Basic Radiohead Formulas Needed

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condor11

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I have searched the web for some basic conversion techniques, but would like to know from some of the knowledgable radio heads, their methods for determining values needed.


i.e. formulas geared towards radio antenna lengths.

I will quote one conversion to start thread:

To convert any frequency to a wavelength, divide the speed of light by it.

So, it's: wavelength = 299792458 divided by frequency.


Any basic radiohead type of conversions, or quick cookbook methods of finding necessary values for pragmatic purposes would be the basis of this thread.
 
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ka3jjz

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Not a bad wiki topic, either. Let me see if I can dig up my General class manual - there are several formulae in there....73s Mike
 

rescuecomm

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For a vertical antenna over a ground plane, the formula is 239 divided by the frequency in megahertz. For a dipole, the formula is 492 divided by the frequency in megahertz. If I remember correctly.
 

jerseymilair

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I think this would be a real good topic. I am in the process of building my own discone antenna and plan on posting a couple of pictures of the finished product. I would like to see some info on the lengths of the horizontal elements and 60º vertical elements for a desired frequency range of 225-400 MHz.
 

prcguy

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A dipole in free space is 468/frequency with the result in ft and 1/10th of a ft. For a 1/4 wave length radiator as in a ground plane its 234/frequency. A Discone skirt will be 1/4 wavelength long at the lowest frequency and the top hat is between .67 and .7 of the skirt length. At best you can expect an 8:1 frequency range but probably less without equipment to tweak and measure. Ideally a Discone has a solid cone and disk. If you use elements, they have to have no more than .02 wavelength spacing at the ends to have the same performance as a solid cone and disk. This would be 16 elements each, which is difficult to produce. Most commercial Discones have 12 elements each which is a good compromise and less (like 8 in most scanner Discones) means a further drop in performance.
prcguy
 

prcguy

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One of the best performing Discones for 225-400MHz is the military AS-197, which has a low cutoff freq of 175MHz. It goes much higher than 400MHz but its optimized for the best radiation pattern over 225-400. I could measure the dimensions if you want to try and duplicate one.
prcguy
jerseymilair said:
I think this would be a real good topic. I am in the process of building my own discone antenna and plan on posting a couple of pictures of the finished product. I would like to see some info on the lengths of the horizontal elements and 60º vertical elements for a desired frequency range of 225-400 MHz.
 

jerseymilair

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The links seem to just have pictures. I'm looking for actual dimensions. I've already machined a base for a discone using aluminum. I've cut-off some 3/16" stainless steel radials(24) and have threaded one end 10-32. I"m trying to determine the length of the radials, something I've haven't seen on "other" web sites.
 

ka3jjz

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loumaag said:
Mike,

Isn't most of this covered from Links from the Wiki, starting here:
http://wiki.radioreference.com/index.php/Antennas
Quite likely some do, Lou. But exactly where to look for some specific formulae - now that's the rub. Between the BuxComm and AC6V sites, I'd be willing to wager that most of the more common ones would be listed there - gathering them up in one place might be better in the long run in any case. They're pretty much standard through the industry, and it might encourage experimentation. But we do want to exercise a little control here - after all, we don't want to reinvent the ARRL Antenna Handbook - they might get a little PO'ed if we did, hi.
I would say that the standard quarter wave, half wave (dipole), vertical and perhaps the Yagi and full wave loop would be a good start. Probably 80% of the antennas out there - maybe a little more - are pretty much based on these formulae in one way or another.

But that's just my thoughts - the Wiki is for everyone, after all, and if someone wants to build a page of formulae, then go for it. 73s Mike
 
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N_Jay

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jerseymilair said:
The links seem to just have pictures. I'm looking for actual dimensions. I've already machined a base for a discone using aluminum. I've cut-off some 3/16" stainless steel radials(24) and have threaded one end 10-32. I"m trying to determine the length of the radials, something I've haven't seen on "other" web sites.

Now where is that "Spoon Feeding" smiley when you need it?

http://en.wikipedia.org/wiki/Discone_antenna
 

ka3jjz

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Wow, I didn't think the online Wikipedia encyclopedia would even know what a discone is! Fascinating find. Wonder what other antenna related links it has? Something to check out when I get back to work on Monday using a T1 instead of this trashy dialup.

73s Mike
 

ka3jjz

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Oh I'm not going to build this page - heck, I can't have all the glory, hi. We've had a few folks step up to the plate in the recent past (see my news item in the Admin forum), and I'd like to see that continue.

Whoever decides to build it should include Tom's link, as well as the others mentioned in the thread. I said I would look up the formulae in the General class manual - and will do that, once a page has been started.

73s Mike
 

Tweekerbob

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Units please!!

condor11 said:
To convert any frequency to a wavelength, divide the speed of light by it.

So, it's: wavelength = 299792458 divided by frequency.
So is that the speed of light (c) in miles per hour or inches per year or what? What units do you use for frequency? MHz, kHz, Hz, GHz??? Is the resulting quotient in units of miles, cubits, inches, millimeters, etc.??

Everyone who has submitted a formula to this thread has omitted the most important thing about conversions...units! Before we all get off on the wrong foot, please provide the appropriate units so the formulae submitted and the ones to be submitted actually mean something.

If you rewrite the equation to be the speed of light (3x10^8 meters/second)=Wavelength (meters/cycle --lowercase Lambda) times Frequency (cycles/second -- lowercase Omega) then you see when you mulitply the wavelength times the frequency that the "cycles" units cancel, and then further when you divide the speed of light by the frequency, the "seconds" cancel, and you end up with meters as your units.

For example: 3.00 x 10^8 / 1.4562 x 10^8 (the powers of ten cancel) is approximately = 2.048 meters (using the above method to determine the units). 1.4652 x 10^8 (cycles/second) is equivalent to 146.520 MHz.
 
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rescuecomm

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The 234 divided by the frequency in mhz for a quarter wavelength give the result in feet.
for a CB antenna. 234/27.00=8.666 feet or eight feet eight inches. The metric calculations take the speed of light in meter per sec and the frequency in mhz. 300/28=11.1 meters then multiply by 0.25 to get a quarter wave = 2.77 meters.
 

gmclam

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SIMPLE MATH:
Wavelength in meters is 300 divided by the Frequency in MHz (100MHz = 3 meters)
Conversion to feet - 3.28 feet per meter (100 MHz = 9.84 feet)
Quarter wavelength = divide by 4 (100 MHz = 2.46 feet)
 
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