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Wavelength help

B

biferi

Member
Jul 29, 2008
66
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I need help findingout the wavelength to get the Length of wire for an ANT.

I think it goes like this.
300 Divided by FREQ in MHZ will give you the Wavelength in Meters.
So if I wanted to cut an ANT. wire to get the FREQ. 7.05 MHz it will go like this.
300 Divided by 7.05 MHz = 42.55 Meters in Length RIGHT???
 

biferi,
That's how to find the full wave length in meters, you're right. But a full wave length isn't that useful as an antenna length, sort of. A dipole, for instance, is a half wave length long. Another commonly used length is a 1/4 wave length (verticals).
And while the "300 / Mhz" figure is correct, it isn't exactly 'close' to a useful length for when wire is being used as a conductor (or metal tubing either). The reason for that is something called 'velocity factor', or the speed of an electron in a conductor as opposed to 'space'. The number that you come up with will be too large. Cutting it back to a shorter length by way of using some method to determining how well you are hearing things (fancy way of saying that using a meter to determine the 'best' length with the equations length as a starting point) is very common and considered normal, or the "tuning" of an antenna.
Not being familiar with metric measurements, or comfortable with them, I usually deal with feet and inches. The "magic" number for a 1/2 wave length in feet is 468 / Mhz. For a 1/4 wave it's 234 / Mhz. And for a full wave length (at the typical antenna heights I deal with) is 984 / Mhz. These figures are also 'ball-park' numbers, typically a bit too long, not exact by any means. For listening or only receiving, they will certainly work just fine. If you do the converting between meters and feet I think you will find that the results are pretty close (close enough anyway).
- 'Doc


(Always a bunch of "if's", "ands", and "but's" in this stuff, isn't there??)
 
Need help

Ok thanks for help I now know how to findout how long to cut my ANT wire for a given FREQ.

But what if you want to make an ANT for not just one or two FREQ. but for the whole Shortwave Spectrum what is the Math for that?

I now know if you know the FREQ you want to get you Divide that by diferant numbers you gave me and I get that.

But when you want to get a lot of FREQ like the whole Spectrum what do you do??
 
The simplest solution for complete coverage of the whole spectrum is to put up the longest antenna that is practical for you, then, quite worrying about it. There is no one particular antenna that will do everything well. Almost any antenna will work reasonably well for receiving. Receivers are not particularly 'picky' about antennas as transmitters are. Just remember that 'size' of antenna is related to wavelength, not feet or meters. Antennas for lower frequencies tend to 'work' for higher frequencies. Antennas for higher frequencies don't tend to 'work' for lower frequencies to the same extent. There are exceptions to either of those statements, but are generally not worth worrying about.
- 'Doc
 
ANT help

If I cut my wires for the Dipole ANT. but do not Soder them to the SO239 Socket but I use Alligator Clips to connect the wires.

I do not mean the Coax Feed I just meen the Left and Right wire Arms. Or do they have to be Sodered to it?
 
If those connections are not soldered, or made mechanically secure, they tend to weather and corrode. When they do that, it's sort of like either on open circuit (loose that part of the antenna) or the loss of conductivity will make whatever is extending from that point appear to be 'shorter' (introduces resistance into the circuit and that's not going to be any good at all). Lot's of simplification there, not a good explanation at all.
I guess you could use those clip and then weather seal them. That would extend the connections time before corroding, but not as well as soldering would.
- 'Doc
 
I'd use plug connections over 'gators any day of the week. Unless you live in an area that absolutely never ever gets a breeze of any description.

The initial post given I believe was for a 40 meter antenna. You can make an antenna cover the 40 meter spectrum, or a good portion of it, by increasing its 'bandwidth'. The antenna dimension in length remain the same, the math doesn't change. A 'cage' type wire antenna will maximize the bandwidth and will not require the use of a tuner as will simply lofting as much wire as you have space for.

Like Doc said, math is 'ball park' figures. It gives one a rough idea of how much wire to order. If you are going to be building more antennas I'd get a 'noise bridge' or 'antenna analyzer'. It will tell you if your physical antenna is too short/long for resonance. Look out for 'tuner tuners' some folks on the QRZ forum don't know the difference! Something like this http://http://www.universal-radio.com/CATALOG/meters/1320.html
 
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ANT help

I am running into the same prablems again?

Every time I go to Cut a Coax Cable into it's two parts. The center pin and the outer shild it just goes to shit.

And when I go to buy say that SO 239 connector that you just screw the Coax onto and it gives you left and right tap offs.

They tell me I am going about it the wrong way.???????????????????????????
 

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