Antenna impedance

[riccardo manfrin]

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).

Thanks in advance.
RM

 

[amdx]

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).

Thanks in advance.
RM

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

 

[Fred Abse]

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.

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.

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).

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.

 

[Fred Abse]

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.

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

I would think the designer picked the L and C for the 12 in antenna.

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.

 

[Michael Black]

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).

"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

 

[Tom Biasi]

"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'.

 

[Michael Black]

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

 

[Tom Biasi]

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.

 

[Helmut Wabnig]

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.

 

[riccardo manfrin]

Il giorno lunedì 4 novembre 2013 04:38:18 UTC+1, Tom Biasi ha scritto:

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.

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?

 

[Phil Allison]

"riccardo manfrin" = Goggle Groups fuckwit

I've been reading about the loading coil,

** No fooling ??

but I just can't figure out the purpose?

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

I thought that the inductor was there to match the antenna load.

** It might be there match your stupid ass.

Has the "loading coil" this purpose?

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

Least of all him.

I mean, is it just another name for impedance matching?

** Not exactly, actually

Or does it serve different purposes?

** 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

 

[riccardo manfrin]

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

On my way to download that.

 

[riccardo manfrin]

Read "Resonant Antennas" at:
http://en.wikipedia.org/wiki/Antenna_(radio)
for a nice explanation.

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

 

[Phil Allison]

"riccardo manfrin"

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.

** 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

 

[Helmut Wabnig]

"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.

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

w.

 

[Phil Allison]

"Helmut Wabnig"

** **** off retard.

 

[Helmut Wabnig]

"Helmut Wabnig"

** **** off retard.

I bet your antenna radiates into the ground.

w.

 

[Fred Abse]

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

Depends how big your hands are...

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