# Antenna impedance

Discussion in 'Electronic Basics' started by riccardo manfrin, Nov 3, 2013.

1. ### riccardo manfrinGuest

With reference to this schematic:
http://www.pyroelectro.com/projects/pyro_rf_transmitter_27mhz/img/schematic.png

I'd like to know if there is a general rule of thumb for determining the antenna impedance Z(w) and therefore the matching inductance L3.

Also, I'd like to know what is the meaning behind the choice of the "12 inches of AWG 28" (cause 12inches do not seem to me like any fraction of the wavelength lambda).

RM

2. ### amdxGuest

Might be better to ask on,

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

3. ### Fred AbseGuest

A vertical 12 inch piece of #28AWG wire, over ground, at 27MHz, will look
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
radiation from the "antenna" at 27MHz.

4. ### Fred AbseGuest

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

Out of a hat, according to my calculations, and simulation. Preferred
over a meg away. 2N2222 isn't a very good choice, either.

5. ### Michael BlackGuest

"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

6. ### Tom BiasiGuest

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'.

7. ### Michael BlackGuest

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

8. ### Tom BiasiGuest

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.

9. ### Helmut WabnigGuest

Is that another world domination conspiracy or what?

w.

10. ### riccardo manfrinGuest

Il giorno lunedì 4 novembre 2013 04:38:18 UTC+1, Tom Biasi ha scritto:
I've been reading about the loading coil, but I just can't figure out the purpose? I thought that the inductor was there to match the antenna load. Has the "loading coil" this purpose? I mean, is it just another name for impedance matching? Or does it serve different purposes?

11. ### Phil AllisonGuest

"riccardo manfrin" = Goggle Groups fuckwit
** No fooling ??

** How surprising...........

** It might be there match your stupid ass.

** **** knows that this retard thinks "match " actually means

Least of all him.

** Not exactly, actually

** Hmmm, is there a tiny flicker of insight ..........

It might just dawn on this fucking idiot that "match" has
more than its literal meaning.

Like in the old joke:

" Got a match ? "

" Yeah - your face and my ass. "

.... Phil

13. ### riccardo manfrinGuest

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

14. ### Phil AllisonGuest

"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 !!!!!!!!!!!

**** equal value impedance matching nonsense.

.... Phil

15. ### Helmut WabnigGuest

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

w.

16. ### Phil AllisonGuest

"Helmut Wabnig"

** **** off retard.