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Author Topic: Electrical length vs physical length...5/8 wave 4' 10m vertical?  (Read 4880 times)
KB1TXK
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« on: October 04, 2010, 07:11:15 AM »

Quote
5/8 wave vertical radiator for 28.400 = 20.6ft

If I can wrap that around a PVC pipe (ideally 4', no higher than 5'), what diameter should I use and how far about should the turns be?   The lazier the turn the better (wide diameter pipe) or can I keep them tight and close? When adding radials, should I also make those in the same fashion (wire wrapped around PVC) or can I use 20' wire in a straight line?

Wire is coated 14ga solid, matching would be done w/ coax choke balun. It will eventually be on top of a vehicle, but for now (until I replace the vehicle) it will be 25' up at home.  Max wattage would be 25w (RS HTX-10)

Despite the questions, I'm not in search so much for exact figures (I will admit, they'd make the build job quicker)...as I am general advice regarding diameter and turn spacing.

If I'm about to make a totally crap antenna, that would be useful to know too..but I don't mind finding that out for myself either.

Using an inverted vee at the moment. Wanted to compare it to a DIY vertical.

Thanks!
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W5FYI
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« Reply #1 on: October 04, 2010, 07:30:07 AM »

Several rules-of-thumb come to mind. First, the closer to full size, the more efficient the antenna. With that in mind, a 5/8-wavelength antenna on 28.5 MHz should be about 20.5 feet long. Second ROT is that if you wind a helical antenna, figure on using about twice the length of wire you would normally need for a full-size antenna. That means you should count on using 40 to 50 feet of wire.  Third ROT indicates that part of an antenna that radiates the most is the area around the current node, so it is probably better to have the turns spaced out in that area and bunch them up elsewhere. Fourth ROT is that you must consider mutual inductance between the helical windings and the matching coil. If you space out the windings near the coil, there will be less coupling. ROT five is that short antennas have lower radiation resistance, but since you're using a matching coil anyway, it won't be much of a problem finding a match to 50-ohm feedline. In fact, you shouldn't need a unun for matching at all!  Finally, a 5/8-wavelength antenna doesn't need an extensive radial system. The metal in your car should suffice; just make sure the base of the coil is coupled to it well. GL, and let us know how it works out.
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N3OX
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« Reply #2 on: October 04, 2010, 07:32:22 AM »

There is no such thing as a four foot 5/8ths wavelength 10m vertical.

A four foot vertical is a four foot vertical, and with proper design, you can get it to work within maybe a dB or two of a full quarter-wavelength whip.  It will never be better than that, and it will have narrower bandwidth than the 1/4 wave.

You can get a 4-5' tall helical vertical to resonate using a wide variety of lengths of wire.  Some people say you need a half wave... some people say this, that, and the other thing.  The best way to do it is to take enough wire to make the right kind of loading inductor maybe 2/3rds of the way from the bottom and just use straight conductors for the rest, and even better, add a cap hat.  But that's not the only way.  There are lots of helicals that will work okay.  But the amount of wire on it in wavelengths is pretty irrelevant.   How you distribute it matters as much.   There's no point in shooting for a given length of wire, other than as little wire as possible to make it resonate.  More wire just lowers the Q of your short antenna by adding loss.  

"Electrical length" is a useful concept but it is only a concept that applies to the characteristics of the impedance of the antenna.  The radiation properties are set by the physical height and the way the current distributes on the antenna.  If you use too much wire on a helical, it is possible to operate it well above the first resonant frequency.  If you do this, it will be really terrible, because you'll get phase reversals in the current on the antenna packed into a short physical length, making the radiation cancel out.

The best advice for "wind some wire on a stick" helical antenna design is to use the absolute minimum amount of wire you can get away with, and just don't pay any attention at all to the exact length.  The resonant frequency will be set by the particular way you wind the thing, and at best, you'll make it radiate like a pretty good lumped-coil-loaded four or five foot antenna.  It's literally impossible to make it radiate like a 5/8ths wave, so don't worry about using that much wire.

Quote
Despite the questions, I'm not in search so much for exact figures (I will admit, they'd make the build job quicker)...as I am general advice regarding diameter and turn spacing.

Straight wire for most of it.  Build a coil of about 20 turns spaced out over 4 inches on a 1.5 inch form and install that about 2/3rds of the way from the bottom of a straight wire vertical.   That's my recommendation for diameter and turn spacing... Grin  There are probably better coil designs than that but it's the first one that comes to mind.  I'm sure others will have ideas for different helicals, but I thought I'd propose a "ballpark" design for a lumped coil load.
« Last Edit: October 04, 2010, 07:34:41 AM by Dan » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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« Reply #3 on: October 04, 2010, 08:44:12 AM »

Quote from: KB1TXK

If I'm about to make a totally crap antenna, that would be useful to know too..but I don't mind finding that out for myself either.

You are about to make a totally crap antenna.

Not that you can't make a helical loaded vertical for 10m that is 4' long - that's not hard to do.  (Though as Dan
pointed out, it might not be the most efficient solution.)  But it is NOT a 5/8 wave antenna, even if it uses
5/8 wavelength of wire - it is still a 4' antenna, and won't be as good as a full quarter wave vertical wire.

The only place I've seen short helical whips advertised as 5/8 wave antennas is for the CB market.


However, I've done as you are suggesting for 15m before, and it isn't difficult to do.  There are a couple
rules of thumb that I've encountered regarding the required length of wire for a helical antenna.  The ARRL
has suggested about half a wavelength of wire when it is wound with a constant pitch (though this
doesn't take into account the length or diameter of the former, both of affect the resonant frequency.)
The other is that, when the top 1/3 of the form is close wound, it requires about 3/4 wavelength of wire
(which happened to be the case for a particular tapered fishing rod.)  Clearly, somewhere in between, you
can find some combination that uses exactly 5/8 wavelength if you want to.

So here is how to proceed.  Start with a length of wire - 20' is as good as anything else - and wind it
on the former, with the turns further apart at the bottom and closer together at the top.  Set it up
on a pole with 4 radials, each 1/4 wave long sloping down from the feedpoint.  Find the resonant
frequency with an SWR analyzer.  If it is too low, slide some of the turns higher up the pipe.  If it
is resonant too high in frequency, slide some of the windings down towards the feedpoint.  Check
the resonant frequency again.  When you get it where you want it, add some wraps of electrical
tape to hold the turns in place.

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WA3SKN
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« Reply #4 on: October 04, 2010, 10:49:53 AM »

"(ideally 4', no higher than 5')"

I take you have a balcony.  Your shortened antenna WILL NOT work like the 5/8 wavelength antenna you hope for. A 1/2 wavelength dipole will be about 8 ft on a side for a total of 16+ ft.  A vertical 1/4 wavelength (worked against ground) will be about 8 ft.  A Loop will be about 8 ft on each side (or make a rectangle).  When you make them shorter than this, there will be compromises.
So what area do you have, and what limitations do you have?  These are the real questions.
Is there a balcony or HOA issue?

-Mike.
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W5DXP
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« Reply #5 on: October 04, 2010, 11:29:11 AM »

...as I am general advice regarding diameter and turn spacing.

Here is an inductance calculator that should be very useful for you. Since it is in metric, I've included a metric/English converter also. Wide spacing on the turns will be most efficient.

http://hamwaves.com/antennas/inductance.html

http://www.worldwidemetric.com/measurements.html
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W7ETA
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« Reply #6 on: October 04, 2010, 06:45:36 PM »

Check out W4RNL's site.  He has a very good section on verts,
73
Bob
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AA4PB
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« Reply #7 on: October 06, 2010, 06:12:45 AM »

The way that it actually works is that a physically short (in terms of wavelength) antenna has capacitive reactance (Xc). To make it resonant, you have to add a loading inductor with an equal amount of inductive reactance (Xl). The amount of wire needed to create that amount of Xl is not necessarily equal to length of wire that it would take to make a resonant antenna stretched out. If you measure out 1/4 wavelength of wire and wind most of it into a coil it will likely not be resonant anywhere near the original calculated frequency.

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KB1TXK
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« Reply #8 on: November 11, 2010, 10:40:15 AM »

whomever took the time to look up an old article about helical antenna construction, copy it, and mail it to me...thank you! I can't find your callsign otherwise I'd PM you.  Either way I just wanted to make sure I told you how much I appreciated it Smiley
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