A Dipole for your CB Base Station?

[W5LZ]

I've used plain old nylon 'trot-line' cord from Wal-Mart for years. Strung all kinds of wire antennas with it. Makes a pretty good insulator too. Also found that the 'colored' cord, green I think, makes for some really weird things sometimes. (The dye used appears to be conductive when wet, bummer!) It do stretch, and it do contract when it breaks. I'd advise NOT being on the other end when it finally snaps, it's like being on the wrong end of a bull-whip, and hurts like @#$$!
- 'Doc

 

[dudmuck]

balun or not

To answer the question whether you need a balun or not:
Balun - AA5TB

It is often found that a balun is not necessary and everything works just fine feeding the balanced antenna directly with coax cable. When this is possible it may be found that the feedline is an odd multiple of 1/4 wavelength. In this case the transmitter end of the feedline is usually grounded and up from this point on the coax 1/4 wavelength or a multiple thereof will appear as a high impedance. When this high impedance point occurs at the feedpoint chances of common mode currents are low. Rather then take any chances it is often recommended to use a balun.

So since this is CB, you could feed it with 27 feet of coax, or 45 feet, etc, and still be fine without a balun.

 

[BOOTY MONSTER]

how can the length of a wavelength of coax be recomended without knowing what coax is used ? i was under the impression a half wavelength of coax was what works best or multiples of it ?

 

[W5LZ]

Good question. If you'll take a good look at 'AA5TB's statement, it isn't really a statement of 'fact', it's only a '-may be found-' thingy, which is meaningless. It also may be found that using coax of a blue color protects your antenna from pregnancy. I would worry about that much more than using a balun at HF.
Using any particular length of coax is only beneficial if that particular length is useful for some particular reason. One characteristic of an electrical half wave length (any multiple) is that the impedance at one end of it is typically seen at the other end of it, at a particular frequency. In other words, it's like it isn't even there. That 'even/odd' multiple thingy pertains only to an electrical quarter wave length. 'Even' multiples turn that 1/4 wave length into a 1/2 wave length. 'Odd' multiples means it's still a 1/4 wave length. Electrical quarter wave lengths have the opposite characteristics of an electrical half wave length. That make sense?
- 'Doc

 

[BOOTY MONSTER]

maybe something is sinking into my thick head because it made sense to me . thanks doc.

odd 1/4 wave bad !!!!
even 1/4 wave good !!!!!
even 1/4wave = 1/2 wave . good !!!!!
as frankenstein might say .

 

[dudmuck]

how can the length of a wavelength of coax be recomended without knowing what coax is used ? i was under the impression a half wavelength of coax was what works best or multiples of it ?

Thats a really good question you ask.

The answer is that common-mode currents on coax are on the outside of the coax shield, so the kind of coax is irrelevant. Velocity factor only applies to signals on the inside.

Possibly that half wavelength is best you speak of is the 18foot CB coax myth.

 

[Master Chief]

Electrical quarter wave lengths have the opposite characteristics of an electrical half wave length. That make sense?

Nope. Please explain "opposite".

 

[Master Chief]

how can the length of a wavelength of coax be recommended without knowing what coax is used? I was under the impression a half wavelength of coax was what works best or multiples of it.

You DO need to know the specs of the coax you are using. Even after that, you need test equipment to get it right because coax is not perfect and you'll have different lengths, even off the SAME reel!

An electrical half wavelength is frequency dependent and not necessary unless you are testing or phasing. It doesn't work any better than any other similar length of coax.

IF your feedpoint of your antenna does NOT match the impedance of your coax, then different lengths of coax will act as a transformer to trick your radio into thinking it has a good match (if radios could think).

If you really want to know what the true match is (at the feedpoint), then you would use an electrical half wavelength of cable cut for a specific frequency and tested ON that frequency.

Do your self a favor, run the shortest length of coax possible and call it a day.

 

[W5LZ]

M.C.,
One characteristic of an electrical half wave length is that the impedance 'seen' at one end of it, say at the antenna's feed point, is also seen at the other end of that electrical half wave length feed line. It's like the feed line wasn't there, that you were measuring at the feed point. (Just as you suggested.) And electrical quarter wave length of feed line has the 'opposite' affect, what's 'seen' at one end is NOT typically seen at the other end. If the impedance seen at one end is 'high', the other end will show 'low', and visa-versa. A characteristic that's also used in phasing lines. (But you knew that anyway, didn't you. It certainly isn't anything new.)
- 'Doc

 

[Robb]

Materials for a CB dipole antenna?
What kind of wire would be best for optimizing this project? Stranded or solid wire? Copper or stainless steel? What guage would be optimal? How about the kind of feedline - ladder line or coax to a SO-239 connector mounted to a 1" PVC T-fitting? A 1:1 balun isn't necessary, but would it be better for a short run of coax to ensure a good match? Or should one calculate the impedance before choosing the balun?
There's nothing like experimenting - or getting it right the first time and enjoy using it...

 

[W5LZ]

The conductor used for making an antenna ought to be of low resistance in general. Using a very high resistance wire for antennas (nichrome, for instance) is not so good. After that, whatever you happen to have the most of, within reason. Copper is much easier to solder to than steel, but steel wire is usable. It should be of such a size that it is strong enough to hold/support it's self and the feed line with out breaking every time the wind blows. Stranded or solid conductor, doesn't much matter. Bare or insulated? Doesn't much matter (pink insulation is always a good choice!). Does bigger wire make an antenna more broadbanded? Not till you get into the SHF or microwave regions, so no, it doesn't. A 14 ga. insulated wire will work for almost anything.
Does using a balun make all that much difference? It can, but it typically doesn't at HF. Knowing something's impedance never hurts. May not help a lot, but certainly doesn't hurt to know what you're talking about, does it? Otherwise, how do you know which 'size' of impedance transforming type balun to get? If the feed line is unbalanced, and the antenna is unbalanced, why bother with a balun at all? That kind'a thingy is called an 'unun', by the way, "unbalanced" to "unbalanced", and no, I'm not making that up. There's also a device used with balanced antennas and balanced feed lines. Never heard one called a 'balbal', usually just an RF transformer. And just to complicate things just a little bit more, baluns should have a very high reactance at the frequency(s) of use. What works fine at one end of the spectrum ain't gonna be worth a 'hoot' at the other end of the spectrum. (Not band, but spectrum!) They are frequency sensitive. Howz that for something else to worry about?? (Don't waste a huge amount of worry on it, it just isn't that big a deal, sort of, kind'a.)
It's early, couldn't sleep, got the flu I think, and this is the first day of my 'week end' (odd work schedule). I'm going to go back to bed and feel sorry for my self.
- 'Doc

 

[BOOTY MONSTER]

on a vertical dipole the coax is supposed to come off at a 90 degree angel in the center of the antenna . is there a minium length that section has to be straight befor it can begin to droop or sag without affecting the antennas performance much ? 2 foot ? 3 foot ? 4 foot ? the longer the better......to some rediculious point as doc would say ?

im thinking this 10 Meter Dipole - N2UHC's Radio Stuff would be simple and strong . use electrical conduit for the upper and lower sections and a thick wall S40 pvc pipe for the insulator with a T comming off the side . then mount to a fence.......... or maybe hang it from a tree ?

 

[W5LZ]

That's one of 'those' questions that just doesn't have a really good, definite answer. As much as you can get, is about as close as anything else. The problem is that the RF field around the antenna will be affected by tht feed line, and the feed line will be affected by that RF field. The resulting radiation patter can be 'slewed' a bit. Probably not much, but still 'some'. And since that RF field can induce some currents in that feed line, you can have problems with that. The typical solution is to run the feed line at a 90 degree angle (or something sort of close to that) for as far as is practical. then, if that induced current on the feed line is a problem, deal with it. One method of doing tht is to wind a choke in the feed line where it changes directions. Maybe another where it enters the house? The 'best' method is the one that works for you in your particular situation.
And then there's the thingy about running the feed line down through that bottom section of conduit and running it to the house from there. Another one of those chokes at the bottom of the antenna where the coax exits the conduit is a good idea too. Or just bury the coax? And maybe another where the coax enters the house. Again, the 'best' way is whatever way works the 'best' for you.
Talk sweetly to it. If that doesn't work, threaten it. If that don't work, beat it to death with an axe! First, you have to get it's attention...
- 'Doc

 

[BOOTY MONSTER]

"Talk sweetly to it. If that doesn't work, threaten it. If that don't work, beat it to death with an axe! First, you have to get it's attention...
- 'Doc"

Damn !!!! thanks doc . LOL
can i use that as my new signature on here ?

 

[6 Meters Rules]

Sure Did Back in the 70's O.C.D. was my first antenna I used & it worked Great! I Made DX contacts all over the US & Abroad ,& I Currently Use a Rotatable Dipole on 10 meters That I have used for 10 + years & have Got OVER 100+ Countries on it !!!!!! , Direct Fed with a HOME BREW DELTA MATCH 2 mhz Bandwidth under 2:1 SWR , You BET WORKS GREAT FOR BACK SCATTER ! AS WELL YOU BETCHA!!!!!