Antenna effective length

[przemek klosowski]

I helped a friend with a remote flash trigger to extend the range. The device
operates at 433 MHz, and as-built has a 1.25" PCB stub serving as an antenna.
Since the wavelength is 27.26", it's really poorly matched and the range petered
out at under ten meters. We soldered a coiled insulated wire of length 12.4 in
(lambda/2 minus the PCB trace length) to the end of the built-in antenna,
and the range increased tremendously, by a factor of ten. The coiled wire fit
in the original enclosure, so it worked out very well. However, after I thought
about it I realized that I don't understand why this thing works.

My question is: what is the effective length of the antenna when it is coiled
up? Since the E-M field extends outside of the antenna conductor, shouldn't
the effective length be the actual length of the coil, rather than the length
of the wire in the coil? Is there a simple, back-of-the-envelope argument
on how the antenna length/configuration helps to couple the E-M radiation out?

 

[MooseFET]

I helped a friend with a remote flash trigger to extend the range. The device
operates at 433 MHz, and as-built has a 1.25" PCB stub serving as an antenna.
Since the wavelength is 27.26", it's really poorly matched and the range petered
out at under ten meters. We soldered a coiled insulated wire of length 12..4 in
(lambda/2 minus the PCB trace length) to the end of the built-in antenna,
and the range increased tremendously, by a factor of ten. The coiled wirefit
in the original enclosure, so it worked out very well. However, after I thought
about it I realized that I don't understand why this thing works.

My question is: what is the effective length of the antenna when it is coiled
up? Since the E-M field extends outside of the antenna conductor, shouldn't
the effective length be the actual length of the coil, rather than the length
of the wire in the coil? Is there a simple, back-of-the-envelope argument
on how the antenna length/configuration helps to couple the E-M radiationout?

An element that is under 1/4 wavelength looks capacitive. If you
think of your coil of wire as an inductor in series with short
element, you will begin to get a handle on what happens when an open
ended coil is used.

 

[[email protected]]

I helped a friend with a remote flash trigger to extend the range. The device
operates at 433 MHz, and as-built has a 1.25" PCB stub serving as an antenna.
Since the wavelength is 27.26", it's really poorly matched and the range petered
out at under ten meters. We soldered a coiled insulated wire of length 12.4 in
(lambda/2 minus the PCB trace length) to the end of the built-in antenna,
and the range increased tremendously, by a factor of ten. The coiled wire fit
in the original enclosure, so it worked out very well. However, after I thought
about it I realized that I don't understand why this thing works.

My question is: what is the effective length of the antenna when it is coiled
up? Since the E-M field extends outside of the antenna conductor, shouldn't
the effective length be the actual length of the coil, rather than the length
of the wire in the coil? Is there a simple, back-of-the-envelope argument
on how the antenna length/configuration helps to couple the E-M radiation out?

The short whips look like a tiny capacitor in series with a small
radiation resistance. That series capacitive reactance is way bigger
than the bit of resistance that radiates the actual power, so by
potential divider effect most of the available drive voltage is lost
across the capacitance leaving only a tiny part to feed the radiation
resistance, hence little transmitted output power.
Seeing as not a lot can be done about the radiation resistance item,
the next best thing is to try and resonate out the series capacitance
using a series L. This (essentially) leaves only the radiation
resistance part, to power match to the transmitter. (about 4ohms?)
In your case ( 433mc), I make your whip at say 0.2pF which has a
reactance of 1700ohms. Thus you need 1700ohms of series XL to resonate
out the series XC. This is a series inductor of about 0.6uH, which by
chance? works out about 10 turns 1/4" using about a foot of wire.
Looks like you guessed the coil right!.

I've no interest in aerials but have used a couple of equations at
the top of ...
http://my.integritynet.com.au/purdic/activeantenna1.htm
also,
http://www.w8ji.com/antennas.htm
makes for useful reading.
There's heaps of info' out there but the subject invites a feeding
frenzy for mathematicians, hence rare to find any back-of-fag-packet
help.

 

[JeffM]

Their are some antenna experts on rec.amateur.amateur.antenna
who can undoubtedly give you a far better answer than I can, BTW.

....even more on rec.radio.amateur.antenna.

 

[Clifford Heath]

The important point is that making the element shorter ...
causes a major reduction of its radiation resistance,
making it more difficult to match efficiently to electronics.
Enough?

Almost, thanks. Assuming near-direct connection to the transmitter's
output balun (such as in a handheld, where a short whip is desired)
there should be no additional difficulty, right?

But if you need to run co-ax, you'd need a balun at each end?
Just making sure I have this straight.

Clifford Heath, VK3CLF.