Antenna impedance

Might be better to ask on,

rec.radio.amateur.antenna

It is a fraction of the wavelength, a very small one. (1/36) Many remote control toys use 27Mhz and a similar short antenna. I would think the designer picked the L and C for the 12 in antenna.

Mikek

A vertical 12 inch piece of #28AWG wire, over ground, at 27MHz, will look something like 0.3 ohms radiation resistance, in series with about 2.5pF. Required series inductance will be 13.92uH.

Radiotron Designer's Handbook, Chapter 22, Section iii gives:

-jZo cot(2*pi*L/lambda) for the reactive component, where:

Zo=138 log(lambda/d) - 104 ohms, where d = wire diameter.

I do it on a spreadsheet I adapted from an HP41 calculator library program.

Just a convenient piece of wire? No other reason I can see. #28 wire won't self-support very well, anyway.

A quick and dirty simulation using the above antenna model shows resonance at about 28.37MHz, with your published component values. Probably not much radiation from the "antenna" at 27MHz.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

With a properly-designed PA stage, which the quoted example isn't...

Out of a hat, according to my calculations, and simulation. Preferred value components, without any adjustment are a dead giveaway. Resonance is over a meg away. 2N2222 isn't a very good choice, either.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

"License free" transmitters often have quite strong limitations, not just power input, but antenna length too. 12inches seems short for 27MHz, but that may be a factor. I'm not even sure if the laws deal with input power anymore, or just radiated power. The latter is harder to measure for the hobbyist, so things may be kept very simple to avoid too strong a signal.

I can't emember what the antenna length is for the 160 to 190KHz band, but it's extremely short for the frequency.

Michael

Some designs use the inductor series circuit as a loading coil for the antenna. calculations don't always hold up in real life. No idea if this applies here, just sayin'.

Yes, but I can't remember if loading coils are allowed by the rules for unlicensed operation or not. I seem to recall something about "well we'll just use a giant loading coil" but then "no, that's not allowed".

Michael

Back many years ago when I first got my FCC First Class License I think the rule came under part 95, unlicensed transmitters. Then 11 meters was taken from the hams and CB was born. The unlicensed band was limited to

100mW and a 5 foot antenna. I don't recall any restrictions on loading coils but that was back when the Pope was an Alter Boy.

Never heard about "forbidden loading coils" Is that another world domination conspiracy or what?

w.

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,

I've been reading about the loading coil, but I just can't figure out the p urpose? I thought that the inductor was there to match the antenna load. Ha s the "loading coil" this purpose? I mean, is it just another name for impe dance matching? Or does it serve different purposes?

--
Read "Resonant Antennas" at: 

http://en.wikipedia.org/wiki/Antenna_(radio) 

for a nice explanation.

On my way to download that.

It is interesting, but I'm looking for a (possibly simple) mathematical formulation that

- applies to a generic dipole of arbitrary length (not lambda /2 or lambda/4, and in general way smaller than the wavelength), - does not take in integrals nor starts from Maxwell's equations - possibly takes in all conventional approximations/simplifications for far field, isotropic, linear, homogeneous, non-dispersive medium, - binds the antenna complex impedance to the length and lambda

The closest to this was Fred's answer but the source is not exactly an "handbook".

I'm now starting from scratch to understand the current and voltages distribution across the dipole to understand how the ratio between them is defined and therefore the impedance.

R

"riccardo manfrin"

** You are so far off the basic issue.

An antenna radiates RF energy in proportion to the VOLTAGE on its driven elements.

A series inductor operating at resonance with some capacitance *magnifies* the voltage appearing at the output of the driving amplifier.

Soooo - it boosts the energy radiation if the antenna is capacitive.

Which all short antennas are !!!!!!!!!!!

Fuck equal value impedance matching nonsense.

... Phil

I always thougt it is the moving electron which radiates. When you charge a metal object to a million volts, will it radiate?

w.

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Only while it's charging/discharging. 


JF

"Helmut Wabnig"

** Fuck off retard.

I bet your antenna radiates into the ground.

w.

Depends how big your hands are...

My hardback copy is only about 2.5 inches thick ;-)

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)