I'm tracking a high altitude balloon on 2m via a handheld scanner, and
in the past have used the standard rubber-duck antenna that the scanner
came with. However, reception isn't too great at times. What kind of
gain increase could I expect to see if I were to replace the current
rubber-duck antenna with a quarter-wave antenna (designed for 2m and
70cm)?

Thanks in advance for any insight!
Dave

Dave,

The thing to remember at VHF is that the dimensions of the hand-held
transmitter/receiver box are just as important as the dimensions of the
antenna.

For example, a properly-designed hand-held transceiver plus a rubber-duck,
isolated from ground, is equivalent, in overall length, to a half-wave
dipole.

Likewise, a vertical whip mounted on a vehicle behaves as a halfwave antenna
of length crudely equal to whip length + vehicle depth. (As modified by a
loading coil if any.)

If the rubber-duck (which is mostly a helical loading coil) is replaced by a
rod or wire of longer length, two things happen -

First, the resonant frequency of the system (antenna + box) changes.

Secondly, the antenna pure input resistance changes from its optimum
(designed for) value to an entirely different impedance of resistance +
reactance.

In your case, due to its longer length, the antenna + its new smaller
loading coil (if you fit one) will certainly radiate more efficiently. But
the mismatched transmitter power amplifier (which you havn't got) will now
be appreciably less efficient.

With the rubber-duck the antenna will be matched to the receiver input. But
with a different antenna impedance there will be a considerable mismatch
loss and receiver sensitivity will also be reduced.

Ideally, if the antenna is changed then both Tx and Rx impedance matching
arrangements should be changed. Which you can't do!

So the BEST you can do is change the antenna rod or wire length such that
the overall length of the system (antenna + box) is crudely 1/2-wavelength
at the working frequency and HOPE that you gain more on the swings than you
lose on the roundabouts.

You will likely be more successful on receive than on transmit. But it is
quite possible for performance, especially on transmit, to be worse.

The most likely result is that you will hardly notice any difference. At
best, if you could measure it, you can expect a few dB improvement. Depends
on how big is the rubber-duck you begin with. Don't throw it away. You may
wish you hadn't.

Another thing to remember is that the extremely uncertain hand and arm which
holds the scanner forms an integral part of the radiating/receiving system.
A sweaty palm and an arm held over your head can seriously affect tuning.

Orientation (polarisation) of the antenna relative to balloon antenna
orientation can also make a great difference.

Exact length of antenna (resonance) is a very non-critical matter. Just
connect some wire to the antenna socket and be prepared to prune it.

What does your "Reception is not too great at times" mean. To effect a
noticeable improvement, a 4-times increase in power from the balloon
transmitter would be noticeable. (A 4-times increase in power is equivalent
to twice the height or distance.)

Great fun is available just from all the uncertainty involved.

Happy experimenting!
----
Reg, G4FGQ

===============================

I'm tracking a high altitude balloon on 2m via a handheld scanner, and
in the past have used the standard rubber-duck antenna that the scanner
came with. However, reception isn't too great at times. What kind of
gain increase could I expect to see if I were to replace the current
rubber-duck antenna with a quarter-wave antenna (designed for 2m and
70cm)?

Thanks in advance for any insight!
Dave

Reg,
Thanks for the feedback. I really appreciate it! If I could, I had a
few additional questions regarding some of your points:


Reg Edwards wrote:

Secondly, the antenna pure input resistance changes from its optimum
(designed for) value to an entirely different impedance of resistance
+
reactance.

With the rubber-duck the antenna will be matched to the receiver
input. But
with a different antenna impedance there will be a considerable
mismatch
loss and receiver sensitivity will also be reduced.

Ideally, if the antenna is changed then both Tx and Rx impedance
matching
arrangements should be changed. Which you can't do!

The antenna I had in mind was made for Radio Shack handheld scanners,
and I have a Uniden handheld, which both have a 50 Ohm impedence I
believe... this would eliminate the impedance mismatch, wouldn't it?

The most likely result is that you will hardly notice any difference.
At
best, if you could measure it, you can expect a few dB improvement.
Depends
on how big is the rubber-duck you begin with. Don't throw it away.
You may
wish you hadn't.

The antenna it came with is about 6" long, and I assume it's made to be
a decent antenna for all frequencies the scanner is designed for (29MHz
to 900+Mhz). Just FYI, in case that changes anything...?

Orientation (polarisation) of the antenna relative to balloon antenna
orientation can also make a great difference.

That would be great if I could control the orientation of the
transmitting antenna. It can vary +/- 45 degrees or so during the
flight... the transmitting antenna is an inverted standard 2m handheld
antenna (8" or 9" long?). I've assumed in the past that if I held the
recieve antenna vertical, that's about as good as I can get...?

What does your "Reception is not too great at times" mean.

On the way down mainly, when it's decending between 30 and 100 mph.
I'm assuming this is due to antenna swing.

To effect a noticeable improvement, a 4-times increase in power from
the balloon transmitter would be noticeable.

I'd love to, but due to battery capacity and time of operation, .5W is
what I'm trying to stick with.

Thanks again for the insight! I very much appreciate it.

Dave

On 22 Feb 2005 06:38:53 -0800, wrote:

I'm tracking a high altitude balloon on 2m via a handheld scanner, and
in the past have used the standard rubber-duck antenna that the scanner
came with. However, reception isn't too great at times. What kind of
gain increase could I expect to see if I were to replace the current
rubber-duck antenna with a quarter-wave antenna (designed for 2m and
70cm)?

Significant. I don't have a number, but it will definitely improve.

On the balloon, is the antenna vertically polarized? I would assume
so. If not, consider a dipole.

Also, to improve the reception further, build a 'rat-tail'. Its a 1/4
wave wire that hangs from the ground of the rig making your 1/4 wave
antenna more of a 1/2 wave. You may find adding that will help even
with the duck.


--
73 for now
Buck
N4PGW

Buck wrote:
On 22 Feb 2005 06:38:53 -0800, wrote:

I'm tracking a high altitude balloon on 2m via a handheld scanner,
and
in the past have used the standard rubber-duck antenna that the
scanner
came with. However, reception isn't too great at times. What kind
of
gain increase could I expect to see if I were to replace the current
rubber-duck antenna with a quarter-wave antenna (designed for 2m and
70cm)?

Significant. I don't have a number, but it will definitely improve.


On the balloon, is the antenna vertically polarized? I would assume
so. If not, consider a dipole.

Also, to improve the reception further, build a 'rat-tail'. Its a
1/4
wave wire that hangs from the ground of the rig making your 1/4 wave
antenna more of a 1/2 wave. You may find adding that will help even
with the duck.

Really? Any point on the ground? Or does it need to be connected
somewhere on the antenna?

Thanks for the advise!
Dave

On 22 Feb 2005 10:41:15 -0800, "dave.harper"
wrote:


Buck wrote:
On 22 Feb 2005 06:38:53 -0800, wrote:

I'm tracking a high altitude balloon on 2m via a handheld scanner,
and
in the past have used the standard rubber-duck antenna that the
scanner
came with. However, reception isn't too great at times. What kind
of
gain increase could I expect to see if I were to replace the current
rubber-duck antenna with a quarter-wave antenna (designed for 2m and
70cm)?

Significant. I don't have a number, but it will definitely improve.


On the balloon, is the antenna vertically polarized? I would assume
so. If not, consider a dipole.

Also, to improve the reception further, build a 'rat-tail'. Its a
1/4
wave wire that hangs from the ground of the rig making your 1/4 wave
antenna more of a 1/2 wave. You may find adding that will help even
with the duck.

Really? Any point on the ground? Or does it need to be connected
somewhere on the antenna?

Thanks for the advise!
Dave

most commonly they are connected to the bnc ground, but anywhere that
is grounded to that connector will work. Since this is a scanner, you
might consider a piece of computer ribbon cable. Cut each wire in the
ribbon to 1/4 of your favorite bands or frequencies until you run out
of wire. If you have left-over strips, just remove them or leave them
for later use.

Just a thought.

Buck

--
73 for now
Buck
N4PGW

Hi Dave

Assuming you build a 1/4GP with a large enough counterpoise (ie dont
rely on the case of the scanner) I think the figure of merit is around
6-12dB. You may however be better off making a turnstile about 3/8
wavelength over a mesh ground so the thing points roughly where you want
it. That will also help with polarization changes as the baloon rotates.
Maybe a similar antenna on the ballon as well?

Cheers Bob Vk2YQA

wrote:
I'm tracking a high altitude balloon on 2m via a handheld scanner, and
in the past have used the standard rubber-duck antenna that the scanner
came with. However, reception isn't too great at times. What kind of
gain increase could I expect to see if I were to replace the current
rubber-duck antenna with a quarter-wave antenna (designed for 2m and
70cm)?

Thanks in advance for any insight!
Dave

On 22 Feb 2005 06:38:53 -0800, wrote:

I'm tracking a high altitude balloon on 2m via a handheld scanner, and
in the past have used the standard rubber-duck antenna that the scanner
came with. However, reception isn't too great at times. What kind of
gain increase could I expect to see if I were to replace the current
rubber-duck antenna with a quarter-wave antenna (designed for 2m and
70cm)?

Thanks in advance for any insight!
Dave

Hi Dave,

Well, I see you got advice, but unrelated to the problem you offered.

The standard rubber duck and a quarterwave are going to have the same
failings: they both have the same response curve in space. Basically
they both are designed to work with transmitters located at right
angles to them. And if you stand in the conventional upright manner
of a biped, then those antennas are going to be vertical with the
favored direction lying along the horizon.

Now, if your balloon is tracking along the curve of the earth's limb,
some several hundred miles out, then this might work admirably (it
doesn't take all that much power to hear this distance when nothing is
in the way). However, as balloons go, they are generally above you
and your antenna is pointed toward them, or nearly so (certainly more
so than at the horizon). This is NOT a favored direction for
communication and antennas when they are small (and a rubber duck, a
quarterwave, or a halfwave easily meet that dismal prospect).

To obtain overhead coverage for communication (and this presumes that
the balloon antenna is also vertical - which means it will suffer from
the same geometry); then you need an antenna that is at least one
wavelength long, or longer (1¼ wavelength would be a nice way to go).

Long antennas have more sensitivity in the direction along their
length.

Now, if your tracking pastime includes all angles from the horizon to
directly above, then you need to consider designs that are more
isotropic (and none of the suggestions offered by everyone, including
me, come anywhere close).

73's
Richard Clark, KB7QHC

Richard Clark wrote:

The standard rubber duck and a quarterwave are going to have the same
failings: they both have the same response curve in space. Basically
they both are designed to work with transmitters located at right
angles to them. And if you stand in the conventional upright manner
of a biped, then those antennas are going to be vertical with the
favored direction lying along the horizon.

Richard,
Thanks for the advise. However, I thought 1/4 wave antennas were more
isotropic than 'most' other antennas... (see link below)

http://www.astrosurf.com/lombry/Radio/dipole-height.gif

What am I missing?

Basically, all I'm trying to do is increase the sensitivity of my
handheld scanner for the 2m range.

Thanks,
Dave

Dave, I have seen you get some good information and some bad
information. The 1/4 wave will give you better results over the short
rubber duck.
There will be not much difference in the pattern between the original
antenna and the 1/4 wave. There will be an improvement in signal level
by about 10 dB. One poster said that you were in trouble because your
antenna was vertical. I am willing to bet you can turn your head and
look upward with the scanner turning upward as well.

Your radio is designed for a 50 Ohm antenna so half or more of the
information given by "Reg Edwards" is bogus as well. I don't think he
tried to mislead you, but may not have experience in real life experiments.
The case of the radio is important only in how it capacitively couples
with your hand or body.
A short vertical has an impedance far below a normal quarter wave. That
means your idea matches better to your radio.
As a sort of ground wire, a 1/4 wavelength long counterpoise, (as
someone said a rat tail) will also help.



Dave wrote:
Richard Clark wrote:


The standard rubber duck and a quarterwave are going to have the same
failings: they both have the same response curve in space. Basically
they both are designed to work with transmitters located at right
angles to them. And if you stand in the conventional upright manner
of a biped, then those antennas are going to be vertical with the
favored direction lying along the horizon.


Richard,
Thanks for the advise. However, I thought 1/4 wave antennas were more
isotropic than 'most' other antennas... (see link below)

http://www.astrosurf.com/lombry/Radio/dipole-height.gif

What am I missing?

Basically, all I'm trying to do is increase the sensitivity of my
handheld scanner for the 2m range.

Thanks,
Dave