1/4 wave vs. 1/2 wave antenna...
Steve Osmon
Is it frequency dependent as to which one to use? Is it transmitter output
power dependent? Maybe Larry could weigh in with expert explaination.
Thanks, Steve Osmon
william...@gmail.com
A half wave antenna is a dipole (the antenna wire meets the elements
and the shield goes 1/4 wave in one direction and the center wire goes
1/4 the other). A 1/4 wave is half a dipole that uses the mass of the
receiver to balance the signal (Larry, did I massacre that?). The 1/4
is the least efficient but most practical antenna since it's just a
short whip, the length being dependent on the frequency. Our wireless
transmitters always use a 1/4 wave. Dipoles are somewhat directional
since they favor signals hitting them broadside, so you can consider a
dipole similar to a figure 8 microphone. A 1/4 is more omni. Since
it's directional a dipole offers more gain over an omni (I think 3db
but I'm pulling that out of my butt cheeks). More directional
antennas such as yagis and log periodic arrays use the dipole as their
driver element and offer more gain by using additional elements to
focus the rf signal. If you are working off a sound cart you are
much better off using a dipole, yagi or lp antenna rather than the
whips on your recievers. One caveat, you do lose a little signal in
the cable from the antenna to the receiver, so keep them short in most
cases.
Best,
Billy Sarokin
Dave Liquorice
> A half wave antenna is a dipole (the antenna wire meets the elements
> and the shield goes 1/4 wave in one direction and the center wire goes
> 1/4 the other). A 1/4 wave is half a dipole that uses the mass of the
> receiver to balance the signal (Larry, did I massacre that?).
It's good enough, apart from the 1/4 wave really works against a ground
plane that ideally the screen of the feeder is electrically connected to.
> The 1/4 is the least efficient but most practical antenna since it's
> just a short whip, the length being dependent on the frequency.
The optimum size of all antennas is determined by the required operating
frequency and other things but it is mainly down to the frequency.
> Dipoles are somewhat directional since they favor signals hitting them
> broadside, so you can consider a dipole similar to a figure 8
> microphone.
With the very sharp null in line with the elements. Can be useful to aim
the null at sources of interference.
> A 1/4 is more omni.
Not really, with a decent ground plane they are, hum, how to describe the
shape... They don't get much signal from below the ground plane and
reception along the axis of the element isn't good either, so you have a
sort of tilted up squashed torus shape. How tilted up the lobe of maximum
sensitiviity is depends on the size of the ground plane, the larger it is
the more the lobe is "pulled down" (IIRC)...
> Since it's directional a dipole offers more gain over an omni (I think
> 3db but I'm pulling that out of my butt cheeks).
3dB over the perfect ommi radiator, that is isotropic one, equal in all
directions. Note that isn't what a 1/4 wave ground plane is...
> If you are working off a sound cart you are much better off using a
> dipole, yagi or lp antenna rather than the whips on your recievers.
Almost certainly, getting reception antennas in the clear and above
obstructions (anything from trees to people) will improve things greatly.
1/2 wave dipoles are a bit fussy about nearby objects, make sure your
support pole doesn't come between the dipole and transmitter and the
spacing of dipole from mast can have interesting effects (that's how yagis
and LPs work...)
> One caveat, you do lose a little signal in the cable from the antenna to
> the receiver, so keep them short in most
Yep no point in connecting your remote antenna with 6db of gain over a
length of feeder with 9dB of loss...
--
Cheers
Dave.
johnpaul215
kind of off topic, but why do some receivers use one antenna vertical
and one horizontal? it doesn't seem to fit with what i am used to. i'm
talking about a standard bag situation with the transmitters clipped
to a belt, or back pocket (not any kind of crazy hiding technique).
Larry Fisher
The thought is that you increase the diversity action of the receiver
by picking up vertically polarized on one antenna and horizontally
polarized on the other. You always want independent signals at the
antennas for best diversity performance. That is to say, you don't
want identical signals since if you have a dropout at one antenna then
the identical dropout will occur at the other antenna. This is one way
of increasing the independence.
We tried several days of walk tests with different people walking
indoors and outside, in the open and around cars in the parking lot
and we found, much to our surprise, that two vertical antennas on the
UCR411 and UCR401 worked as well or BETTER than one vertical and one
horizontal. Further, angling the antennas left and right 45 degrees so
that they were at right angles also gave no improvement. We had
already designed a little plastic doo dad to hold the antennas at the
+- 45 degrees in anticipation of good walk test results. Said project
was consigned to the dustbin where it found good company.
In general, if the transmitter antenna is vertical then so should be
the receiver antennas.
Your mileage may vary.
Best Regards,
Larry Fisher
Lectrosonics
william...@gmail.com
received by each of 2 antennas and selecting the strongest signal.
The idea being that if there is phase cancellation (radio signals are
bouncing all over, when 2 signals hit your antenna at slightly
different times they can cancel each other out), it is unlikely to
happen at both antennas simultaneously. If you are using the whips
mounted on the receiver they are only a couple if inches apart which
is not ideal. typically you want them to be a minimum 1/4 wavelength
apart. By placing one antenna horizontal and the other vertical you
are effectively increasing the distance between the two and increasing
their diversity performance.
Am I getting any better Dave :-)
Dave Liquorice
> We tried several days of walk tests with different people walking
> indoors and outside, in the open and around cars in the parking lot
> and we found, much to our surprise, that two vertical antennas on the
> UCR411 and UCR401 worked as well or BETTER than one vertical and one
> horizontal. Further, angling the antennas left and right 45 degrees so
> that they were at right angles also gave no improvement.
Thanks for posting that Larry, confirms what I have suspected for a long
while. With diversity reception the important thing is that the aerials
just separated, angling or polarity(*) doesn't really matter. The phase
cancellations at each aerial at any given moment it time are going to be
different simply by virtue of not being in the exact same bit of space.
I don't even think the distance between the aerials matters that much,
provided they aren't very close. ie less than a cm at UHF frequencies.
(*) I'd still have the elements vertical as more often than not the
transmit aerial will be vertical but I'm not overly convinced it makes
much difference by the time the RF has escaped from some ones back pocket
and bounced around a room...
--
Cheers
Dave.
johnpaul215
>
> >kind of off topic, but why do some receivers use one antenna vertical
> >and one horizontal? it doesn't seem to fit with what i am used to. i'm
> >talking about a standard bag situation with the transmitters clipped
> >to a belt, or back pocket (not any kind of crazy hiding technique).
Thanks Larry, and everyone else.
I'm in the "..... and here is the sound kit" situation with Micron 700
series receivers. Not the Lectrosonics i am used to using (or
owning).
The receivers have the vert+horiz setup. We have been having issues
with reception. I am leaning towards the block not being ideal for the
city, but i wondered if the antennas were not helping either. I don't
know anyone using that block in the city, so i can't do a scanning
test to prove what we think (the microns lack any scanning feature). I
know it doesn't help that the transmitter antennas are a little used
and may get stuffed in a pocket or something less than ideal.
It sounds like each sound kit will be getting a Lectro set to test
out, but in the meantime i may try making one receiver with 2 vertical
antennae.
-johnpaul
Steve Osmon
Best, Steve Osmon
<william...@gmail.com> wrote in message
news:76f1795f-3793-44ce...@l42g2000hsc.googlegroups.com...
Larry Fisher
I agree completely except I would always like to see at least a
quarter wavelength between antennas. Note that our UCR receivers
violate that 1/4 wavelength and seem to work fine.
Cheers,
Larry F