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Author Topic: Really new kid question... please  (Read 2398 times)
K7AOZ
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Posts: 6




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« on: September 23, 2012, 01:55:21 PM »

When building a wire dipole and measuring the length of the legs, do you measure :
the length to the eye of the egg insulator at the ends of the dipole;
the length to the start of the egg insulator at the ends of the dipole;
the length to point where the wire stops after returning from the insulator and has been wrapped around the 'incoming' wire;
-or- am I being too critical / OCD about my measurements?  Smiley

I think the latter is the case as I have seen lengths at the feed end that range from inches to several feet and no one seems too concerned about this extra wire that surely must be radiating.

Thanks OM's , 'preciate it.
Al ... soon to be on 20 meters ... I hope...
K7AOZ ( KD7PML )
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N4UM
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Posts: 480




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« Reply #1 on: September 23, 2012, 02:23:38 PM »

Measure from the center of the center insulator to the hole in the far end of the end insulator.  In any case, make the wire 4 or 5 percent LONGER than the formula calls for and prune it very judiciously to get it tuned right where you want it.  I often make the distance between the center of the center insulator and the end loop of the end insulator a little on the short side but leave plenty of wire on the ends hanging down from the end insulator for trimming (adjustment) purposes.
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K7AOZ
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Posts: 6




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« Reply #2 on: September 23, 2012, 03:08:25 PM »

Thanks for the quick reply. So, if I am using insulated wire, the portion that has already gone through the eye of the insulator and is then wound around the actual antenna part to secure it is still radiating. If I were to use non-insulated wire and do as some videos suggest and solder the securing wraps then the bitter ends of the wire are no longer radiating?

A followup question: you say to "prune it judiciously", are you pruning the bitter ends of the wire, or are you unwrapping the securing windings and repositioning the egg / ceramic / PVC insulator ?

e.g. I have the wire going to the insulator and back to the antenna, I make a half dozen wraps to secure it and then have say 10" left hanging. Do I undo the wraps and reset the position of the insulator to then have only 6" wire left hanging. Or do I just cut the 10" down to 6" ?

I think that I understand that it's the dangly bits that get trimmed.
al
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WB6BYU
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Posts: 13458




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« Reply #3 on: September 23, 2012, 04:49:33 PM »

The reason for leaving the dangly bits in the first place is so you can trim them without
needing to retie the wire to the insulator.

The first thing to remember is that the traditional formula is only an estimate.  The actual
length required for resonance depends on the height above ground, wire diameter, insulation
type and thickness, how the wires are tied to the insulator, what type of balun you are
using, etc.  It's easier to cut it a bit long and prune off (or fold back) the ends than to
try to account for every variable to get it right the first time.
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WB2WIK
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Posts: 20633




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« Reply #4 on: September 23, 2012, 06:05:41 PM »

I'd measure it several times and use a micrometer. Wink

As Dale said, all the formulas are just estimates to get you in the general ballpark.  Rare is the case where that will predict the correct length for your actual installation.
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N8CMQ
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Posts: 386




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« Reply #5 on: September 24, 2012, 07:18:10 AM »

The formula 468/f=1/2wavelength in feet for free space.
Depending on frequency, height above ground and nearby structures, that can change the resonant frequency. That is why antenna analyzers are popular.
160M 80M, 40M, and other low band antennas are usually compromises due to length and height requirements, if horizontal...
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K5TR
Administrator

Posts: 387


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« Reply #6 on: September 25, 2012, 06:50:16 PM »

The formula 468/f=1/2wavelength in feet for free space.

Yep.

N0AX wrote a nice article about this awhile back.

http://www.eham.net/articles/23802
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George
K5TR
K0ZN
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Posts: 1560




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« Reply #7 on: September 25, 2012, 08:26:20 PM »

Hi.

Not sure one of your questions was answered. Re: mechanical construction of the end of a dipole.  

(A.)  Assuming you used NON-insulated (bare) wire (my recommendation for a couple of reasons), then the "extra" wire that has gone though the insulator eye and is "folded back" and attached to the antenna wire simply becomes PART of the antenna. i.e. that 'extra' wire is obviously connected to the antenna and is just a conductor in parallel with the other wire. A handy way to terminate non-insulated wire is the use of smaller "split bolt" electrical connectors, available in the electrical dept. of almost any good hardware store or the big box home supply stores.
The advantage of using the split bolts is that you can easily loosen them to change the length of the antenna to tweak for lowest SWR. if you solder the end
termination it is kind of "permanent". The voltage is very high at the end of a dipole, so there is no real driving need to solder. I have used split bolts
at both the feed point and ends of dipoles and other antennas for years and have never, ever had a problem of any type.....and, again, they make assembly
and adjustment much easier. Length is measured to the point the wire passes through the eye of the end insulator.

(B.)  If you use INSULATED wire, it often shortens the electrical length somewhat from the formula length....could be a couple of percent.  If you use insulated wire for the antenna, you have a little more work and a little more challenge in mechanical construction since you need to remove the
insulation at the feedpoint so you can connect the transmission line/coax wire(s) to the antenna wire.  At the far end insulator, you will NOT have a connection between the 'extra' wire and the main antenna line, so the 'extra' wire  IS  a part of the antenna radiating section and must be considered part of the TOTAL antenna length.
 
I have never used insulated wire, so I am not sure what the best way to mechanically secure it at an insulator is to insure mechanical integrity. If you leave wire dangling, I guess that would make for fairly easy trimming, etc.  I still think that in the end, the cleanest looking
and easiest to adjust method is bare wire and split bolts, but I am sure there are other good methods too. FYI: there is NO electrical advantage or disadvantage to
either bare or insulated wire. The "weathering" of bare wire does not hurt its ability to function in the least. I have been around a lot of military and commercial
HF installations and I have NEVER seen insulated wire used for antennas; it may have happened, but I never saw it. Ocean going ships with wire antennas do not use
insulated wire and they are in an extreme salt water environment. I suppose the insulation does add some weight and slightly increase wind resistance due to the slightly larger diameter of the wire so that is a fractional disadvantage to insulated wire.

Bottomline: either bare or insulated wire will work just fine. There is just some minor differences in methods of connection and termination.

Good luck,

73,  K0ZN
« Last Edit: September 25, 2012, 08:36:50 PM by K0ZN » Logged
W5WSS
Member

Posts: 1772




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« Reply #8 on: September 26, 2012, 08:46:10 AM »

Yeah when the far end of the wire is folded back upon itself the point of fold is the end of the element and the extra wire left over and parallel is not contributing to the length generally speaking.

Therefore the length is measured from where the antenna feedline connects at the center to the center hole of an end insulator.

critical distinction when one is entering the final (after tuning completion) leg lengths into a log for reference later when one needs to exact a rebuild after high winds or something.

Have fun and congratulations on your upgrade!
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K5KNE
Member

Posts: 65




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« Reply #9 on: September 28, 2012, 08:04:16 AM »

Look at "Dipole and Inverted V Antenna Basics" on Youtube. It has a good explanation of dipole antennas.

Good Luck   K5KNE
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WB6BYU
Member

Posts: 13458




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« Reply #10 on: September 28, 2012, 12:51:17 PM »

Quote from: K0ZN

...At the far end insulator, you will NOT have a connection between the 'extra' wire and the main antenna line, so the 'extra' wire  IS  a part of the antenna radiating section and must be considered part of the TOTAL antenna length.
 


It actually is a bit more complex than that:  folding an insulated wire back does raise the
resonant frequency, but not by as much as it would if the wire were bare and connected back
to itself.  (But even that would actually lower the frequency slightly, as the end section is
thicker with the folded back wire, so has more capacitance.)

This is another problem with using the "traditional formula" that seems to assume that wire
length is constant for different antenna shapes:  it isn't.

I built an 80m dipole with about 5' of wire hanging down at the ends, and put an insulated
clip on the end.  If I clip it out the rope as far as it reaches (so the wire is fully straight)
I can get down to about 3750.  By folding it back up the wire the resonant frequency
moves to about 3960, allowing me to operate all the way up to 4 MHz.  I can adjust the
location of the clip for resonance anywhere in between, all with the same length of wire.
(This is convenient for portable operation, as it allows me to make minor adjustments to
the antenna tuning as needed each time I set it up.)


Most of my antennas use stranded, insulated wire, because I get spools of hookup wire
cheap for experimentation.  It is also much more practical for portable antennas, as it
doesn't kink nearly as much as solid wire and is easier to wind up and put in my pocket. 
With thick wire you can just twist the wire end around the standing part as you would
with solid wire.  With hook-up wire I usually tie a loop knot in the end of the wire (a
bowline or a figure-8-followthrough works well.)
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