for a long time.
 
yeah i was using a piss poor example of an antenna that didnt work on a car and hardly worked as a base station antenna. it was an 500w weather band by some off company. high swr and nothing seemed to fix it. so instead i got a firestick II for my firebird 1.3 on the radio and i have great range with my cb. needs an amplifier though i had a HR2510 but its not feeling good right now so i went from 10watt to 4watt. :bored:

my base station right now i have a uniden 510XL :laugh: with an base loaded mag mount antenna stuck onto a metal chair outside my window. that i pushed the legs down a little with. then thats attached to an archer antenna matcher. then thats attached to a SWR meter THEN attached to the 510XL..... i have a 1.3 SWR at the meter and a hot 3.4 from the matcher to the antenna.

that antenna was a 1.5 swr antenna stuck on 79 camaro but because it doesnt have "its other half" its outa whack, but the matcher gives it the correct 50ohm load to the radio so the radio is safe and i got a 30db signal at my friends house. about half a mile away but were excited to make our dipoles and see what we can do.
 
you will have fun and definately get beter reports than with the mag mount tuner.

Get the dipole as high in the air as you can, and most of all have fun
 
yeah it will be fun. we plan on making these as cheap as we can but usable for our short mileage use. we just want correct swr readings and the ease of mounting them like cloths lines.
the area we live in is CB dead basically. and down the road we have i dont even know.... its a dead spot for everything, cell phones, cb, radio anything... like a huge magnetic chunk of iron under the ground for all we know its just dead, but for what were doing that crap mag mount on the chair lower than my house and covered by other buildings still gets 30db at his house so any dipole will work.

i bet if down the road a with a 200w amplifier we could get some better use because we are on a ridge line overlooking 2 different valleys and can see straight line 4 different towns but we only receive each other right now.

were going to get a 50' spool of 14ga speaker wire for 12$ and make our selfs 2 sets of dipoles. being speaker wires are 2 sets of 50' so we have 100' of wire that is insulated. then i have plenty of rubber bushings and materials to make a set of dipoles.
 
A 'choke' 'cleans' the CMC (un-wanted) currents from the outside of coax. (CMC = common mode currents, unwanted 'stuff' except in particular circumstances.)
A coaxial choke is just a coil that inhibits the passage of alternating currents, which is what those CMC's are. The size of that coil is frequency dependent so the number of turns in that coil/choke will determine if there's any 'inhibiting' or not. That choke or coil only affects what's on the outside of the coax, not on what's inside that coax. That means that they typically don't make any difference in what's transmitted.
That's not a very comprehensive, or definite answer, but it's understandable to most people where the math might not be.
Any help?
- 'Doc
 
To continue on to what DOC said,
Coax is an unbalanced transmission line ( carries thr RF from your radio to the antenna) Your antenna is a balanced load, each side is equal in length, or should be.

to transfer RF from an unbalanced to balanced load their is this wonderful device called a BALUN (BAL= balanced UN= unbalanced)

Thsi does exactly like the previous post stated, stops the unwanted CMC on the shield of the coax, major cause of RFI and TVI.

Google is a great tool google BALUN and see how many thousands of hits you get, you can read till your eye's turn red.

Long story short, is it needed? Nope, but neither is changing the oil in your vehicle, it stills shows oil on the dipstick right? Why bother changing it?

The balun does all that hs been stated , so best bet, put one in your dipole or buy a commercial one 1:1 current balun. the ugly choke will work just fine and costs less than the commercial version.
 

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